Statistical failure analysis of metallic U-10ZR/HT9 fast reactor fuel pin by considering the Weibull distribution and cumulative damage fraction

A statistical failure analysis is performed to obtain some insight into the rupture behavior of the metallic U-10Zr/HT-9 fast reactor fuel pins which were irradiated in X447 subassembly of EER-II reactor. Generally, the primary factor that contributes to metallic fuel pin failure is believed to be plenum pressure buildup due to fission gas release. However, the calculated cumulative damage fraction (CDF), by considering plenum pressure buildup as the sole contributor for cladding stress, are lower than the expected value by an order of magnitude. When fuel/cladding mechanical interaction is considered in CDF calculations, the calculated values of failed pins are closer to the expected values. The remained difference may be due to the effects of temperature/stress fluctuation and neutron irradiation during operation, which are hard to be quantitatively modeled. The failure probability functions are determined in terms of CDF and bumup, respectively, based upon the known Weibull statistical failure model. The failure density function based on CDF exhibits wide distribution (shape parameter, beta is similar to 1.5), while the function on bumup shows sharp distribution shape (beta is similar to 17). The calculated limit on peak burnup and CDF to meet 99.99% reliability for X447 pins are 6.6 at.% and 2.2 x 10(-4), respectively. The CDF limit for 99.99% reliability is similar to 0.3% of their mean value to failure. The CDF based failure function agrees relatively well with the strain based failure function in literature. (C) 1998 Elsevier Science Ltd. All rights reserved.