Because of the complex characteristics of the irra­diated fuel, the thermal conductivity is often deduced from correlations using global parameters summariz­ing the state of the fuel: for instance, the burnup and the irradiation temperature. Some fuel characteristics are implicitly taken into account. For instance, the grain size is a parameter having a small impact on the conductivity of fresh fuels but a large impact on the conductivity of irradiated fuels. This is because grain boundaries are sinks for point defects and there­fore a smaller grain size induces reduced concentra­tions of point defects.

2.17.2.3.1 Irradiation temperature

The irradiation temperature has an impact on the state of the fuel. During long irradiations, tempera­ture has an effect on the microstructure of the fuel and on the concentration of radiation damage accu­mulated. During a short annealing (transient or laboratory annealing), the effect is mainly restricted to the radiation damage concentration change and to the redistribution of some fission products: pre­cipitation of atoms that were distributed as single atoms and formation of fission gas bubbles. How­ever, the definition of this parameter is vague because the irradiation temperature at a given radial position is not constant throughout the irradiation and the real relevant parameter is the irradiation temperature history. The out-of-pile measurements must be interpreted by considering the irradiation temperature at end of life (EOL).