NATURE'S UNCOMMON ELEMENTS: PLUTONIUM AND TECHNETIUM

Abstract

Natural 99Tc and 239Pu 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 99Tc/U and 2.4 x 10-12 to 44 x 10-12 for 239Pu/U. Measured concentrations are compared to those expected if the ores had behaved as closed systems with respect to U and its products. Under conditions of secular equilibrium in closed systems, 99Tc and 239Pu concentrations are solely a function of the neutron flux, U content, and decay rates. The neutron production rate and physical and chemical 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 99Tc and 239Pu beyond the amounts predicted for secular equilibrium. Although production by neutron-capture on 98Mo complicates the accuracy of the 99Tc predictions, the excess quantities nonetheless strongly suggest the redistribution of these elements within the deposit. The failure to observe complementary deficiencies within the deposit suggests that the redistribution processes enriched the elements, removing small proportions of the elements from large uraniferous masses of rock, and concentrating them in smaller, less uraniferous, masses. The consistency of 239Pu/U and 99Tc/U ratios 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 gains of these nuclear products from the deposit as a whole.