INSIGHTS FROM THE ANALYSES OF RISK-INFORMEDEXTENSION OF DIESEL GENERATOR ALLOWED OUTAGE TIME

摘要

In recent years, many U.S. nuclear plants have applied and received approval for the risk-informed extensionof the Allowed Outage Time (AOT) for Emergency Diesel Generators (EDGs). These risk-informed applicationsneed to meet the regulatory guidance on the risk criteria. This paper discusses in detail insights derived from therisk-informed analyses performed to support these applications.The risk criteria on ?CDF/?LERF evaluate the increase in average risk by extending the AOT for EDGs,induced primarily by an increase in EDG maintenance unavailability due to the introduction of additional EDGpreventive maintenance. By performing this preventive maintenance work on-line, the outage duration can beshortened. With proper refinement of the risk model, most plants can meet the ?CDF/?LERF criteria for extendingthe EDGAOT from, for example, 3 days to 14 days. The key areas for model enhancements to meet these criteriainclude offsite/onsite power recovery, LERF modeling, etc. The most important LERF model enhancementsconsist of refinement of the penetrations included in the containment isolation model for the consideration of alarge release, and taking credit for operator vessel depressurization during the time period between core damageand vessel failure. A recent study showed that although the frequency of loss of offsite power (LOSP) hasdecreased, the duration of offsite power recovery has actually increased. However, many of the events used toderive this conclusion may not be applicable to PRAs. One approach develops the offsite power non-recoveryfactor by first screening the LOSP events for applicability to the plant being analyzed, power operation, and LOSPinitiating event, then using the remaining events data for the derivation based on the fraction of events withrecovery duration longer than the time window allowed.The risk criteria on ICCDP/ICLERP examine the increase in risk from the average CDF/LERF, based on theincreased maintenance unavailability, to the instantaneous CDF/LERF caused by the removal of an EDG fromservice. Some plants have implemented a requirement to disallow any planned maintenance on other accidentinitiation/mitigation equipment during the time period when an EDG is out of service (OOS) for maintenance.Nevertheless, emergent failures may still occur during the time when an EDG is removed from service. Anotherrisk management strategy that some plants have adopted is to require a specific set of electrical and systemalignments be implemented during the time when each EDG is OOS. These are the alignments that would yield thelowest risk when a particular EDG is unavailable.The risk-informed application for EDGAOT extension must also address risks associated with external events.Fires and seismic events are the most important events to evaluate. For the fire events, only plant locations that,due to fire damage, could result in an LOSP are critical. These generally include switchgear rooms, cable spreadingroom, control room, etc. The likelihood of a fire-induced LOSP event in the control room and cable spreadingroom is significantly less than that in the 4kV switchgear rooms because at least two separate circuit shorts arerequired. For the seismic events, offsite power usually is the weakest link from the fragility standpoint. Seismicrisk is therefore generally sensitive to the increase in EDG maintenance unavailability as long as the seismic risk isa significant contributor to the total risk.