Document: NRC Regulatory Guide
Document ID: 2559e7e3-22aa-4dda-8fee-ac029c2a69e4
Document Type: regulatory_guide
Title: Guidance for Residual Heat Removal
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0407/ML040750334.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.139
CFR Part: 
CFR Title: 

Content:
n their hot-standby condition values. It is the intent of this guide to place the same degree of emphasis on the entire range of reactor coolant temperatures and pressures, including the range between hot standby and RHR operation conditions. The importance of reliable systems that remove decay heat from the reactor coolant system (RCS) while the latter is at or near normal operating temperatures is indicated by the results of WASH-1400, "Reactor Safety Study" (RSS). The capability of a typical PWR plant and a typical BWR plant to remove decay heat following a plant trip was evaluated in the RSS on a probabilistic basis. The evaluation included both those events in which the reactor protection system (RPS) failed (Anticipated Transients Without Scram) and events in which the RPS func- tioned as designed. For these types of events, it was considered acceptable to maintain the reactor at or near normal operating temperature and pressure for a long time. However, in the event of a plant trip even with a successful operation of the RPS, systems or equipment failures that led to the inability to remove decay heat resulted in a higher proba- bility of a core melt than that predicted for a large LOCA for both PWRs and BWRs. Conse- quently, a significant safety benefit will be gained by upgrading those systems and equipment needed to maintain the RCS at the hot-standby condition for extended periods or those needed to cool and depressurize the RCS so that the RHR system can be operated. 1. 139-1 Furthermore, even though it may generally be considered safe to maintain a reactor in a hot-standby condition for a long time, experience shows that thei-e have been events that required eventual cooldown and long-term cooling until the RCS was cold enough to perform inspection and repairs. It is therefore obvious that the ability to transfer heat from the reactor to the environment after a shutdown is an important safety function for both PWRs and BWRs. Consequently, it is