Document: NUREG-0800
Document ID: b8d2b8f5-432a-4c0a-81ab-20231f7d0f28
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1508/ML15089A068.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
environmental conditions prevent closure. The staff reviews the low power and shutdown internal flood and internal fire analysis using methods similar to the at power assessment, considering (1) each defined plant operational state, (2) the impact of breached or failed fire barriers that could impact a fire 19.0-24 Revision 3 – December 2015 area, and (3) the impact of impaired or disabled flood barriers that could impact a flood zone. 6. Accidents during non-power modes of operation generally are not part of the design bases of the facility. Consequently, non-power operations, associated accident sequences and specific accident phenomenology are not considered in the review of the accident analyses provided in Chapter 15 of the FSAR. Indeed, the staff’s review of the level of safety during non-power modes of operation provided by the design of the facility and operating procedures and controls in place is limited to the review of the PRA for non-power modes of operation. This puts additional burden on the PRA reviewer to pursue issues, as necessary, to assure that the PRA model has fidelity and the assumptions in the risk analyses are justified. In some cases the reviewer may need to engage reviewers from other technical branches that have expertise in particular areas (e.g., systems operation, T-H performance, operating experience). Reviewers should therefore be aware of the following issues related to safety during non-power modes of operation: A. Based on previous PRAs, studies by the EPRI and studies performed by the staff, roughly 80 percent of risk for traditional PWR designs occurs during periods when the reactor coolant system is drained and open (midloop operation is a subset of this condition). B. The time it takes to reach boiling in the reactor vessel following loss of the decay heat removal function can be very short during PWR midloop operation (e.g., 12 minutes). Steaming into the containment will lead to intolerable conditions that could