Carbon and oxygen impurities in UN and PuN

Atomic scale computer simulations, based on density functional theory (DFT), are used to predict the energies and structures of the impurity defects carbon (C) and oxygen (O) in uranium mononitride (UN) and plutonium mononitride (PuN). It is found that ½O 2 will be incorporated at vacant nitrogen sites in hypo-stoichiometric UN 1-x and PuN 1-x until these sites are occupied. At this point, as in stoichiometric UN and PuN, the reaction proceeds to form uranium dioxide (UO 2 ) and plutonium dioxide (PuO 2 ), liberating nitrogen gas (N 2 ), in preference to O being accommodated at interstitial positions. If the reference states are UO 2 or PuO 2 (instead of ½O 2 ), negligible solid solubility is predicted into UN and PuN. It is possible UO 2 will be partly accommodated in UN 1-x . C (from the graphite reference state) exhibits different defect energetics, with interstitial incorporation unfavourable in UN and PuN; and while C is readily incorporated into a vacant nitrogen site in UN, it is not in PuN. UC will demonstrate extensive solid solubility with UN. Conversely, stoichiometric plutonium monocarbide (PuC) is not an observed phase, with PuC 1-x and plutonium sesquicarbide (Pu 2 C 3 ) the stable phases, which exhibit negligible solubility in PuN.