Swelling due to the partition of soluble fission products between the grey phase and uranium dioxide

• Small tetravalent cations are predicted to cause a contraction of both phases as they segregate from UO2 into (Ba, Sr)ZrO3.
• Cr3+, Fe3+, Gd3+, Sm3+ and Nd3+ are predicted to cause a contraction of both phases by segregation to (Ba,Sr)ZrO3.
• Pr3+ and La3+ were predicted to cause a combined contraction of UO2 and (Ba,Sr)ZrO3 through segregation to the grey phase.
• With excess Cr3+ all trivalent cations are predicted to segregate to the grey phase causing a contraction of both phases.

The change in volume associated with the partition of soluble cations from uranium dioxide into the (Ba,Sr)ZrO3 grey phase has been investigated using atomic scale simulations. Here past work on the thermodynamic drive for the segregation of trivalent and tetravalent cations from uranium dioxide is built upon in the context of fuel swelling. Only small tetravalent cations segregate into the grey phase and this is predicted to result in an overall reduction in fuel volume. Individual trivalent cations that segregate, can cause either a contraction or an expansion of the overall fuel volume. Cr2O3 doped UO2 promotes co-partition forming mixed cation clusters in the grey phase and causing an overall reduction in fuel volume for all trivalent cations. This may have implications for fuel performance and may alter other fuel swelling mechanisms.