Completion of Containment Performance Improvement Program and Forwarding of Insights for Use in the Individual Plant Examination for Severe Accident Vulnerabilities (Generic Letter No. 88-20, Supplement No. 3)

                                July 6, 1990

TO:         ALL LICENSEES HOLDING OPERATING LICENSES AND CONSTRUCTION 
            PERMITS FOR NUCLEAR POWER REACTOR FACILITIES EXCEPT LICENSEES 
            FOR BOILING WATER REACTORS WITH MARK I CONTAINMENTS

SUBJECT:    COMPLETION OF CONTAINMENT PERFORMANCE IMPROVEMENT PROGRAM AND 
            FORWARDING OF INSIGHTS FOR USE IN THE INDIVIDUAL PLANT 
            EXAMINATION FOR SEVERE ACCIDENT VULNERABILITIES - GENERIC LETTER 
            NO. 88-20, SUPPLEMENT NO. 3


This letter announces the completion of the NRC staff's Containment 
Performance Improvement (CPI) program.  Technical insights arising from this 
effort for PWR containments and for BWR Mark II and Mark III containments 
are being forwarded via this letter for use in licensee efforts as part of 
the Individual Plant Examination (IPE) effort described in Generic Letter 
88-20.  No regulatory requirements have resulted from the CPI program for 
these containment types.   Similar technical information for BWR Mark I 
containments was discussed in SECY 89-017, "Mark I Containment Performance 
Improvement Program", dated January 23, 1989, and summarized in an enclosure 
to Generic Letter 88-20, Supplement 1, dated August 29, 1989.  The technical 
information may be useful to licensees during their examinations of their 
plants for vulnerabilities to severe accidents.

Four specific insights are believed by the staff to be important enough to 
bring to the attention of licensees for use as they determine appropriate in 
the IPE for the plant types to which they apply.  These insights are briefly 
summarized below.  As final technical reports providing additional detail 
are published, they will be made available to all licensees.  

Licensees should bear in mind that the insights listed below are not all 
inclusive and unique plant features may exist that also warrant 
consideration in the IPE.  Licensees should search for possible "outliers" 
that might be missed absent a systematic search in areas of both mitigation 
and prevention.

      Mark II Containments

      For events where inadequate containment heat removal could cause core 
      degradation, additional containment heat removal capability using 
      plant-specific hardware procedures is expected to be considered as 
      part of the IPE process.  Potential methods of removing heat from 
      containment include, but are not limited to, using a hardened vent or 
      other means of improving reliability of suppression pool cooling.  It 
      is expected that the negative as well as the positive benefits of the 
      enhanced containment heat removal capability will be considered.  For 
      example, for those events where venting is initiated after core melt 
      and subsequent vessel failure have occurred, the benefit of scrubbing 
      of fission products cannot be assured for Mark II containments to the 
      same degree as in Mark I plants.  This is because molten core 
      materials on the floor of the containment may fail downcomers or drain 
      lines and result in suppression pool bypass.

9007020114 
.

                                    - 2 -


      In addition, the Mark I improvements contained in Supplement 1 to 
      Generic Letter 88-20 dated August 29, 1989 are expected to be 
      considered for applicability  to Mark II containments. 

      Mark III Containments

      A potential vulnerability for Mark III plants involves station 
      blackout, during which the hydrogen igniters would be inoperable.  
      Under these conditions, a detonable mixture of hydrogen could develop 
      which could be ignited upon restoration of power.  Licensees with Mark 
      III containments are expected to evaluate the vulnerability to 
      interruption of power to the hydrogen igniters as part of the IPE.  A 
      backup power supply meeting the requirements for the Alternate AC 
      option of the Station Blackout Rule would be one method of ensuring 
      uninterrupted operation of the hydrogen igniters.

      In addition, the Mark I improvements contained in Supplement 1 to 
      Generic Letter 88-20 dated August 29, 1989, as well as containment 
      heat removal as discussed for Mark II containments, are expected to be 
      considered for applicability to Mark III containments. 

      PWR Ice Condenser Containments

      The same situation could occur in ice condenser containments as in 
      Mark III containments relative to hydrogen detonations following 
      restoration of power.  Therefore, licensees with ice condenser 
      containments are expected to evaluate the vulnerability to 
      interruption of power to the hydrogen igniters as part of the IPE. 

      PWR Dry Containments

      Depending on the degree of compartmentalization and the release point 
      of the hydrogen from the vessel, local detonable mixtures of hydrogen 
      could be formed during a severe accident and important equipment, if 
      any is nearby, could be damaged following a detonation.  In addition, 
      smaller subatmospheric containments may develop detonable mixtures of 
      hydrogen on a global basis.  Licensees with dry containments are 
      expected to evaluate containment and equipment vulnerabilities to 
      localized hydrogen combustion and the need for improvements (including 
      accident management procedures) as part of the IPE.

      It should be noted that currently available computer codes have been 
      shown to overestimate mixing of hydrogen in the containment and may 
      not be adequate to evaluate the potential for high local 
      concentrations of hydrogen (e.g., ANS Proceedings, 1989 National Heat 
      Transfer Conference, August 6-9, 1989, Philadelphia, PA, Page 
      233-241).  Thus any analyses should be supplemented by judgement as to 
      the adequacy of the results and consideration of the impact of higher 
      than predicted hydrogen concentration due to stratification.  

.

                                    - 3 -


      Given an estimate of local concentration of hydrogen, NUREG/CR-5275 
      provides a discussion of one method that has been used to evaluate the 
      potential for local hydrogen detonations.

This generic letter provides information that may assist licensees in 
performing their Individual Plant Examination pursuant to Generic Letter 
88-20.  It does not contain any new requirements and no reply to this 
generic letter is required.

Generic Letter 88-20 was issued pursuant to 10 CFR 50.54(f).  A copy of the 
10 CFR 50.54(f) evaluation which justified issuance of Generic Letter 88-20 
is in the Public Document Room.  This supplement does not change the scope 
of Generic Letter 88-20.  Therefore, there is no additional burden 
associated with this letter, and a separate OMB clearance is not required. 

                                Sincerely,




                                James G. Partlow
                                Associate Director for Projects
                                Office of Nuclear Reactor Regulation

Enclosure:
List of Most Recently Issued Generic Letters