Reliability of CRBR primary piping: critique of stress-strength overlap method for cold-leg inlet downcomer

A critique is presented of the strength-stress overlap method for the reliability of the CRBR primary heat transport system piping. The report addresses, in particular, the reliability assessment of WARD-D-0127 (Piping Integrity Status Report), which is part of the CRBR PSAR docket. It was found that the reliability assessment is extremely sensitive to the assumed shape for the probability density function for the strength (regarded as a random variable) of the cold-leg inlet downcomer section of the primary piping. Based on the rigorous Chebyschev inequality, it is shown that the piping failure probability is less than 10/sup -2/. On the other hand, it is shown that the failure probability can be much larger than approximately 10/sup -13/, the typical value put forth in WARD-D-0127.