Report of the United Nations Scientific Committee on the Effects of Atomic Radiation. General Assembly 19. Session. Supplement No. 14(A/5814)

The nuclear explosions carried out between September 1961 and December 1962 sharply increased the radio-active contamination of the environment and consequently the doses of radiation that human populations will receive. However, the Committee notes that after the cessation of nuclear test explosions in the atmosphere, in outer space and under water, and in view of the propitious circumstances prevailing, further contribution from these sources to the radio-active contamination of the environment has ceased. Information on the amounts of various radio-nuclides and on the rates at which they deposit on the earth's surface and enter the food chain is necessary in order to compute the doses to human tissues. Since the cessation of atmospheric tests in December 1962, the Committee has been able to collect sufficient information to enable it to up-date adequately the estimates of the resulting radiation doses. Almost all of the fission products from the 1961-1962 explosions have been introduced into the stratosphere. The strontium-90 from these tests increased the stratospheric inventory at the end of 1962 by about 5 megacuries over the level in mid-1961. The rate of transfer from the stratosphere to ground level depends upon the altitude to which the products rise in the atmosphere and the latitude at which the explosions occur. For example, the mean residence time of material in the stratosphere above 100 km exceeds five years while in the lower stratosphere it is less than one year. Assessment of the experimental data has led the Committee to adopt an over-all mean residence time for the composite stratospheric fission products of two years. While this time is shorter than that used in the 1962 report, the predicted deposition of strontium-90 and caesium-137 is not appreciably altered by the change in the mean residence time. The fall-out rate of long-lived radio-activity in 1962 was three times that for the period 1960-1961 and during the year 1963 the fall-out exceeded that in any previous year. The Committee envisages that in 1964 the fall-out rate may be some two-thirds of that during 1963 and will continue to decrease progressively in future years