Conservative Width of High-Burnup Structure in Light Water Reactor UO₂ Fuel as a Function of Pellet Average Burnup

Based on the high-burnup fuel data available in open literature, a conservative width of high-burnup structure (HBS) in light water reactor UO₂ fuel, which can be used for fuel performance and accident analysis or assessment of spent fuel under geological disposal conditions, is proposed as a function of pellet average burnup. For pellet average burnup of 30 to 60 GWd/t U, where the HBS generally increases with burnup because of the accumulation of irradiation damage, a conservative HBS width is given by w_HBS = 13.3 (bu_avg - 30), where w_HBS is the HBS width in m and bu_avg is the pellet average burnup in GWd/t U. For pellet average burnup of 60 to 75 GWd/t U, where microstructural damage caused by irradiation is partly annealed, a conservative HBS width is expressed by w_HBS = 2.02 exp(bu_avg /11.35). In the case of pellet average burnup above 75 GWd/t U up to at least 100 GWd/t U, the HBS width does not exceed some limiting value of 1.5 mm because high temperature in the central region of the fuel pellet has caused an extensive annealing of irradiation damage. In addition, because of significant fission gas release during irradiation up to high burnup, HBS formation might not have expanded to the pellet region whose temperature was lower than the threshold one. Therefore, for this burnup range, a conservative HBS width is given as w_HBS = 1500 m.