Document: NRC Regulatory Guide
Document ID: de79d411-4fc9-456e-a5a0-f4a910ca4c9a
Document Type: regulatory_guide
Title: Assumptions Used for Evaluating a Control Rod Ejection Accident for Pressurized Water Reactors (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2111/ML21119A157.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.77
CFR Part: 
CFR Title: 

Content:
osed application. If neither toxic information nor detection instruments are available, human detection, such as unpleasant smell, burning odor, irritated eyes, and choking, may be useful as a warning of a dangerous condition and a signal to don PPE. Quick-response detectors should be placed in the fresh air inlets (both normal and emergency air intakes). Depending on the design, it may also be appropriate to have separate channels of detectors for fresh air inlets and to have detectors in the CR envelope ventilation system recirculation lines. The system response time, which incorporates the detection response time, the valve closure time, and associated instrument delays, should be less than or equal to the required isolation time based on the IDLH value. Remote detectors may be located at storage and unloading locations. These detectors may be placed, and the detector trip points adjusted, to ensure detection of either a leak or a container rupture. A detector trip signal should isolate the CR before toxic chemical concentration within the CR exceeds the chemical’s IDLH value. The detector trip signal should also set off an alarm and provide a readout in the CR. An alternative to the installation of remote detectors would be an isolation system that uses local detectors with a very short isolation time. 4.2 Isolation System The evaluation should consider the capability to close the CR air ducts with dampers and thus isolate the CR. For onsite storage, measures should be in place to manually isolate the CR. Upon detection of a toxic chemical, a detector should initiate complete closure of isolation dampers to the CR with minimal delay. The isolation time is a function of the CR design, in particular, the inleakage characteristics. If the detectors are upstream from the isolation dampers, then credit will be allowed for the travel time between the detectors and the dampers. The isolation system and its components, the recirculating filter system, and the air