Long Term Low Power Operation in Pressurized Water Reactors (Generic Letter No. 84-21)

                                UNITED STATES
                          WASHINGTON, D.C. 20555  

                              October 16, 1984 



          (Generic Letter 84-21) 

In June 1983, the NRC staff received a Licensee Event Report from a  
Pressurized Water Reactor (PWR) licensee concerning extended operation at 
low power followed by a return to full power operation. The licensee had not
contemplated this type of plant operation in performing their safety 
analysis for the cycle of operation in question. Based on the results of a 
subsequent reanalysis, the licensee concluded that load follow operations in 
conjunction with this type of plant operation and with a return to full 
power could result in core peaking factors being greater than those 
resulting from the original safety analysis. This unanticipated increase in 
the core peaking factors was caused by a burnup distribution produced by the 
extended low power operation which was different than the full power burnup 
distribution. Upon return to full power, this burnup distribution, now 
representative of the low power operation and interacting with the moderator 
temperature coefficient and the moderator density variation, would produce 
this unanticipated increase in the core peaking factor. Consequently, this 
type of plant operation could result in a potential unreviewed safety 
question and would be reportable under the provisions of 10 CFR Part 50.59. 

The fuel supplier for this particular licensee has performed an evaluation 
of extended, low power operation followed by a return to full power. They 
have evaluated first and reload cycle cores as a function of time in cycle. 
They have also evaluated, parametrically, the effect of reduced power, time 
at reduced power, and a control rod bank insertion strategy and worth on the
core total peaking factor and radial peaking factor. The outcome of this 
analysis is a set of procedures that the plant, using this fuel supplier's 
safety analysis, should follow when extended, low power operation occurs. 

All other PWR fuel suppliers have provided responses to an NRC request for 
information on how extended, low power operation is treated in their safety 
analyses. Each of these fuel suppliers has provided an adequate response in 
terms of either directly including extended, low power operation in their 
safety analysis; using incore measurements; or providing plant operation 
instructions for a given fuel cycle. 

Accordingly, PWR licensees and applicants for an operating license are 
reminded that core safety analyses involve a number of assumptions 
concerning plant operation throughout a given fuel cycle. If, for any 
reason, a plant is not operated as planned (e.g., extended, low power 
operation as described above), the licensee should review the cycle safety 
analysis to verify its applicability. If parts of the safety analysis are 
not applicable, these should be reanalyzed and the applicability of the 
Technical Specifications should be confirmed. 

                                    - 2 - 

This letter is provided for information purposes only. Actions based on the 
recommendations provided herein are voluntary and no response to the NRC or 
review by the NRC is required. Therefore, no clearance from the Office of 
Management and Budget is required. 

                              Darrell G. Eisenhut, Director 
                              Division of Licensing  

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