Document: NRC Regulatory Guide
Document ID: cf1737d6-2a8a-4765-aaf5-f5f2a617ef91
Document Type: regulatory_guide
Title: Design of Main Steam Isolation Valve Leakage Control Systems for Boiling Water Reactor Nuclear Power Plants
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740263.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.96
CFR Part: 
CFR Title: 

Content:
the main steam isolation valves in postulated design-basis LOCA situations would be a small fraction of the 10 CFR Part 100 guidelines, 2 provided the main steam system from the isolation valves up to and including the turbine condenser remains intact. However, results of staff analyses on some typical plants using the standard conservative assumptions for considering the offsite consequences of a postulated design-basis LOCA (e.g., loss of leak-tightness beyond the turbine stop valve, uncontrolled leakage of the main steam isolation valves at or above current typical technical specification limits of 11.5 standard cubic feet per hour per valve at typical calculated containment pressures combined. with site meteorological data typical of that being presently submitted with license applications) have indicated that the calculated doses would be in excess of Part 100 guidelines. The position of the staff with respect to the seismic design classification of the steam system does not require Seismic Category I design requirements for the turbine stop and control valves, steam line piping beyond the stop valve, the turbine, the turbine con denser, or connecting piping of less than 2% inches in diameter. However, there is a need for design improve ments to provide appropriate safety margins for the large numbers of plants now planned. The staff believes that, unless systems can be relied on to remain intact and capable of providing significant dose reduction factors in postulated accident conditions, a leakage control system for main steam isolation valves should be provided for new boiling water reactor plants3 to supplement the isolation function of the main steam isolation valves and reduce uncontrolled or untreated leakage from the steam line valves. It has been proposed that dose reduction factors due to the transport delay time of the containment atmos phere in passing through the main steam lines within containment or through the main steam lines from the isolation