Defect Chemistry and Radiation Stability of (Gd & Zr) Co-Doped UO₂ Solid Solutions

The irradiation behaviour of pure UO₂ is well experimentally characterised, but little is known about the irradiation of UO₂ doped with fission products such as rare-earth elements that are formed during in-reactor operations where the fuels are exposed to extreme environments of intense radiation and high temperature. For this purpose, this study was carried out to investigate the effect of heavy Kr-ion irradiation of UO₂ (to simulate radiation damage within a reactor) when doped with both trivalent Gd (a common burnable poison) and tetravalent Zr (a high concentration fission product) at varied concentrations.<br/>The mechanisms by which the dopants incorporate the UO₂ lattice were explored experimentally. XRD measurements showed that Gd-addition creates an oxygen vacancy environment comparable to hypo-stoichiometric UO_2-x, and HERFD-XANES revealed the presence of U(V) species in proportion quantities to the trivalent dopant to compensate for the charge. Zr doping, on the other hand, induced a contraction of the unit cell and Raman characterisations showed the induction of a new defect species with O<i>_h</i>symmetry that includes the Zr cation in the 8-fold coordination of O^2- in the form of ZrO₈-complex clusters.<br/>Kr-irradiation of the pristine samples showed that dopants modify the irradiation character of the UO₂ structure, with the undoped pellet accumulating the highest concentration of lattice disorder, indicating that the co-doped solid solutions are more radiation resistant. The cation Frenkel pair formation enthalpy was calculated using DFT+U, which revealed that uranium interstitials formed as part of displacement cascades in doped samples have a significant thermodynamic drive to displace Zr/Gd substitutional defects, increasing the defect recombination volume. This mechanism, however, is not observed in undoped UO₂. In this talk, the key mechanisms that occur near defect-boundary interfaces will be discussed to highlight the precise role of dopants in changing the lattice defects mechanisms in nuclear fuels.