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The Great East Japan Earthquake

The Fukushima Daiichi Nuclear Plant Accident Caused by the Massive Tsunami

Yoshiaki Oka
Professor (Joint Department of Nuclear Energy), Faculty of Science and Engineering, Waseda University

Occurrence of Accident: Unexpectedly Large Tsunami

A severe earthquake and massive tsunami hit the Pacific coastline of eastern Japan on March 11, 2011. A total of six nuclear power plants automatically shut down because of the earthquake. Power from outside the plant was also lost. Emergency diesel generators started operating. Emergency reactor cooling systems also began to operate. However, the massive tsunami that struck about an hour later damaged the pumps of the seawater cooling system so the last-resort means of dissipating heat into the sea was lost. The diesel generators also stopped working due to electrical insulation failure so the entire AC power source was lost. The result was a severe accident exceeding the plant's design standards. Systems capable of supplying water to the reactors without using AC power operated, but finally these shut down when their control DC power supply was lost between several hours and two days after the accident.

Without the last-resort means of heat dissipation, the temperature and pressure of the water in the reactors rose, due to post-shutdown heat generation from fuel radiation (decay heat) and water levels in the reactor vessels began to fall. The loss of both AC power and final means of heat dissipation occurred in all of the reactors (common cause failure) so reactors 1 to 4 plunged into crisis one after the other. Reactors 5 and 6 were not at risk because their emergency diesel generators kept working and supplying electricity for their cooling systems to work.

Fire extinguishing equipment was used to douse the reactors in water in an attempt to cool them with the steam generated from the nuclear fuel. These severe accident response measures were established in the 1990s. (Figure 1)

Figure 1 Severe Accident Management Systems

Accident development: Common cause failure of loss of power and loss of ultimate heat sink

Because the reactor pressure was higher than the capacity of the pumps in the fire extinguishing system, water could not be fed in properly from outside. While work for pressure reduction was repeatedly held up by power outages, the fuel rods in the reactor (Unit 1) became exposed, their temperature rose, and oxidation of the zirconium at fuel cladding tubes caused water reduction resulting in the generation of hydrogen. Radiation doses were also high, which further hampered work activity.

Hydrogen is noncondensable so the pressure within the containment vessels rose. To reduce the pressure in the containment vessels (and thereby prevent damage to them and maintain their containment function), gas was released from inside. As the pressure in the reactors and containment vessels fell, hydrogen collected in the top of the reactor buildings. On March 12 and 14 respectively, the hydrogen in Units 1 and 3 ignited explosively and the two reactor buildings were destroyed. The reactor pressure vessels and their containment vessels are housed within concrete 2 meters thick so they were not damaged. (The reactors did not explode. They shut down.) In Unit 2 there was an explosion in the bottom of the containment vessel, which damaged the reactor pressure vessel and the containment vessel. (Figure 2)

Figure 2 Boiling water reactor

Accident resolution: Cooling by injecting water

Water has being injected into the reactors to cool them. The most important thing for cooling the fuel and preventing the release of radioactive materials is to keep the fuel covered in water. Although the cooling method of injecting water and condensing the resulting steam is unstable, the external power supply has been restored so there is only a low possibility of failure to the extent that there would be long-term fuel uncover.

Restoration of the pump and power supply in Unit 2 is taking a long time because of the high radiation level. The contaminated water injected into the reactor flows from the reactor building to the turbine building and has a high radiation dose, and the pump and power supply are positioned in these buildings. The water injected into the unit was flowing from the pit in through a crack caused by the earthquake and into the ocean. The flow from the pit in Unit 2 has been stopped. Leaking and cooling can be resolved by returning the leaking water to the reactor and connecting a heat exchanger along the way to achieve stable cooling.

(The path to accident resolution was shown by TEPCO on April 17. Simultaneous measures include stable cooling of the reactor and spent fuel pool, the containment, treatment, and storage of contaminated water and its reuse for cooling, and the control of radioactive material.)

Atmospherically released radiation: Far less than Chernobyl

Radioactive materials were released in the air together mainly with steam in the explosions on March 12 and 14. The amount of radioactive materials released was extremely low compared with the accident at Chernobyl in which the reactor itself exploded. (The level of radioactive iodine released-the IAEA's accident evaluation standard-was estimated at between one tenth and one hundredth that of Chernobyl. In Fukushima, hardly any nuclear fuel itself has been released.) Most of the radiation is inside the reactor pressure vessels, containment vessels and reactor buildings. The released substances are noble gases and volatile fission products. Noble gases are dispersed by the wind and disappear. They are not reactive so even if inhaled they are then exhaled. For the volatile fission products of consequence, iodine and cesium are mainly released as cesium iodide. Being water soluble, cesium iodide released into the atmosphere exists in dust and water droplets, and it fell to the ground in rain on around March 20. Water and vegetables became contaminated.

Iodine accumulates in the thyroid gland of infants. Cesium does not collect in any particular organ. The respective half-lives of iodine and cesium are eight days and 30 years while their respective elimination half-lives when taken into the human body are 5 days and 80 days. Contaminated vegetables and milk have been taken off the market. Their permissible radiation amount is set to the low exposure level due to naturally occurring radiation. This does not pose any health hazard.

To be able to store the highly contaminated water that had collected in the turbine building and trench, the water in the radwaste building tank was released into the ocean. The radioactive concentration of the latter was 1/10000th that of the highly contaminated water in the turbine building and trench, so this release was an unavoidable emergency measure. The permissible radiation concentration for seafood is controlled at the same level as for vegetables and other agricultural produce, and fish exceeding that level are taken off the market. This, along with harmful rumors, has had a significant impact on fishermen. Contaminated water flowing out of the plant is dispersed in the eastern Pacific by the Oyashio Current colliding with the Kuroshio Current. Measurement and control of seafood particularly along the coast is important. Long-term research into the accumulation of radiation in marine life is being conducted. Fisherman cannot operate in the seas around the nuclear plant because fishing rights for that area have been originally revoked.

Radioactivity in the environment: Natural and manmade radioactivity

Radioactive materials existed in the environment before the accident. It includes naturally occurring radioactivity as well as manmade radioactive materials emitted from nuclear weapons testing above ground and so on. Natural radioactivity consists of the atomic nuclei of elements which were formed when the universe was created and which have an extremely long half-life (hundreds of millions of years). They include nuclides formed by the α-decay and β-decay of uranium and thorium. One of these is radon, a gaseous nuclide that makes up half of naturally occurring radiation exposure. Another is potassium-40, which exists in human muscles.

For more than 50 years, observations of manmade radiation have been conducted in Japan and the reports published (http://www.mri-jma.go.jp/Dep/ge/2007Artifi_Radio_report/Chapter5.htm). Large amounts of radiation were released in the atmosphere at nuclear weapons testing above ground in the 1960s (and in China until the 1980s). This radioactivity has reduced to about one thousandth of the amount at that time, but still exists in the world. Even plutonium, in addition to cesium and strontium are measurable. After the end of nuclear testing in the air, radioactivity is blown to Japan mainly accompanying yellow sand from the Asian continent. It is re-suspension of the radioactivity of nuclear testing in the ground of the continent. Minute traces of plutonium have been detected in soil from the Fukushima Daiichi nuclear plant, but the amounts are small and at the same level as those originating from past nuclear testing. Moreover, nuclear tests were conducted using various isotopes of plutonium so it is difficult to judge only from the isotopic ratio of plutonium whether the traces are from nuclear testing or the recent accident.

Strontium radiation also needs to be reported so that readers can understand the relationship of the detected strontium with the background of past manmade radiation and seasonal changes (yellow sand).

Radiation detection includes statistical errors. The measurement results of small amounts of radiation should also consider and present the margin of error.

Impact of radiation on the body: No need to fear low dose exposure

The effects of radiation on the human body are acute disorders such as leukopenia and latent (late-onset) disorders such as leukemia and cancer. None of these are a concern with a dose of 100mSv or less. Genetic disorders have not been observed in humans. The health impact of radiation in the high-dose region is well known from data from Hiroshima and Nagasaki, medical and scientific data on x-rays from the last 100 years, and studies of the Chernobyl accident.

The latent disorders have been found by comparisons of incidence frequency in exposed populations and non-exposed populations (so-called epidemiological surveys). No significant difference is seen at low radiation doses. However, the effects of low doses on latent disorders need to be determined for regulations and radiation control. Extrapolating the effects from a fundamental point, regulations adopt a linear model without a threshold dose that considers the effect on health to be in proportion to the size of the dose (Figure 3). The International Commission on Radiological Protection (ICRP) recommends the principle of ALARA (as low as reasonably achievable with regard to radiation exposure). According to the linear no-threshold model, even the lowest amounts of radiation exposure exert a bad effect, which causes fear among the public about radiation and radioactivity. Such fear is also triggered by ambiguous remarks from experts such as, "Exposure to radiation is OK, but you should avoid it as much as possible."

Figure 3 Effect of low dose radiation on health and the linear no-threshold model (hypothetical)

We are exposed to natural radiation in our daily lives. We receive an annual dose of around 2.4mSv. Levels of atmospheric radiation measured far from the site such as in Tokyo are about the same as annual natural radiation levels so there is no need to worry about the impact on people's health. The good health effects of low doses of radiation have traditionally been used in Japan by people in the form of radium hot springs and radon spa. There is no need to be afraid of exposure to low doses of radiation.

The permissible concentrations in items such as vegetables, milk, and water have also been determined from the permissible radiation dose, which is within the variations of annual levels of natural radiation. The hypothetical linear no-threshold model is explained at the following site: http://en.wikipedia.org/wiki/Linear_no-threshold_model

In Radiation and Reason published in 2009, an Oxford University professor specializing in radiotherapy states that the permissible radiation exposure limits used until now are extremely conservative, and even twice 100mSv at once, 100mSv accumulated in one month, and 5,000mSv over a lifetime respectively, are still conservative.

In 2007 the ICRP set the limit for ordinary people in an emergency to from 20 to 100mSv as a standard in the event of a nuclear accident, and set the permissible dose if the effect of radioactive material remained after the accident to 1 to 20mSv. The upper limit for the currently enforced 20km radius evacuation zone has been set to 50mSv, and that for the next zone between 20 and 30km from the plant in which people have been told to remain indoors has been set to 10mSv.

Evacuation zone and vicinity: Detailed measures required

In the evacuation and indoor restriction zones and their surrounding areas there are hotspots of high radiation levels. In these areas, doses are thought to be high due to the accumulation of radioactive cesium in the soil deposited by rain, because the direction of the wind had been in the path of radioactive material dispersed (at the time of the hydrogen explosions). One strategy is to dig up and replace the topsoil.

We know from previous accidents that hotspots exist. In these areas, there is a risk of long-term doses exceeding the national standard of 20mSv. It is known that exposure over a long period of time has less effect on health (due to a degree of recovery from physical damage) than short-term (temporary) exposure, a fact which is utilized in radiotherapy. There should be a study of the length of time in those areas considering their accumulated doses. According to the standard given by the Oxford professor mentioned above, for example, long-term exposure will not exceed 100mSv over a period of one month. Adopting this figure would greatly reduce the number of evacuated residents and the length of time of the evacuation.

Furthermore, rather than whether to make the assessment period one year or not, reducing radiation by replacing topsoil may be an urgently required measure, according to ALARA. This would reduce local residents' exposure to radiation in the future. It should be acceptable to cover the residents' expenses so that they can carry out the work. In hotspot areas, measures such as replacing topsoil can meet the national standard of 20mSv. Respecting local residents, various other community based measures will probably be discovered from their own perspective.

In any case, TEPCO and the government need to do their utmost to resolve the accident immediately, avoid further leaks of radioactivity, and reduce radiation exposure doses so that they can discontinue the evacuation zone and indoor restriction zone and put an end to harmful rumors.

Issues to be examined: Conflict between conservatism in safety measures and impact on people at major disasters

Evacuation places a huge burden on residents with the possibility of disrupting and separating families. Harmful rumors also jeopardize the livelihood of fishermen and farmers. Safety is always considered in a conservative light, but the number of residents forced to evacuate varies greatly depending on the figures used as the basis for evacuation. The same is true of harmful rumors. A detailed response from the government is required. In a major disaster such as this, being conservative may not necessarily help the residents of the affected region. Disaster evacuation can be conducted in the way similar to the evacuation of children during wartime. Perhaps the conflicting issues of conservatism in safety measures and the impact on people need to be examined going forward.

For factors that are difficult to determine with certainty such as the effects of low dose radiation, the government should not simply respond with conservative figures such as zero or one. Rather, a new kind of relationship between the government and the public may need to be examined which could, for example, take the form of a policy of approval for the return of residents to their homes if they submit a letter of understanding of the potential risk (thereby consenting not to take legal action against the government.)

Although the crisis has not yet been resolved, progress is being made even if it will take a long time. Power and water sources are being restored and there is little possibility of their being lost for a prolonged time. Furthermore, half of the nuclear fuel has already destroyed, and radioactive iodine's half-life of eight days means that most of it has already decayed and disappeared. So there is no possibility of more radioactivity than at the time of the initial hydrogen explosion being emitted going forward. Cesium 137 has a half-life of about 30 years and will now become the main constituent of environmental radioactivity. Radiation damage that can be attributed to the effects of cesium has not been measured in the Chernobyl accident. Results of tests on soil contaminated by radioactivity emitted from above ground nuclear testing indicate that after two to three years, cesium had adhered to the soil and hardly any was being absorbed by rice. Also, the half-life of cesium in the earth's atmospheric circulation is 1.1 years, so even if radioactivity from Fukushima is observed overseas, such observed values are short-term and only about 1/100th of the values seen in the era of above ground nuclear testing.

Of course long-term follow-up studies and research will be required into the effects of cesium not only on the earth's surface but also on marine life.

Conclusion: Accident caused by massive tsunami; drastic improvements to safety mechanism needed

The cause of the accident was an unexpectedly large tsunami. Although the earthquake was the largest on record, the plant actually withstood it and the plant's emergency equipment did function. Possible future measures include installing an emergency power supply or cooling type heat exchanger at an altitude that would not be reached by a tsunami, improvements that have already begun at other nuclear power plants. Many costly lessons have been learned from this accident, but there is a risk of the institutions concerned moving too far towards a detailed tightening of regulations. First, there is a need to drastically improve the country's safety mechanism which was not based on an evaluation of safety against the kind of massive tsunami that only occurs once every thousand years or so. Detailed regulatory tightening has been continually conducted in Japan since the country began to use nuclear power. Although a lot of effort was put in, some essential points were missed. Since the Tokaimura nuclear accident, there has been wide-ranging reinforcement of nuclear regulatory bodies. This latest accident illustrates the shortcomings of these regulatory bodies and the problems with the safety mechanisms. However, although a drastic overhaul of safety organizations is required, improving the mechanisms does not mean creating another organization.

With the aim of improving global competitiveness, the overriding proposition is for Japan to build the very best national response mechanisms in order to survive the 21st century, not only in terms of safety. Countries and organizations that have built such top rate mechanisms are the ones that have survived.

Yoshiaki Oka
Professor (Joint Department of Nuclear Energy), Faculty of Science and Engineering, Waseda University

Born in 1946, Dr. Oka is an emeritus professor at the University of Tokyo and a past president of the Atomic Energy Society of Japan. He specializes in nuclear reactor engineering and his major publications include a series of nuclear studies textbooks titled Nuclear Reactor Design [Genshiro Sekkei] (Ohmsha) as well as Super Light Water Reactors and Super Fast Reactors and Advances in Light Water Reactor Technologies (Springer). His invention, the super light water reactor, has been studied worldwide as a fourth generation nuclear power reactor.

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Tokyo City University and Waseda University, Joint Department of Nuclear Energy