Studies on the radioactive contamination due to nuclear detonations I. Studies on the Radioactive dust due to nuclear detonation in Bikini on March 1, 1954

A study has been made, from the health physics standpoint, of the radioactivity emitted from the dust collected from No. 5 Fukuryu Maru, which was showered by the strong radioactive ash at about 0-90 miles east of Bikini on March 1, 1954. The probable dose of external gamma radiation the crew might have received during their two weeks voyage may be estimated roughly about 500-800 rad. However,, judging from the strong radioactive contamination of the boat, it may be inferred that the crew might have received a considerable-degree of internal irradiation besides the external whole body gamma irradiation and the local beta irradiation on the skin where the radioactive dust directly contacted, The specific activity of the dust when it fell on the boat a few hours after the nuclear detonation may be estimated to be roughly about one curie per gram. From the radiochemical analysis and the beta-ray analysis, the major part of the radioactivity included in-the dust was found to be due to a mixture of various fission products, while the main chemical component of the dust (Bikini ash) itself consisted of a calcium compound. The alpha-ray was also detected by the use of an ionization chamber in that portion where transuranium elements, if present, were collected. The tuna fish and the shark fins which were brought back to Japan by No.5 Fukuryu Maru in the middle of March, 1954 were most strongly contaminated on the skin, but the fish caught later in the South Pacific were found to be contaminated more strongly in the internal organs rather than on the skin. The area in the Pacific where the radioactive contaminated fish were caught seemed to expand gradually with time. In the latter half of 1954, besides the ordinary fission products, the radioactive Zn65 which was not found in appreciable amount in the original Bikini ash has been detected from the internal organs of the contaminated fish. A possible production of Zn65 by the neutron activation of some metallic part of the bomb devices at the time of nuclear detonation may be inferred,, Based upon the results of the biological experiments conducted with the extract of Bikini ash using rats and the result of the radioactive analysis of some of the organs of Mr. Kuboyama, the chief wireless operator of No. 5 Fukuryu Maru who died at about 206 days after the nuclear detonation on March 1954, an attempt has been made to estimate the internal dose of radiation. Although the accurate estimation seems to be extremely difficult, judging from the degree of strong radioactive fallout and the various conditions of the boat and the crew, the upper limit of the integral internal dose for the liver might be estimated in the range of the order of magnitude of about 102-104 rads, while that of the bone in the range of about 1-300.rads. From the degree of strong radioactive contamination of the boat and the results of the radiochemical analysis of the Bikini ash, the size of the nuclear detonation conducted on March 1, 1954 in Bikini was estimated to be roughly about the order of magnitude of 10 megaton TNT equivalent fast neutron fission of the natural uranium or uranium 238. (author)