The performance of irradiated Boraflex under seismic conditions

The flexural strength and Young's Modulus of irradiated Boraflex have been measured on specimens having been exposed to a range of gamma doses up to >3 x 1010 rads. The measurements were performed on specimens prepared from small coupons irradiated in a Co-60 facility as well as material destructively removed from fuel racks at two PWRs. The material taken from fuel racks shows no decrease in flexural strength at the higher doses whereas the samples prepared from small coupons do. The latter is attributed to accelerated aging in small coupons. Weibull analyses of the data provides threshold stresses for failure below which the failure probability is essentially zero. Spent fuel racks structural design/licensing reports and other information were reviewed for six spent fuel pools. The cases evaluated were in the East, Midwest, and West Coast regions of the US. These represented nine distinct fuel rack designs by four rack manufacturers. The fuel racks were designed for BWR and PWR fuel including fuel manufactured by ABB Combustion Engineering, General Electric and Westinghouse. From the licensing reports peak strains in the stainless steel rack structure during the limiting safe shutdown earthquake were developed. Conservative assumptions were applied in determining how the strains in the structural stainless steel are transferred to the Boraflex using experimentally determined values of Young's Modulus, the peak stresses in the Boraflex were computed. In all cases the calculated Boraflex stresses were less than the threshold failure stress by a substantial margin