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:
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 essential that a power plant have the capability to go from hot-standby to cold-shutdown conditions (when this is determined to be the safest course of action) under any accident conditions. These accident conditions can conceivably include a safe shutdown earthquake (SSE) and an extended loss of offsite power that may have resulted from that SSE. In that case, all components and equipment that are not seismic Category I and all systems or parts of systems that depend solely on offsite power sources for their operation would be assumed inoperable. Under these circumstances, a plant safe shutdown (including cooldown) within a reasonable time requires systems designed to safety grade standards and operable from the control room. However, limited operator actions outside the control room may be permitted if suitably justified. Four processes are necessary to achieve a cold shutdown in a power plant: (1) the inser- tion of the control rods, with or without boration to the cold shutdown concentration, (2) heat rejection to the surroundings, (3) depressurization, and (4) long-term cooling. These processes are discussed below. 1. Boration of the RCS to the required cold shutdown concentration provides an addi- tional reactivity control measure to ensure that the reactor will not become critical during and after the RCS cooling. a. For pressurized water reactors (PWRs), the boration of the RCS is used in addi- tion to the insertion of the control rods. Boration is achieved by the chemical and volume control system (CVCS). It is important that this safety function can be achieved in all accident conditions, including an SSE and an extended loss of offsite power. In case of a loss of offsite power, the only means of mixing the injected boron solution with the reactor coolant is