Identification and assessment of BWR in-vessel severe accident mitigation strategies

Abstract

This report provides the results of work carried out in support of the U.S. Nuclear Regulatory Commission Accident Management Research Program to develop a technical basis for evaluating the effectiveness and feasibility of current and proposed strategies for boiling water reactor (BWR) severe accident managemenl First. the findings of an assessment of the current status of accident management strategies for the mitigation of in-vessel events for BWR severe accident sequences are described. This includes a review of the BWR Owners' Group Emergency Procedure Guidelines (EPGs) to detennine the extent to which they currently address the characteristic events of an unmitigated severe accident and to provide the basis for 111 recommendations for enhancement of accident management procedures. Second. where considered necessary. new candidate accident management strategies are proposed for mitigation of the late-phase (after core damage has occurred) events. Finally. recommendations are made for consideration of additional strategies where warranted. and two of the four candidate strategies identified by this effort are assessed in detail: (1) preparation of a boron solution for reactor vessel refill should control blade damage occur during a period of temporary core dryout and (2) containment flooding to maintain the core debris within the reactor vessel if the injection systems cannot be restored.

Severe Accident Issues Raised by the Fukushima Accident and Improvements Suggested

“…For a typical BWR, depressurization of the reactor pressure vessel is recommended in the severe accident management guideline [12,13,14] to enable alternate water injection into the core to arrest core damage progression. It would prevent reactor vessel failure.…”

“…However, it is very difficult to implement depressurization without a power supply, because the valves need to be operated at high pressure. Also, as the timely operation of depressurization is very critical for the timing of core damage and vessel failure [14,16], the power supply to the SRVs should be secured. A mobile alternative AC power source or battery power could be prepared to enable activation of the depressurization, even in the event of SBO.…”

In-Vessel Core Degradation

“…On the other hand, since the lower drywell (the cavity below the lower head) of a BWR is much larger in depth and diameter than the reactor pit of a PWR, it will take a much longer time for the water to reach the lower head by flooding the cavity. So, the answer to in-vessel coolability of a BWR via external vessel cooling as a SAM measure is not straightforward [100]. More research work is needed in this respect.…”

Possible Methods to Estimate Core Location in a Beyond-Design-Basis Accident at a GE BWR with a Mark I Containment Stucture

“…This design is shown in Figure 1. The station blackout accident scenario has been consistently identified as the leading contributor to calculated probabilities for core damage [1].…”