VALIDATING THE FILL GAS THERMAL CONDUCTIVITY MODELS FOR GAP CONDUCTANCE CALCULATIONS IN NUCLEAR FUEL PERFORMANCE CODES

摘要

The gap between fuel and cladding is initially pressurizedto prevent unstable thermal behavior and to maintain claddingintegrity. Helium is often chosen as the initial fill gas in nuclearfuel rods to improve the heat transfer through the gapbetween the fuel and the cladding. In light water reactors,this initial pressure ranges from 0.3 to 3.45 MPa and tendsto increase throughout the life of the reactor as a result ofexpansion/contraction of the fuel and cladding, fission gasrelease, and so on. In some cases, the rod internal pressureapproaches the external pressure or exceeds it. However, thepressure dependence of the thermal conductivity is neglectedin nuclear fuel performance codes at present. Current codesuse correlations that use only temperature and are calibratedto data at pressures below 0.1 MPa, but the pressure e ect issignificant on the thermal conductivity for the pressures above1.0 MPa. Therefore, the use of the current models in nuclearapplications, particularly in gap conductance calculations, isquestionable. This is examined herein. Theories related to gasproperties are presented, along with their validation againstliterature data for selected inert gases. Underlying assumptionsare clearly described for each model, and their possibleimpacts on gap conductance calculations are discussed. It isconcluded that the current approach in nuclear applicationsis only valid for helium; however, it is not valid in most gapconductance calculations once the gap is occupied mostly witheither lower conductivity gaseous fission products or an initialfill gas other than helium.