Book Description

The objective of this study is to evaluate the relative impact of the design parameters on bundle uranium mass and sheath strain and to re-evaluate the basis for the limitation on bundle mass due to an increase in bundle subchannel cross-sections. bundle uranium mass is determined by parameters that in turn affect the sheath strain during operation. this might affect sub-channel flow areas and affect the chf-ccp. the bundle uranium mass was assessed with electres and resulting sheath strains estimated for a candu 6 fuel channel operating at overpowers just at the trip set point of the reactor (onset of sheath dryout), a 14% power increase. the electres fuel modeling code is used to determine the relative impact on sheath strain of the design parameters that control uranium mass, namely, pellet density, diametral clearance, axial gap, and pellet face geometry (chamfer, dish depth, and land width). a limitation was placed on bundle uranium mass by new brunswick power. this came from a ccp evaluation showing that a candu 6 reactor, fuelled with bundles having average masses greater than 19.25 kg u, would have a net positive sheath strain over a fuel channel at the power for the onset of dryout, and therefore a ccp penalty. the calculations were based on steady bundle powers, operating in a fuel channel at ccp to a burnup of 168 mw middle dot h/kg u. at this burnup the strain calculation included a 14% power boost. these are indeed very conservative assumptions with a view to maximizing calculated sheath strains, without regard for fuel defect probability. for comparison, this study has produced electres strain calculations for high power channel power histories representative of 8 bundle shifts, also with a 14% power boost, operating at dryout.