Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
October 17, 1986
TO ALL LICENSEES OF BOILING WATER REACTORS AND APPLICANTS
Gentlemen:
SUBJECT: 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 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
considerations.
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.
Sincerely,
Robert M. Bernero, Director
Division of BWR Licensing
Office of Nuclear Reactor Regulation
