United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-90: Main Steam Line Break Effect on Environmental Qualification of Equipment

                                                         SSINS No.:  6835  
                                                         IN 84-90          

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                   OFFICE OF INSPECTION AND ENFORCEMENT  
                          WASHINGTON, D.C. 20555  

                             December 7, 1984  

Information Notice No. 84-90:   MAIN STEAM LINE BREAK EFFECT ON 
                                   ENVIRONMENTAL QUALIFICATION OF EQUIPMENT 

Addressees: 

All pressurized water reactor and gas cooled nuclear power plants holding an
operating license (OL) or construction permit (CP). 

Purpose: 

This information notice is provided as a notification of a potential problem
pertaining to plant analysis and equipment qualification with respect to a 
postulated main steam line break with releases of superheated steam. It is 
expected that recipients will review this information for applicability to 
their facilities and consider actions, if appropriate, to ensure similar 
problems do not exist at their facilities. However, suggestions contained in
this information notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

Westinghouse methodology for computing mass and energy releases for 
postulated main steam line break accidents is described in References 1 and 
2 (Attachment. 1).  An earlier version of Reference 1 was used on an interim
basis for the licensing of most Westinghouse-designed plants. Interim use of
the earlier version of Reference 1 was judged acceptable, on the belief that
the computed mass and energy releases would result in conservative estimates
of pressure and temperature effects if no entrainment was assumed. 

In the course of the NRC staff review of a later version of Reference 1, it 
was noted that even though entrainment and other refinements had been 
included, the steam generator blowdown model did not account for the heat 
transfer from the uncovered portion of the steam generator tube bundle to 
the escaping steam. Staff acceptance of the current version of Reference 1 
required, in part, that Westinghouse include a model to account for this 
heat transfer mechanism, i.e., steam superheating. 

As a result of the modeling change to account for superheating of the steam 
exiting the steam generator, Westinghouse has revised the mass and energy 
releases from main steam line breaks. For some break sizes, locations, and 
plant conditions these releases, when applied to plant-specific analyses, 
may 




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                                                        IN 84-90          
                                                        December 7, 1984  
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produce thermal environments more severe than previously prescribed for 
environmental qualification of safety-related equipment. Consequently, in 
June 1984, Westinghouse Electric Corporation notified each of the 
Westinghouse plant owners of a possible unreviewed safety question 
concerning the temperature envelope used for the environmental qualification 
'of equipment. On July 26, 1984, the Westinghouse Regulatory Response Group 
issued a letter addressing the action taken to date by the Westinghouse 
Owners Group on this issue (Reference 3). On August 20 and October 24, 1984,
the Westinghouse Owners Group further responded by letters on the proposed 
program to address the issue (References 4 and 5). 

Discussion: 

The analyses performed to date indicate that the significance of the issue 
is quite plant specific. Components inside or outside of containment, can be
affected, depending on the postulated break location. 

The NRC staff considers steam superheating during steam generator tube 
bundle uncovery as a result of postulated main steam line breaks and 
subsequent release to compartments, if omitted from plant analyses, to 
represent a potential deficiency in the equipment environmental 
qualification required by 10 CFR 50.49. The information discussed above is 
based on preliminary NRC staff review. It is not yet clear to what extent 
this issue may affect equipment qualification at all main steam line break 
locations in Westinghouse plants or in non-Westinghouse plants. Accordingly, 
it is suggested that licensees review their main steam line break analyses 
with regard to this issue. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 


                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contacts: C. V. Hodge, IE 
                    (301) 492-7275 

                    D. L. Wigginton, NRR 
                    (301) 492-7354 

Attachments: 
1.   References 
2.   List of Recently Issued IE Information Notices
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