Laboratory actinide partitioning: Whitlockite/liquid and influence of actinide concentration levels

Abstract

Fission and alpha track radiography techniques have been used to measure partition coefficients ( D) at trace (ppm) concentration levels for the actinide elements Th, U, and Pu between synthetic whitlockite and coexisting “haplobasaltic” silicate liquid at 1 bar pressure and 1250°C at oxygen fugacities from 10 −8.5 and 10 −0.7 bars. Pu is much more readily incorporated into crystalline phases than is U or Th under reducing conditions (fO 2 = 10 −8.5), because Pu is primarily trivalent, whereas U and Th are tetravalent. Definitive valence state assignments cannot be made, but our best estimates of corrected partition coefficients for Pu +3, Pu +4, Th +4, U +4, and U +6 are, for whitlockite 3.6/<⩽0.6/1.2/0.5/⩽0.002. The effect of changing pressure and liquidus temperature is relatively small, which probably reflects a weak temperature dependence for D (whitlockite) but possibly could be due to cancellation of opposing temperature and pressure effects. Comparison of experiments at trace U levels with those containing percent concentrations of UO 2 indicate that Si is involved in the substitution of U in whitlockite with U + 2 Si ⇄ Ca + 2 P being the most likely mechanism. D u is lower. 0.3 vs 0.5. at percent levels compared to 20 ppm. This is best explained by the effect of U on melt structure or by a decrease in the fraction of tetravalent U at high U concentrations.