Effect of Pressure Tube Creep on the Void Coefficient of a CANDU Fuel Bundle

Abstract

The results of a study of the effect of an increase in pressure tube diameter due to radiation induced creep on the total void effect of a CANDU 37-element fuel bundle are reported here. The calculation presupposes that the pressure tube diameter increases by ~6%, and considers how the lattice reactivity and nuclide components of reactivity change between the two extremes in both voided and cooled conditions. MCNP4A is used as the basis for predictions. Fresh fuel nuclide compositions are used throughout. For the model considered, MCNP4A predicts that the void effect increases by about 3 mk, due to a decrease in lattice reactivity for the case of the cooled channel as the pressure tube diameter increases by 6% and the flux distribution varies. This is due to the fact that the yield rate from U235 decreases because the fuel is in a lower flux region. No significant reactivity differences are predicted to occur for the voided channel in the presence of diametral creep. An experimental test of the predictions is proposed.