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RADIO-ACTIVE FISSION-PRODUCTS IN THE 

 HUMAN FOOD CHAIN 



J. F. LOUTIT 



Medical Research Council Radiobiological Research Unit, 

 Atomic Energy Research Establishment, Harwell, England 



It is now of course common knowledge that small amounts of fission-products 

 from nuclear fission are widely distributed around the earth and in the 

 atmosphere. Most of these radio-active materials result from the testing of 

 nuclear weapons; but, as the accident in 1957 at Windscale showed, a 

 contribution also comes from I'eactors and their associated plant. There are 

 two fundamental differences — firstly, in the case of weapons there is instan- 

 taneous fission so that, for each, the fission-products are a more or less pre- 

 dictable mixture of the short, medium and long-lived nucleides. In the 

 reactor, on the other hand, the process of fission has been going on for days, 

 weeks or months. Thus the composition of the mixture varies with time: 

 the short-lived nucleides will have largely decayed and the longer the charge 

 has been exposed to the neutron flux, the greater the accumulation of the 

 more long-lived products. Secondly, in the explosion of weapons there is a 

 deliberate i^elease not only of energy but of all by-products : whereas in civil 

 practice every effort is made to contain the radio-active materials, so that 

 in normal operations there is an escape only of unavoidable traces of radio- 

 active material. On a world-wide basis we are concerned more with the 

 deposition of products from weapons. The hazards from reactors and their 

 associated plant are confined to the local and moderately remote areas. 



I shall be concerned mostly with the fission-products from weapons and 

 the world-wide fall-out. Furthermore, my remarks will be chiefly concerned 

 with the landmass that I know, the United Kingdom; the global problem 

 has been considered and reported on recently by the United Nations 

 Scientific Committee. 



At the Atomic Energy Research Establishment at Harwell, the Health 

 Physics Division has for many years been recording the radio-activity in the 

 atmosphere, in rainfall and on the ground resulting from explosion of 

 nuclear weapons. In the earlier years these explosions were mainly of 

 weapons of kiloton size exploded in Nevada. Later there were contributions 

 from the U.S.S.R. and from the British tests in Australia. Weapons of this 

 size throw up debris to a varying height but this seldom exceeds about 

 40,000 to 50,000 feet, the region of the so-called tropopause between the 

 lower troposphere and the overlying stratosphere. Stewart and his col- 

 leagues at Harwell^ find that the radio-active fission-products from such 

 explosions are deposited relatively rapidly, the half-time of deposition being 

 about three weeks. During this time of course they are carried I'ound the 

 globe with the prevailing air-currents. 



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