Effect of Pellet Cracking on Light Water Reactor Fuel Temperatures

It is postulated that typical light water reactor (LWR) fuel pellets will crack after a few power cycles and that the majority of the pellet segments will lie against the cladding. When there is a nominal cladding-to-pellet gap at operating conditions, pellet cracking will improve the fuel-to-cladding gap conductance but will reduce the fuel thermal conductivity. A model that accounts for the effects of fuel pellet cracking on both fuel conductivity and gap conductance has been formulated. Fuel centerline temperature measurements were made during the steady-state irradiation in the Halden Heavy Boiling Water Reactor of four typical LWR-type test rods with varying fuel density and pellet-to-cladding gap sizes. Calculations using the cracked pellet model were compared to the in-pile temperature measurements, and good agreement was obtained.