Abstract
On the morning of Thursday, 30 September 1999, a criticality accident occurred in a uranium processing plant at Tokai-mura, Japan. In the News and Information section of this issue of the Journal, Dr Kenzo Fujimoto, one of the Journal's International Advisors who has been closely involved with the subsequent investigations, describes the accident and its aftermath, as currently understood. An IAEA report is expected soon, and may be available by the time this editorial is published. However, it is clear that three workers were seriously exposed during the accident, and that two of them have received doses that are life-threatening due to deterministic effects. A criticality accident, an uncontrolled, self-sustaining, neutron-induced nuclear fission chain reaction that generates an intense burst of radiation, is an esoteric event that is unique to the nuclear industry, and careful design and operational controls are necessary to ensure that such accidents do not occur. Obviously, something went very wrong at Tokai-mura. Criticality accidents do not, in general, have major off-site consequences, although there may be releases of short-lived fission products produced by the criticality and, in lightly shielded facilities, direct exposure of members of the public to the radiation burst could be a problem for buildings situated close to the site boundary. It is the workers, however, who bear the brunt of criticality accidents, particularly in plants without heavy radiation shielding as at Tokai-mura, and it would seem that only intensive medical intervention prevented the deaths of two Japanese workers in the days following the accident. The results of the investigations are awaited with interest.
Coincidentally, in this issue Jean-Marc Cosset and his colleagues describe the experience of the Insitut Curie Radiopathology Unit, Paris, in treating patients involved in radiation accidents, including four (from outside France) who eventually died as a result of their exposures. It is instructive that many of the accidents involved industrial radiography, reminding us that occupational radiation protection extends much more widely than the nuclear industry.
An accident that most certainly had off-site radiological consequences was the explosion at the Chernobyl nuclear power station, Ukraine, in April 1986. The marked excess of childhood thyroid cancer in those areas of the former USSR that received the highest levels of contamination with radioiodine is well established, but there is little evidence as yet for other radiation-induced adverse health effects, either in the general population or in the workers involved in the recovery operations in the wake of the accident. In this issue, Victor Ivanov and his colleagues from Obninsk, Russia, present the results of their analysis of thyroid cancer in the territories of Russia most affected by the Chernobyl accident. Once again, strong evidence for an increase of childhood thyroid cancer since 1991 is presented by these authors, especially in those under 5 years of age at exposure, that cannot be accounted for by enhanced detection efficiency through medical screening. Further work is in progress in an attempt to establish thyroid doses and the resultant risk coefficient, and also what factors might modify the effect of exposure to radioiodine.
Also in the News and Information section is a report of an epidemiological study by Louise Parker and her colleagues [1], who found a statistically significant increasing trend in the stillbirth rate for births to partners of male Sellafield workers with the cumulative external radiation dose received by the fathers before the conception of their children. This study is based upon a sound database of all births in Cumbria during 1950-1989, good linkage between fathers and Sellafield workers, doses based on occupational dosimetry records, and a sophisticated statistical analysis. Parental irradiation is predicted to increase the risk of health effects due to heritable genetic damage in subsequently conceived offspring, including congenital malformations, some of which will result in stillbirth. And yet the magnitude of the trend is considerably greater than would be expected from current genetic risk estimates [2], which are generally regarded as conservative, and, as Mark Little shows in a paper in this issue, the trend is statistically incompatible with the level of adverse effects experienced by children born to Japanese parents exposed during the atomic bomb explosions at Hiroshima and Nagasaki. As Hazel Inskip has noted [3], the trend is based on small numbers, which is always a concern. Stillbirth is a difficult endpoint to study in this respect because the genetic component (the only component that can be affected by paternal exposures of any kind) is small and this is embedded in a background of factors - year of birth, social class, birth order and maternal age - that have a major impact upon stillbirth risk. For example, during 1950-1989 the Cumbrian stillbirth rate fell fivefold. All of these risk factors will be associated, to some extent, with paternal dose and there must remain a question over whether the analysis has adequately accounted for the influence of these background factors, especially when information on maternal factors (such as maternal age), which are so important to the risk of stillbirth, was not available to the study. Some indication that insufficient data on other risk factors could be a problem is the association of borderline statistical significance between paternal dose and specified causes of stillbirth other than congenital malformations, since it is difficult to envisage how these causes could be strongly linked to any sort of paternal exposure. Appreciable care is in order when interpreting this trend and no doubt further work will shed more light on its meaning.
Editor's note. On 15 November 1999, the International Atomic Energy Agency published the report entitled ``Report on the Preliminary Fact Finding Mission Following the Accident at the Nuclear Fuel Processing Facility in Tokaimura, Japan''. Details can be found at the IAEA website (www.iaea.org). The report will be reviewed in the next issue of the Journal.
References
[1] Parker L, Pearce M S, Dickinson H O, Aitkin M, Craft A W 1999 Stillbirths among offspring of male radiation workers at Sellafield nuclear reprocessing plant Lancet 354 1407-14
[2] Wakeford R, Tawn E J, McElvenny D M, Scott L E, Binks K, Parker L, Dickinson H and Smith J 1994 The descriptive statistics and health implications of occupational radiation doses received by men at the Sellafield nuclear installation before the conception of their children J. Radiol. Prot. 14 3-16
[3] Inskip H 1999 Stillbirth and paternal preconceptional radiation exposure Lancet 354 1400-1