Inferential-Statistical Reevaluation of Spent Fuel Zircaloy Cladding Integrity

From the published results of experiments investigating the effects of delayed hydride cracking (DHC) on spent fuel Zircaloy cladding integrity, relevant data have been extracted and re-analyzed, taking advantage of inferential statistics and an information-theoretic model selection criterion. Statistical tolerance intervals, the method of maximum likelihood estimation, and the Akaike information criterion, corrected for small sample size, were applied to a small sample of measured values of the threshold stress-intensity factor $K_{IH}$. The purpose was to create a well-grounded probability density function for use in a mathematical model correlating random variates of $K_{IH}$ with important conditions for the initiation of crack growth by DHC, specifically, cladding hoop stress and the depth and shape of surface flaws. A selection criterion purposely designed for small sample sizes and the robust nature of inferential statistics were ideally suited for the intended reevaluation. The fidelity of the mathematical model was protected by the exclusion of any simplifying approximations, e.g., substitution of constants or single-valued descriptive statistics for variables. The probabilistic effect of the random variable $K_{IH}$ was thereby precisely mapped onto the linearly correlated variable, threshold cladding hoop stress, as a function of surface flaw depth and shape. Contour plots of the results constitute significant improvements over previous quantitative single-point estimates of the effects of DHC on spent fuel Zircaloy cladding integrity.