United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-75: Minimum Temperature for Criticality

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

                               October 14, 1994


NRC INFORMATION NOTICE 94-75:  MINIMUM TEMPERATURE FOR CRITICALITY


Addressees

All holders of operating licenses or construction permits for pressurized-
water reactors (PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potentially non-conservative initial conditions
that were used in the analysis of some design-basis transients.  As a result,
some plant technical specifications for minimum temperature for criticality
may not be adequately conservative.  It is expected that recipients will
review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems.  However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.

Description of Circumstances

Commonwealth Edison Company (Zion Nuclear Station Units 1 and 2) and New York
Power Authority (Indian Point Nuclear Power Plant Unit 3) have informed the
NRC that their technical specifications for minimum temperature for
criticality were not supported by the safety analyses for their plants.  The
licensing analysis performed by Westinghouse for Zion assumed a nominal hot-
zero-power (no-load) operating temperature of 286 oC [547 oF], but Zion's
technical specifications allow criticality if the average reactor coolant
system temperature is greater then 260 oC [500 oF].  The safety analysis for
Indian Point 3 was also performed at 286 oC [547 oF], but its technical
specifications allow criticality at 232 oC [450 oF], a limit which was set by
reactor vessel material considerations.  A review of Indian Point records
indicated that the reactor was brought critical below 286 oC [547 oF] several
times in the early life of plant operations (before 1988).  The lowest
temperature during these instances was 272 oC [521 oF].  

After discovering a potential to operate the plant in a region outside that
analyzed, both licensees instituted administrative controls to ensure that the
minimum temperatures for criticality are bound by the safety analyses
performed for their plants.  In addition, both licensees submitted license
amendments to NRC to revise the minimum temperature for criticality.  

Further details concerning these events are in a 10 CFR Part 21 report to NRC
prepared for the Zion plant, dated March 18, 1993, and in Indian Point 3
Licensee Event Report 93-046-00, dated December 1, 1993.    

9410070143.                                                            IN 94-75
                                                            October 14, 1994
                                                            Page 2 of 2


Discussion

For transient analysis performed for hot-zero-power cases, small changes
(e.g., 3 oC [6 oF]) in initial conditions such as allowed by standard
technical specifications would have a negligible impact on analysis results. 
However, if PWRs are allowed to achieve criticality significantly below the
temperature that was previously analyzed at hot-zero-power, the following
safety concerns would be raised: 

(1)   The transient analyses, such as "rod withdrawal from subcritical," "rod
      ejection," "zero power feedwater malfunction," and "boron dilution
      event" documented in the Final Safety Analysis Report, might not have
      been analyzed at temperatures below hot-zero-power and could be non-
      conservative.  This could cause the analyses results with small margins
      to violate specified fuel design limits (i.e., centerline fuel melt or
      departure from nucleate boiling) for one or more of these postulated
      transients.

(2)   The response of the power range ex-core nuclear instrumentation may be   
      adversely effected by the increased density of the reactor coolant at    
      lower temperatures.  This could result in a higher power being reached   
      before a power range reactor trip occurs which might violate specified   
      fuel design limits for transients that rely on this trip. 

(3)   The moderator temperature coefficient will become more positive, perhaps
      causing a violation of existing technical specifications.  Higher values
      of moderator temperature coefficient could exceed those used in some of
      the transient safety analyses.  A more positive moderator temperature
      coefficient at power would result in reactivity insertion that could
      increase the consequences of an anticipated-transient-without-scram
      event.
 
New analyses may justify criticality at somewhat lower temperatures.  For
example, Commonwealth Edison Company performed the necessary safety analyses
for the Zion station and the staff approved a technical specification
amendment to change the minimum temperature for criticality to 277 oC 
[530 oF].

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


                                      original signed by

                                    Brian K. Grimes, Director
                                    Division of Project Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  George A. Schwenk, NRR
                    (301) 504-2814

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