Fuel thermal behaviour

The interest in the thermal behaviour of the fuel mainly comes from the safety criterion which prohibit any fuel melting in the pins. Due to the gap lying between fuel and cladding, the highest temperatures are most probably occurring at Beginning Of Life (BOL) before completion of the central hole formation and before any substantial gap closure has taken place. However it cannot be ruled out that after some burn up the thermal transfer between fuel and cladding becomes worse than it was at BOL Then if the power does not decrease the maximum temperature might become higher than at (BOL). In order to get an overall experimental validation of our thermal calculations we need to cover the entire range of the pin life. Actually the method cannot be the same for BOL and end of life (EOL). For BOL it is possible to get a direct thermal measure through thermocouples, but this method is no longer practical after some days due to the failure of the thermocouples under neutrons flux at the temperatures of interest. This failure may happen before or after complete gap closure is reached and the rate of gap closure is especially meaningful for the BOL thermal behaviour. Another aspect of the thermal behaviour is the statistical one which may be obtained by the post-irradiation examination of the fuel microstructure, although it is not a proper way to get the absolute temperatures in the fuel, it is one of the most direct ones to have an insight in fuel thermal dispersion at BOL and over-heating at EOL