THE BOMB TESTS FALL-OUT AND RADIO-STRONTIUM

Press, Volume XCV, Issue 28261, 26 April 1957, Page 10

THE BOMB TESTS FALL-OUT AND RADIO-STRONTIUM

[By Professor i

E. W. TITTERTON,

Professor of Nuclear Physics, Australian

National University, in the ‘’Sydney Morning Herald.'')

In an earlier article the main features of fall-out from various types of nuclear bomb tests were discussed and the results of assessments of the level of “external exposure” of the world’s population stated. The levels were found to be so small, that the genetic damage resulting was negligible, in comparison with the background radiation to which everyone is exposed or as compared with the average exposure to medical X-rays which is standard practice in the western world. But what of “internal exposure?” Would not radioactive ity taken into the human system be more dangerous? From the internal point of view the biologically important element in fall-out is strontium--90. Like calcium and radium, strontium is a bone-seeker and if it is taken into the body it ends up in the bones and can irradiate these for a long period of time; its radioactive half-life is 28 years, but it is not retained by bone for so long, the biological half-life being about eight years. Such irradiation, if it were intense enough, could produce bone tumours. The route of strontium-90 from the time of the explosion to its uptake in human bones is known in broad outline. Being out of the longest-lived fission products it is a significant component of “old” fall-out (that is, fall-out taking place months and years after the actual H-bomb detonation) and when it reaches the earth, it becomes incorporated into the upper few inches of soil. “Comforting” Discrimination Plants growing in soil take up radioactive strontium along with their necessary calcium, but it is found that the plants discriminate against strontium in favour of calcium by a factor of about 1.4 times. Cows, therefore, consume plants which contain calcium and strontium and both pass into milk. Again, fortunately, there is discrimination against radioactive strontium—this time by a large factor of seven times, so that the milk contains less strontium-90 per unit of calcium. Eventually human beings take strontium into their bodies by eating both vegetables and milk products and the strontium behaves in the body like calcium. But again calcium is deposited in bone in preference to strontium-90 —the discrimination against strontium in going from food to bone being about a factor of eight. Nature, therefore, reduced the concentration of strontium-90 in calcium by a factor of nearly 100 times in passing from soil to human bone via plants and cows’ milk, and this is a comforting fact. Nevertheless, the question arises whether sufficient of the material could enter a human body and be deposited in bone, leading to ill effects.

The answer is obtained by making measurements of the actual amount of material in the bones of man and animals. Programmes for such work have been under way for some time in the United States and the United Kingdom. As of July, 1956 (the latest date for which overall figures are available), the worldwide average strontium-90 content of man was about one-fifth of a micro-microcurie per gram of calcium.

Big Safety Factors This is one five-thousandth part of the permissible body burden of this material which the International Commission of Radiological Protection regards as safe—that is, it would not produce bone cancers in those who carry it. It should, however, be ‘stated that in the case of young children where growth is occurring and there is a much higher proportion of -active bone, experiments show such bones to have three to four times more strontium-90 per gram of calcium than the average adult. In these cases, therefore, the safety factor is reduced to some 1200 times.

Such large safety factors hardly warrant hysterical propaganda against test programmes at the present time, and, moreover, there is other important scientific evidence available which bears on the matter. Ever since the beginning of time man has been living with just such a problem. Radium, like strontium-90, is also radioactive and a bone-seeker and the upper 12 inches of the earth’s crust contains radium in considerable amounts. It, too, is absorbed into all living things and all human beings. Every adult man contains an amount of radium half a million times greater than the activity of the present stron-tium-90 burden. In fact, it is onethousandth part of the maximum permissible radium burden considered by the International Commission for Radiological Protection to be safe, and this, in turn, is approximately one -ten-thou-sandth part of the minimum amount that is known to have produced injury to human beings.

One cannot conclude that for any given soil content of stron-tium-90 the equilibrium human burden would be the same as the equilibrium body burden of radium of the same soil level. However, it is worth noting the smallness of the present stron-tium-90 values in relation to the amount of radium present in all soils, and more particularly in relation to the very much larger concentrations of radium that could be safely tolerated. It can be stated that careful measurements made in Australia, coupled with the world-wide surveys, show that both the external gamma ray dose and also the strontium-90 fall-out in this country are below the averages of world figures. The figures for Japan are nearly twice as great, because that island gets the full benefit not only of global fall-out. but also of local fall-outs from Soviet tests. Even so, Japanese figures do not give cause for alarm, and, in fact, they are smaller than the equivalent figures for the U.S.A. Future Risks Assessed A question which might reasonably be asked is how the stron-tium-90 burden will increase if weapon tests continue. If it is assumed that future weapon tests result in an annual fall-out equal to the highest amount experienced in any one year up to the present time, then an equilibrium value will be reached in roughly 100 years’ time which would be about 50 times as high as at present. This would still be many times smaller than the amount estimated to lead to the human body accumu-

lating a maximum permissible body burden of strontium-90. But such a whittling away of the presently large safety factor would make consideration of some restriction on the testing of highyield “dirty” bombs necessary in the next few decades. These possible trends, however, are continually checked by people responsible for weapon testing, and this situation would be reviewed many times before such levels were actually reached. It was probably considerations of this type which led the British Prime Minister, Mr Harold Macmillan, and President Eisenhower at Bermuda to suggest that all future weapon tests be registered with an international agency. The facts quoted above, based on careful world-wide measurements both of the external and internal hazards from weapon testing, allow the present situation to be summarised as follows: “Fear War: Not Weapon Tests’* Weapon tests of the type carried out within continental Australia and continental North America are trivial as sources of radio activity of world significance. The high-yield weapon of the fission-fusion-fission type is the one which produces problems on a global scale. Even including such weapons there is, however, at the present time a very large margin of safety, and weapon testing on the scale practised over the last few years and with the same mixture of weapon types could be continued for many years without serious worry. Development of true thermonuclear weapons, rather than the “dirty’’ fission - fusion - fission weapons, would lead to an improvement in the situation by adding a further factor of safety of about 100 times. Strontium fall-out is the factor which imposes a limit to the number of fission-fusion-fission bombs which should be fired in the next few decades. It is not too much to hope that the deliberations of the United Nations Disarmament Committee will eventually lead to the restriction of the number of weapons of this type which can be fired in any given year. There is a vast difference between a carefully prepared programme for testing a few nuclear weapons under well-controlled conditions and in such a way as to minimise the radiological hazard and an all-out war with both sides using nuclear bombs of all sizes in the most effective and damaging ways possible. This is a time when no nation can afford to have vital issues obscured by emotional outbursts and political manoeuvring®. Clear heads and unbiased judgments are essential if we are to pick our way through the pitfalls which might lead to world war. Mankind need not fear nuclear weapon tests; but it should certainly fear all-out nuclear warfare. (Concluded)