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Nov 04, 2011

Lessons from Japan’s nuclear crisis

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At 14:46 Japanese Standard Time on March 11 this year, the north-east of Japan was hit by an earthquake followed, between 40 and 60 minutes later, by a tsunami. The earthquake registered nine on the Richter Scale and was the fourth-largest earthquake recorded since 1900. It was also the most powerful earthquake to hit Japan since modern detection and recording instruments began to be used, 130 years ago. It occurred under the Pacific seabed, at a depth of 30 km, near the east coast of the main Japanese island of Honshu. The epicentre (which is not the same as the centre) of the earthquake was 177 km north-east of Fukushima.

On its website, the US Geological Survey sums up the combined consequences of the earthquake and tsunami. “At least 15 703 people killed, 4 647 missing, 5 314 injured, 130 927 displaced and at least 332 395 buildings, 2 126 roads, 56 bridges and 26 railways destroyed or damaged by the earthquake and tsunami along the entire east coast of Honshu from Chiba to Aomori. The majority of casualties and damage occurred in Iwate, Miyagi and Fukushima from a Pacificwide tsunami with a maximum run-up height of 37.88 m at Miyako. The total economic loss in Japan was estimated at $309-billion. . . . The tsunami destroyed or severely damaged many coastal towns in the Kuji-Minamisanriku-Namie area.”

In the affected area, there were five nuclear power stations (NPSes) with 15 reactors between them. These were the Higashidori NPS (one reactor), the Onagawa NPS (three reactors), the Fukushima Dai-ichi NPS (six reactors), the Fukushima Daini NPS (four reactors) and the Tokai Daini NPS (one reactor).

All were affected, to a greater degree or lesser degree, by the earthquake. Three reactors (Onagawa Units 2 and 3 and Fukushima Daini Unit 3) experienced no problems and achieved cold shutdown on March 12 (the day after the earthquake). Three reactors (Fukushima Daini 1, 2 and 4) lost their residual heat removal (RHR) systems, but quickly had temporary cables installed to restore RHR capacity and they reached cold shutdown on March 14 and 15. Two reactors – Onagawa 1 and Tokai Daini – lost their external alternating current (ac) power supply, but switched to emergency diesel generators, and achieved cold shutdown on March 12 and 15. One reactor (Higashidori) was shut down for routine inspection, with its fuel outside the reactor; it lost its external ac supply but switched on its emergency diesel generators. All reactors which had either external ac or emergency diesel generator power achieved cold shutdown.

The Crisis at Fukushima Dai-ichi

Things went differently at Fukushima Dai-ichi, which is one of Japan’s oldest NPSes. On the day of the earthquake, three of its reactors were operating normally (Units 1, 2 and 3) and three were undergoing routine inspection (Units 4, 5 and 6). Its oldest reactor, Unit 1, had been commissioned in 1971 and its youngest, Unit 6, in late 1979. All six were boiling water reactors (BWRs), but Unit 1 was a BWR3 model with an electrical output of 460 MWe, while Units 2, 3 and 4 were BWR4 models with outputs of 784 MWe each, and Units 5 and 6 were BWR5 models – Unit 5’s output being 784 MWe and Unit 6’s 1 100 MWe.

BWRs are similar to the more common pressurised water reactor (PWR) design. However, in a PWR, there is a primary cooling circuit which pipes water under very high pressure (to stop it from boiling) through the reactor core; the water becomes extremely hot (about 325 ˚C) and is pumped to a heat exchanger where it transfers its heat to water in a separate, secondary circuit. The water in the secondary circuit turns to steam which is used to drive turbines and generate power.

By contrast, in a BWR, there is only one water circuit and the water is at a lower (but still high) pressure, allowing it to boil (at about 285 ˚C) and form steam; as the steam leaves the reactor core, remaining water droplets within it are separated out and the steam itself is used to drive the turbines. In BWRs, the turbines are part of the reactor loop, and so are exposed to some radiation and need to be shielded. Maintenance personnel need radiological protection (although most of the radionuclides in the steam have a half life of only seven seconds).

When the earthquake happened, the three operational reactors at Fukushima Dai-ichi automatically shut down (but were still hot), all external power was lost and the emergency diesel generators were started, as were the emergency cooling systems. But, between 40 and 60 minutes after the earthquake, the NPS was hit by the tsunami. Although the NPS was protected by a 10-m-high sea wall, the tsunami at this point was 15 m high. It swept over the seawall and knocked out the emergency diesel generators, electrical systems and batteries for Units 1, 2, 3 and 4. But the air-cooled diesel generator supplying Units 5 and 6 was not damaged and both these reactors reached cold shutdown status on March 20.

Following the tsunami, Fukushima Dai-ichi Unit 1 lost all electrical power, its isolation condenser (part of its emergency cooling system) stopped and its terminal heat sink capability was lost (meaning it could not transfer heat out of the reactor). Deprived of cooling, the temperaturs within the Unit 1 reactor soared. At temperatures of 1 000 ˚C to 1 300 ˚C, the zirconium from the tubes containing the fuel started to react with the water and steam within the reactors (seawater was pumped into the reactor using mobile fire pumps as an improvised emergency cooling measure) and so generated hydrogen, which increased the pressure within the reactor vessel. At 2 300 ˚C to 2 500 ˚C, the nuclear fuel, control rods and fuel assembly structures melted, forming a mixture known as corium; this melting is estimated to have begun about eighty minutes after the tsunami struck.

The corium pooled in the bottom of the reactor vessel and some of it may have leaked into the dry well floor (the dry well is part of the containment system for a BWR). The reaction with the water was accelerated and further increased the pressure. To relieve this pressure, steam and hydrogen had to be vented out into the containment structure and then the atmosphere. This started at 14:30 local time on March 12, but lack of power meant that it could not be directed, as desired, to the external chimney stack and much of the steam/hydrogen cloud actually flowed back into Unit 1’s service floor on the top of the reactor building. At 15:36 on March 12 the hydrogen exploded, destroying the reactor building.

Unit 2 lost all electrical power and all terminal heat sink capability, but its reactor core isolation cooling system (RCIC) continued to function until 13:25 on March 14. For some six-and-a-half hours after that the reactor received no cooling water and its fuel began to melt at about 18:00 that day, with the resultant corium collecting at the bottom of the reactor vessel. Seawater began to be injected into the reactor around 20:00 that evening. Again, pressure within the reactor vessel increased, requiring venting at 11:00 on March 13. At 06:00 on March 15, there was the sound of an explosion in the torus room within the containment structure. The reactor building itself appears not to have been damaged.

Unit 3 lost all electrical power and all terminal heat sink capability, but its RCIC system was activated at 16:03 on March 11 to inject water into the reactor. However, the RCIC stopped at 11:36 on March 12 owing to steam pressure problems (the RCIC is operated by steam pressure). The reactor’s high-pressure injection cooling (HPIC) system was started just under an hour later at 12:35, only to stop at 02:42 on March 13, again owing to pressure problems (the HPIC is also operated by steam pressure).

Unit 3 then received no cooling water for six hours and forty minutes, when seawater began to be pumped in. Again, its nuclear fuel, control rods and fuel assembly structures melted (starting about 08:00 on March 13), became corium and moved to the bottom of the reactor vessel. Again pressure built up and venting was required at 05:20 on March 14, followed by a hydrogen explosion at 11:01 that same day, which destroyed the reactor building.

Unit 4 was shut down and its fuel was stored in the spent fuel pond. But, with the loss of all electrical power, the ability to circulate cool water through the fuel pond was lost. And, at about 06:00 on March 15, an explosion occurred. It is believed that this was caused by hydrogen leaking into Unit 4’s building through pipes from Unit 3. Water was sprayed into the fuel pond, initially with fire pumps, but subsequently with a concrete pump feeding a hose on a 58 m boom, which allowed much greater precision in the aiming of the water through the damaged walls of the building.

All explosions at Fukushima were caused by hydrogen; there were no nuclear explosions.
These hydrogen explosions released con- siderable amounts of radioactive material, especially iodine and caesium, into the atmosphere. In addition, the debris from the explosion at Unit 3, which fell to the ground near the reactor building, was highly radio-active. From April 1 to 6, some 520 m3 of contaminated water leaked into the sea, while, with government approval, about 10 400 m3 of slightly contaminated water was released into the sea during the period April 4 to 10.

Meanwhile, at 19:03 on March 11, a nuclear emergency was declared and at 20:50 the Fukushima Prefecture government ordered the evacuation of all people living within a 2 km radius of the NPS. This radius was increased to 3 km at 21:23 by the country’s Prime Minister, who extended it again, to 10 km, at 05:44 on March 12 and yet again, at 18:25 that same day, to 20 km. As the main contamination zone from Fukushima Dai-ichi was found to extend to the north-west of the site, in May, people living 20 km to 40 km from the NPS were ordered to be evacuated, taking the total number of evacuees to some 100 000.

After the Crisis

So far, there have been no radiation casualties among either workers at, or residents around, the NPS (that is, no one has developed immediate radiation sickness). Three workers at the NPS were killed by the earthquake and tsunami, and two more have died, for reasons unconnected with radiation. Japanese health authorities are pretty confident that the long-term death toll from radiation will be zero.

External ac power was restored to the NPS on March 22. Stable cooling has now been achieved at the NPS, with temperatures at the bottom of the reactor pressure vessels being below 100 ˚C since the end of September and the radiation dose on the site declining. Cold shutdown of all three reactors is expected to be achieved by the end of this year. All three, plus Unit 4, will be decommissioned over the coming decades.

Release of radioactive materials from the NPS site has dropped by a huge amount – the intensity now eight-million times less than it was at the height of the crisis. A new building, covering Unit 1, has been completed to contain radioactivity, and 128 140 t of contaminated water has been treated. A lot of the radio- active debris has been removed using remotely controlled front-end loaders. Extensive decontamination activities are being conducted in the evacuated areas and it is hoped people will be able to start going home early next year.

Japan has learnt five major lessons from the Fukushima nuclear accident, reports Professor Hideki Nariai of the Science Council of Japan. The first of these lessons is the need to strengthen the measures needed to deal with severe accidents, such as earthquakes and tsunamis. The second lesson is that severe accident response measures need to be enhanced – measures, for example, which would prevent hydrogen explosions and the venting of gases from the containment vessel. The third is that nuclear emergency responses need to be strengthened to be able to handle simultaneous large-scale natural disasters and prolonged nuclear accidents; the fourth that the safety infrastructure needs to be reinforced, in particular the strengthening of the nuclear safety regulatory agencies, strengthening the legal framework and criteria and reinforcing safety guidance; and the fifth, but not the least, is to ensure that a safety culture is thoroughly instilled in the nuclear industry.

Since the accident, the Japanese authorities have evaluated the robustness of their country’s NPSes concerning external shocks, revised safety guidelines regarding seismic events and major accidents, and decided to overhaul the regulatory system, with three agencies and offices to be merged into one. A committee of investigation into the Fukushima Dai-ichi accident has also been set up.

Nariai cites other points as well. Science and technology (S&T) have developed enormously since Fukushima Unit 1 was commissioned 40 years ago – how do you, how will you, incorporate S&T advances in NPSs, which have a lifetime of decades? Nuclear energy today involves questions which cover a wide range of scientific and technological fields, and cooperation between these disciplines is necessary. And, in this crisis, Japan benefited from being a high-technology society with high-technology companies. But what about other countries? Those adopting nuclear energy need to develop nuclear expertise and, ideally, some nuclear industries as well. “The Fukushima accident was very severe but we are recovering from this accident,” affirms Nariai. “It is now on its way to settlement.”

Further Lessons and New Knowledge

“The Fukushima accident has confirmed what was known about the behaviour of non-cooled nuclear fuel and has given the kinetics of the irreversible processes that can occur,” points out Professor Robert Guillaumont of the French Academy of Sciences. “Each accident boosts the research in the field of safety because it brings to light new scientific and societal phenomena.” Among these new phenomena were the reaction between the hot fuel and water, between the melted fuel, control rod and fuel assemblies at the bottom of the reactors and water and the disassociation of water molecules by radiation (known as radiolysis).

One striking feature of Fukushima is that a number of separate events, each with a very low probability of happening, occurred almost at the same time. As a result, the methodology of the safety case analysis (SCA), which evaluates risks and incident and accident countermeasures in the nuclear industry, needs to be re-examined.

The cooling problems (of both operating reactors and fuel ponds) at Fukushima show a need to improve the defence in depth of NPSes, by providing greater water reserves. In addition, the numbers of fuel assemblies held in each fuel pond should be limited and consider- ation should be given to creating containment systems for radioactive emissions from these ponds.

Further, radioactive waste created by an accident, including contaminated water, can be significant and creates a big social problem. These wastes will have to be swiftly dealt with to prevent the spread of radiation, and the water decontaminated so that it can be recycled.

Fukushima also revealed the difficulty in controlling chemical (not just nuclear) hazards in damaged reactors, such as the build-up of steam and hydrogen, leading to explosions. There will be a need to install the means of controlling hydrogen and to cool the melted nuclear fuel in a damaged reactor.

Moreover, the leaking of radioactive materials into the environment after an accident can go on for a sustained period, but the material may remain in the area of the NPS and so the area may not experience the dilution that happens if the material is spread over a large area. Better modelling is needed of short range transport and deposition of radio- active particles. In addition, rapid means of decontaminating large areas, without spreading radioactivity elsewhere, are needed, so that people may return to their homes as soon as possible.

The capacity of reactors to withstand previously unconsidered events should be examined at the design stage. “Fukushima does not call into question NSS (Nuclear Safety Standards) but it calls first to apply NSS with increased rigour and then to improve some standards,” asserts Guillaumont. “Fukushima gives the opportunity to increase the pool of national and international independent experts; to connect national safety organisations to enhance and share expertise, as proposed by the [International Atomic Energy Agency] in 2010; to increase the credibility of the SCA and make it comprehensible and known to the public; and to give high priority and support to basic applied and technological research in nuclear safety, combining both operators, specific safety organisations and academic research organisations.”

(Professors Guillaumont and Nariai recently addressed the Academy of Science of South Africa Nuclear Energy Safety Symposium, in Pretoria.)

Edited by: Martin Zhuwakinyu
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