Tuesday, May 15, 2012

Fukushima Accident Summary (world-nuclear.org)

This is such as good description of the accident from a no-need-to-panic perspective, I believe it should be more widely distributed. It has a good description of plans to clean up the plants and remove fuel, but it is from an industry-apologist perspective.

Fukushima Accident 2011

(updated 27 April 2012)
  • Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011. 
  • All three cores largely melted in the first three days.  
  • The accident was rated 7 on the INES scale, due to high radioactive releases in the first few days. 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 they were being cooled with recycled water from the new treatment plant. Reactor temperatures had fallen to below 80C at the end of October, and official 'cold shutdown condition' was announced in mid December.  
  • Apart from cooling, the basic ongoing task is to prevent release of radioactive materials, particularly in contaminated water leaked from the three units.  
  • There have been no deaths or cases of radiation sickness from the nuclear accident, but over 100,000 people have had to be evacuated from their homes to ensure this. 
The Great East Japan Earthquake of 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 19,000 and much damage to coastal ports and towns with over a million buildings destroyed or partly collapsed.

Eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the quake hit. Subsequent inspection showed no significant damage to any from the earthquake. The operating units which shut down were Tokyo Electric Power Company's (Tepco) Fukushima Daiichi 1, 2, 3, and Fukushima Daini 1, 2, 3, 4, Tohoku's Onagawa 1, 2, 3, and Japco's Tokai, total 9377 MWe net. Fukushima Daiichi units 4, 5 & 6 were not operating at the time, but were affected. 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 the entire site was flooded by the 15-metre tsunami. This disabled 12 of 13 back-up generators on site and also the heat exchangers for dumping reactor waste heat and decay heat to the sea. The three units lost the ability to maintain proper reactor cooling and water circulation functions. 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. A hardened emergency response centre on site was unable to be used in grappling with the situation due to radioactive contamination.

Three Tepco employees at the Daiichi and Daini plants were killed directly by the earthquake and tsunami, but there have been no fatalities from the nuclear accident.

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. 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 two Fukushima plants and their siting

The Daiichi (first) and Daini (second) Fukushima plants are sited about 11 km apart on the coast, Daini to the south.

The recorded seismic data for both plants - some 180 km from the epicentre - shows that 550 Gal (0.56 g) was the maximum ground acceleration for Daiichi, and 254 Gal was maximum for Daini. Daiichi units 2, 3 and 5 exceeded their maximum response acceleration design basis in E-W direction by about 20%. The 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 original design basis tsunami height was 3.1 m for Daiichi based on assessment of the 1960 Chile tsunami and so the plant had been built about 10 metres above sea level with the seawater pumps 4 m above sea level. The Daini plant as built 13 metres above sea level. In 2002 the design basis was revised to 5.7 metres above, and the seawater pumps were sealed. Tsunami heights coming ashore were about 15 metres, and the Daiichi turbine halls were under some 5 metres of seawater until levels subsided. Daini was less affected.  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. The June 1896 earthquake of estimated magnitude 7.6 produced a tsunami with run-up height of 38 metres in Tohoku region, killing 27,000 people.

The tsunami countermeasures taken when Fukushima Daiichi was designed and sited in the 1960s were considered acceptable in relation to the scientific knowledge then, with low recorded run-up heights for that particular coastline. But through to the 2011 disaster, new scientific knowledge emerged about the likelihood of a large earthquake and resulting major tsunami. However, this did not lead to any major action by either the plant operator, Tepco, or government regulators, notably the Nuclear & Industrial Safety Agency (NISA). The tsunami countermeasures could also have been reviewed in accordance with IAEA guidelines which required taking into account high tsunami levels, but NISA continued to allow the Fukushima plant to operate without sufficient countermeasures, despite clear warnings.

A report from the Japanese government's Earthquake Research Committee on earthquakes and tsunamis off the Pacific coastline of northeastern Japan in February 2011 was due for release in April, and might have brought about changes. The document includes analysis of a magnitude 8.3 earthquake that is known to have struck the region more than 1140 years ago, 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.
(See also background on Earthquakes and Seismic Protection for Nuclear Power Plants in Japan)

Events at Fukushima Daiichi 1-3 & 4

It appears that no serious damage was done to the reactors by the earthquake, and the operating units 1-3 were automatically shut down in response to it, as designed. 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. The 125-volt DC batteries for units 1 & 2 were flooded and failed, leaving them without instrumentation, control or lighting. Unit 3 had battery power for about 30 hours.

At 7.03 pm Friday 11 March 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. On Saturday 12th he extended the evacuation zone to 20 km.

Inside the Fukushima Daiichi reactors

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.

 BWR 3

When the power failed at 3.42 pm, about one hour after shutdown of the fission reactions, 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.  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 primary containment (PCV) through safety valves.  Later this was accompanied by hydrogen, produced by the interaction of the fuel's very hot zirconium cladding with steam after the water level dropped. 
As pressure started to rise here, the steam was directed into the suppression chamber under the reactor, within the containment, but the internal temperature and pressure nevertheless rose quite rapidly.  Water injection commenced, using the various systems provide for this and finally the Emergency Core Cooling System (ECCS).  These systems progressively failed over three days, so from early Saturday water injection to the reactor pressure vessel (RPV) was with fire pumps, but this required the internal pressures to be relieved initially by venting into the suppression chamber/ wetwell.
Inside unit 1, it is 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 rose to some 2800°C so that the central part started to melt after a few hours 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 steadily. 
As pressure rose, 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 Saturday.  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, representing a serious failure of this sytem.  The vented steam, noble gases and aerosols were accompanied by hydrogen.  At 3.36 pm on Saturday 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 the hydrogen mixed with air and ignited.  (Oxidation of the zirconium cladding at high temperatures in the presence of steam produces hydrogen exothermically, with this exacerbating the fuel decay heat problem.)
In unit 1 most of the core - as corium comprised of melted fuel and control rods - was assumed to be in the bottom of the RPV, but later it appeared that it had mostly gone through the bottom of the RPV and some 70 cm into the drywell concrete below.  This reduced the intensity of the heat and enabled the mass to solidify.
Much of the fuel in units 2 & 3 also apparently 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 units 2 & 3 would be producing about 3.0 MW each. 
In unit 2, water injection using the steam-driven back-up water injection system failed on Monday 14th, and it was about six hours before a fire pump started injecting seawater into the RPV.  Before the fire pump could be used RPV pressure had to be relieved via the wetwell, which required power and nitrogen, hence the delay.  Meanwhile the reactor water level dropped rapidly after back-up 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 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 seemed to rupture, possibly due to a hydrogen explosion there, and the drywell containment pressure inside dropped. However, subsequent inspection of the suppression chamber did not support the rupture interpretation.   Later analysis suggested that a leak of the PCV developed about midday Saturday 12th.
In Unit 3, the main back-up water injection system failed at 11 am on Saturday 12th and early on Sunday 13th, water injection using the high pressure system failed also 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.  Early on Sunday venting the suppression chamber and containment was successfully undertaken.  It is now 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.
Early on Monday 14th PCV venting was repeated, and this evidently backflowed to the service floor of the building, so that at 11 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 some of that on the ground near unit 3 was very radioactive.
In defueled 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.
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 was boiling off for some months.  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.
There was a peak of radioactive release on 15th, but the source remains uncertain. Due to volatile and easily-airborne fission products being carried with the hydrogen and steam, the venting and hydrogen 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.
Nitrogen is being injected into the containment vessels of all three reactors to remove concerns about further hydrogen explosions, and in December this was started also for the pressure vessels.  Gas control systems which extract and clean the gas from the PCV to avoid leakage of caesium have been commissioned for all three units.
Through 2011 injection into the RPVs of water circulated through the new water treatment plant achieved relatively effective cooling, and temperatures at the bottom of the RPVs were stable in the range 60-76°C at the end of October, and 27-54°C in mid January.  RPV pressures ranged from atmospheric to slightly above (102-109 kPa) in January, due to water and nitrogen injection.  However, since they are leaking, the normal definition of "cold shutdown" does not apply, and Tepco waited to bring radioactive releases under control before declaring "cold shutdown condition" in mid December, with NISA's approval.  This, with the prime minister's announcement of it, formally brought to a close the 'accident' phase of events.
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.
 Event sequence following earthquake (timing from it)
  Unit 1 Unit 2 Unit 3
Loss of AC power + 51 min + 54 min + 52 min
Loss of cooling + 1 hour + 70 hours + 36 hours
Water level down to top of fuel* + 3 hours + 74 hours + 40 hours
Core damage starts* + 4 hours + 77 hours + 42 hours
Fire pumps with fresh water + 15 hours   + 43 hours
Hydrogen explosion (not confirmed for unit 2) + 25 hours
service floor
+ 87 hours
suppression chamber
+ 68 hours
service floor
Fire pumps with seawater + 28 hours + 77 hours + 46 hours
Off-site electrical supply + 11-15 days
Fresh water cooling + 14-15 days

 * estimated. 
Tepco has said that the three reactors, with unit 4, are written off and will be decommissioned.

Summary: 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 were 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, 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 to well below boiling point 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 and pressure vessels. Tepco declared "cold shutdown condition" in mid December when radioactive releases had reduced to minimal levels.
(See also background on  nuclear reactors at Fukushima Daiichi)

Fuel ponds: developing problems

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 Daiichi 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.) 
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. 
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. 
After the hydrogen explosion in unit 4 early on Tuesday 15 March, Tepco was told to implement injection of water to unit 4 pond which had a particularly high heat load (3 MW) from 1331 used fuel assemblies in it, so it was the main focus of concern. It needed the addition of about 100 m3/day to replenish it after circulation ceased. 
From Tuesday 15 March attention was given to replenishing the water in the ponds of units 1, 2, 3 as well. 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 might be damaged, but the majority were intact. 
There was concern about structural strength of unit 4 building, so support for the pond was reinforced by the end of July.
New cooling circuits with heat exchangers adjacent to the reactor buildings for all four ponds were commissioned after a few months, and each 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.
The next task is to remove the salt from those ponds which had seawater added, to reduce corrosion. 
The central spent fuel pool on site holds about 60% of the Daiichi used fuel, 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 after two weeks.
Summary: The new cooling circuits with external heat exchangers for the four ponds are working well.  Temperatures are normal.  Analysis of water in mid August confirmed that most fuel rods are intact.
(See also background on  Fukushima Fuel Ponds)

Radioactive releases to air

Regarding releases to air and also 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. 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.

As cooling failed on the first day, evacuations were progressively ordered. By the evening of Saturday 12 March the evacuation zone had been extended to 20 km from the plant. Since then, evacuated residents have been allowed to return home for brief visits. The government is undertaking detailed radiation monitoring in the evacuation area to ensure the safety of those returning. Permanent return of most evacuees is envisaged from April 2012. From 20 to 30 km from the plant, the criterion of 20 mSv/yr dose rate was applied to determine evacuation, and is now the criterion for return being allowed in 2012. 20 mSv/yr was also the general limit set for children's dose rate related to outdoor activities, but there were calls to reduce this. In areas with 20-50 mSv/yr from April 2012 residency is restricted, with remediation action to be completed in March 2014.

A significant problem in tracking radioactive release was that 23 out of the 24 radiation monitoring stations on the plant site were disabled by the tsunami.

After the hydrogen explosion in unit 1 on 12 March, some radioactive caesium and iodine were detected in the vicinity of the plant, having 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 hydrogen explosion at unit 3 on 14th and in unit 4 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. However, it is not clear that this was implemented. On 11 April the government suggested that those outside the 20km zone who were likely to accumulate 20 mSv total dose should move out within a month. Data at the end of May (with most I-131 gone by decay) showed that about half of the 20 km evacuation zone and a similar area to the NW, total about 1000 sq km, would give an annual dose of 20 mSv to March 2012.

France's Institute for Radiological Protection & Nuclear Safety (IRSN) estimated that maximum external doses to people living around the plant were unlikely to exceed 30 mSv/yr in the first year. This was 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 elswhere.

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. 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 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.

NISA estimated that about 130 PBq of iodine-131 was released from the reactors, mostly around 15 March and the two days following - 0.16% of the total inventory. In 32 days this released iodine would have diminished to one sixteenth of original activity - 8 PBq. NISA's report to IAEA said that this 130 PBq of I-131 together with 6 PBq of caesium-137* released gave an "iodine-131 equivalent" figure of 370 PBq, 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. Japan's Nuclear Safety Commission (NSC, a policy body) 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 release of 5200 PBq iodine-131 equivalent. 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. 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. In addition it 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.

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.

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. The frame was assembled 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. The cover was completed with ventilation systems working by the end of October 2011. It is expected to be needed for two years. Similar covers are being designed to fit around unit 3 & 4 reactor buildings once the top floors are cleared up about mid 2012. Work started on the 69 x 31 m cover (53 m high) for unit 4 in April 2012.

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: 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 to stabilise dust. The main source of radioactive releases was the apparent hydrogen explosion in the suppression chamber of unit 2 on 15 March. A cover building for unit 1 reactor has been built and commissioned. 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.
Sequence of evacuation orders based on the report by the Independent Investigation Commission on the Fukushima Nuclear Accident:

11 March 

14:46 JST The earthquake occurred. 

15:42 TEPCO made the first emergency report to the government. 

19:03 The government announced nuclear emergency. 

20:50 The Fukushima Prefecture Office ordered 2km evacuation. 

21:23 The government ordered 3km evacuation and to keep staying inside buildings in the area of 3-10km. 

12 March

05:44 The government ordered 10km evacuation. 

18:25 The government ordered 20km evacuation. 

15 March

11:01 The government ordered to keep staying inside buildings in the area of 20-30km. 

25 March
  The government requested voluntary evacuation in the area of 20-30km.

21 April  
The government set the 20km no-go area.

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. 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 total) in order to free up storage for more highly-contaminated water from unit 2 reactor and turbine buildings which needed to be removed to make safe working conditions. Unit 2 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 contained 3 TBq/m3 of I-131 and 13 TBq/m3 of Cs-137. Some120 m3/day of fresh water was being injected into unit 2 reactor core and this replenished the contaminated water being removed, as in the other units.

Tepco built a new wastewater treatment facility to treat contaminated water. The company used both US proprietary adsorbtion and French conventional technologies in the new 1200 m3/day treatment plant.  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. Desalination is necessary on account of the seawater earlier used for cooling, and the 1200 m3/day desal plant produces 480 m3 of clean water while 720 m3 goes to storage. By mid March 2012, over 250,000 m3 of water had been treated.

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 was 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.

Summary: A large amount of contaminated water had accumulated on site, but with the commissioning of a new treatment plant in June this was progressively being treated and recycled for reactor cooling. However, the main plant is not performing as well as expected, and a supplementary plant was installed. 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 December, Tepco had checked the radiation exposure of 19,594 people who had worked on the site since 11 March, for many of these considering both external dose and internal doses (measured with whole-body counters). It reported that 167 workers had received doses over 100 mSv. Of these 135 had received 100 to 150 mSv, 23 150-200 mSv, three more 200-250 mSv, and six had received over 250 mSv (309 to 678 mSv) apparently due to inhaling iodine-131 fume early on. The latter included the two unit 3-4 control room operators in the first two days who had not been wearing breathing apparatus. There were 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 3500 of some 3700 workers at the damaged Daiichi plant have received internal check-ups for radiation exposure, giving whole body count estimates. 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".

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 hindered access there through into 2012.

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. With an 8-day half-life, most I-131 had gone by the end of April.

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 was 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 by May 31. All the 1,080 children tested for thyroid gland exposure showed results within safe limits, according to the report submitted to IAEA in June. By December, government health checks of some 1700 residents who were evacuated from three municipalities showed that two-thirds received an external radiation dose within the normal international limit of 1 mSv/yr, 98% were below 5 mSv/yr, and ten people were exposed to more than 10 mSv.

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.

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 transient 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: 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, nor any doses approaching harmful levels. However, some 160,000 people were evacuated from their homes and only in 2012 are allowed limited return.

Fukushima Daiichi 5 & 6

Units 5 & 6, in a separate building, also lost power on 11 March 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. They were restored to cold shutdown by the normal recirculating system on 20th, and mains power was restored on 21-22nd.

Remediation plans

Tepco published a 6- to 9-month plan on 17 April for dealing with the disabled Fukushima reactors, and updated this several times subsequently. Remediation is proceeding approximately as planned.

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.

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 reinstalled. Casks to transfer the removed fuel to the central spent fuel facility will also be designed and manufactured using existing cask technology. In December Tepco estimated that used fuel would be removed from the storage pools within two years.

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 analyse 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. Then 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 PCVs will be ascertained, and it will be sampled and analysed. 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. In December Tepco estimated that the fuel would be removed from the reactors within 25 years - in line with US experience at Three Mile Island, though other estimates suggest ten years.
The four reactors will be completely demolished in 30-40 years - much the same time frame as for any nuclear plant.

Preparing for return of evacuees: This is a high priority and the evacuation zone will be decontaminated where required and possible, so that evacuees (reportedly some 160,000) can return without undue delay. In December the government said that where annual radiation dose would be below 20 mSv/yr, the government would help residents return home as soon as possible and assist local municipalities with decontamination and repair of infrastructure. In areas where radiation levels are over 20 mSv/yr evacuees will be asked to continue living elsewhere for “a few years” until the government completes decontamination and recovery work. The government will consider purchasing land and houses from residents of these areas if the evacuees wish to sell them. Early in 2012 the Environment Ministry said that contaminated areas would be re-categorised from March: below 20 mSv/yr, evacuation called off; 20-50 mSv/yr "restrict residency" with remediation action to be completed in March 2014; and over 50 mSv/yr "difficulty of return", and remediation deferred.  Such areas add to those devastated by the tsunami, where rebuilding is very uncertain.

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 and then sealing them for a further decade or two while the activation products in the steel of the reactor pressure vessels decay. They can then be demolished. As noted above, 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. In January it was reported that an industry consortium (Hitachi GE Nuclear Energy, Mitsubishi Heavy Industries and Toshiba) would determine how to locate fuel debris inside units 1-3 and how to fill the pressure vessels with water.

Tepco has allocated ¥207 billion ($2.53 billion) in its accounts for decommissioning units 1 to 4. The government has allocated ¥1150 billion ($15 billion) for decontamination in the region, with the promise of more if needed.

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 focus on actual dose reduction. They should "avoid over-conservatism" which "could not effectively contribute to the reduction of exposure doses" to people. It warned the government against being preoccupied with "contamination concentrations rather than dose levels," since this "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."

Fukushima Daini plant

Units 1-4 were shut down automatically due to the earthquake, but there was major interruption to cooling 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.

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.

There is no technical reason for the Fukushima Daini plant not to restart.

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 radioactive releases suggested that some 630 PBq of I-131 equivalent had been discharged, mostly in the first week. This then matched the criterion for level 7. In early June NISA increased its estimate of releases to 770 PBq, from about half that, though in August the NSC lowered this estimate to 570 PBq

For Fukushima Daini, NISA declared INES Level 3 for units 1, 2, 4 - each a serious incident.

Accident liability

Beyond 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” (which it did not do here), 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 established 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 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.

This 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 830 billion ($10.7 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 Compensation Facilitation Corporation started by working with Tepco to compile a business plan for the next decade. This was approved by the Ministry of Economy, Trade and Industry (METI) so that some JPY 900 billion ($11.5 billion) could be released to the company through bonds issued to the Nuclear Damage Compensation Fund to cover compensation payments to March 2012. The plan also involves Tepco reducing its own costs by JPY 2545 billion ($32.6 billion) over the next ten years, including shedding 7400 jobs. This special business plan will be superseded by a more comprehensive business plan in March 2012, which was expected to involve compensation payments of JPY 910 billion ($11.6 billion) annually. Tepco wants to include an electricity rate increase of 17% in the plan, to cover the additional annual fuel costs for thermal power generation to make up for lost capacity at idled nuclear power plants. In February 2012 METI approved a further JPY 690 billion ($8.9 billion) in compensation support from the Nuclear Damage Liability Compensation Fund, subject to Tepco's business plan giving the government voting rights.

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.
By the first anniversary of the accident, Tepco had paid JPY 446 billion ($5.4 billion) in compensation, based on decisions of the Nuclear Damage Compensation Facilitation Corporation. At that point, some 40% of the people entitled to apply for compensation had done so.

Inquiries and reports

In May a team of 18 experts from 12 countries spent a week at the plant on behalf of the International Atomic Energy Agency (IAEA), and its final report was presented to the IAEA Ministerial Conference in Vienna in June.

Early in June the independent Investigation Committee, a panel of ten experts, mostly academics and appointed by the Japanese cabinet, began meeting. It has two technological advisers. An initial report was published in December 2011 and a final report is due in mid 2012. The panel set up four teams to undertake investigations, but not to pursue the question of responsibility for the accident.
The national Diet later set up a legally-constituted Nuclear Accident Independent Investigation Commission (NAIIC) of ten members which started its work in December 2011. One of the purposes of NAIIC is to provide suggestions including the “re-examination of an optimal administrative organization” for nuclear safety regulation based on its investigation of the accident.

On 7 June 2011 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.

In November 2011 the US Institute of Nuclear Power Operators (INPO) released its Special Report on the Nuclear Accident at the Fukushima Daiichi Nuclear Power Station, with timeline. This 97-page report gives a valuable and detailed account of events.

Also in November 2011 the Japan Nuclear Technology Institute published a 280-page report on the accident, with proposals to be addressed in the future.

On 2 December 2011 Tepco released its interim investigation report on the accident (in Japanese).

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) is undertaking 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.
An analysis by the Carnegie Endowment in March 2012 said that if best practices from other countries had been adopted by Tepco and NISA at Fukushima, the serious accident would not have happened, underlining the need for greater international regulatory collaboration.
In April 2012 the US Electric Power Research Institute (EPRI) published Fukushima Daiichi Accident – Technical Causal Factor Analysis, which identified the root cause beyond the flooding and its effects as a failure to consider the possibility of the rupture of combinations of geological fault segments in the vicinity of the plant.

"Stress Tests" on Japanese reactors

The government has ordered so-called "stress tests" based on those in the EU for all Japan's nuclear reactors except Fukushima's before they restart following any shutdown, including for routine checks. After some confusion the government decided that these would be in two stages.

In the primary stage, 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 as problems developed. Stage 1 tests must be approved before reactors are restarted.

In the second stage even more severe events are 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. A 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.
The stage 1 results for individual plants are considered first by NISA and then by the Nuclear Safety Commission before being forwarded to the prime minister's office for final approval. Local government must the approve restart. Late in March NISA had received stage 1 assessments for 17 reactors - 12 PWRs and 5 BWRs. Three of these had been approved by NISA and two confirmed by NSC.

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, which was due to be commissioned 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.

Acton J.M. & Hibbs M, Why Fukushima was preventable, March 2012 Carnegie Paper.

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