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:
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 natural convection circulation. b. For boiling water reactors (BWRs), the boration of the RCS is achieved by the standby liquid control system (SLCS). However, that system is activated only if the control rods fail to shut down the reactor. 2. Heat rejection to the surroundings is the only way to avoid a core melt under normal or accident shutdown conditions. a. For PWRs, heat rejection is achieved by the main steam system and either the normal or the auxiliary feedwater system. In case of an SSE, only seismic Category I components and equipment are assumed operable. During a loss of offsite power, the auxiliary feedwater system provides cooling water to the steam generators. Without offsite power, 1.139-2 reactor cooling depends solely on natural convection circulation induced by the cooling effect of heat transfer to the steam generators. Natural circulation may lead to slow and unequal cooling. Slow cooling will result in longer cooldown times, which, in turn, require a larger clean feedwater inventory. Unequal cooling may lead to hot spots and high vessel stresses. It is essential that adequate cooling be provided under these circumstances to keep the integrity of the reactor coolant pressure boundary and maintain the reactor core in a coolable form. b. For BWRs, heat rejection is achieved by the main steam system and either (1) the normal feedwater system in conjunction with the main condenser or (2) the reactor core isolation cooling (RCIC) system in conjunction with the condensate storage tank, the residual neat removal heat exchangers in the steam condensing mode, and the pressure suppression pool. Further heat rejection is achieved by the RHR system after the RCS has been sufficiently depressurized. 3. For all current designs of PWRs and BWRs, depressurization