Chemical Geology, 88 (1990) 301-315 30 I Elsevier Science Publishers B.V., Amsterdam [2] A search for fossil nuclear reactors in the Alligator River Uranium Field, Australia: Constraints from Sm, Gd and Nd isotopic studies R. Maas" and M.T. McCulloch Research School of Earth Sciences, The Australian National University, Canberra, A. C. 72 260I (Australia) (Received June 14, 1989; revised and accepted July 10, 1990 ) ABSTRACT Maas, R. and McCulloch, M.T., 1990. A search for fossil nuclear reactions in the Alligator River Uranium Field, Australia: Constraints from Sm, Gd and Nd isotopic studies. Chem. Geol., 88:301-315. We report Gd, Sm and Nd isotope ratios from high-grade U ores and uraninites from the Alligator River Uranium Field, Northern Territory, Australia. Relatively large variations (up to 0.26%) are found in (normally invariant) t SSGd/~SVGd and ~5°Sm/~49Sm ratios due to neutron capture on ~57Gd and 149Sm. Smaller variations (0.03%) are represent in the isotopic abundances of t45Nd and 142Nd (and by implication ~43Nd) which are produced by admixture of isotopically exotic Nd from fission of 23sU and 238U. Correlations in ~SGd/~57Gd-U/Gd, ~49Sm/~5°Sm-U/Sm and ~42,145Nd / ~44Nd_ U/Nd plots indicate that U-rare-earth element systems were approximately closed. This suggests that post-reactor con- lamination was not important and that maximum thermal neutron fluences of ~ 10 ~6 n cm 2 derived from Gd and Sm isotope anomalies are a true record of the fission activity at Nabarlek. These fluences are lower by ~ 10 4 than those re- ported for the Oklo, Gabon, natural reactor. The low neutron fluence at Nabarlek is consistent with neutron production from spontaneous fission of 238U, (a,n) reactions and slow neutron-induced fission of 235U. The latter, although important in the overall neutron budget, was not adequate for the U ore to become critical and hence induce a self-sustaining nuclear reaction. The main difference between the Oklo and Alligator River U ore are the higher HREE contents in the latter, which reduced the number of available thermal neutrons to levels below those necessary for a sustained nuclear chain reaction. 1. Introduction The discovery of natural nuclear reactor zones in the Lower Proterozoic U deposit at Oklo, Gabon (I.A.E.A., 1975 ) has encouraged the search for similar phenomena in other Pre- cambrian U deposits. U ores of age ~ 1500 Ma or more would have contained U considerably more enriched in the reactor fuel isotope 235U (>/2.45% of natural U) than modern U (235U~0.72 atom%). This would have in- "Present address: Department of Applied Physics, Curtin University, G.P.O. Box U 1987, Perth, W.A. 6001, Australia. creased the likelihood of a sustained nuclear chain reaction at that time dramatically, given that other conditions were favourable. These conditions include the availability of a critical mass of high-grade ore, light-element neutron moderators such as water, graphite, etc., and low abundances of strong neutron absorbers such as the rare-earth elements (REE), and boron. Evidence for fossil nuclear reactor ac- tivity at Oklo includes major changes in the isotopic abundances of the REE (Loubet and All6gre, 1977), trace alkali and earth-alkali elements (Brookins et al., 1975), noble gases (Drozd et al., 1974) and many other elements (Loss et al., 1984), caused by sustained ther- 0009-2541/90/$03.50 © 1990 -- Elsevier Science Publishers B.V.