Fukushima Accident 2011

Fukushima Accident 2011


Fukushima Accident 2011
(updated 2 November 2011)
Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, hence causing a nuclear accident on 11 March. All three cores largely melted in the first three days.
Concerns were also raised by used fuel storage at a fourth reactor losing water.
The accident was rated 7 on the INES scale, due to the high radioactive releases in the first few days. All four reactors are written off - 2719 MWe net.
After two weeks the three reactors (units 1-3) were stable with water addition but no proper heat sink for removal of decay heat from fuel. By July all were being cooled with recycled water from the new treatment plant. Achievement of 'cold shutdown' with new atmospheric heat exchangers is still not achieved, though temperatures had fallen to below 80°C at the end of October.
Apart from cooling, the basic ongoing task is to prevent release of radioactive materials, particularly in contaminated water leaked from the three units, with about 90,000 m3 still awaiting reatment.
There have been no deaths or radiatoin sickness from the nuclear accident.
The Great East Japan Earthquake* with magnitude 9.0 at 2.46 pm on Friday 11 March did considerable damage in the region, and the large tsunami it created caused very much more. The earthquake was centred 130 km offshore the city of Sendai in Miyagi prefecture on the eastern cost of Honshu Island (the main part of Japan), and was a rare and complex double quake giving a severe duration of about 3 minutes. Japan moved a few metres east and the local coastline subsided half a metre. The tsunami inundated about 560 sq km and resulted in a human death toll of over 20,000.
* originally, and more specifically, the Tohoku-Chihou-Taiheiyo-Oki Earthquake.
Eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the quake hit. The operating units which shut down were Tepco's Fukushima Daiichi 1, 2, 3, Fukushima Daini 1, 2, 3, 4, Tohoku's Onagawa 1, 2, 3, and Japco's Tokai, total 9377 MWe net. Fukushima Daiichi units 4-6 were not operating at the time, but were affected, total 2587 MWe net (units 4-6). Onagawa 1 briefly suffered a fire in the turbine building, but the main problem initially centred on Fukushima Daiichi units 1-3. Unit 4 became a problem on day five.
The reactors proved robust seismically, but vulnerable to the tsunami. Power, from grid or backup generators, was available to run the Residual Heat Removal (RHR) system cooling pumps at eight of the eleven units, and despite some problems they achieved 'cold shutdown' within about four days. The other three, at Fukushima Daiichi, lost power at 3.42 pm, almost an hour after the quake, when almost the entire site lost the ability to maintain proper reactor cooling and water circulation functions due to being flooded by the 15-metre tsunami. This disabled 12 of 13 back-up generators on site, located in the basements of the turbine buildings, and also the heat exchangers for dumping reactor heat to the sea. Electrical switchgear was also disabled.
Thereafter, many weeks of focused work centred on restoring heat removal from the reactors and coping with overheated spent fuel ponds. This was undertaken by hundreds of Tepco employees as well as some contractors, supported by firefighting and military personnel. Some of the Tepco staff had lost homes, and even families, in the tsunami, and were initially living in temporary accommodation under great difficulties and privation, with some personal risk. Media coverage of the Fukushima drama often ignored the context of the enormous natural disaster which greatly affected how it played out. A hardened emergency response centre on site proved very helpful in grappling with the situation.
Three Tepco employees at the Daiichi plant were killed directly by the earthquake and tsunami.
Among hundreds of aftershocks, an earthquake with magnitude 7.1, closer to Fukushima than the 11 March one, was experienced on 7 April, but without further damage to the plant. The epicenter was 120 km from Fukushima but only 20 km from Onagawa, where power supply was affected. On 11 April a magnitude 7.1 earthquake and on 12 April a magnitude 6.3 earthquake, both with epicenter at Fukushima-Hamadori, caused no further problems.
The following information remains provisional, especially regarding what is inside the reactor pressure vessels. Pressures are quoted in absolute terms, ie atmospheric (101 kPa) plus 'gauge' unless indicated otherwise (1 kPa = 0.145 psi).
Organisational acronyms:
Tepco = Tokyo Electric Power CompanyNISA = Nuclear & Industrial Safety Agency (Japan, regulator), NSC = Nuclear Safety Commission (Japan, policy body)METI = Ministry of Trade, Economy & Industry (Japan),IAEA = International Atomic Energy Agency (UN body),JAIF = Japan Atomic Industrial Forum (industry body),JNTI = Japan Nuclear Technology InstituteISRN = Institute for Radiological Protection & Nuclear Safety (France)
Site, earthquakes and tsunamis: background
The Daiichi (first) and Daini (second) Fukushima plants are sited about 11 km apart on the coast, Daini to the south.
Japanese nuclear power plants are designed to withstand specified earthquake intensities evident in ground motion. If they register ground acceleration of a set level, systems will be activated to automatically bring the plant to an immediate safe shutdown. In this case the set scram level was 135 Gal (150 Gal at Daini)*. The maximum response acceleration against design basis ground motion for both Fukushima plants had been upgraded since 2006, and is now quoted at horizontal 438-489 Gal for Daiichi and 415-434 Gal for Daini. At this level they must retain their safety functions. In 2008 Tepco upgraded its estimates of likely Design Basis Earthquake Ground Motion Ss for Fukushima to 600 Gal, and other Japanese operators have adopted the same figure.
* 1 Gal = 1 cm/sec2 , 980 Gal = gravity.
The recorded data for both plants some 180 km from the epicentre shows that 550 Gal (0.56 g) was the maximum ground acceleration for Daiichi, in the foundation of unit 2 (other figures 281-548 Gal), and 254 Gal was maximum for Daini. Units 2, 3 and 5 exceeded their maximum response acceleration design basis in E-W direction by about 20%. Recording was over 130-150 seconds. All nuclear plants in Japan are built on rock (ground acceleration was around 2000 Gal a few kilometres north, on sediments).
The design basis tsunami height was 5.7 m for Daiichi and 5.2 m for Daini, though the Daiichi plant was built about 10 metres above sea level and Daini 13 metres above. Tsunami heights coming ashore were more than 14 metres for both plants, and the Daiichi turbine halls were under some 5 metres of seawater until levels subsided. The maximum amplitude of this tsunami was 23 metres at point of origin, about 180 km from Fukushima. In the last century there have been eight tsunamis in the region with maximum amplitudes at origin above 10 metres (some much more), these having arisen from earthquakes of magnitude 7.7 to 8.4, on average one every 12 years. Those in 1983 and in 1993 were the most recent affecting Japan, with maximum heights at origin of 14.5 metres and 31 metres respectively, both induced by magnitude 7.7 earthquakes. It is not clear why the design basis tsunami height for the two plants was set so low in the 1960s.
A Japanese government's Earthquake Research Committee had put together a report on earthquakes and tsunamis off the Pacific coastline of northeastern Japan in February and was planning to release it in April. The document includes the committee's analysis of a magnitude 8.3 earthquake that is known to have struck the region more than 1140 years ago. This was apparently caused when three sections of the seabed shifted simultaneously, triggering enormous tsunamis that flooded vast areas of Miyagi and Fukushima prefectures. The report concludes that the region should be alerted of the risk of a similar disaster striking again. The 11 March earthquake measured magnitude 9.0 and involved substantial shifting of multiple sections of seabed over a source area of 200 x 400 km. Tsunami waves devastated wide areas of Miyagi, Iwate and Fukushima prefectures.

Fukushima Daiichi 1-3 & 4
It appears that no serious damage was done to reactors 1-3 by the earthquake, but they were automatically shut down in response to it, as designed, since the ground acceleration was about 500 Gal. At the same time all six external power supply sources were lost due to earthquake damage, so the emergency diesel generators located in the basements of the turbine buildings started up. Initially cooling would have been maintained through the main steam circuit bypassing the turbine and going through the condensers. Then 41 minutes later the first tsunami wave hit, followed by a second 8 minutes later. These submerged and damaged the seawater pumps for both the main condenser circuits and the auxiliary cooling circuits, notably the Residual Heat Removal (RHR) cooling system. They also drowned the diesel generators and inundated the electrical switchgear and batteries, all located in the basements of the turbine buildings (the one surviving air-cooled generator was serving units 5 & 6). So there was a station blackout, and the reactors were isolated from their ultimate heat sink. The tsunamis also damaged and obstructed roads, making outside access difficult.
All this put those reactors 1-3 in a dire situation and led the authorities to order, and subsequently extend, an evacuation while engineers worked to restore power and cooling. About nine hours later mobile power supply units had reached the plant and were being connected. The capacity of these would be less than the main plant diesel system, and in any case the main electrical switchgear had been drowned. The 125 volt DC batteries for units 1 & 2 were flooded and failed, leaving them without instrumentation, control or lighting. Unit 3 did have battery power, but it was insufficient to drive the main RHR cooling system, and these batteries were apparently depleted in about 30 hours.
At 7.03 pm Friday a Nuclear Emergency was declared, and at 8.50pm the Fukushima Prefecture issued an evacuation order for people within 2 km of the plant. At 9.23 pm the Prime Minister extended this to 3 km, and at 5.44 am on 12th he extended it to 10 km. He visited the plant soon after. At 6.25 pm on Saturday 12th he extended the evacuation zone to 20 km.
Reactors: background
The Fukushima Daiichi reactors are GE boiling water reactors (BWR) of an early (1960s) design supplied by GE, Toshiba and Hitachi, with what is known as a Mark I containment. Reactors 1-3 came into commercial operation 1971-75. Reactor power is 460 MWe for unit 1, 784 MWe for units 2-5, and 1100 MWe for unit 6. The fuel assemblies are about 4 m long, and there are 400 in unit 1, 548 in units 2-5, and 764 in unit 6. Each assembly has 60 fuel rods containing the uranium oxide fuel within zirconium alloy cladding. Unit 3 has a partial core of mixed-oxide (MOX) fuel (32 MOX assemblies, 516 LEU). They all operate normally at 286°C at core outlet under a pressure of 6930 kPa and with 115-130 kPa pressure in dry containment. The operating pressure is about half that in a PWR. NISA says maximum design base pressure for reactor pressure vessels (RPV) is 8240 kPa at 300°C, and for containment (PCV) is about 500 kPa*. At low containment pressures hydrogen and other gases are routinely vented through charcoal filters which trap most radionuclides.
* NISA gives 430 kPa for unit 1 and 380 kPa for 2-3 at 140°C as 'maximum', apparently gauge pressure, so add 101 for absolute: 530 and 480 kPa. Before venting, unit 1 RPV got to 900 kPa and PCV to 850 kPa early on 12th.
The BWR Mark I has a Primary Containment system comprising a free-standing bulb-shaped drywell of 30 mm steel backed by a reinforced concrete shell, and connected to a torus-shaped wetwell beneath it containing the suppression pool (with 3000 m3 of water in units 2-5). The drywell, also known as the Primary Containment Vessel (PCV), contains the reactor pressure vessel (RPV). For simplicity, we will use the term 'dry containment' here. The water in the suppression pool acts as an energy-absorbing medium in the event of an accident. The wetwell is connected to the dry containment by a system of vents, which discharge under the suppression pool water in the event of high pressure in the dry containment. The function of the primary containment system is to contain the energy released during any loss-of-coolant accident (LOCA) of any size reactor coolant pipe, and to protect the reactor from external assaults. The Japanese version of the Mark I is slightly larger than the original GE version.
During normal operation, the dry containment atmosphere and the wetwell atmosphere are filled with inert nitrogen, and the wetwell water is at ambient temperature. A small amount of hydrogen is routinely formed by radiolytic decay of water, and this is normally dealt with by recombiners in the containment vessel. They would be insufficient for countering major hydrogen formation due to oxidation of zirconium fuel cladding.
If a loss of coolant accident (LOCA) occurs, steam flows from the dry containment (drywell) through a set of vent lines and pipes into the suppression pool, where the steam is condensed. Steam can also be released from the reactor vessel through the safety relief valves and associated piping directly into the suppression pool. Steam will be condensed in the wetwell, but hydrogen and noble gases are not condensable and will pressurise the system, as will steam if the wetwell water is boiling. In this case emergency systems will activate to cool the wetwell, see below. Excess pressure from the wetwell (above 300 kPa) can be vented through the 120 m emission stack via a hardened pipe or into the secondary containment above the reactor service floor of the building. If there has been fuel damage, vented gases will include noble gases (krypton & xenon), iodine and caesium, the latter being scrubbed in some scenarios. Less volatile elements in any fission product release will plate out in the containment. (The later Mark II containments are similar to Mark I, but both are much smaller than the Mark III and those which became standard in PWRs.)
The secondary containment houses the emergency core cooling systems and the spent/ used fuel pool. It is not designed to contain high pressure.

The primary cooling circuit of the BWR takes steam from above the core, in the reactor pressure vessel, to the turbine in an adjacent building. After driving the turbines it is condensed and the water is returned to the pressure vessel by powerful steam-driven pumps. There are also two powerful jet-pump recirculation systems forcing water down around the reactor core and shroud. When the reactor is shut down, the steam in the main circuit is diverted via a bypass line directly to the condensers, and the heat is dumped there, to the sea. In both situations a steam-driven turbine drives the pumps, at least until the pressure drops to about 450 kPa (50 psig), but condenser function depends on large electrically-driven pumps for the seawater which are not backed up by the diesel generators.
In shutdown mode at low pressure, the Residual Heat Removal (RHR) system then operates in a secondary circuit (RHR is connected into the two jet-pump recirculation circuits), driven by smaller electric pumps, and circulates water from the pressure vessel to RHR heat exchangers which dump the heat to the sea, using external electric pumps in the secondary circuit. This RHR system is fully supported by the diesel generators. Unit 1 had an isolation condenser (IC) for passive core cooling, with reactor steam going to an external condenser, and it needed only DC battery power to operate. Units 2-5 have a Reactor Core Isolation Cooling (RCIC) actuated automatically which can provide make-up water to the reactor vessel (without any heat removal circuit). It is driven by a small steam turbine using steam from decay heat, injecting water from a condensate storage tank or the suppression pool and controlled by the DC battery system. The RCIC systems played a helpful role in units 2 & 3 until the suppression pool water boiled, to 11am on 12th in unit 3, and to 2pm on 14th in unit 2.
Then there is an Emergency Core Cooling System (ECCS) as further back-up for loss of coolant. It has high-pressure and low-pressure elements. The high pressure coolant injection (HPCI) system in units 1-3 has pumps powered by steam turbines which are deigned to work over a wide pressure range. The HPCI draws water from the large torus suppression chamber beneath the reactor as well as a water storage tank, and requires only DC battery power. For use below about 700 kPa, there is also a Low-Pressure Coolant Injection (LPCI) mode through the RHR system but utilising suppression pool water, and a core spray system, all electrically-driven. All ECCS sub-systems require some power to operate valves etc, and the battery back-up to generators may provide this.
Beyond these original systems, Tepco in 1990s installed provision for water injection via the fire extinguisher system through the RHR system (injecting via the jet-pump nozzles) as part of it Severe Accident Management (SAM) countermeasures. Air-cooled diesel generators were installed at Daiichi 2, 4 & 6 - the last being the only one to survive the tsunami.
The Fukushima reactors have much of their switchgear on the ground floor in the turbine buildings rather than elevated, as at some similar US plants. Also they have control rooms with analogue instrumentation typical of the period, so not only did many instruments fail, but data could not be downloaded and accessed remotely to assist diagnosis and remedial action.

A frequently-voiced concern during the first week of the accident was regarding fuel meltdown. This would start to occur if the fuel itself reached 2500°C (or more, up to 2800°C, depending on make-up). At this point, the fuel rods slump within the assemblies. Conceivably, the “corium” (a mixture of molten cladding, fuel, and structural steel) drops to the bottom of the reactor vessel. If the hot fuel or cladding is exposed to cooling water en route, it may solidify and fracture, falling to the bottom of the reactor vessel. Given that the melting point of the steel reactor vessel is about 1500°C, there is an obvious possibility on the corium penetrating the steel if it remains hot enough.* In any case the BWR pressure vessels have numerous penetrations at the bottom for control rods and instrumentation, so any corium, to the extent it remained molten, would shower into the bottom of the drywell containment. The whole fuel melt scenario is much more probable with a sudden major loss of coolant when the reactor is at full power than in the Fukushima situation, at least beyond the first few days. Before fuel melting, cladding cracks at about 1200°C, its oxidation begins at about 1300°C (releasing hydrogen) and the zirconium cladding melts at about 1850°C and reacts with uranium oxide to form a molten eutectic, which would release volatile fission products such as caesium. These temperatures are quite possible days after shutdown in the absence of cooling.
* In the 1979 US Three Mile Island accident, the corium didn't breach the pressure vessel, though about half the core melted and it went 15 mm into the 225 mm thick pressure vessel steel. The pressure vessel glowed red-hot for an hour.
Oxidation of the zirconium cladding in the presence of steam produces hydrogen exothermically, with 5.8 MJ/kg of Zr from this exacerbating the fuel decay heat problem. There is some potential for this to become self-sustaining at high temperatures, giving rise to a zirconium cladding fire with a burn front along the axis of the fuel rods. Such a fire is possible in a spent fuel pond following major loss of coolant from leakage or boiling.

When the power failed at 3.42 pm, about one hour after shutdown, the reactor cores would still be producing about 1.5% of their nominal thermal power, from fission product decay - about 22 MW in unit 1 and 33 MW in units 2 & 3, according to calculations from well-established models, though the exact amount depends on average burn-up in each core. Without heat removal by circulation to an outside heat exchanger, this produced a lot of steam in the reactor pressure vessels housing the cores, and this was released into the dry containment (PCV) through safety valves, accompanied by hydrogen, produced by the exothermic interaction of the fuel's very hot zirconium cladding with steam after the water level dropped. As pressure rose here, the steam was directed into the suppression chamber under the reactor, within the containment, but the internal temperature and pressure rose quite rapidly. Water injection using the IC or RCIC and Emergency Core Cooling System (ECCS), commenced. The IC system in unit 1 failed apparently late on day one, the RCIC system in unit 2 ran for about three days, that in unit 3 failed after 19 hours but the HPCI system then started automatically due to low water level and it ran for 14 hours.After the built-in systems failed, water injection to the RPVs was with fire pumps, but this required the internal pressures to be relieved by venting into the suppression chamber/ wetwell. Water injection was apparently via the Low Pressure Coolant Injection (LPCI) system, connected into the Residual Heat Removal (RHR) system. On 25-26 March, water from a nearby dam started to be used instead of seawater. Boron was added to the injected water, to guard against any criticality.
Inside unit 1, it is now provisionally understood that the water level dropped to the top of the fuel about three hours after the scram (6 pm) and the bottom of the fuel 1.5 hours later (7.30 pm). The temperature of the exposed fuel rode to some 2800°C so that the central part started to melt and by 16 hours after the scram (7 am Saturday) most of it had fallen into the water at the bottom of the RPV. Since then RPV temperatures have decreased, and since early April have been under 120°C. NISA estimates put the time scale shorter, with core melt by 8pm Friday.
In unit 1 RPV pressure at 8 pm on the Friday was 6900 kPa - normal operating pressure. On Saturday 12th at 2.45 am it had come down to about 800 kPa, with similar in the PCV - twice the PCV design level (compared with 'maximum' 430 kPa gauge pressure), and 300°C in PCV instead of the designed 138°C maximum). Attempts were made to vent the containment, and when external power and compressed air sources were harnessed this was successful, by about 2.30 pm. The venting was designed to be through an external stack, but in the absence of power much of it backflowed to the service floor at the top of the reactor building. The vented steam, noble gases and aerosols were accompanied by hydrogen, produced by the exothermic interaction of the fuel's very hot zirconium cladding with steam and water. At 3.36 pm on 12th, there was a hydrogen explosion on the service floor of the building above unit 1 reactor containment, blowing off the roof and cladding on the top part of the building, after vented hydrogen mixed with air and ignited.
Following the unit 1 meltdown (not then recognized), fresh water was injected into the reactor pressure vessels (RPV) using fire pumps, starting from 5.46 am on 12th. About 80 m3 was injected into unit 1 RPV over 9 hours until the supply ran out. Seawater was then substituted, from about 7 pm.
It is now understood that much of the fuel in units 2 & 3 also melted to some degree, but to a lesser extent than in unit 1, and a day or two later. In mid May the unit 1 core would still be producing 1.8 MW of heat, and most of it - as corium comprised of melted fuel and control rods - is assumed to be in the bottom of the RPV. The heat output from fuel in units 2 & 3 will depend on the average burn-up of the fuel assemblies, but is likely greater (see graph below).
In unit 2, as mentioned above, water injection using the steam-driven Reactor Core Isolation Cooling (RCIC) system failed on 14th, and it was about six hours before a fire pump started injecting seawater into the RPV - about 8 pm. Before the fire pump could be used RPV pressure had to be relieved via the wetwell, which required power and nitrogen, hence the delay. On 14th, the reactor water level dropped rapidly after RCIC cooling was lost, so that core damage started about 8 pm, and it is now provisionally understood that much of the fuel then melted and probably fell into the water at the bottom of the RPV. NISA similarly estimates that much of the fuel had melted about 100 hours after the scram. Pressure was vented on 13th and again on 15th, and meanwhile the blowout panel near the top of the building was opened to avoid a repetition of unit 1 hydrogen explosion. Early on Tuesday 15th, the pressure suppression chamber under the actual reactor apparently ruptured, evidently due to a hydrogen explosion there, releasing significant radioactivity and dropping the drywell containment pressure inside. Later analysis suggested that a leak of the PCV developed about midday 12th, 21 hours after the quake.
In unit 3, the RCIC system failed at 11 am on Saturday 12th and venting the containment was done late in the day. On Sunday 13th, at 5.10 am, water injection using the high pressure coolant injection (HPCI) system failed and water levels dropped dramatically. RPV pressure was reduced by venting steam into the wetwell, allowing injection of seawater using a fire pump from just before noon. At 9.20 am venting the suppression chamber and containment was successfully undertaken. It is now provisionally understood that core damage started about 9 am and much or all of the fuel melted on the morning of Sunday 13th and possibly fell into the water at the bottom of the RPV, or was retained on the core support plate within the shroud, and NISA agrees this timing.
On Monday 14th at 5.20 am PCV venting was repeated, though both RPV and drywell pressure remained at about 500 kPa. The venting evidently backflowed to the service floor of the building, then at 11.01 am a very large hydrogen explosion here above unit 3 reactor containment blew off much of the roof and walls and demolished the top part of the building. This explosion created a lot of debris, and that on the ground near unit 3 was extremely radioactive. The extent of RPV and containment damage is uncertain.
Water has been injected into each of the three reactor units more or less continuously, and in the absence of normal heat removal via external heat exchanger this water has been boiling off. In the government report to IAEA in June it was estimated that to the end of May about 40% of the injected water boiled off, and 60% leaked out the bottom. In June this was adding to the contaminated water on site by about 500 m3 per day.
In unit 4, at about 6 am on Tuesday 15 March, there was an explosion which destroyed the top of the building and damaged unit 3's superstructure further. This was apparently from hydrogen arising in unit 3 and reaching unit 4 by backflow in shared ducts when vented from unit 3. This interpretation was confirmed in August by measurement of radiation from filters in the standby gas treatment system between the two units.
Due to volatile and easily-airborne fission products being carried with the hydrogen and steam, these explosions discharged a lot of radioactive material into the atmosphere, notably iodine and caesium. NISA said in June that it estimated that 800-1000 kg of hydrogen had been produced in each of the units.
In mid May the unit 1 core would still be producing 1.8 MW of heat, and most of it - as corium comprised of melted fuel and control rods - is assumed to be in the RPV, though some has possibly gone further into the bottom of the containment drywell through the numerous penetrations. The fuel and corium was evidently being cooled satisfactorily through water injection, since the RPV temperature (under pressure) was 100-120°C. The 85 m3 per day of water is evidently both evaporating and leaking into the bottom of the building. Nitrogen is being injected into the containment vessel. There was much the same picture in the larger units 2 & 3, with temperature ranging up to 130°C. All three PCV pressures are atmospheric, according to JNTI. Nitrogen injection to unit 2 started at the end of June, and for reactor 3 in mid July. This removed concerns about possible hydrogen explosions. In October a gas control system was commissioned for unit 2. This extracts and cleans the gas from the PCV to avoid leakage of caesium. Similar units are envisaged for units 1 and 3 by year end.
The heat from the fuel in the reactor cores was estimated by France's ISRN as 2.5 MW thermal in unit 1 and 4.2 MW in units 2 & 3 after almost three weeks from 11 March. Tepco's estimate is 1.5 to 1.8 MW in unit 1 at 8 weeks. The IRSN estimates represent enough heat to boil away 95 cubic metres and 160 m3 of water per day respectively if all external circulation to heat sinks ceases, indicating the need for constant top up while the heat is not being dumped in the normal or replacement heat exchanger circuits. From 30 March, 3 to 11 m3/hour has been injected into each reactor. The heat production levels are diminishing slowly. Tepco intends to rig up cooling circulation for each reactor to an atmospheric heat exchanger, similar to those being used for the fuel ponds. Work has started for unit 1. Meanwhile injection into the RPVs of water circulated through the new water treatment plant is achieving relatively 'stable cooling', and temperatures at the bottom of the reactor pressure vessels have decreased and were stable in the range 60-76°C at the end of October. Pressures ranged from atmospheric to slightly above (100-125 kPa), due to water and nitrogen injection.
Tepco is injecting water using the Core Spray lines in addition to the current Feed Water System at units 2 & 3, where relevant valves have been confirmed as operable. The core spray system delivers water inside the shroud, and has been more effective in cooling.
Since 20 March JAIF has said that the containment vessel of unit 2 is thought to be damaged, and that of unit 3 could possibly be also, but unit 1 and 4 were intact. (Unit 4 has no fuel in it.). At the end of August workers were able to enter the containment of unit 4 to check damage and radiation levels.

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The AC electricity supply from external source was connected to all units by 22 March. Power was restored to instrumentation in all units except unit 3 by 25 March. However, radiation levels inside the plant were so high that normal access was impossible until June.
Tepco has said that the three reactors, with unit 4, will be written off and decommissioned.
Accident sequence following earthquake

Unit 1
Unit 2
Unit 3
Loss of AC power
+ 51 min
+ 51 min
+ 51 min
Loss of cooling
+ 1 hour
+ 70 hours
+ 36 hours
Water level down to top of fuel
+ 3 hours
+ 74 hours
+ 37 hours
Core damage starts
+ 5 hours
+ 87 hours
+ 62 hours
Fire pumps with fresh water
+ 15 hours

+ 42 hours
Hydrogen explosion
+ 24 hoursservice floor
+ 87 hourssuppression chamber
+ 68 hoursservice floor
Fire pumps with seawater
+ 28 hours
+ 78 hours
+ 46 hours
Off-site electrical supply
+ 10 days
Fresh water cooling
+ 12-13 days
source: Tony Irwin
A levee has been built on the seaward side of the plant to protect it against further tsunamis up to 10 metres high.
Further details of the reactors are in an Appendix, but the September METI report to IAEA will be more precise and authoritative, even in its Summary.
Summary to date: Major fuel melting occurred early on in all three units, though the fuel remains essentially contained except for some volatile fission products vented early on, or released from unit 2 in mid March, and some soluble ones which are leaking with the water, especially from unit 2, where the containment is evidently breached. Cooling still needs to be provided from external sources, now using treated recycled water and relying on some evaporation, while work continues to establish a stable heat removal path from the actual reactors to external heat sinks. Temperatures at the bottom of the reactor pressure vessels have decreased and are stable. Access has been gained to all three reactor buildings, but dose rates remain high inside. Nitrogen is being injected into all three containment vessels.
Fuel ponds: background
Used fuel needs to be cooled and shielded. This is initially by water, in ponds. After about three years under water, used fuel can be transferred to dry storage, with air ventilation simply by convection. Used fuel generates heat, so the water is circulated by electric pumps through external heat exchangers, so that the heat is dumped and a low temperature maintained.
There are fuel ponds near the top of all six reactor buildings at the plant, adjacent to the top of each reactor so that the fuel can be unloaded under water, when the top is off the reactor pressure vessel and it is flooded. The ponds hold some fresh fuel and some used fuel, pending its transfer to the central used/spent fuel storage on site. (There is some dry storage on site to extend the plant's capacity.)
From the plant the used fuel is intended to be shipped periodically for recycling. Tepco and JAPC are building a Recyclable Fuel Storage Centre in Mutsu, operating from mid 2012 with 5000 t capacity. The JPY 100 billion facility will provide interim storage for up to 50 years before used fuel is reprocessed at Rokkasho. NISA approved this in August 2010. Until it is finished and operational in 2012 there has been a build-up of used fuel at reactor sites. The Rokkasho plant has been much delayed, and is now expected in commercial operation in October 2012. There is some storage capacity there, though this may be full.
At the time of the accident, in addition to a large number of used fuel assemblies, unit 4's pond also held a full core load of 548 fuel assemblies while the reactor was undergoing maintenance, these having been removed at the end of November.
The temperature of these ponds is normally low, around 30°C when circulation is maintained with the Fuel Pool Circulation and Clean-up (FCP) system, but they are designed to be safe at about 85°C in the absence of pumped circulation (and presumably with moderate fuel load). They are about 12 metres deep, so the fuel is normally covered by 7 metres of water.
Unit 2, 3 & 4 ponds are about 12 x 10 metres, with 1240, 1220 and 1590 assemblies capacity respectively (unit 1 is about 12 x 7 m, 900 assemblies). Unit 4 pond contains a total 1331 used assemblies (783 plus full fuel load of 548), giving it a heat load of about 3 MW thermal, according to France's IRSN, which in that case could lead to 115 cubic metres of water boiling off per day, or about one tenth of its volume. Other estimates put the heat load at 2 MW. Unit 3's pool contains 514 fuel assemblies, unit 1 has 292 and unit 2 has 587, giving it a heat load of 1 MW. There is no MOX fuel in any of the ponds.
Two of the reactor unit ponds (2 & 4) were unusually full even before unit 4 core was unloaded in November, since there was little spare space (only for 465 assemblies) in the central fuel storage pond on site. Thus there was a lot more fuel in the reactor ponds with correspondingly high heat loads and cooling requirements than might have been the case.
Moving the used fuel from reactor ponds to central storage involves loading it under water into casks which are lowered down and trucked the short distance (see RH side of cutaway diagram above). It requires access from the service floor and the use of cranes which were damaged in the hydrogen explosions.
The central fuel storage on site near unit 4 has a pond about 12 x 29 metres, 11 m deep, with capacity of 3828 m3 and able to hold 6840 fuel assemblies. Its building is about 55 x 73 m. (There are 6375 assemblies in the undamaged central pool storage on site, with very low decay heat, and 408 in dry cask storage - utilized since 1995 for used fuel no longer needing much cooling.)
See also comments in Reactor Background above regarding oxidation of zirconium cladding. A further concern raised during the accident was regarding criticality in the spent fuel ponds. Studies of safety and security of spent fuel storage have noted this possibility but not analysed it, pointing out that no previous criticality accidents have resulted in significant radioactive releases outside the plants, since the criticality itself immediately disperses the source material.

Fuel ponds: developing problems
A separate set of problems arose as the fuel ponds, holding fresh and used fuel, in the upper part of the reactor structures were found to be depleted in water. The primary cause of the low water levels was loss of cooling circulation to external heat exchangers, leading to elevated temperatures and probably boiling, especially in heavily-loaded unit 4. On 13 March Tepco said it was consulting with NISA and METI regarding cooling of the ponds
In unit 4, at about 6 am on Tuesday 15 March, there was an explosion in the top part of the building, near the fuel pond, which destroyed the top of the building and damaged unit 3's superstructure further. This was apparently from hydrogen arising in unit 3 and by backflow in shared ducts when vented from unit 3. Then there was a fire and soon after the radiation level near the building reached 400 mSv/hr, apparently from this source. The fire was extinguished in three hours. At 10 pm on 15th Tepco was told to implement injection of water to unit 4 pond. Some leakage may have been caused by the earthquake. This pond had a particularly high heat load (3 MW) from 1331 used fuel assemblies in it, so needed the addition of about 100 m3/day to replenish it after circulation ceased.
The focus from Tuesday 15 March was on replenishing the water in the ponds of units 1, 2, 3 and 4. Initially this was attempted with fire pumps but from 22 March a concrete pump with 58-metre boom enabled more precise targeting of water through the damaged walls of the service floors. There was some use of built-in plumbing for unit 2. Analysis of radionuclides in water from the used fuel ponds suggested that some of the fuel assemblies may be damaged, but the majority are intact. This was confirmed in mid August, though Cs-137 levels were up to 110 MBq/L, and Cs-134 similar.
The pond at unit 4 has been the main focus of concern due to its high heat load. It needs continual top-up with water, and over 100 m3 of water was being added daily, but at the same time there is concern about the structural strength of the building, which had been weakened either by the earthquake or the hydrogen explosion. The fuel pond when full of water is a mass of some 2000 tonnes high up in this, supported by the thick reinforced concrete containment and the outside wall of the building, which is considerably damaged. Structural support for the pond was reinforced by the end of July. Temperature had been typically more than 80ºC, with water level 5 metres down. It seems that the main water loss was from evaporation rather than leakage.
New cooling circuits with heat exchangers adjacent to the reactor buildings for all four ponds have been commissioned. They reduced the pool temperature from 70°C to normal in a few days. Each has a primary circuit within the reactor and waste treatment buildings and a secondary circuit dumping heat through a small dry cooling tower outside the building. Temperatures are now normal, in the range 22-26°C for ponds 1-3 and 31ºC for pond 4. Unit 4 posed a particular challenge due to plumbing damage.
The next task is to remove the salt from those ponds which had seawater added. Tepco is installing desalination equipment first at pond 4, and will follow this with 2 & 3.
The central spent fuel pool holds about 60% of the used fuel on site, and is immediately west (inland) of unit 4. It lost circulation with the power outage, and temperature increased to 73°C by the time mains power and cooling were restored on 24 March. The pool was topped up on 21st.
Summary to date: The new cooling circuits with external heat exchangers for the four ponds are working well. Temperatures are all below 40°C. Analysis of water in mid August confirmed that most fuel rods are intact. The unit 4 building supporting its pond has been reinforced.
Radioactivity
Radioactive releases are measured by the amount of (radio)activity in the material, and quoted in Becquerels. Whether this is in the air or settled on the ground, it may expose people to ionizing radiation, and the effect of this is measured in Sieverts, or more typically milliSieverts (mSv). Exposure to ionizing radiation can also be by direct radiation from the plants and fuels themselves, though not released to the environment. This is only a hazard for those on the plant site, and the level diminishes with distance from the radioactive source. It is the chief hazard for the plant workers, who wear film badges so that the dose can be monitored. A short-term dose of 1000 mSv is about the threshold of acute radiation syndrome (sickness). An instant dose of 100-250 mSv can slightly increase the risk of later developing cancer, but if this dose is spread over time there is less risk of any effect. On 17 March NISA set 250 mSv as the maximum allowable dose for Fukushima recovery workers, under health physics controls. At the end of October this was reduced to 100 mSv for new workers. The International Commission on Radiological Protection (ICRP) allows up to 500 mSv for workers in emergency rescue operations.
Radioactivity in the cooling water flowing through the core is mainly the activation product nitrogen-16, formed by neutron capture from oxygen. N-16 has a half-life on only 7 seconds but produces high-energy gamma radiation during decay. (It is the reason that access to a BWR turbine hall is restricted during actual operation.) There is also often some leakage from fuel elements of fission products, including noble gases and iodine-131.
Regarding releases to air and water leakage from Fukushima, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine-131, which has a half-life of 8 days. Iodine-131 decays to inert and stable xenon-131. It is readily taken up by the body and accumulates in the thyroid gland. Three months after the accident (after fission ceased) I-131 has virtually disappeared as a problem.
The other main radionuclide is caesium-137, which has a 30-year half-life, is easily carried in a plume, and when it lands it may contaminate land for some time. It is a strong gamma-emitter in its decay. Cs-134 is also produced and dispersed, it has a 2-year half-life. Caesium is soluble and can be taken into the body, but does not concentrate in any particular organs, and has a biological half-life of about 70 days. In assessing the significance of atmospheric releases, the Cs-137 figure is multiplied by 40 and added to the I-131 number to give an "iodine-131 equivalent" figure.
Radioactive releases to air
As cooling failed on the first day, evacuations were progressively ordered. By the evening of 12 March the evacuation zone had been extended to 20 km. Since then, those resident 3-20 km from the plant have been allowed to return home for brief visits, and those within 3 km are now being allowed to do so. The government is to begin detailed radiation monitoring in the evacuation area to ensure the safety of those returning. Permanent return of most evacuees is envisaged early in 2012. From 20 to 30 km from the plant, the criterion of 20 mSv/yr dose rate was applied to determine evacuation. 20 mSv/yr was also the general limit set for children's dose rate related to outdoor activities, but there are calls to reduce this.
After the hydrogen explosion in unit 1 on 12 March, some radioactive caesium and iodine were detected in the vicinity of the plant, indicating fuel damage. This material had been released via the venting. Further I-131 and Cs-137 and Cs-134 were apparently released during the following few days, particularly following the explosion at unit 3 on 14th and the apparent rupture of suppression chamber of unit 2 on 15th. Considerable amounts of xenon-133 and iodine-131 were vented, but most of the caesium-137 (14 out of 15 PBq total) along with most of the Cs-134 apparently came from unit 2 on or after the 15th. Also ten times more iodine is attributed to unit 2 than unit 1, while unit 3 produced half as much as unit 1. However, there remains some uncertainty about the exact sources and timings of the radioactive releases.
On 16 March, Japan’s Nuclear Safety Commission recommended local authorities to instruct evacuees under 40 years of age leaving the 20 km zone to ingest stable iodine as a precaution against ingestion (eg via milk) of radioactive iodine-131. The pills and syrup (for children) had been pre-positioned at evacuation centers. The order recommended taking a single dose, with an amount dependent on age. On 11 April the government suggested that those outside the 20km zone who were likely to accumulate 20 mSv dose should move out within a month. Data at the end of May (with most I-131 gone) showed that about half of the 20 km evacuation zone and a similar area to the northwest, total about 1000 sq km, would give an annual dose of 20 mSv to March 2012.
France's IRSN estimated that maximum external doses to people living around the plant were unlikely to exceed 30 mSv/yr in the first year. This is based on airborne measurements between 30 March and 4 April, and appears to be confirmed by the above figures. It compares with natural background levels mostly 2-3 mSv/yr, but ranging up to 50 mSv/yr.
The main concentration of radioactive pollution stretches northwest from the plant, and levels of Cs-137 reached over 3 MBq/m2 in soil here, out to 35km away. The Cabinet Secretary said that the 84,000 displaced residents will not be able to return home until all three units are in cold shutdown, now expected by year end. On 21 April the government said that "any residents who chose to remain in the zone would be required to leave because the damaged reactors have not been stabilized," and it apparently perceived an ongoing risk from them. In mid May about 15,000 residents in a contaminated area 20-40 km northwest of the plant were evacuated, making a total of about 100,000 displaced persons.
The venting and unit 2 containment rupture pushed the on-site dose rates from about 1 mSv/day to about one thousand times that, though by the end of May the caesium was at low levels (about two orders of magnitude less than the iodine). Gamma radiation on site close to the reactors decreased greatly when unit 3 fuel pond was replenished with water on 19th. On Sunday 20th, levels were mostly below 3 mSv/hr and on 21st were nearly down to 2 mSv/hr about 500 m north on unit 3.
The IAEA reported on 19 March that airborne radiation levels had spiked three times since the earthquake, notably early on 15th (400 mSv/hr near unit 3), but had stabilized since 16th at levels significantly higher than the normal levels, but within the range that allows workers to continue on-site recovery measures. For instance, NISA reported 12 mSv/hr dose rate on the site boundary early on 14th, then 3.4 mSv/hr mid 16th, dropping to 0.65 mSv/hr 13 hours later at the same point. Late on 24th it was about 0.2 mSv/hr at the front gate, having been ten times that a few days earlier. Steady decrease has been maintained - on 4 April it was 0.12 mSv/hr at the front gate and 0.05 mSv/hr at the west gate. On 17 April dose rates at eight monitoring points around the boundary ranged from 0.01 at the north end to 0.19 mSv/hr at the south.
Tepco said on 13 April that it estimated the total inventory upon shutdown as 81 EBq of I-131, of which 130 PBq is estimated by NISA to have been released from the reactors, mostly around 15 March and the two days following - 0.16% of total. In 32 days this released iodine would have diminished to one sixteenth of original activity - 8 PBq, while Tepco estimated that 440 PBq remained inside. NISA's report to IAEA said 130 PBq of I-131 and 6 PBq of caesium-137 had been actually released, giving an "iodine-131 equivalent" figure of 370 PBq, a figure which resulted in the re-rating of the accident to INES level 7. NISA in June increased this estimate to 770 PBq, being 160 PBq of I-131 and 15 PBq of Cs-137.* The NSC estimated that 12 PBq of Cs-137 had been released, giving an "iodine-131 equivalent" figure of 630 PBq to 5 April, but in August lowered this estimate to 570 PBq. The 770 PBq figure is about 15% of the Chernobyl figure of 5.2 EBq iodine-131 equivalent.This is about ten percent of the Chernobyl figure. The NSC said that most radioactive material was released from the unit 2 suppression chamber during two days from its apparent rupture early on 15 March, and the later estimate put the peak between 1pm and 5pm on 15 March. It said that about 154 TBq/day was being released on 5 April, but that this had dropped to about 24 TBq/d over three weeks to 26 April and to about 24 GBq/d in mid July. In mid August the estimate from all three reactors together was about 5 GBq/d.
* The Cs-137 figure is multiplied by 40 in arriving at an "iodine-131 equivalent" figure, due to its much longer half-life.
Tepco sprayed a dust-suppressing polymer resin around the plant to ensure that fallout from mid March was not mobilized by wind or rain. By mid July some 40 hectares had been covered. Tepco says that radiation levels around the plant site remain relatively low. In addition it has removed a lot of rubble with remote control front-end loaders, and this further reduced ambient radiation levels, halving them near unit 1. The highest radiation levels on site came from debris left on the ground after the explosions at units 3 & 4. Some rubble beside unit 3 was giving the highest dose rate of some 1000 mSv/hr, while other debris patches are at 30-40 mSv/hr. Some 500 skips of rubble, each about 5.6 m3 have been removed, allowing much improved access around the plant. Dose rate at the plant boundary from current releases was 1.7 mSv/yr in mid July.
Radioactivity, primarily from caesium-137, in the evacuation zone and other areas beyond it has been reported in terms of kBq/kg (compared with kBq/m2 around Chernobyl). However the main measure has been presumed doses in mSv/yr. The government appears to have adopted 20 mSv/yr as its goal for the evacuation zone and more contaminated areas outside it, but will support municipal government work to halve levels ranging from 1 to 20 mSv/yr by August 2013. The total area under consideration for attention is 13,000 sq km. The Environment ministry has set 8 kBq/kg Cs-137 as exemption level for sewage sludge, ash etc, and disposal of anything less than this is unrestricted.In mid May work started towards constructing a cover over unit 1 to reduce airborne radioactive releases from the site, to keep out the rain, and to enable measurement of radioactive releases within the structure through its ventilation system. This was fabricated and trial assembled at Onahama, then disassembled and brought to Fukushima by barge. The frame was reassembled over the reactor, enclosing an area 42 x 47 m, and 54 m high. The sections of the steel frame fitted together remotely without the use of screws and bolts. All the wall panels have a flameproof coating, and the structure has a filtered ventilation system capable of handling 40,000 cubic metres of air per hour through six lines, including two backup lines. The cover structure is fitted with internal monitoring cameras, radiation and hydrogen detectors, thermometers and a pipe for water injection. It will be able to handle accumulated snow loads of 30 centimetres and wind speeds of up to 90 kilometres per hour. The cover was completed with ventilation systems working by the end of October. It is expected to be needed for two years. Similar covers will be designed to fit around unit 3 & 4 reactor buildings once the top floors are cleared up about mid 2012.
In August the government said it would begin full-scale decontamination of the 20 km evacuation zone so that residents could return early in 2012. Meanwhile the access restrictions on areas 20-30 km from the plant have been relaxed.
Tests on radioactivity in rice have been made and caesium was found in a few of them. The highest levels were about one quarter of the allowable limit of 500 Bq/kg, so shipments to market are permitted.Summary to date: Major releases of radionuclides, including long-lived caesium, occurred to air, mainly in mid March. The population within a 20km radius had been evacuated three days earlier. Considerable work was done to reduce the amount of radioactive debris on site and stabilise dust. The main source of radioactive releases was the hydrogen explosion in the suppression chamber of unit 2 on 15 March. A cover building for unit 1 reactor is under construction. Radioactive releases in mid July had reduced to 1 GBq/hr, and dose rate from these at the plant boundary was 1.7 mSv/yr. See also WNN report on this.
Managing contaminated water
Removing contaminated water from the reactor and turbine buildings had become the main challenge in week 3, along with contaminated water in trenches carrying cabling and pipework. This was both from the tsunami inundation and leakage from reactors. Run-off from the site into the sea was also carrying radionuclides well in excess of allowable levels. Over 1-6 April some 520 m3 of contaminated water from unit 2 with 4700 TBq of activity leaked into the sea until the source was sealed. By the end of March all storages around the four units - basically the main condenser units and condensate tanks - were largely full of contaminated water pumped from the buildings.
Accordingly, with government approval, Tepco over 4-10 April released to the sea about 10,400 cubic metres of slightly contaminated water (0.15 TBq) in order to free up storage for more highly-contaminated water from unit 2 reactor and turbine buildings which needs to be removed to make safe working conditions. This is the main source of contaminated water, though some of it comes from drainage pits. NISA confirmed that there was no significant change in radioactivity levels in the sea as a result of the 0.15 TBq discharge.
Tepco then began transferring highly-radioactive water from the basement of unit 2 turbine hall and cabling trench to the holding tank and waste treatment plant just south of unit 4. The water contains 3 TBq/m3 of I-131 and 13 TBq/m3 of Cs-137. Some 120 m3/day of fresh water was being injected into unit 2 reactor core and it was assumed that this replenishes the contaminated water that is being removed, as in the other units. By 21 May about 8000 m3 had been pumped to the treatment plant from unit 2.
Tepco is now operating a new wastewater treatment facility to treat 1200 m3 per day. In mid June there was 110,000 m3 of contaminated water awaiting treatment and re-use, containing about 750,000 TBq. On 21 May a steel barge 136 metres long and 46 metres wide, the "mega-float", arrived at the site to augment the water storage capacity by 10,000 m3 pending treatment.
The company is using both US proprietary adsorbtion and French conventional technologies in the new 1200 m3/day treatment plant. This first removes any oil, then caesium by adsorbtion on to zeolite (Kurion process), then uses reagents to precipitate other impurities (Areva-Veolia plant). Separately, this stored water then undergoes reverse osmosis (RO) desalination (in a Hitachi plant) producing 480 m3/day, following which it is being used as fresh water for reactor cooling. Desalination is necessary on account of the seawater earlier used for cooling. The 720 m3/day of concentrated saltwater is stored, and 250 m3/day is further treated. The loaded zeolite from the Kurion process will be vitrified to reduce volume and stabilise the material. The sludge from the Areva plant is stored on site. A supplementary and simpler SARRY plant to remove caesium using Japanese technology and made by Toshiba and Shaw Group was installed and commissioned in August.
At mid September, 81,000 m3 of highly-contaminated water was awaiting treatment, and by mid October 128,000 m3 had been treated, with much of this desalinated and recycled for reactor cooling. Treatment is proceeding at over 80% of capacity of 1200 m3/day. Earlier there had been some stoppages, so that average to late July was only 750 m3/day. and as of mid August the Kurion plant had only operated at 66% of capacity.
In October Tepco started spraying 17,000 m3 of treated and desalinated water on the ground inside its plant boundary, both as irrigation and dust suppressant. The original source of this was both groundwater and seawater from 11 March inundation of units 5 & 6.
By the end of June, Tepco had installed 109 concrete panels to seal the water intakes of units 1-4, preventing contaminated water leaking to the sea. From mid June some treatment with zeolite of seawater at 30 m3/hr was being undertaken near the water intakes for units 2 & 3, inside submerged barriers installed in April. From October, a steel water shield wall is being built on the sea frontage of units 1-4. It extends about one kilometre, and down to an impermeable layer beneath two permeable strata which potentially leak contaminated groundwater to the sea.
A 4-year international survey assessing radiological pollution of the marine environment near the plant commenced in July, under IAEA auspices and led by Australia, South Korea and Indonesia. In September, researchers at the Japan Atomic Energy Agency, Kyoto University and other institutes estimated that about 15 PBq of radioactivity (I-131 and Cs-137) had been released into the sea from late March through April, including substantial airborne fallout. Another study, by France's IRSN, put the discharge at 27 PBq of Cs-137.
Summary to date: A large amount of contaminated water had accumulated on site, but with the commissioning of a new treatment plant in June this is now progressively being treated and recycled for reactor cooling. However, the main plant is not performing as well as expected, and a supplementary plant is being installed at the end of July. Some radioactivity has been released to the sea, but this has mostly been low-level and it has not had any major impact beyond the immediate plant structures. Concentrations outside these have been below regulatory levels since April.
Radiation exposure in plant and beyond
To 31 July, Tepco had checked the radiation exposure of 7500 people who had worked on the site since 11 March, considering both external dose and internal doses (measured with whole-body counters). On 8 August it reported that 103 workers had received doses over 100 mSv. Of these 95 had received 100 to 200 mSv, two more 200-250 mSv, and six had received over 250 mSv (309 to 678 mSv) apparently due to inhaling iodine-131 fume. The latter included the two unit 3-4 control room operators in the first few days who had not been wearing breathing apparatus*. There are up to 200 workers on site each day. Recovery workers are wearing personal monitors, with breathing apparatus and protective clothing which protect against alpha and beta radiation. So far over 3,500 of some 3,700 workers at the damaged Daiichi plant have received internal check-ups for radiation exposure. On the basis of these whole body count estimates, 6 Tepco staff had received over 200 mSv before 31 May, and another 3 had received 200-250 mSv, in addition to their external doses. Taking those into account, 8 staff had received more than 250 mSv and 4 more 200-250 mSv. The level of 250 mSv is the allowable maximum short-term dose for Fukushima accident clean-up workers, 500 mSv is the international allowable short-term dose "for emergency workers taking life-saving actions".
* Tepco reported that the committed effective dose internally for these two was 540 and 590 mSv - "whole body dose over 50 years" though in reality much shorter for iodine, in addition to external exposure of 103 and 88 mSv respectively.
No radiation casualties (acute radiation syndrome) have been reported, and few other injuries, though higher than normal doses are being accumulated by several hundred workers on site. High radiation levels in the three reactor buildings still hinder access there. On 24 March three contractors laying cable in unit 3 received a dose of more than 170 mSv, two suffering beta radiation burns on their legs from contaminated water. The Fukushima Labour Bureau issued a stern rebuke to Tepco.
Monitoring of seawater, soil and atmosphere is at 25 locations on the plant site, 12 locations on the boundary, and others further afield. Government and IAEA monitoring of air and seawater is ongoing, with high but not health-threatening levels of iodine-131 being found in March. Environmental levels are decreasing. With an 8-day half-life, most I-131 had gone by the end of April. However a radiological hotpsot was identified near Iitate village 30 km northeast of the plant.
On 4 April radiation levels of 0.06 mSv/day were recorded in Fukushima city, 65 km northwest of the plant, about 60 times higher than normal but posing no health risk according to authorities. Monitoring beyond the 20 km evacuation radius to 13 April showed one location - around Iitate - with up to 0.266 mSv/day dose rate, but elsewhere no more than one tenth of this. At the end of July the highest level measured within 30km radius was 0.84 mSv/day in Namie town, 24 km away. The safety limit set by the central government in mid April for public recreation areas is 3.8 microsieverts per hour (0.09 mSv/day).
No harmful health effects were found in 195,345 residents living in the vicinity of the plant who were screened as of May 31. All the 1,080 children tested for thyroid gland exposure showed results within safe limits, according to the report submitted to IAEA on 7 June.
Japan's health ministry has set up a special office to monitor the health of workers at the plant. The new office will compile data on radiation exposure for workers for long-term monitoring purposes, and inspect daily work schedules in advance.
As access to the three destroyed reactors improves, a number of highly radioactive hot spots are being discovered, notably in the Standby Gas Treatment System (SGTS), through which steam was vented to relieve reactor pressure during the first days of accident in mid March. It is highly likely that further radiation hotspots are discovered, particularly in the filtered vent systems, as Tepco conducts stabilisation work inside the reactor buildings. These will then need to be shielded. In mid October airborne radioactivity in unit 1 was reported to be about one tenth the legal limit for nuclear workers.
Media reports have referred to "nuclear gypsies" - casual workers employed by subcontractors on short-term basis, and allegedly prone to receiving higher and unsupervised radiation doses. This workforce has been part of the nuclear scene for at least four decades, and at Fukushima their doses are very rigorously monitored. If they reach certain levels, eg 30 mSv, but varying according to circumstance, they are reassigned to lower-exposure areas.
Summary to date: Six workers have received radiation doses apparently over the 250 mSv level set by NISA, but at levels below those which would cause radiation sickness. There have been no harmful effects from radiation on local people, or any doses approaching harmful levels
Fukushima Daiichi 5 & 6
Units 5 & 6, in a separate building, also lost power on 11th due to the tsunami. They were in 'cold shutdown' at the time, but still requiring pumped cooling. One air-cooled diesel generator at Daiichi 6 was located higher and so survived the tsunami and enabled repairs on Saturday 19th, allowing full restoration of cooling for units 5 and 6. While the power was off their core temperature had risen to over 100°C (128°C in unit 5) under pressure, and they had been cooled with normal water injection, presumably with the RCIC system. They were restored to cold shutdown by the RHR system on 20th, and mains power was restored on 21-22nd. Tepco said that local people would be consulted on whether the reactors might be restarted.
Tepco's remediation plan
Tepco published a 6- to 9-month plan on 17 April for dealing with the disabled Fukushima reactors, and updated this subsequently. See also Tepco's 19 July report.
Much of this has either been achieved or is well in progress, and the elements are described above.
At the end of August Tepco announced its general plan for proceeding with removing fuel from the four units, initially from the spent fuel ponds and then from the actual reactors. This was detailed further in METI's September report to IAEA.Storage ponds: First, debris will be removed from the upper parts of the reactor buildings using large cranes and heavy machinery. Covers will be built, and overhead cranes and fuel handling machines necessary to remove the spent fuel assemblies will be installed. Casks to transfer the removed fuel to the central spent fuel facility will also be designed/ manufactured using existing cask technology. Reactors: First it will be necessary to identify the locations of leaks from the primary containment vessels (PCVs) and reactor buildings using manual and remotely controlled dosimeters, cameras, etc., and indirectly analyze conditions inside the PCVs from the outside via measurements of gamma rays, echo soundings, etc. Any leakage points will be repaired and both reactor vessels (RPVs) and PCVs filled with water sufficient to achieve shielding, and the vessel heads will be removed. The location of melted fuel and corium will then be established. In particular, the distribution of damaged fuel believed to have flowed out from the reactor pressure vessels (RPVs) into PVCs will be ascertained, and they will be sampled, etc. After examination of the inside of the reactors, states of the damaged fuel rods and reactor core internals, sampling will be done and the damaged rods will be removed from the RPVs as well as from the PCVs. The whole process will be complex and slow, since safety remains paramount.Preparing for return of evacuees: This is a high priority and the evacuation zone will be decontaminated where required and possible, so that evacuees can return as soon as possible.
Earlier, consortia led by both Hitachi-GE and Toshiba submitted proposals to Tepco for decommissioning units 1-4. This would generally involve removing the fuel in about ten years and then sealing them for a further decade or two while the activation products in the steel of the reactor pressure vessel decay. They can then be demolished. Removal of the very degraded fuel will be a long process in units 1-3, but will draw on experience at Three Mile Island in USA. The Hitachi-GE group includes Bechtel and Exelon, the Toshiba team includes Babcock & Wilcox and Shaw. Areva was also planning to submit a proposal.
Tepco has allocated ¥207 billion ($2.53 billion) in its accounts for decommissioning units 1 to 4.
A 12-member international expert team assembled by the IAEA at the request of the Japanese government reported in October on remediation strategies for contaminated land. The mission focused on the remediation of the affected areas outside of the 20 km restricted area. The team said that it agreed with the prioritization and the general strategy being implemented, but advised the government to "cautiously balance the different factors that influence the net benefit of the remediation measures to ensure dose reduction." They should "avoid over-conservatism" which "could not effectively contribute to the reduction of exposure doses" in efforts to remediate large areas of contaminated land. The team also warned the Japanese government of the "potential risk of misunderstandings that could arise if the population is only or mainly concerned with contamination concentrations rather than dose levels." It added, "The investment of time and effort in removing contamination concentrations from everywhere, such as all forest areas and areas where the additional exposure is relatively low, does not automatically lead to reduction of doses for the public." The team's report calls on the Japanese authorities to "maintain their focus on remediation activities that bring best results in reducing the doses to the public."
There is no technical reason for the Fukushima Daini plant not to restart.
Fukushima Daini plant
Units 1-4 were shut down automatically due to the earthquake, but there was major interruption to cooling occurred due to the tsunami - here only 9 m high - damaging heat exchangers, so the reactors were almost completely isolated from their ultimate heat sink. Damage to the diesel generators was limited and also the earthquake left one of the external power lines intact, avoiding a station blackout as at Daiichi 1-4. This generator was supplemented by a 9 km power cable installed by the company in 16 hours. Nevertheless, the government ordered an evacuation within 10 km (in practice this was within Daiichi 20 km zone).
In units 1, 2 & 4 there were cooling problems still evident on Tuesday 15th. Unit 3 was undamaged and continued to 'cold shutdown' status on 12th, but the other units suffered flooding to pump rooms where the equipment transfers heat from the reactor heat removal circuit to the sea.
All units achieved 'cold shutdown' by16 March, meaning core temperature less than 100°C at atmospheric pressure (101 kPa), but still requiring some water circulation. The almost complete loss of ultimate heat sink proved a major challenge, but the cores were kept fully covered.
Radiation monitoring figures remained at low levels, little above background.
By 20 May Tepco had made significant modifications to the plant to guard against loss of all power and loss of cooling to both reactors and spent fuel ponds. It had also installed embankments and other protection against large tsunamis.
International Nuclear Event Scale assessment
Japan's Nuclear & Industrial Safety Agency originally declared the Fukushima Daiichi 1-3 accident as Level 5 on the International Nuclear Events Scale (INES) - an accident with wider consequences, the same level as Three Mile Island in 1979. The sequence of events relating to the fuel pond at unit 4 was rated INES Level 3 - a serious incident.
However, a month after the tsunami the NSC raised the rating to 7 for units 1-3 together, 'a major accident', saying that a re-evaluation of early releases suggested that some 630 PBq of I-131 equivalent had been discharged. This included 130 PBq of I-131, and 6.1 PBq of Cs-137, presumably with significant amounts of noble gases. This then matched the criterion of "environmental release corresponding to a quantity of radioactivity equivalent to a release to the atmosphere of more than several tens of thousands of terabequerels (ie tens of PBq) of iodine-131", most of this being in the first week. The IAEA quotes a more specific 50 PBq of I-131 equivalent. In early June NISA increased its estimate of releases to 770 PBq, from about half that.
For Fukushima Daini, NISA declared INES Level 3 for units 1, 2, 4 - each a serious incident.
Accident liabilityBeyond whatever insurance Tepco might carry for its reactors is the question of third party liability for the accident. Japan is not party to any international liability convention but its law generally conforms to them, notably strict and exclusive liability for the operator. Two laws governing them are revised about every ten years: the Law on Compensation for Nuclear Damage and Law on Contract for Liability Insurance for Nuclear Damage. Plant operator liability is exclusive and absolute (regardless of fault), and power plant operators must provide a financial security amount of JPY 120 billion (US$ 1.46 billion) - it was half that to 2010. The government may relieve the operator of liability if it determines that damage results from “a grave natural disaster of an exceptional character”, and in any case total liability is unlimited.
In mid April, the first meeting was held of a panel to address compensation for nuclear-related damage. The panel will establish guidelines for determining the scope of compensation for damage caused by the accident, and to act as an intermediary. It was established within the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and is led by Law Professor Yoshihisa Nomi of Gakushuin, University in Tokyo.
On 11 May, Tepco accepted terms established by the Japanese government for state support to compensate those affected by the accident at the Fukushima Daiichi plant. The scheme includes a new state-backed institution, the Nuclear Damage Compensation Facilitation Corporation, to expedite payments to those affected by the Fukushima accident. The body would receive financial contributions from electric power companies with nuclear power plants in Japan, and from the government through special bonds that can be cashed whenever necessary. The government bonds total JPY 5 trillion ($62 billion). Tepco accepted the conditions imposed on the company as part of the package. That included not setting an upper limit on compensation payments to those affected, making maximum efforts to reduce costs, and an agreement to cooperate with an independent panel set up to investigate its management. The Nuclear Damage Compensation Facilitation Corporation, established by government and nuclear plant operators, includes representatives from other nuclear generators and will also operate as an insurer for the industry, being responsible to have plans in place for any future nuclear accidents. The provision for contributions from other nuclear operators is similar to that in the USA. The government estimates that Tepco will be able to complete its repayments in 10 to 13 years, after which it will revert to a fully private company with no government involvement. Meanwhile it will pay an annual fee for the government support, maintain adequate power supplies and ensure plant safety. Tepco has estimated its extra costs for fossil fuels in 2011-12 (April-March) will be about JPY 1 trillion ($12.4 billion).
On 14 June, Japan's cabinet passed the Nuclear Disaster Compensation Bill, and a related budget to fund post-tsunami reconstruction was also passed subsequently.
In September the Nuclear Damage Compensation Facilitation Corporation started by working with Tepco to compile a business plan for the next decade. This needs to be approved by government for Tepco to receive assistance. Tepco wants to include an electricity rate increase of about 15% in the plan, saying it is necessary to cover about JPY 1 trillion ($13 billion) in additional annual fuel costs for thermal power generation to make up for lost capacity at idle nuclear power plants. Cost cutting measures will also be needed.
The government and 12 utilities are contributing funds into the new institution to pay compensation to individuals and businesses claiming damages caused by the accident. It will receive JPY 7 billion ($91 million) in public funds as well as a total of JPY 7 billion from 12 nuclear plant operators, the Tepco share of JPY 2379 million ($30 million) being largest. The percentage of utility contributions is fixed in proportion to the power output of their plants, so Kansai Electric Power Co. will provide JPY 1229 million, followed by JPY 660 million by Kyushu Electric Power Co. and JPY 622 million by Chubu Electric Power Co. Japan Nuclear Fuel Ltd., which owns a spent nuclear fuel reprocessing plant in Aomori Prefecture, will provide JPY 117 million to the entity. The utility companies will also pay annual contributions to the body. Tepco is required to make extra contributions, with the specific amount to be decided later.
A government panel reviewing Tepco’s finances said the utility is expected to pay $13.2 billion through March 2012 in compensation for damages and then $11.6 billion annually. The panel also said that Tepco will have to cut 7400 jobs and reduce costs by $32.5 billion over the next ten years.
Inquiries and reportsIn May a team of 18 experts from 12 countries spent a week at the plant on behalf or the International Atomic Energy Agency (IAEA). Its preliminary report was delivered at the end of May, and a final report in June, presented to the IAEA Ministerial Conference in Vienna.Early in June the independent panel of ten experts, mostly academics, appointed by the Japanese government began meeting. It has two technological advisers. An initial report is due at the end of 2011 and a final report in mid 2012. The panel has set up four teams to pursue investigations, but not to pursue the question of responsibility for the accident.On 7 June the government submitted a 750-page report to IAEA compiled by the nuclear emergency taskforce, acknowledging reactor design inadequacies and systemic shortcomings. It said that "In light of the lessons learned from the accident, Japan has recognized that a fundamental revision of its nuclear safety preparedness and response is inevitable."
On 11 September a second report was issued by the government and submitted to the IAEA, summarising both on-site work and progress and off-site responses. It contained further analysis of the earthquake and tsunami, the initial responses to manage and cool the reactors, the state of spent fuel ponds and the state of reactor pressure vessels. It also summarised radioactive releases and their effects.
Meanwhile a July report from MIT's Centre for Advanced Nuclear Energy Systems provided a useful series of observations, questions raised, and suggestions. Its Appendix has some constructive comment on radiation exposure and balancing the costs of dose avoidance in circumstances of environmental contamination.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has started a 12-month study on the magnitude of radioactive releases to the atmosphere and ocean, and the range of radiation doses received by the public and workers.
"Stress Tests" on Japanese reactorsThe government has ordered "stress tests" based on those in the EU for all Japan's nuclear reactors except Fukushima's before they restart following any shutdown. This means most will be shut down until late in 2011 (though the EU stress tests do not envisage shutting reactors down). After some confusion the government decided that these would be in two stages.
In the primary tests, plant operators assessed whether main safety systems could be damaged or disabled by natural disasters beyond the plant design basis. This identified the sheer magnitude of events that could cause damage to nuclear fuel, as well as any weak points in reactor design. The 'tests' started from an extreme plant condition, such as operating at full power while used fuel ponds are full. From there, a range of accident progressions such as earthquakes, tsunamis and loss of off-site power were computer simulated using event trees, addressing the effectiveness of available protective measures at each stage. In the second stage even more severe events will be considered, with a focus on identifying 'cliff-edge effects' - points in a potential accident sequence beyond which it would be impossible to avoid a serious accident. This stage will include the effects of simultaneous natural disasters. Of particular focus will be the fundamental safety systems that were disabled by the tsunami of 11 March, leading to the Fukushima accident: back-up diesel generators and seawater pumps that provide the ultimate heat sink for a power plant. All power plants need to have been subject to this secondary analysis by the end of the year. The results will be considered by NISA as well as the Nuclear Safety Commission.
The government has confirmed the creation of a separate Nuclear Safety Agency under the authority of the environment ministry and combining the roles of NISA and NSC by April 2012. As an expression of its determination to strengthen nuclear safety regulation it plans to receive an IAEA Integrated Regulatory Review Service mission in 2012.
Lessons learned: summary• The severity of external events was underestimated - tsunami height relative to siting plant close to sea level - vulnerability of electrical grid system to earthquake damage• Essential to maintain instrumentation, lighting and communications. - locate back-up generators and main electrical switchgear in secure place• Essential to maintain core cooling post-shutdown - Need diverse and physically-separated cooling systems with heat sinks - Need reliable AC electric supply• Essential to protect the containment system - from risk of hydrogen explosion - from possibility of backflow in vent systems• Ensure safety of spent fuel in storage - ready provision of make-up water• Accident management for multiple and/or prolonged reactor events
Sources:
Tepco NISA IAEA METI JAIFNSC