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, March 25, 2021