Performance of two CANDU-6 fuel bundles containing elements with pellet-density and clearance variances

Two CANDU-6 geometry bundles were manufactured with controlled variances in pellet density and internal pellet-to-sheath clearances close to the upper and lower limits of the design specification. These were irradiated in the Point Lepreau Generating Station during 1994 and recently examined in the hot cells at AECL Chalk River. The bundles achieved bundle-average burnups of 150 and 161 MWh/kgU, and operated at peak outer-element powers of ∼50 kW/m. The primary objective of the investigation was to evaluate the effect of variation in pellet density and clearances on fuel performance. Internal diametral clearance variations were shown to have a dominant effect on circumferential sheath strain (typically -0.05% per 0.01 mm increment in diametral clearance at the midpellet location). When clearances were close to the upper limit of the specification, outer-element midpellet residual sheath strain was predominantly compressive; when clearances were close to the lower limit of the specification, midpellet sheath strain was tensile. Similar trends were observed in pellet interface (ridge) strain. Pellet-density variations had a secondary effect on sheath strain (typically 0.01% per 0.01 Mg/m3 increment at the midpellet location). High-density fuel exhibited slightly less fission-gas release than did low-density fuel (2% vs. 3%). (author)