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 r eported. These show that also at burnups as high as 4.5% FIMA the onse t temperature for thermal gas release is close to 1200-degrees-C. At c onstant linear power the 1200-degrees-C isotherm moves outwards with b urnup due to the development of a.thermal barrier at the pellet rim an d a general degradation in the fuel thermal conductivity. This causes the observed burnup enhancement of gas release . Using radial temperat ure profiles constructed from experimental data, grain growth and Xe d iffusion during a power transient at high burnup are reexamined. It is found that grain growth is slower than predicted for steady-state irr adiation conditions. The effective Xe diffusion coefficient, D(We), in the transient tests with hold times of 40, 42 and 62 h is given by D( Xe) = 1.9 X 10(-8) exp(- 21890/T) cm2 s-1, T in K. For the tests with a 4 h hold time, D(Xe) is given by D(Xe) = 2.5 X 10(-9) exp(- 21 0501 T) below 1570-degrees-C, but by the former equation above 1600-degrees -C. In the long transient tests, the increase in the D(Xe) below 1570- degrees-C is attributed to the annealing of gas traps in the oxide lat tice.