United States Nuclear Regulatory Commission - Protecting People and the Environment

Potential for Loss of Post-LOCA Recirculation Capability Due to Insulation Debris Blockage (Generic Letter No. 85-22)



                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                           WASHINGTON, D. C. 20555

                              December 3, 1985

TO ALL LICENSEES OF OPERATING REACTORS, APPLICANTS FOR OPERATING LICENSEES, 
AND HOLDERS OF CONSTRUCTION PERMITS. 

Gentlemen: 

SUBJECT:  POTENTIAL FOR LOSS OF POST-LOCA RECIRCULATION CAPABILITY DUE TO 
          INSULATION DEBRIS BLOCKAGE (Generic Letter 85-22 ) 

This letter is to inform you about a generic safety concern regarding LOCA 
generated debris that could block PWR containment emergency sump screens or 
BWR RHR suction strainers, thus resulting in a loss of recirculation or 
containment spray pump net positive suction head (NPSH) margin. 

The potential exists for a primary coolant pipe break to damage thermal 
insulation on the piping as well as that on nearby components. Insulation 
debris could be transported to water sources used for long-term post-LOCA 
recirculation and containment sprays (i,e., PWR containment emergency sumps 
and BWR suction intakes in the suppression pools) and deposited on debris 
screens or suction strainers. This could reduce the NPSH margin below that 
required for recirculation pumps to maintain long-term cooling. 

This concern has been addressed as part of the efforts undertaken to resolve
USI A-43, "Containment Emergency Sump Performance."  The staff's technical 
findings contain the following main points. 

o    Plant insulation surveys, development of methods for estimating debris 
     generation and transport, debris transport experiments, and information
     provided as public comments on the findings have shown that debris 
     blockage effects are dependent on the types and quantities of 
     insulation employed, the primary system layout within containment, 
     post-LOCA recirculation patterns and velocities, and the post-LOCA 
     recirculation flow rates. It was concluded that a single generic 
     solution is not possible, but rather that debris blockage effects are 
     governed by plant specific design features and post-loca recirculation 
     flow requirement.

o    The current 50% screen blockage assumption identified in Regulatory 
     Guide (RG) 1.82, "Sumps for Emergency Core Cooling and Containment 
     Spray Systems," should be replaced with a more comprehensive 
     requirement to assess debris effects on a plant-specific basis. The 50% 
     screen blockage assumption does not require a plant-specific evaluation 
     of the debris-blockage potential and usually will result in a 
     non-conservative analysis for screen blockage effects.

The staff has revised Regulatory Guide (RG) 1.82, Revision O, "Sumps for 
Emergency Core Cooling and Containment Spray Systems" and the Standard 
Review Plan Section 6.2.2, "Containment Heat Removal Systems" based on the  


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above technical findings. However, the staff's regulatory analysis 
(NUREG-0869, Revision 1, "USI A-43 Regulatory Analysis") evaluated (1) 
containment designs and their survivability should loss of recirculation 
occur, (2) alternate means to remove decay heat, (3) release consequences 
(which were based on pipe break probabilities which did not incorporate 
insights gained from recent pipe fracture mechanics analyses), and (4) cost 
estimates for backfits considered (i.e., reinsulating). This regulatory 
analysis did not support a generic backfit action and resulted in the 
decision that this revised regulatory guidance will not be applied to any 
plant now licensed to operate or that is under construction. The revised 
guidance will be used on Construction Permit Applications, Preliminary 
Design Approval (PDA) applications, and applications for licenses to 
manufacture that are docketed after six (6) months following issuance of RG 
1.82, Revision 1, and Final Design Approval (FDA) applications, for 
standardized designs which are intended for referencing in future 
Construction Permit Applications, that have not received approval at six (6) 
months following issuance of the RG 1.82, Revision 1. 

Although the staff has concluded that no new requirements need be imposed on
licensees and construction permit holders as a result of our concluding 
analyses dealing with the resolution of USI A-43, we do recommend that RG 
1.82, Revision 1 be used as guidance for the conduct of 10 CFR 50.59 reviews
dealing with the changeout and/or modification of thermal insulation 
installed on primary coolant system piping and components. RG 1.82, Revision
provides guidance for estimating potential debris blockage effects. If, as a 
result of NRC staff review of licensee actions associated with the changeout 
or modification of thermal insulation, the staff decides that Standard 
Review Plan Section 6.2.2, Revision 4 and/or RG 1.82, Revision 1 should be 
(or should have been) applied to the rework by the licensee, and the staff 
seeks to impose these criteria, then the NRC will treat such an action as a 
plant-specific backfit pursuant to 10 CFR 50.109. It is expected that those 
plants with small debris screen areas (less than 100 ft2), high ECCS 
recirculation pumping requirements (greater than 8000 gpm), and small NPSH 
margins (less than 1 to 2 ft of water) would benefit the most from this type 
of assessment in the event of a future insulation change. RG 1.82, Revision 
O with its 50% blockage criteria does not adequately address this issue and 
is inconsistent with the technical findings developed for the resolution of 
USI A-43. 

This information letter along with enclosed copies of NUREG-0897, Revision 
1, RG 1.82, Revision 1 and SRP Section 6.2.2, Revision 4 should be directed 
to the appropriate groups within your organization who are responsible for 
conducting 10 CFR 50.59 reviews. 
.

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No written response or specific action is required by this letter. 
Therefore, no clearance from the Office of Management and Budget is 
required. If you have any questions on this matter, please contact your 
project manager. 


                              Hugh L. Thompson, Jr., Director 
                              Division of Licensing 

Enclosure: 
NUREG-0897, Revision 1 
RG 1.82, Revision 1 
SRP Section 6.2.2, Revision 4
Page Last Reviewed/Updated Thursday, June 27, 2013