Statistical Characterization of Cable Electrical Failure Temperatures Due to Fire for Nuclear Power Plant Risk Applications

Abstract

Single-value failure temperatures for fire loss of electrical cable functionality have been the norm for Fire Probabilistic Risk Assessments since the publication in 2005 of NUREG/CR-6850. If the calculated exposure temperature matches or exceeds the cable failure temperature, electrical failure is always assumed; if not, no failure is assumed. While this can be relaxed somewhat if a distribution for the exposure temperature is estimated, use of a distribution on the cable failure temperature itself more readily enables such relaxation and, therefore, a more realistic assessment. This paper develops probability distributions for different generic cable types (based on insulation) using data from the US Nuclear Regulatory Commission tests. Results indicate mean failure temperatures considerably higher than those used deterministically, 252°C, 421°C and 383°C, respectively for thermoplastic, thermoset and Kerite-FR®. This suggests considerable relaxation from the conservatism inherent using the deterministic failure temperatures could be achieved. The paper then postulates two hypothetical distributions on the exposure temperature from applying a fire phenomenological model in a statistical way to estimate the possible relaxation using the distributed cable failure temperatures to enhance the realism of the assessment. Examples show that use of probabilistically-distributed cable failure temperatures (in conjunction with similar for exposure temperatures) can reduce the probability of electrical failure for a normally-distributed exposure temperature with a mean of 350°C and standard deviation of 58.3°C by factors of approximately three and eight for Kerite-FR® and thermoset cables, respectively. The reduction would be less pronounced for thermoplastic cables, although larger reductions would be possible here as well for lower exposure temperatures (e.g., a factor of two).