Development of a 3-D Finite Element Model to Examine Fuel Bundle Behaviour under Post-Dryout Heat Transfer Conditions

Abstract

Elements in a CANDU® fuel bundle can deform (i.e., deflect or bow) during their residence in the fuel channel by thermally-induced phenomena. Although small deflections are warranted, there is a risk that under post-dryout conditions, the induced thermal gradient may increase this effect and put the integrity of the fuel and fuel channel at risk. With reactor ageing effects such as pressure tube creep becoming more significant, there is an increased risk of dryout which further impacts the reactor operating power. To prevent dryout, conservative parameters are used for the trip parameter acceptance criteria as stated in CNSC regulatory guide G-144. Limited in-reactor experiments have indicated that operational margins exist when compared to those stated in G-144. A 3-D model utilizing the finite-element method to examine fuel deformation under post-dryout heat transfer conditions could be used as a tool to help better understand the effects of thermally-induced phenomena. A fully coupled thermal-mechanical model is being developed to simulate the deformation behaviour of a section of a bundle at the onset of dryout. Preliminary results of the model development and validation will be presented.