The Fukushima accident and nuclear energy

Gerald Lalor and Charles Grant , Guest Columnists

Jamaica has little to fear from the recent nuclear accident in Japan but it is nevertheless of much interest because of the expanding role of nuclear energy as one component of the effort to provide for global needs without increasing carbon dioxide levels in the atmosphere and the important lessons that can be learned from an event as large as the destruction of much of the Fukushima Reactor Complex by the tsunami resulting from the earthquake of March 11. An effect which, though understandable, was somewhat surprising because Japan has had so great a reputation for designing for such natural disasters.

Because Japan has few natural resources and uses large quantities of energy, it has become the world's third-largest user of nuclear power, producing 34.5 per cent of its electricity from 55 nuclear reactors. In addition, two nuclear plants are under construction, and 11 are in advanced planning stages. The country has also consistently expanded and improved its nuclear technology and influence. For example, Japan Steel Works Ltd is the only plant capable of forging the giant ingots for making the central part of the core containment vessel of a large reactor in a single piece. It has also developed the ability to reprocess spent fuel, and Toshiba, for example, has become one of the world's most important manufacturers of nuclear reactors and a leader in the thrust to develop the new smaller reactors.

The country has won its spurs and its commitment to nuclear energy appears irrevocable. In all these circumstances, a full report of the explanation of the events of March 11 when it is prepared and the necessary follow-up to improve safety will be of great interest, but even now the reports attract full media attention, as numerous countries consider what seemed to have happened and the impact on the future of nuclear energy.

The Fukushima Daiichi nuclear reactor site

The site is located on the northeast coast of Japan and contains six boiling water reactors of United States design (General Electric and Westinghouse), ranging in size from 460-1,100 megawatts, that were installed between March 1971 and September 1979. It is operated by the Tokyo Electric Power Company.

Figure 1. Fukushima Reactor Complex

A schematic of a typical boiling water reactor system is outlined in Figure 2.

Figure 2. A typical boiling water nuclear reactor

In principle, the reactor core is a powerful heat source which produces steam for the electricity-generating turbines. The zirconium cladding of the fuel rods is the first line of defence against radiation leakage; the second and most critical defence is the reactor core containment vessel, which is made of strengthened steel, four to eight inches thick. The final defence is the larger containment building made of steel and concrete. A breach of the containment building under accident conditions would release radiation into the atmosphere.

Performance of the reactors after the earthquake

Reactors 1, 2 and 3 of the Fukushima Daiichi's six reactors were at the full power rating of 1100 MWe when the earthquake struck. They shut down automatically, and since the input power lines were wrecked, the emergency diesel generators were used to begin removal of the residual heat that used fuel rods to generate even on shutdown as the radioactivity decays. This amounts to as much as seven per cent of the nominal power prior to shutdown, dropping to about two per cent within two hours and one per cent in a day, but even this is a lot of heat in a large core and can cause the water to boil building up the pressure in the coolant circuit. This can be managed through pressure-release valves, but venting increases pressure within the reactor building containment and eventually releases radioactive products like tritium, nitrogen, caesium and iodine to the atmosphere. This procedure seems to have succeeded in reactors 1 and 3 but there was a leak in reactor 2. In addition, the spent fuel casks in which the used fuel rods are stored lost some of their water.

The diesels worked for about an hour then quit. The backup battery system was not designed to handle the pumping load on its own. The owners notified the government of a technical emergency situation which allows officials to take additional precautionary measures. At this stage, without coolant, there was the danger of 'meltdown', a situation in which the reactor core melts, falls to the floor of the container and reacts with the concrete and other materials. This would destroy the reactor, likely leading to massive emissions of radiation. Attempts were made, with some success, to inject water into the reactor, but this involved a controlled slow release of air and water vapour to the atmosphere to release some of the pressure build-up and the radiation levels at the site boundary increased to 0.5 millisieverts per hour.

Water for injection was trucked in, helicopter spraying was used and, eventually, resort was made to the use of seawater. It is uncertain how much salt crust is forming on the fuel rods. Crusts would insulate the rods from the water and allow them to heat up. If they are thick enough, they can block water circulation between the fuel rods. The heating could rupture their zirconium cladding and release radioactive iodine and other material.

Explosions

As the temperature increased, some water would have decomposed to hydrogen, raising the likelihood of explosions, of which there were at least two which also contributed to release of radiation.

Ionising Radiation Leakage and health risk

Radiation is termed ionising if it can disrupt molecules and form charged particles or ions. This includes X-rays, the similar but even more penetrating gamma and a number of charged particles: beta rays which are very energetic electrons and alpha particles which are the nuclei of helium atoms are the most important of these.

Radiation Doses

The modern unit of radiation dose is the sievert (Sv), the dose equivalent from one joule (equivalent to 0.24 calories) of energy deposited by ionising radiation in one kilogram of tissue, making appropriate allowance for radiation type and the way in which the energy is deposited in human tissue.

The effects of high doses of radiation on humans are shown in Table 1.

The effects of radiation exposure are classified as deterministic, where the effect is certain to occur under given conditions, and stochastic, where the effect may or may not occur in an individual but is detectable in a sufficiently large population that received above a certain level of exposure.

Dose limits

The accepted dose limits are in the ranges of thousandths of a sievert (mSv), very much smaller than the levels at which effects on health are observed. Radiation workers are limited to less than 50mSv per year, and practices in the nuclear industry that would give doses greater than 5 mSv per year to any member of the public, or 1mSv per year to any group for an extended period, are not allowed. Medical fluoroscopy workers may receive up to 5mSv per year but most receive less than three, and many have doses too small to be measured. People living in areas of low natural radioactivity receive a dose of about 1mSv per year.

Some countries have areas with high background counts; these include Brazil, France, India, Niue Island, and Egypt. The highest recorded background levels are in Kerala and the Madras states of India where more than 100,000 people are exposed to an annual dose rate which averages 13 millisieverts per year. (The world average is less than one.) Residents in these areas show no clear evidence of increases in genetic damage or cancers. Data interpretation is complex, but at least it appears that man-made radiation in such quantities is unlikely to produce statistically significant harmful biological effects. This may be somewhat reassuring, but for whatever reasons; probably tracing back to nuclear weapons and to the propaganda of the Cold War, most of us have a deep-seated fear of ionising radiation. And there is a good old maxim - accept no more than the minimum possible exposures.

Foods contain small amounts of radioactivity because of potassium-40 and carbon-14, and frequently also small quantities of uranium, thorium, and their daughter products. The amounts depend on a number of factors including the soils from which the food originated. Normally, there needs be no concern. However, a large-scale nuclear accident can lead to a global food problem because of the movement of radioisotopes in the atmosphere and the extent of international trade.

The health concern with iodine-131 is that the body draws it to the thyroid gland, which can result in the development of thyroid cancer. However, the isotope has a half-life of about eight days, meaning its risk reduces fairly quickly after a release. Caesium-137, with a half life of almost 31 years, presents a longer-term concern, although particulates of the isotope will be dispersed more quickly by rainfall and ground penetration. Strontium can be particularly dangerous because it follows calcium into the bone.

A curious feature of radiation is how little of it, in terms of total energy, is required to harm. A dose of X-rays that would undoubtedly kill a man if given as whole body exposure is only equivalent to enough heat to hardly warm a cup of coffee. This is because the radiation is absorbed in discrete amounts, a lot is given to some cells doing great harm but others escape.

See Part Two in next Sunday's In Focus. Email feedback to columns@gleanerjm.com.

• Table 1. Effects of high doses of radiation on humans

Dose (Sv) -

Effects

>100 -

Most unpleasant: death occurs in a few fours due to massive organ disruption, especially of the nervous system A few such accidents have been observed.

>10 -

Death occurs after a few days and is preceded by nausea, vomiting and diarrhoea. The cause of death is the sterilisation of the cells whose reproduction renews the lining of the gut, resulting in holes in the lining.

Few Sv -

The blood forming organs are damaged but death is no longer inevitable. The person may be saved by transfusions, isolation in sterile conditions, antibiotics and careful nursing until the body can renew its capability to make the necessary blood cells again. There have been survivals from accident in this range but it is touch and go.