Time assessment of instrumentation survivability and severe accident guidelines application

Resumen

The Instrumentation performance during a Severe Accident (SA) is currently one of the key identified gaps in Nuclear Safety. The instrumentation is licensed using the pressure and temperature reached during a design basis accident, but SA phenomena are not considered on its design limits. During a SA, it is likely that the pressure or temperature range of the instrumentation is exceeded, and therefore, their measurements may not be reliable throughout the remaining accident development. As those measurements may be used to mitigate the consequences of the SA, its correct use or interpretation can make a difference in the accident management. For this reason, a methodology for instrumentation survival and assessment is proposed. It consists of the identification of the mitigation actions to perform together with the parameters that trigger its start and the instrumentation required for its development. The damage condition of the instruments is assessed in a continuous manner during the accident, rather than a binary state (function/not function). In this sense, a MELCOR computational simulation of a short-term SBO sequence in a 3 loop Westinghouse PWR with a large dry containment is used as an example. It takes into account the possible degradation of the instrumentation due to pressure-temperature increases. Then, the conditions of pressure, temperature and humidity are used to estimate the instrumentation survivability during this accident. The best instrumentation option for a specific variable of interest is assessed for all SA guidelines, together with the level of hazard associated with them. This methodology is applicable to other sequences to show the reliability of measurements and actions. Depending on the instrument availability, different alternatives are suggested that can be considered to manage the accidental sequence.