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Ten years ago, on April 26, 1986, at 1.23 A.M. a major accident
occurred at the No. 4 reactor of the Chernobyl Power Station. It is now
generally recognized (1,2) that this was the greatest catastrophe of a
civil nuclear power stationin the history of mankind. The accident was
characterized by two explosions and a subsequent fire which damaged both
the reactor and the containment structure. As a result there was a prolonged
release into the atmosphere of large quantities of radioactive material
including gases, halogens and particulate matter. The majority of the release
occurred over a period of about 10 days with two distinct peaks in release
rate (first on April 26 and second on May 6). In following days due to
a variety of mitigative measures the situation was put under control and,
although the release continued for several weeks, the rate of it was much
lower (3). The expelled fission products and fuel elements released during
the first peak (characterized in fact by the two explosions, the first
due to steam and a second due to hydrogen) accumulated in a cloud which
centered at approximately 4,500 m altitude. A complex and varying set of
meteorological conditions present in April and May 1986 in Europe dispersed
the radioactive cloud over the whole Northern Hemisphere (4,5). Initially,
as wind direction was north-west, the cloud of radioactive material was
carried over the Baltic Sea to Scandinavia. Within the next 24 hrs this
cloud split into three segments: one started to travel east, across northern
former USSR finally reaching China and Japan, the second travelled across
Norway being finally detected in the United States and the third turned
south-westwards pushing clouds over central Europe. By the end of April
the wind direction rotated clockwise, pushing incoming clouds first eastwards
across former USSR and then south finally reaching Turkey. During the first
week of May the direction of the wind changed again, this time to south-east
carrying radionuclides over eastern Mediterranean. The meteorological situation
between April 26 and May 6 was also characterized by a number of rainstorms
which resulted in numerous, still poorly identified "hot spots" where deposition
of radioactive material could be more than 100 times greater than in the
surrounding areas (6). All together the deposition of radionuclides in
a given country was a consequence of the distance from the place of the
accident, wind direction and rainfalls at the time of passage of the radioactive
cloud. Although, several nuclides were present in contaminated air masses,
this review will for obvious reasons concentrate on radioiodines. Preliminary
Russian estimates of 131I released to atmosphere amounted to about 0.264
EBq (=1018 Bq) (3), while more recent calculations suggest (7,8,9), that
131I release could be as high as 1.3 EBq, or even 1.7-1.8 EBq. Data on
other radioiodine isotopes are sparse. It is well recognized (7,8) that
they are present with higher core inventories and as their half-life is
short they are of special significance in first days and near the place
of nuclear accident. Due to a very unusual position taken by local and
central authorities of the former USSR in the first days of the Chernobyl
catastrophe (LA Ilyin - personal communication) spectroscopy of the environmental
radionuclides was not immediately performed. The characteristics of the
nuclear fuel of reactor No 4 in Chernobyl may suggest however (10) that
132Te and 133I released in the accident amounted to about 3.5 EBq. In Warsaw,
Poland, on April 28 and 29 approximately 28% of the air radioactivity was
present as short-lived isotopes of iodine (11). In Seibersdorf, Austria,
on May 1 the activity of 132I at ground level was over twice the activity
of 131I (12) and in the United Kingdom on May 2 activities of 131I and
132I in the air were equal (13). It can be therefore suggested, that in
some parts of Belarus or Ukraine the radioactivity due to short-lived radioiodines
could be higher than due to 131I.
All these data suggest, that the total release of radioiodines from Chernobyl which occurred between April 26 and May 6 was only about 100 times lower than the respective total release of radioiodines from all 77 nuclear tests performed above the ground by the USA and former USSR in the record year of 1962 (2). Available estimates also show, that the release of radioiodines during the Chernobyl catastrophe was approximately 2000 times higher than the release during the Windscale accident and about 3000000 times higher than the release of 131I during the Three Mile Island accident (2). It is estimated that the released radioactivity in Chernobyl was 200 times greater than the combined release of the atomic bombs at Hiroshima and Nagasaki. It should be pointed out that Chernobyl catastrophe was also unique in a sense, that the release of radioactivity was prolonged and that at least in areas closer to the burning reactor (but also elsewhere), short-lived iodine isotopes could significantly contribute to the total radioactive contamination. |
