Catastrophic breaches of a reactor vessel and its reinforced concrete containment by an MFCI, a disintegrating plant fragment or a hydrogen explosion are circumspectly engineered to be highly improbable. From all precursors the aggregate probability is typically no greater than 10“7 per operating year, or assuming a Poissonian distribution there is an expectation of one such rare event in 10 million years. However, granted such an event, the release of fission products would pose the principal danger to public health and the environment. In this context, the hazard is represented by the expected increased6 number of cancers induced in the surrounding population. The fission products from U-235 and Pu — 239 are quite similar, and toward the end of a 3-year fuel cycle the actual fission product inventories of fast and thermal reactors are broadly the same. Consequently the hazards from both reactor types are similar. With regard to plutonium it is neither a significant chemical poison nor a radiological hazard because

i. Its principal emission is a-particles which pose no threat outside the human body.

ii. The half-lives of Pu-239 and Pu-240 are about 24,000 and 6500 years respectively [76], so the radiation dose per unit of absorbed mass is relatively low.

iii. Though inhalation constitutes the most serious hazard, reten­tion in the lungs occurs in common with other aerosols for sizes 1 to 5 mm only. Particles below 1 mm tend to be exhaled while those above 5 mm are expelled in phlegm.

Those additional to the natural incidence.

Releases of some nuclides like Strontium and Caesium rapidly plateout, so serious effects on public health can be prevented by isolating the surrounding area from the food chain for a limited period of time. Noble gases like Krypton and Xenon form a significant portion of the fission product inventory, but their hazard is markedly reduced because there is negligible lung absorption of these gases. On the other hand wind-borne aerosols of radioiodides or their chemical salts[56] are readily absorbed, and this effect is aggravated by a selective accumulation in the thyroid. For these reasons, radioiodides[57] or their compounds are generally considered [161,162] to be the major hazard to public health in the unlikely catastrophic failure of a reactor and its containment system. Indeed, Farmer’s Reactor Safety Criterion [157] is based on the likely number of additional thyroid cancers if the iodides in a fission product inventory were to be released at ground-level. Developments in digital computer hardware and simulation techniques now allow dose-rates and induced cancer statistics to be calculated for many components of a fission product release and with more detailed representations of local population density and dispersion due to weather conditions. Even so, the predictions still over-estimate the hazards (grossly so according to some opinions [104]) by neglecting important alleviating factors. For example

1. With a water reactor, the mass of iodides available for dispersion would be markedly reduced by dissolution in the large quantities of water and vapor in the reactor and its containment [104]. With fast reactors, their sodium coolant would operate in a similar manner.

2. It seems likely that fission products would be released into the atmosphere as a hot rising plume. This effect, which promotes their dispersion and lowers the dose rate, is not represented in some computer simulations.

3. The hazard is defined in terms of additional induced cancers, whereas owing to highly effective surgery actual fatalities for the thyroid condition are only about 10 to 20% of all presentations [163,164].

4. Data on radiation-induced cancers is available only from the high- dose cases of Japanese A-bomb survivors. Predicting the incidence of cancers caused by an atmospheric release of reactor fission products necessitates a linear extrapolation to much lower dose — levels. However, no observable human health effects have been demonstrated below about 20mSv, due almost certainly to the regular natural replacement of body cells. Indeed, the Chernobyl exclusion zone is now populated by normal healthy families of wolves and bears [322]. This linear dependence can therefore be reasonably assumed to be pessimistic [163].