United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-57: Risk Impact Study Regarding Maintenance During Low-Power Operation and Shutdown

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                               December 13, 1995


NRC INFORMATION NOTICE 95-57:  RISK IMPACT STUDY REGARDING MAINTENANCE         
                               DURING LOW-POWER OPERATION AND SHUTDOWN


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to inform licensees of an NRC Office of Research study on the effect of
system or equipment maintenance on boiling water reactor low-power and
shutdown core damage frequency.  It is expected that licensees will review the
information for applicability to their facilities and consider actions, as
appropriate, to avoid risk-significant situations.  However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.  

Discussion

The details of the study were published in October of 1994 in NUREG/CR-6166,
"Risk Impact of BWR Technical Specifications Requirements During Shutdown,"
(Accession No. 9411040066) which evaluated the risk impact of limiting
conditions for operation at low power and shutdown in the current technical
specifications for Grand Gulf Nuclear Station.  These limiting conditions for
operation include allowed outage times, that is, the time allotted for system
maintenance outages.  

A probabilistic model was developed for each of the eight plant operational
states, which are a reclassification of the five modes of operation defined in
the technical specifications.  These eight plant operational states address
the range of plant conditions from refueling with the vessel head off and the
upper pool filled to full-power operation.  The models were developed in
sufficient detail to allow for comparisons among the plant operational states. 
Figure-of-merit measures included increase in conditional core damage
frequency.  The measures explicitly accounted for the impact of test and
maintenance activities (e.g., allowed outage times and surveillance time
intervals) for both power and shutdown conditions.

NUREG/CR-6166 indicates that the increase in conditional core damage frequency
for taking a single train of standby service water out of service during
different plant operational states in an operating cycle can be substantial. 
Single-train standby service water maintenance outages have been shown to
result in an increase in conditional core damage frequency during low power 

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and the first few days of hot shutdown (plant operational states 1, 2, and 3) 
that is comparable to the increase in conditional core damage frequency at
full-power operation (plant operational state 0).  During plant operational
state 4 (hot shutdown with shutdown cooling operating down to 0 psig) and
plant operational state 5 (cold shutdown with temperatures � 200 �F), the
study indicates that the increase in conditional core damage frequency could
exceed that at full-power operation.  NUREG/CR-6166 states, "Based on the
Increase in Conditional CDF {core damage frequency}, single and multiple train
maintenance on SSW {standby service water} should probably not be scheduled
for the late phases of hot shutdown and the early phases of cold shutdown,
when the Alternate Decay Heat Removal System (ADHRS) is not an alternative for
decay heat removal."

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                      /s/'d by DMCrutchfield


                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Samuel S. Lee, NRR
                     (301) 415-1061

                     Arthur J. Buslik, RES
                     (301) 415-6184

                     John H. Flack, NRR
                     (301) 415-1094

                     Neal K. Hunemuller, NRR
                     (301) 415-1152
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