United States Nuclear Regulatory Commission - Protecting People and the Environment

Availability of NUREG-1169, "Technical Findings Related to Generic Issue C-8; Boiling Water Reactor Main Steam Isolation Valve Leakage and Leakage Treatment Methods" (Generic Letter No. 86-17)

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION  
                          WASHINGTON, D. C. 20555 

                              October 17, 1986



          LEAKAGE AND LEAKAGE TREATMENT METHODS" (Generic Letter 86-17) 

This letter is to inform you that NUREG-1169, "Technical Findings Related to
Generic Issue C-8; Boiling Water Reactor Main Steam Isolation Valve Leakage 
and Leakage Treatment Methods", was published in June, 1986. This NUREG 
presents the results of the NRC staff and contractor efforts to assess 
Generic Issue C-8, "MSIV Leakage and LCS Failures". This issue deals with 
the difficulty of some main steam isolation valves (MSIVs) in Boil in Water 
Reactors (BWRs) to meet Technical Specification leakage rate limits (which 
are typically 11.5 SCFH at a 25 psig test pressure) and with the need for 
leakage control systems. The leakage rates are based on a large 
loss-of-coolant accident LOCA), a specified design basis source term from 
the core (TID 14844), the worst single active failure, and no credit for any
non-seismic Category I equipment, components, and structures. To ensure 
acceptable performance guidelines for engineered safety features, leakage 
control systems (LCS) have been installed on most BWRs to direct any leakage
past MSIVS during the design basis LOCA to an area served by the Standby Gas
Treatment System (SGTS). If the leakage rate past an MSIV significantly 
exceeds the Technical Specification value, the LCS may not be effective 
because of limitations in its design. 

As a result of these concerns, the staff prioritized the MSIV leakage and 
LCS failures as a high priority Generic Issue in January 1983. 
Independently, the BWR Owners Group (BWROG) formed the MSIV Leakage Control 
Committee to determine the cause of high leakage rates associated with many 
of the MSIVs, and to develop recommendations for reducing the leakage rates. 
The BWROG Committee completed its effort and provided recommendations and 
comments to the staff in February, 1984 and April, 1986. 

In evaluating MSIV leakage with respect to offsite doses following a LOCA, 
the technical evaluation of this Generic Issue has used realistic 
assumptions concerning the equipment, facilities and site characteristics 
available to mitigate the effects of a LOCA. The specific elements of the 
effort were: 

     o    To evaluate the BWROG recommendations associated with reducing 
          leakage past the MSIVs and assess the effectiveness of the 
          recommendations as implemented by licensees. 

     o    To evaluate the existing safety-related LCS comparing it's 
          effectiveness with that of other methods of handling the leakage 
          likely to be available following a LOCA. 


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     o    To perform a probabilistic risk assessment (PRA) to evaluate the 
          reliability and relative risks associated with the different 
          methods of mitigating the effects of a LOCA. 

     o    To evaluate the use of alternate equipment to mitigate the effects
          of a LOCA, and potential changes in allowable MSIV leakage rates. 

     o    To identify areas in current licensing guidance (the Regulations, 
          the  Standard Review Plan, the Regulatory Guides, and the 
          Technical Specifications) that are related to MSIVs and LCSs that 
          will be considered in the second, or regulatory assessment, phase 
          of resolution of the generic issue. 

A safety-related LCS was evaluated by comparing it's effectiveness with 
other methods of handling leakage that could be available following a LOCA. 
Several ways to treat leakage which make use of the holdup volume and 
surfaces of the main steam lines (MSL) and condenser and fission product 
attenuation elsewhere, were considered. A PRA considering the reliability 
and relative risks associated with the different methods of mitigating the 
effects of a LOCA was performed. The overall risks from the accident 
sequences in which MSIV leakage could be a significant factor were found to 
be low without an LCS, and alternate fission product handling techniques 
were shown to produce significant potential dose reductions. 

NUREG-1169 represents technical findings associated with Generic Issue C-8 
and the staff expects it to be used in the regulatory resolution of the 
issue. The latter is anticipated to be complete in another year in 
coordination with  ongoing generic activities relating to accident source 
terms, to overall containment leak rate and leak rate testing 

Neither this letter nor NUREG-1169 constitute a change in the existing 
generic staff positions or allowable MSIV leakage rates, nor the 
specifications for MSIV leakage control systems. Therefore, no action is 
being requested of applicants or licensees. The information contained in 
NUREG-1169, however, including the recommendations of the BWROG, may be 
useful guidance for some licensees contemplating plant specific change 
requests related to this issue pending final generic resolution. 

NUREG-1169 may be purchased by calling (202) 275-2060 or (202) 275-2171 or 
by writing to the Superintendent of Documents, U.S. Government Printing 
Office, Post Office Box Number 37082, Washington, D. C. 20013, or the 
National Technical Information Service, Department of Commerce, 5258 Port 
Royal Road, Springfield, Virginia 22161. 

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This letter is provided for information only. It does not contain any new 
requirements or guidance for licensees of boiling water reactors. Actions 
based on the contents provided herein are voluntary and no reply to the NRC 
is required. 


                                   Robert M. Bernero, Director 
                                   Division of BWR Licensing  
                                   Office of Nuclear Reactor Regulation
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