United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-69: Errors in Main Steam Line Break Analyses for Determining Containment Parameters

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

                              November 1, 1991


NRC INFORMATION NOTICE 91-69:  ERRORS IN MAIN STEAM LINE BREAK ANALYSES 
                               FOR DETERMINING CONTAINMENT PARAMETERS


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 a safety problem that may result from not 
considering the possibility that feedwater could continue to flow following 
a main steam line break (MSLB) inside the containment.  This problem is 
applicable to plants equipped with feedwater isolation susceptible to single 
failure.  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

On October 18, 1991, the Northeast Nuclear Energy Company notified the NRC 
of a problem at the Millstone Nuclear Power Station, Unit 2, that could 
cause the facility to exceed containment design limits.  The licensee found 
this concern during MSLB analyses for the forthcoming replacement of the 
steam generators.  In performing the analyses, the licensee identified that 
during full power operation, with a break on a main steam line, and with 
offsite power available, a single failure of a feedwater control valve to 
close could result in higher peak containment temperature and pressure than 
the design limits.  The containment design limits for temperature and 
pressure were 289�F and 54 psig; the new analysis shows preliminary values 
of 420�F and 68 psig, respectively.  Previously, the licensee's worst case 
analysis assumed loss of offsite power with a MSLB.

Discussion

NRC Bulletin No. 80-04, "Analysis of a PWR Main Steam Line Break with 
Continued Feedwater Addition" and Standard Review Plan (NUREG 800) Section 
6.2.1.4 provide guidance for assuming a single failure of any active 
component, with and without offsite power available, and discuss the effect 
of such a failure on the ability to ensure containment integrity following a 
MSLB.  The Bulletin also addresses the possibility of the reactor returning 
to power.  Offsite power remaining available could be more limiting for 
containment parameters.

9110290103
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                                                       IN 91-69 
                                                       November 1, 1991
                                                       Page 2 of 2 


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.




                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation


Technical contacts:  Thomas Koshy, NRR
                     (301) 492-1176

                     William LeFave, NRR
                     (301) 492-2385


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