Advanced digital I ampersand C systems in nuclear power plants: Risk- sensitivities to environmental stressors

Microprocessor-based advanced digital systems are being used for upgrading analog instrumentation and control (I ampersand C) systems in nuclear power plants (NPPs) in the United States. A concern with using such advanced systems for safety-related applications in NPPs is the limited experience with this equipment in these environments. In this study, we investigate the risk effects of environmental stressors by quantifying the plant's risk-sensitivities to them. The risk- sensitivities are changes in plant risk caused by the stressors, and are quantified by estimating their effects on I ampersand C failure occurrences and the consequent increase in risk in terms of core damage frequency (CDF). We used available data, including military and NPP operating experience, on the effects of environmental stressors on the reliability of digital I ampersand C equipment. The methods developed are applied to determine and compare risk-sensitivities to temperature, humidity, vibration, EMI (electromagnetic interference) from lightning and smoke as stressors in an example plant using a PRA (Probabilistic Risk Assessment). Uncertainties in the estimates of the stressor effects on the equipment's reliability are expressed in terms of ranges for risk-sensitivities. The results show that environmental stressors potentially can cause a significant increase in I ampersand C contributions to the CDF. Further, considerable variations can be expected in some stressor effects, depending on where the equipment is located