Anticipated transients without scram for light water reactors: implications for liquid metal fast breeder reactors

The proposed ATWS acceptance criteria for LWRs (as specified in NUREG-0460) are in principle, applicable to LMFBRs. For LMFBRs, the major criterion will be the assurance that the plant protection system (shutdown or scram) has a sufficiently high reliability (low failure rate) so that core disruptive accidents (as currently defined) will lie outside the design basis. For early plants, however, mitigating systems may also be required. Alternative accident scenarios for LMFBRs, which are initiated from the shutdown state or may lead to potential core disruptive accidents even following scram, need to be examined in greater detail. The proposed LWR-ATWS criteria do not appear to present any new or unforeseen design and/or safety questions for LMFBRs. They do, however, specify design goals for mitigating systems which may insure conformance with NRC policy. Preliminary recommendations are made for future research and evaluation.