Combined Effects of Graphite Discs and Central Holes on Pellet Temperatures in CANDU Fuel

Abstract

Studies are currently underway to increase the average burnup in CANDU81' reactors while maintaining a low probability of fuel failure due to stress-corrosion cracking. This paper presents the results of a feasibility study on a design modification of the CANDU fuel involving annular UO, pellets separated by graphite discs. Slightly enriched uranium is used. The two-dimensional computer program FEAT was used to calculate the temperature distribution in the pellet, sheath, and graphite discs. The classical equation for steady-state non-linear conduction of heat is solved in the program by the finite element technique. A sensitivity analysis was carried out to examine the effects of fuel element linear power, fuel element burnup, pellet central hole diameter, and heat transfer coefficients at various interfaces of the pellet/sheath/disc; specifically, to investigate their impacts on fuel temperature distribution. The results confirm that, compared to the conventional solid pellets, annular pellets reduce the temperature at the pellet disc interface. However, the annular pellets have little effect on the volume-average temperature of the pellet.