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:
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 valves to the turbine stop valves should be included in staff calculations of postulated accident effects. Analyses by some applicants, based on assump tions different from those used by the staff, have indicated that long transport delays might occur. On that basis, it has been argued that a leakage control system is not necessary to reduce potential leakage from the steam systems of boiling water reactor plants. The staff has considered these analyses and has concluded that, although they are useful in making so-called 2 Part 100 guidelines, as used in this guide, refer to the radiation dose limits used in determining the boundaries of the exclusion area and the low population zone pursuant to 10 CFR Part 100. 3The staff defimes "new" boiling water plants to be those plants utilizing the General Electric Company's BWR 6/Mark IlI design or subsequent BWR designs. "realistic" or "best-estimate" dose calculations and hence in showing margins that might exist above the limit-type calculations of the staff, a more positive method of reducing the radiological effects of potential leakage of the main steam system isolation valves should be provided. The staff also has concluded that some limited credit for transport delay effects is appropriate in determining the design basis for such leakage control systems. Staff analyses of the contribution of main steam isolation valve leakage to total calculated offsite doses in postulated design-basis loss-of-coolant accidents made with conservative allowances for transport delay effects show that the 2-hour site boundary- dose is not affected by the subject leakage. The long-term dose in the low population zone, however, is affected for uncontrolled Isolation valve leakage rates typical of current technical specification values. Thus