IN WAKE OF FUKUSHIMA DISASTER , JAPPAN TO END NUCLEAR POWERE BY 2030
The Japanese government announced a dramatic turn in its energy policy Friday, vowing to make the densely populated island nation nuclear-free by the 2030s.
Last year's tsunami-triggered disaster at the Fukushima nuclear power complex forced evacuation of more than 160,000 and contaminated huge swaths of territory north of Tokyo. Prior to the accident, nuclear plants provided nearly a third of Japan's power generation, and the government had planned to increase that proportion to more than half.
In unveiling the new policy, Prime Minister Yoshihiko Noda acknowledged that the vast majority of Japanese support the zero option on nuclear power. The new blueprint calls for investing almost $500 billion over the next two decades to expand renewable sources like wind and solar power, the NHK broadcast network reported.
The energy plan sets forward a three-pronged approach to phasing out nuclear power generation after the March 11, 2011, earthquake and tsunami that inundated the Fukushima plant and set off the worst nuclear accident since the Chernobyl meltdown in 1986 in Ukraine.
A 40-year limit has been set on operation of existing plants, construction of new nuclear-generation facilities is prohibited and any further restarts of Japan's 48 idled nuclear plants will be contingent on their meeting strengthened safety standards to be adopted by a newly created independent regulatory agency.
Japan's anti-nuclear movement swelled after the Fukushima disaster. Opposition to nuclear power became more vocal this summer when the government authorized the restart of two plants in Ohi, serving the populous areas around Kyoto and Osaka. A nationwide shutdown after the Fukushima accident put all 50 remaining nuclear plants offline pending safety improvements and inspections.
Public opinion polls have indicated that more than 70% of Japanese support ending nuclear energy production in Japan, and public hearings in recent months were dominated by voices against resuming nuclear production. Large crowds of anti-nuclear demonstrators have also appeared outside the prime minister's home and offices on a weekly basis.
Business leaders opposed a change in policy. The Federation of Electric Power Companies deemed the phase-out decision "very regrettable," warning that it will raise energy prices for business and consumers and, at least in the near future, increase greenhouse gas emissions as the country relies more on fossil fuels until renewables can be expanded.
Greenpeace Japan urged the government to give the new energy policy the force of law, to avoid it becoming mere "lip service" to appease the public ahead of elections expected later this year.
Japan is already the world's largest importer of liquefied natural gas and is expected to increase reliance on that source as it phases out nuclear energy.
Germany abandons nuclear power programme in wake of Fukushima disaster
Germany's coalition government agreed early on Monday to shut down all the country's nuclear power plants by 2022, the environment minister said, making it the first major industrialised nation in the last quarter century to announce plans to go nuclear-free.
The country's seven oldest reactors already taken off the grid pending safety inspections following the catastrophe at Japan's Fukushima nuclear power plant in March will remain offline permanently, Norbert Roettgen added. The country has 17 reactors in total.
Roettgen praised the coalition agreement after negotiations through the night between the governing parties.
"This is coherent. It is clear," he told reporters in Berlin. "That's why it is a good result."
Chancellor Angela Merkel pushed through measures in 2010 to extend the lifespan of the country's 17 reactors, with the last one scheduled to go offline in 2036, but she reversed her policy in the wake of the Japanese disaster.
"We want the electricity of the future to be safe, reliable and economically viable," Merkel told reporters on Monday.
Germany's energy supply chain "needs a new architecture," necessitating huge efforts in boosting renewable energies, efficiency gains and overhauling the electricity grid, she added.
"We have to follow a new path," Merkel said.
Germany, Europe's biggest economy, stands alone among the world's major industrialised nations still using nuclear power in its determination to gradually replace it with renewable energy sources.
Italy decided to stop producing nuclear power after the 1986 Chernobyl disaster.
Through March - before the seven reactors were taken offline - just under a quarter of Germany's electricity was produced by nuclear power, about the same share as in the US.
Energy from wind, solar and hydroelectric power currently produces about 17 per cent of the country's electricity, but the government aims to boost its share to around 50 per cent in the coming decades.
Many Germans have been vehemently opposed to nuclear power since Chernobyl sent radioactive fallout over the country. Tens of thousands repeatedly took to the streets in the wake of Fukushima to urge the government to shut all reactors.
A center-left government a decade ago first penned a plan to abandon the technology for good because of its inherent risks by 2021. But Merkel's government last year amended it to extend the plants' lifetime by an average of 12 years.
But the conservative chancellor reversed her pro-nuclear stance after the earthquake and tsunami that crippled the Fukushima Dai-ichi plant on March 11, triggering nuclear meltdowns.
Merkel's government ordered the country's seven oldest reactors, all built before 1980, shut down four days after the Fukushima incident. The plants accounted for about 40 per cent of the country's nuclear power capacity.
Germany used to be a net energy exporter, and the agency overseeing its electricity grid said on Friday that the country remains self-sufficient even without the seven reactors and another plant that has already been offline for more than a year for maintenance work.
The coalition government's decision broadly follows the conclusions of a government-mandated commission on the ethics of nuclear power, which delivered its recommendation to abolish the technology by 2022 on Saturday. Details of the final report are to be presented later on Monday.
Shutting down even more reactors, however, will require billions of euros of investment in renewable energies, more natural gas power plants and an overhaul of the country's electricity grid.
The government of neighboring Switzerland, where nuclear power produces 40 per cent of the country's electricity, also announced last week that it plans to shut down its reactors gradually once they reach their average lifespan of 50 years - which would mean taking the last plant off the grid in 2034.
Kudankulam Nuclear Power Plant (KKNPP).
The mess that Kudankulam now is entirely the creation of Government of India and Government of Tamil Nadu due to their arrogant, arbitrary and autocratic methods giving scant respect and regard to the voices if the very people who voted them to power. Instead of entering into dialogue with the protesting public to “ally their fears ” as promised, these governments sent ‘Sarkari Scientists’ who only printed reams of paper and gave it as expert opinion. When people asked for their voice to be heard it was responded with hundreds of criminal cases of sedition and ‘waging war against the state’. When people still persisted police brutality was let loose on women and children sitting on beach sands, peacefully protesting against ‘fuel loading’ in the Kudankulam Nuclear Power Plant (KKNPP).
Actually the three officers in charge of the situation – the DIG, the SP and the District Collector – showed restraint. It was IG Police Rajesh Das who landed up out of nowhere, gave the protesters 10 minutes to disperse and when they refused, ordered lathi charge and tear gas shelling. The men jumped into the sea; but women and children could not and were beaten up. This IG is the same man responsible for the firing on innocent Dalits at Paramakudi last year. He is stated to be incapable of handling sensitive situations.Yet he has been unleashing terror among the harmless villagers and hunting down SP Udayakumar, the PMANE coordinator, as if he is an international terrorist. One wonders as to whether Rajesh Das is taking orders from the autocratic Russian masters or the democratically elected Government of Tamil Nadu.
Police is using the High Court order as a cover for indulging in brutality. The fact is that there have been several bizarre, irregular and illegal happenings connected with KKNPP from the very beginning. What baffles me is the clearance given for fuel loading and the nuclear establishment going ahead with this ‘commissioning process’ despite the scathing report of Comptroller & Auditor General of India about ‘lapses in safety measures’ by the Atomic Energy Regulatory Board (AERB) posing “grave threats.” CAG, the highly regarded constitutional watchdog has highlighted several lapses by AERB: Non-preparation of a nuclear and radiation policy; No safety documents as recommended by two expert committees; No decommissioning plan which is extremely critical for public safety; Non-adoption of international safety standards and practices.The Nuclear Power Corporation of India themselves have said that they are yet to implement the 17 safety measures recommended by the post-Fukushima taskforce. The environment clearance was given way back in 1988. There’s no fresh water near the site. Yet High Court has passed the order ignoring all these critical issues. The case is now in the Supreme Court
Fact of the matter is that Nuclear power is neither the cleanest nor the safest. The human cost is too high. There’s been no new plant commissioned in the US for the last 20 years not only because of environmental concerns, but also because no bank is willing to fund it. No one is willing to insure a nuclear plant because of the risks involved. There are many hidden costs, among them the cost of decommissioning it after 40 years and storage of spent fuel for several centuries!
We don’t need nuclear power. There are better alternatives available. We have no uranium, but the Government of India is sabotaging the development of new coal and thorium-based technology. The PM talks of energy security, but how can there be any security if we have to import the reactors and fuel from abroad? The pressure from the superpowers has led to major efforts to sabotage all alternatives. Australia doesn’t have a single nuclear plant and wants to sell us uranium! The policy is driven by foreign interests. And where’s the land to establish new nuclear plants of 2,50,000 MW capacity by 2050?
In 1969, it was announced that by 2000, the country would have 43,500 mega watts of nuclear power. Today, we have just 4860 mega watts of nuclear power installed, of which barely 50 % is generated. It’s been a colossal failure. We have such fine IITs, they could have done R & D in solar and wind energy to reduce cost and improve efficiency. But such R & D was deliberately killed. Our Minister for New and Renewable Energy says India can’t survive without nuclear power!
Regarding the allegation of foreign funds and NGOs, all I would say is that a jaundiced eye sees everything yellow. Protestors, mostly fishermen are Christians. The Church does get foreign funds, for welfare projects but that’s through legal channels. I don’t think any of that money has gone into this protest movement. Fundamentalist elements branded Muslims as terrorists; now Christians are being condemned as traitors? What kind of ‘secularism’ is this?
The struggle in Kudankulam is indeed very unique. It is a community movement led by the people, not any particular leader. The 100-year-old St Lourdes church in Idinthkarai village provides space to the protesters, a majority of whom are fishermen and Christians. A few Hindu families residing in the village present a united front against the 2000 MW nuclear power plant. A majority of these fishermen know a lot about the harmful effects that a nuclear power plant might have on their environment and their livelihoods and about the Fukushima disaster.
The protest against KKNPP has been going on ever since May 1989 when the then Prime Minister Rajiv Gandhi signed an agreement with the Russians to build the nuclear plant in Kudankulam. Since then the locals have been raising their voices against the power plant, but nobody bothers to listen to them. Apart from Idinthakarai, 30 other villages around Kudankulam are fighting for the same cause. KKNPP would destroy the livelihood of most of these villagers who depend on fishing on the seas.
It is audacious to say that KKNPP is perfectly safe and nuclear power should be pushed at any cost. Let us hear from Mikhail Gorbachev, former president of USSR who signed the 1989 Indo-Russian Nuclear Agreement that brought the KKNPP: “Today we know that about 77,000 square miles of territory in Europe and the former Soviet Union has been contaminated with radioactive fallout, leaving long-term challenges for flora, fauna, water, the environment, and human health. Tens of billions of dollars have already been spent in trying to contain and remediate the disaster, with a new containment shell now being constructed over the 1986 sarcophagus and what’s left of the reactor. The material damage inflicted by Chernobyl, although enormous, pales in significance when compared to the ongoing human costs. The true scope of the tragedy still remains beyond comprehension and is a shocking reminder of the reality of the nuclear threat. The closed nature and secrecy of the nuclear power industry, which had already experienced some 150 significant radiation leaks at nuclear power stations throughout the world before the Chernobyl fire, greatly contributed to the accident and response difficulties. As the global population continues to expand, and the demand for energy production grows, we must invest in alternative and more sustainable sources of energy—wind, solar, geothermal, hydro—and widespread conservation and energy efficiency initiatives as safer, more efficient, and more affordable avenues for meeting both energy demands and conserving our fragile planet.”
Former Japanese Prime Minister Naoto Kan, nearly a year after he oversaw his government’s widely criticized handling of the Fukushima Daiichi accident in a recent interview with The Wall Street Journal has said: “I would like to tell the world that we should aim for a society that can function without nuclear energy.” Accordingly Japan cabinet has decided to phase out nuclear power in three stages and join Germany in being a totally nuclear free country. France, that has a dependency of 74% on nuclear energy is scaling it down to 50% and may further reduce it. Many other countries are following suit. India’s dependency on this source is a paltry 2.5% and this is the time to cry a halt starting with KKNPP.
What is the way out, many people ask. The answer is that Government can still reverse their decision. PMANE Expert Team has suggested fuel-switching and make KKNPP a liquefied gas-based power plant. This can be supplemented with wind, solar and tidal power for which there is huge potential in Kudankulam and adjoining areas. India’s leadership should plan country’s energy security taking into account these natural resources and not by imitating other countries which lack the natural resources India has.
Central Government and State Government of Tamilnadu should stop immediately the oppressive methods unleashed on the people agitating against KKNPP. An informed public debate and also a meaningful and an in-depth debate in Indian Parliament on the need for nuclear power will be in the interest of India’s future.
KKNPP should not be commissioned till a full and proper independent examination of the safety aspects of all the existing nuclear installations and nuclear power plants is carried out including the damage caused to the environment around them which includes water, soil and life forms of all types. Indian people have a constitutional right to know all the things that have been pushed under the carpet by Department of Atomic Energy by misusing the provisions of the Atomic Energy Act, 1962. This should be complied with, lest the NUCLEARGATE, possibly the most massive of all scams could unfold very soon as predicted by Dr. A.Gopalakrishnan, eminent nuclear scientist and former Chairman of Atomic Energy Regulatory Board!
FUKUSHIMA DISASTER ON 11.03.2011
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 80ºC at the end of October, and official 'cold shutdown condition' was announced in mid December.
• Apart from cooling, the basic ongoing task was 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 had to be evacuated from their homes to ensure this. Government nervousness has delayed their return.
The Great East Japan Earthquake of magnitude 9.0 at 2.46 pm on Friday 11 March 2011 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.
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 eroded about 65 cm into the drywell concrete below (which is 2.6 m thick). 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 on Tuesday 15th.
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.
