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Plutonium-244 in the the Pre-Fermi Early Solar System and Natural

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Plutonium-244 in the the Pre-Fermi Early Solar System and Natural Geochemical Journal, Vol. 26, pp. 1 to 20, 1992 The Shibata Prize Awardee's Lecture Plutonium-244 in the early solar system and the Pre-Fermi natural reactor' P. K. KURODA2 4191 Del Rosa Court, Las Vegas, Nevada 89121, U.S.A. (Received December 2, 1991; Accepted February 17, 1992) system and the natural reactors, many years later INTRODUCTION in the United States. It is therefore a great honor I express my deep gratitude to the and pleasure for me to have this opportunity to Geochemical Society of Japan for bestowing me speak to you on the current status of these ongo the highest award, which carries the name of the ing investigations. late Professor Yuji Shibata of the Imperial Uni versity of Tokyo, the founding father of THE PRE-FERMI NATURAL REACTOR Geochemistry in Japan. When I entered the University in 1936 and I graduated from the Imperial University of began attending a two-year course in Inorganic Tokyo in the same year Hahn and Strassmann Chemistry taught by Professor Shibata, what im (1939) discovered fission. Soon thereafter, Pro pressed me most was that a large-scale research fessor Kenjiro Kimura, under whom I did my project on volcanoes in Japan, supported by the thesis work, told me to initiate a research work Imperial Academy of Science, was being main on the radioactivity of the water of Masutomi tained in his laboratories in the Chemistry Radium Springs in Yamanashi Prefecture, joint Department, when it was a general custom in ly with another graduate student named Shinya those days that chemistry professors seldom car Oana, who was doing his thesis work under the ried out researches based heavily on field work. I supervision of Professor Yuji Shibata on the den had the great fortune of being able to participate sity variations of natural waters associated with in Professor Shibata's research project as the volcanoes. Our goal was to see if there was a cer youngest and most inexperienced member of his tain relationship between the radioactivity and group for two years just before his retirement the heavy water concentrations of the water. from the University, which coincided with the We found no clear relationship between them beginning of the war in the Pacific. after studying the water of Masutomi Radium What I learned from Professor Shibata and Springs for a period of about two years from his co-workers about geochemistry more than 1939 through 1941, when the war began and our half a century ago gave me a strong incentive to joint work was terminated (Oana and Kuroda, undertake my studies on 244Pu in the early solar 1940, 1942). Although no definite conclusion ')This paper is a condensed version of the acceptance speech delivered by the author on the occasion of his being awarded the Shibata Prize, at the Annual Meeting of the Geochemical Society of Japan, held in Matsuyama, Ehime Prefecture, Japan, on October 1, 1991. 2jDistinguished Professor Emeritus , Department of Chemistry, University of Arkansas, Fayetteville, Arkansas, U. S.A. 1 2 P. K. Kuroda could be drawn from the early studies on the in the remote past. The reaction site consisted of radioactive springs at Masutomi, I often several bodies of very rich uranium ore, and wondered in those days whether or not a more than 500 tons of uranium had been involv uranium ore deposit located deep under the ed in the reactions with a quantity of energy ground might have been involved in some released equal to about 100 x 109 kWh. The in nuclear reactions in which the heavy water tegrated neutron flux at certain points exceeded played an important role (for more details con 1.5 x 1021neutrons per cm2 (for more details, see "The Oklo Phenomenon" cerning the early history, see Kuroda, 1979, , Proceedings of a 1982, 1983, 1989, 1990a, b, 1991). Symposium, Libreville, 23-27 June 1975, In 1952, I moved to the University of Arkan published by the International Atomic Energy sas at Fayetteville, Arkansas, and started my Agency, Vienna, 1975; excellent review articles own research project, which was to measure the published by Roth, 1977; and by Hagemann and contents of various radioactive isotopes pro Roth, 1978; a book written in the Japanese duced by the spontaneous fission of 238Uin pit language by Fujii, 1985; and several review ar chblende ores (Kuroda and Edwards, 1954; ticles by Kuroda, 1975, 1982, 1983). Kuroda et al., 1956, 1957; Ashizawa and In 1976, Shukolyukov and co-workers Kuroda, 1957; Parker and Kuroda, 1958; (Shukolyukov et al., 1976; Shukolyukov and Heydegger and Kuroda, 1959; Kenna and Min, 1977; Shukolyukov and Minh, 1977) Kuroda, 1960, 1961, 1964; Kuroda and Menon, reported that they found an anomalous fission 1961; Kuroda and Arino, 1964), with the xenon component in the Oklo reactors. The ultimate objectives to establish the natural occur isotopic composition of this anomalous fission rences of element 43 (technetium) and element xenon is such that the abundances of 131Xeand 61 (promethium), both of which had so far been 132Xerelative to 136Xe are markedly enhanced regarded as "artificial" elements (Kenna and when compared to the relative fission yields from Kuroda, 1961, 1964; Attrep and Kuroda, 1968). the thermal neutron-induced fission of 235U.The The papers concerning the theory of natural reac nature of this anomalous xenon from the Oklo tors (Kuroda, 1956a, b) were by-products of the reactor remained unexplained for many years, extensive radiochemical studies which were be but Kuroda (1990a, b) pointed out that it can be ing carried out at the University of Arkansas dur attributed to the fact that 131Xe,132Xe and 134Xe ing the 1950's. have fairly long-lived precursors: 8.04-day 1311, In 1956, I predicted that a large uranium ore 78.2-hour 132Te and 42-minute 134Te, respec deposit could have become an operating pile dur tively. ing the geological history of the earth (Kuroda, Radionuclides which have been retained and 1956a,b). During the 1950's and 1960's, the idea preserved at the site of nuclear reactors have of a critical uranium chain reaction occurring in high melting and boiling points. Gaseous nature was not taken seriously and it soon elements and elements with melting points lower became almost completely forgotten, but sixteen than that of tellurium (452°C) appear to have years later, on September 25, 1972, the world's mostly migrated out of the reactor. About one scientific community learned of an extraordinary percent of fission-produced xenon isotopes have discovery made by research workers at the been retained, however, and the isotopic com French Atomic Energy Establishment (Bodu et positions of small amounts of xenon released al., 1972; Neuilly et al., 1972; Baudin et al., from the Oklo reactors were found to be abnor 1972) that uranium had been found, in the mal in that the relative abundances of 131Xeand 132Xewere markedly enhanced deposit at Oklo in the Republic of Gabon, . Africa, with an abnormal isotopic composition The origin of this anomalous xenon can be at that led one to arrive at the conclusion that self tributed to a non-linear oscillatory mode of sustaining nuclear chain reactions had occurred operation (Bilanovic and Harms, 1985) of the 244pu in the early solar system and natural reactors 3 reactors at temperatures of about 400°C, from the chondritic (stone) meteorite Richard periodically being turned on and off, in a manner ton was heavily enriched in 129Xe. He concluded quite similar to the present-day geysers or inter that this isotope almost certainly was formed mittent hot springs. The time period during from the radioactive decay of 1291with a half-life which the reactor was turned off was calculated of 16 million years, now extinct as a natural to be about 3 hours from the observed ratios of radioactivity but not so at the time of formation 132Xe , 134Xe and 136Xe in the anomalous xenon of the meteorite: (for more details, see Kuroda, 1990a, b; 1991). 1291 f _ 129Xe (stable) 1.6 x 107 year PLUTONIUM-244 IN METEORITES Immediately after the discovery of 1291in the In 1957, the Soviet Union launched the Sput early solar system, Kuroda (1960, 1961) pointed nik and newly elected President John F. Ken out that 244Puwith a half-life of 82 million years nedy declared in 1960 that the United States should have also been present in the early solar would send Man to the moon before the decade system and the experimental evidence for its ex was over. Reynolds (1960a, b) at Berkeley then istence could be secured by searching for the made the important discovery that the xenon presence in meteorites of excess heavy xenon 252Cf oG 2.55 Y 248 Um 0G 4.7 x 105 y 244Pu 240PU OL 8.2 x 107 y 7.3 m 60 m 240NP oC 6760 y P 14.lh 240U 2360 232Th J 2.39x107 y Spontaneous Fission (6.55 x 10 la y) 0(, 1.41 x 1010 y (Thorium Series Decay Chain) I Stable Xenon Isotopes I (131X,' 132X,, 134X,, 136X,) 208Pb1 (stable) Fig. 1. The beginning of the thorium series decay chain. 4 P. K. Kuroda Table 1. The experimental confirmation of the ex (1964) reported that they discovered the presence istence of 244Pu spontaneous fission xenon in of roughly 50 x 10-12 (cc STP / g) of excess fission meteorites (Alexander et al., 1971) xenon (136fXe)in the carbonaceous chondrite Fission yield Renazzo and, although the amount of excess Source 131Xe 132Xe 134Xe 136Xe xenon found in Renazzo appeared to be greater than expected by at least a factor of 2, they Pasamonte 33 ± 3 93±8 91±2.5 100 noted that "...this discrepancy alone is not Pasamonte 25 ± 3 88.5±3 94±5 100 Whitlokite 31±8 97 ± 8 93 ± 1 100 sufficient to rule against the possibility of 244Pu from fission gas in Renazzo." St.
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