Document: NRC Regulatory Guide
Document ID: 8e45dce1-e1e7-4415-b1dd-7e2a610e545b
Document Type: regulatory_guide
Title: Fire Protection for Nuclear Power Plants (Rev. 4)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2023/ML20231A835.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.189
CFR Part: 
CFR Title: 

Content:
SISBO may exceed the actual risk posed by the fire, and the licensee should consider the risk carefully when evaluating the safe-shutdown design and procedures. A plant typically uses this approach to avoid or minimize the need for operator manual actions after a fire. However, acceptable operator manual actions that are implemented in accordance with Regulatory Position 5.3.1.3 of this guide and NUREG-1852 may present a lower risk than the SISBO approach. 5.4.3 Associated Circuits of Concern When alternative or dedicated shutdown systems are credited for achieving post-fire safe shutdown, a specific category of circuits has been defined (referred to as “associated circuits of concern”), and acceptable approaches to mitigating the consequences of fire-induced failure of these circuits have been identified. The licensee should evaluate these circuits, which are nonsafety or safety circuits that could adversely affect the identified shutdown equipment by feeding back potentially disabling conditions (e.g., hot shorts or shorts to ground) to power supplies or control circuits of that equipment. Such disabling conditions should be prevented to ensure that the identified safe-shutdown equipment will function as designed. Associated circuits of concern are defined as those cables (safety-related, nonsafety-related Class 1E and non-Class 1E) outside containment that have a physical separation less than that specified in Regulatory Positions 5.3.1.1.a, b, and c of this guide (or less than that specified in Regulatory Position 6.1.1.1 for cables inside a noninerted containment) and that meet one of the following criteria: a. A common power source with the shutdown equipment (redundant or alternative) is not electrically protected from the circuit of concern by coordinated breakers, fuses, or similar devices. b. A connection to circuits of equipment would adversely affect the shutdown capability if spuriously operated (e.g., RHR or reactor coolant system isolation valves,