THE BOMB TESTS FALL-OUT AND THE GENETIC PROBLEM

Press, Volume XCV, Issue 28260, 24 April 1957, Page 12

THE BOMB TESTS FALL-OUT AND THE GENETIC PROBLEM

(By

Professor E. W. TITTERTON

in the "Sydney Morning Herald”)

(Reprinted by Arrangement)

I

Hydrogen bomb tests have bes come the centre of political controversy again—mainly due to ! Japanese propaganda against i Britain’s forthcoming Christmas Island tests. Some Japanese themselves regard the fuss and bother as absurd and one newspaper, “Asahi,” went, so far as to say that the matter had reached the proportions of a new disease which they referred to as “radioactivity neurosis”! But, nevertheless, the layman may wonder what is the truth of the matter and this can best be reached by a sober, unemotional study of the facts. Let me start by clearing up the confusion about the types of bombs which are tested. (1) NUCLEAR FISSION BOMBS (often called atom bombs): .These weapons have yields ranging from a few thousand tons T.N.T. equivalent to an upper limit of perhaps 200,000 tons of T.N.T. equivalent. They can be made to work only by using highly fissile material such as plutonium or uranium-235. When such bombs are tested radioactive fission products are produced and give rise to fallout, but not on a global scale. The energy of explosions of this type is too small to allow the radioactive cloud to penetrate through the tropopause, which lies about 40,000 feet above the surface of the earth. The activity is confined below this level and is blown by prevailing winds so that it moves around the earth at approximately the same latitude. Fall-out from it therefore occurs over a relatively narrow zone of the earth. Bombs of this type are tested in Nevada in the U.S.A, and at the testing sites in Australia as well as in ' the U.S.S.R. The radioactive fallout which they produce is trivial ' in comparison to the hydrogen ; (high yield) weapons tests. (2) THE TRUE THERMO- J NUCLEAR BOMBS (often called 1 fusion or hydrogen bombs): i In this device a nuclear fission 1 bomb of the type described above < is used to ignite a nuclear fuel ; which may be one of several i types but most probably involves heavy hydrogen (deuterium). The energy-producing reactions are then the fusion of mixtures of ! light elements with helium as the j end product and they create no < radio-active fission products what- ‘ so-ever. This statement is true 1

irrespective of the size of the bomb and it is believed that such bombs can be made as big as desired up to, .say, 30 million tons (30 megatons) T.N.T. equivalent. “Relatively Clean” Some radioactivity is associated with such explosions; it comes from the fission bomb detonator and is more than in the usual cases of fission bomb explosions because the vast production of neutrons i vaulting from the fusion process causes almost complete fission of the detonator core. But, irrespective of the size of the fusion bomb, the radioactivity involved will be not more than some ten times greater than that of a conventional fission bomb of type 1. This type of high-yield hydrogen bomb therefore is, relatively

speaking, a radio-actively “clean” weapon and does not contribute alarmingly large quantities of radioactive material to man’s

environment. In particular the fusion reactions can never produce strontium-90 which, as will be discussed later, is a biologically undesirable product.

Without doubt some bombs of the thermo-nuclear type haVe been tested and It is to be expected that bomb designers will be moving towards improved versions of such weapons. (3) THE FISSION-FUSION-FISSION BOMB (Often incorrectly called a fusion, a hydrogen, and even a thermo-nuclear bomb): «

In this type of weapon there are three energy-producing stages. A nuclear fission bomb is used as a detonator to start fusion reactions going in a second stage. The neutrons produced in reactions of this second stage are then used in a third stage to produce fission in natural uranium, which is cheap and plentiful. The bulk of the energy is actually produced in the third stage by the fission process and therefore correspondingly increased quantities of radioactive fission products (including strontium-90) are formed. In this type of weapon the quantity of radioactive fission products is proportional to the size of the explosion—rif the energy yield is doubled the radioactivity is doubled.

“Global Fallout” Neither the U.S.A. nor the U.S.S.R. has stated definitely that it has used this type of weapon, but it is commonly believed that the majority of so-called hydrogen bomb tests carried out by both nations have been of this type. From the radioactive fall-out point of view this is a “dirty” weapon, and it is the radioactivity from such weapons which has given rise to problems associated with what has come to be known as “global fall-out.”

To emphasise the point it is only necessary to state that a 20 megaton weapon of this type would produce about 100 times as much fission radioactivity as a 20 megaton weapon of type 2 and no less than 1000 times as much radioactivity as the test of a 20 kiloton nuclear fission bombWith these ideas clear in our minds, let us examine how global fall-out comes about. It results entirely from the firing of highyield bombs of types 2 and 3. Such bombs are so powerful that the great boiling mass of radioactive material formed during the explosion is carried up through the tropopause into the stratosphere, sometimes as high as 100,000 feet, and there it spreads but in a layer above the tropopause forming a sort of weakly radioactive skin to the earth. This skin, however, is some 40,000 feet above the earth’s surface at a level far higher than the region in which aircraft fly. Because the stratosphere is a highly stable region, much of this radioactive material remains there for many years, possibly 10 or 20, during which time it decays continuously and steadily gets less

- potent, but some slowly drifts . through the tropopause and once it reaches the rain-forming layers 3 of the atmosphere it is brought t down to earth in a few weeks s by the scavenging action of falling rain. Fall-out from this cause is found to be distributed more r or less uniformly over the whole •, of the earth's surface, and hence f its name “global.” s A Necessary Distinction 5 There is, therefore, a most important distinction between nuclear fission bombs (type I—-so-called atom bombs) and high yield i bombs (so-called hydrogen types » 2 and 3 bombs). The former produce very small quantities of ; radioactivity compared with the 1 latter, and the fall-out from their t clouds is confined to a strip t around the world at approximate- * ly the latitude of the test site. High-yield weapon explosions, however, Itead to falj-out over the 1 whole surface of the globe—-no i corner escapes—and this fall-out continues for many years after J the detonation. Finally, the ; quantities of radioactivity involved • in the case of fission-fusion-fission E bombs is equivalent to firing of » some 1000 of nominal fission bombs ; (type 1). ; It is well known that exposure to radiation can have deleterious i effects on health if the level, is > sufficiently high, and therefore . those people whose duty it is to > carry out weapons tests on behalf of their governments have taken all possible steps to keep t radiation exposure as small as possible and to determine by ; physical measurement exactly what exposures do occur. Global fall-out (that is, fall-out from types 2 and 3 weapons) has ’ turned out to be most significant biologically because, although it is at a very low level, every single member of the human family is ’ exposed to it. Exposure can be of two types, one of which we might call “external” and the other we might call “internal,” to denote radioactivity taken into the human system. External exposure results from radiations emitted from fall-out material on the ground and is mainly due to Exposure of this type could lead to genetic difficulties if it were high enough and the difficulties depend on the number of people exposed. Reassuring Reports For this reason careful fall-out measurements have been made all over the world. Both in the Vnited Kingdom and in the United States non-Govemmental committees of leading scientists, including geneticists, have examined the relevant data. In England the Medical Research Council appointed an independent committee which reported that “the present and foreseeable hazards from external radiation due to fall-out from the test explosions from nuclear weapons, fired at the present rate and in the present proportion of the different kinds, are negligible.” The American Academy of Science, which carried out the investigation in the United States, reported that “the fall-out dose to date (and its continuing value if it is assumed that the weapons testing will not be substantially increased) is a small one as compared with background radiation, or as compared with the average exposure in the United States to medical X-rays.” The Indian Government, which has always opposed nuclear weapon manufacture and testing on moral grounds, asked for an assessment of the position from its biological scientists. The results are given in the Indian Ministry of Information publication “Nuclear Explosions” (1956) where the statement is made that “the genetic harm already done to mankind from test explosions is probably negligible.” These statements, made after careful investigation by responsible scientists, are in striking agreement. They indicate that the hazard from external radiation as a result of all weapon tests made to date is negligible both from the genetic and general biological points of view. Restrictions on Tests

The external radiation exposure as it is at present or is likely to be in the foreseeable future is far to small to require a restriction of weapons test programmes.

The situation in regard to internal exposure is involved and will be discussed in a following article. But it can be said here that a similar conclusion is reached on the evidence available—the internal exposure is still so low that there is no sound scientific reason for cancelling the British H-bomb tests at Christmas Island or for cancelling any other weapon test for that matter. The over-all situation relative to nuclear weapons testing, including safety defence, and international aspects was well summarised in a “Sydney Morning Herald” editorial on June 22, 1956, which said: “It would be little short of calamitous were pressure to discontinue these tests allowed to develop . . . whether we like it or not, we are living in an atomic age, and an age in which the only sure guarantee of security is possession of these terrible weapons. It is in Australia’s own interest—as was rightly recognised when Australia agreed to co-operate with Britain—that the British Commonwealth should have them at its command. “The risk involved is negligible compared with the risk of being left behind in the atomic weapons race. That has to be accepted by the Australian people no less than the Australian Government. It will, however, be hoped that the time is very near at hand when weapon tests can be discontinued, and when governments and scientists can concentrate on the peaceful uses of atomic energy.” (To be concluded.)