Performance of Bruce Natural UO2 Fuel Irradiated to Extended Burnups

Abstract

Bruce-type bundles XY, AAH and OF were successfully irradiated in the NRU reactor at Chalk River Laboratories to outer-element burnups of 570-900 MWh/kgU. These bundles were of the Bruce Nuclear Generating Station (NGS)-A "first-charge" design that contained gas plenums in the outer elements. The maximum outer-element linear powers were 33-37 kW/m. Post-irradiation examination of these bundles confirmed that all the elements were intact. Bundles XY and AAH, irradiated to outer-element burnups of 570-700 MWhkgU, experienced low fission-gas release (FOR) (< 3%); bundle GF, irradiated to an outer-element burnup of 900 MWh/kgU, experienced high FOR (1 7%). Gas plenums and lower internal gas pressures appear to have contributed to the relatively low sheath strain observed in bundle GF (0.1 % at midpellet). The analysis in this paper illustrates a dependence of FGR on power, bumup and power-history type. Prior experience with Bruce NGS-A fuel experiencing declining power histories has shown that high FGR and fuel defects may occur at burnups > 500 MWh/kgU (equivalent to bundle-average 450 MWh/kgU) when maximum outer-element linear powers are > 50 kW/m. The analysis in this paper suggests that CANDU 37-element fuel can be successfully irradiated (low- FGR/defect-free) to burnups of at least 700 MWh/kgU, provided maximum powers do not exceed 40 kW/m.