Nature's uncommon elements: Plutonium and technetium

Natural Tc-99 and Pu-239 were measured in ores from the Cigar Lake uranium deposit, in which U concentrations ranged from 0.3 to 55 wt%. Atomic ratios ranged from 1.4 x 10(-12) to 51 x 10(-12) for Tc-99/U and 2.4 x 10(-12) to 44 x 10(-12) for Pu-239/U. Measured concentrations are compared to those e xpected if the ores had behaved as closed systems with respect to U and its products. Under conditions of secular equilibrium in closed systems, Tc-99 and Pu-239 concentrations are solely a function of the neutron flux, U con tent, and decay rates. The neutron production rate and physical and chemica l parameters that control the in situ neutron flux were measured in several samples. Neutron transport modeling of the sample environs using the MCNP code indicated that about half of the samples showed apparent excesses of T c-99 and Pu-239 beyond the amounts predicted for secular equilibrium. Altho ugh production by neutron-capture on Mo-98 complicates the accuracy of the Tc-99 predictions, the excess quantities nonetheless strongly suggest the r edistribution of these elements within the deposit. The failure to observe complementary deficiencies within the deposit suggests that the redistribut ion processes enriched the elements, removing small proportions of the elem ents from large uraniferous masses of rock, and concentrating them in small er, less uraniferous, masses. The consistency of Pu-239/U and Tc-99/U ratio s in bulk rock suggests that the redistribution processes observed at Cigar Lake are highly localized and in no case result in large-scale losses or g ains of these nuclear products from the deposit as a whole. Copyright (C) 1 999 Elsevier Science Ltd.