Temperature measurements in high burnup UO₂ nuclear fuel: Implications for thermal conductivity, grain growth and gas release

Fuel centreline temperatures measured under transient conditions are reported. These show that also at burnups as high as 4.5% FIMA the onset temperature for thermal gas release is close to 1200°C. At constant linear power the 1200°C isotherm moves outwards with burnup due to the development of a thermal barrier at the pellet rim and a general degradation in the fuel thermal conductivity. This causes the observed burnup enhancement of gas release. Using radial temperature profiles constructed from experimental data, grain growth and Xe diffusion during a power transient at high burnup are reexamined. It is found that grain growth is slower than predicted for steady-state irradiation conditions. The effective Xe diffusion coefficient, D_Xe, in the transient tests with hold times of 40, 42 and 62 h is given by D_Xe = 1.9 × 10⁻⁸exp(−21890/T) cm²s−1, T in K. For the tests with a 4 h hold time, D_Xe is giv by D_Xe = 2.5 × 10⁻⁹exp(−21050/T) below 1570°C, but by the former equation above 1600°C. In the long transient tests, the increase in the D_Xe below 1570°C is attributed to the annealing of gas traps in the oxide lattice.