I216 NATIONAL INSTITUTES OF HEALTH CLINICAL STAFF Internal Medicine around it,,.pyoducing a variety of isotopes. significant only in relatively short-term fall- they decay, fequintly in a very confpli- many years it was not considered to be a serious hazard. #°I has about 4.5% yield; it has a 2.5-hr half-life, so it is only im- Most. # these isotopes are radioactive and cated way. The average numberof isotopes in a chain before any-given fission fragment reaches a stable isotope is through six dif- ferent:‘daughter.generations. There are 4 numberof isotopes of major biological importance. First, there are two induc&d_isotopes that should be considered. 14C is a-trivial fission product, but because of the intense neutron source‘there is a nifrogen-neutron reaction giving *C. It was estimated several years ago that.at that time the amount of *G in the atmosphere was roughly 70%, greater than before the first atomic explosion. 2*Na is another isotope that is largely an induced isotope, and it is of particular im- portance in underwater blasts or blasts that are at all close to seawater because of the sodium in the ocean. Under these conditions, a substantial amount of *4Na, which has a 14-hr half-life, can be formed, and in short-term fallout this can be of some importance. There are three or four other isotopes that are of importance for several reasons. Sr, for example, has a 5.3% fission vield; that is, of 100 atoms of uranium, 5.3 atoms (if normalized for mass) end up as Sr, so this is a significant fission product. It also has a 28-yr half-life so that it will persist for a long time. Finally, it localizes in bone so that it can be of importance biologically. The iodine isotopes are clearly of importance. Let me give a few numberstoillustrate variations in fission yield with the type of device and the magnitude of release of radioactive iodine. For *U and_highenergy neutrons as the explosive device, there is a 4.7% yield of 181. With 785U fission the yield is 3%, and for 78U with thermal! neutronsit is 2.9%, so that depending on the device there can be a fairly substantial difference in the amount of production of any given isotope; but in each case 181] is an important product. 1I is clearly out because of its 8-day half-life, and for portant in very short-term fallout. 183] has a substantial yield, 6%, and a 21-hr halflife, so it is of some importance over the course of perhaps a week. 1#5J is another radioisotope of iodine; this has a 6.7-hr half-life and is important only for a few days. There are some other isotopes from fission explosions that must be considered. One is 487Cs, which still can be detected in most exposed individuals in the Marshall Islands. It is also a potential problem because of relatively long-term storage in, muscle. Cesium is further worrisome in? meat eaters such as Eskimos who eat cari-§ bou. There is cerium, which has almost a® 3-yr half-life and a substantial fission yield, and finally barium. The amounts ofthese isotopes formed are perfectly enormous. Just one example: Explosion of a megaton bomb ofthe fission type produces enough radioactivity so that if it is evenly distributed over 1,000 square miles it will give in 1 hr a dose rate of the order of 1,000 rads/hr. Hence, a 30-min exposure to this amount of radiation would, in general, be lethal. This is for a 1-megaton bomb, but you may recall that the Russians exploded a 100-megaton bomb a few years ago. The explosion to which the Marshallese were exposed was of the order of 15 megatons. . The lifetime of these fission products is very complicated because there are literally hundredsof isotopes formed, all of different half-lives. There is, however, an empirical “rule of 7” that states that at any given time for the first 8 or 10 days the amount of radioactivity remaining after a fission explosion decreases by a factor of 10 after 7 br, by a factor of 10? at 7? hr, and by a factor of 108 at 78 hr. Therefore, if at 1 hr there are 100 units of radioactivity, at 7 hr