Risk assessment of isolation devices in safety systems

This report discusses electronic isolators which are used to maintain electrical separation between safety and non-safety systems in nuclear power plants. The concern is that these devices may fail allowing unwanted signals or energy to act upon safety systems, or preventing desired signals from performing their intended function. While operational history shows many isolation device problems requiring adjustments and maintenance, we could not find incidents where there was a safety implication. Even hypothesizing multiple simultaneous failures did not lead to significant contributions to core damage frequency. Although the analyses performed in this study were not extensive or detailed, there seems to be no evidence to suspect that isolation device failure is an issue which should be studied further.