The world-wide deposition of long-lived fission products from nuclear test explosions

摘要

Measurements of the gross fission product activity deposited since 1951 at two stations in the U.K. have been described in an earlier report. A network of six stations in the U.K. and thirteen in other parts of the world has now been set up at which rainwater is collected over monthly or three-monthly periods and analysed for Sr89, Sr90, Cs137 and Ce144. The stations have been commissioned at intervals since the first was set up in May, 1954 and an account is given of the results obtained so far, particularly on Sr90 for which the records are the most complete.nIt is shown that most of the deposited is derived from those nuclear explosions whose clouds enter the stratosphere and return to earth slowly over a number of years. In the successive yearly periods between May 1954 and May 1957 the Sr90 deposition at a representative station in the U.K. amounted to 2.06, 2,24 and 2.55 mc/km2 respectively and the cumulative total in May 1957 was 7°5 mc/km2. Cs137 levels are about 50% higher than those of Sr90. The experimental data indicate that within a given region, fallout is proportional to rainfall. Based on some simple assumptions about the frequency of nuclear weapon tests in the future, estimates are made of the possible future levels of Sr90 in soil in the U.K.