Incorporation of volatile fission products in UN and PuN and comparison to oxides

Abstract

The incorporation of volatile fission products (Kr, Xe, Br, I, Te, Rb and Cs) in a range of interstitial and defect sites in UN and PuN is examined via predictions made using atomic scale density functional theory calculations. Endothermic incorporation energies are predicted for both Kr and Xe at all sites examined although the least unfavorable site is the di-vacancy cluster (since it has the greatest volume). Preference for this nitride incorporation site is maintained for all other species due to their large sizes. Comparing accommodation in UN and PuN, with the exception of Te, all other species show preference for PuN, which may reflect the larger lattice parameter of PuN though bonding differences also have an influence. Comparing corresponding incorporation in oxides and nitrides, overall there are considerable differences, which may lead to distinct fission product retention behavior. Specifically, energies for Kr and Xe are considerably less unfavorable in the oxides than nitrides. The same oxide preference is predicted for I and especially for Rb and Cs and to a lesser extend Br. While Te shows a small preference for UO2 over UN, there is a preference for PuN over PuO2.