Probabilistic Risk Assessment of a Light Water Reactor Coupled with a High-Temperature Electrolysis Hydrogen Production Plant - Part 2: Hazards Assessment and PRA

The profitability of existing nuclear power plants (NPPs) can be enhanced by using excess thermal energy to supply industrial processes. While the decision to modify the NPP to supply thermal energy externally to the plant is an economic one, the licensing permission for the modification is based on safety. To investigate the safety acceptance for such a modification, two generic probabilistic risk assessment (PRA) models were developed in this study to evaluate the effect on safety of the addition of a heat extraction system (HES) to a light water reactor (LWR). The two PRA models are for a pressurized water reactor (PWR) and a boiling water reactor (BWR), respectively. The introduction of a HES has the goal of providing heat that would normally be discarded to be used for new revenue generation through processes such as making hydrogen. For example, this HES module could feed process heat to a High Temperature Electrolysis Facility (HTEF). The PRAs used in this assessment of the HES are generic meaning the details of the analysis are for a hypothetical facility. A Failure Mode and Effects Analysis (FMEA) was performed to identify and screen the possible hazards due to the addition of the hydrogen production system. Hydrogen cloud and high-pressure jet detonation events were studied in relation to their contribution to components/structures fragility. A sensitivity analysis was performed to investigate the effect of several HES design options and distance to the nuclear reactor complex. The results of the PRA indicate that application using the U.S. licensing approach in 10 CFR 50.59 is justified because of the minimal increase in initiating event frequencies for all design basis accidents, none exceeding 6%. The PRA results for core damage frequency (CDF) support the use of Regulatory Guide 1.174 as further risk information that supports a change to the plant. Further insights provided through hazard analysis and sensitivity studies confirm that the safety case for licensing an HES addition and an HTEF sited at one km from the nuclear power plant is strong. Note that unique site-specific and hydrogen production characteristics could alter these conclusions.