Information Notice No. 88-28:Potential for Loss of Post-LOCA Recirculation Capability due to Insulation Debris Blockage
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
May 19, 1988
Information Notice No. 88-28: POTENTIAL FOR LOSS OF POST-LOCA
RECIRCULATION CAPABILITY DUE TO
INSULATION DEBRIS BLOCKAGE
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems regarding debris that could block containment emergency sump screens
in a pressurized water reactor (PWR) or debris that could block emergency core
spray pump or residual heat removal pump strainers in a boiling water reactor
(BWR). 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 do not constitute NRC requirements; therefore, no specific action or
written response is required.
Description of Circumstances:
On March 14, 1988, Pennsylvania Power & Light (the licensee for Susquehanna)
notified the U.S. Nuclear Regulatory Commission of a 10 CFR Part 21 reportable
item concerning deterioration of drywell insulation and the potential for the
aluminum foil coating of the insulation to block emergency core cooling system
(ECCS) strainers during a loss-of-coolant accident (LOCA).
While the unit was in a refueling outage, the licensee inspected the
Susquehanna Unit 2 drywell. The licensee observed extensive delamination of
the aluminum foil coating on the surface of the fiberglass insulation used on
valve bodies and pipe hangers and in other areas that are awkward or difficult
to insulate.
The aluminum foil covering is 1-mil thick and is bonded to the outer covering
of Alpha Maritex fiberglass cloth (style #2025/9480 HT) that is used as a
covering for Temp-Mat insulation. An upper-bound estimate is that 5000 square
feet of this insulation is used in more than 300 different locations within
the drywell. The licensee estimates that 50 percent of the insulation has
undergone some degradation. This is the first time the licensee has observed
degradation to this extent, although some degradation had been noticed
earlier. A different insulation material is used at Susquehanna Unit 1, and
no insulation delamination has been observed at that unit. Alpha Associates,
Inc. supplied the insulation
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May 19, 1988
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to the licensee through Bechtel. However, representatives of Alpha Associates,
Inc., have indicated that other companies supply the same or similar
insulation and that its use may be widespread.
After the event at Susquehanna Unit 2, the licensee for Millstone Unit 1
reported, that in a worst-case scenario, the net positive suction head margin
for ECCS pumps could be compromised because of suction strainer blockage
resulting from LOCA-generated insulation debris. At Millstone Unit 1,
Temp-Mat insulation is used without a foil facing. In this case, the concern
was that the fiberglass cloth covering would be freed during a LOCA and
migrate to the ECCS pump suction strainers. The licensee is evaluating the
feasibility of replacing the existing strainers with strainers that have a
larger area as a possible solution of the problem of transported insulation.
Discussion:
In PWRs the containment emergency sumps provide for the collection of reactor
coolant and chemically reactive spray solutions following a LOCA, thus serving
as water sources to effect long-term recirculation for residual heat removal
and containment atmosphere cleanup. In a BWR, the suppression pool serves as
the water source for effecting long-term recirculation cooling.
Debris, transported as a result of an event, can block sump debris
interceptors and sump outlets, resulting in degradation or loss of
recirculation flow margin. At Susquehanna, the ECCS core spray pump suction
screen has a cross-sectional area of 18.3 square feet, while each residual
heat removal pump suction screen has a cross-sectional area of 43.8 square-ft.
In comparison, the insulation covering that could block recirculation flow
covers an area of several thousand square feet. However, the potential effect
of insulation on sump performance is plant specific. Thus, plant insulation
surveys, methods for estimating debris generation and transport, debris
transport experiments, and other information have shown that the effects of
debris-blockage depend on the types and quantities of insulation used, the
primary system layout within the containment, and post-LOCA recirculation flow
rates. The staff addressed similar concerns in resolving Unresolved Safety
Issue A-43, "Containment Emergency Sump Performance." In particular, the
staff's technical findings concluded that a single generic solution is not
possible, but rather that the effects of debris blockage are governed by
plant-specific design features and post-LOCA recirculation flow requirements.
The NRC staff has investigated the buoyancy, transport, and headloss
characteristics of reflective metallic insulation and construction materials
and the results are summarized in NUREG/CR-3616, "Transport and Screen
Blockage Characteristics of Reflective Metallic Insulation Materials."
Briefly, the tests showed that thin metallic foils could be transported at low
flow velocities and that flow blockage could occur at the lower portion of the
screen.
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Although the exact cause of the degradation of the foil covering on the insu-
lation at Susquehanna is not known, the causes may include temperature,
humidity, and the effects of radiation on the neoprene-type adhesive used in
the bonding process.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con-
tact listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: L. Zerr, NRR
(301) 492-1177
Attachment: List of Recently Issued NRC Information Notices
. Attachment
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Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-27 Deficient Electrical 5/18/88 All holders of OLs
Terminations Identified or CPs for nuclear
in Safety-Related power reactors.
Components
85-35, Failure of Air Check 5/17/88 All holders of OLs
Supplement 1 Valves to Seat or CPs for nuclear
power reactors.
88-26 Falsified Pre-Employment 5/16/88 All holders of OLs
Screening Records or CPs for nuclear
power reactors and
all major fuel
facility
licensees.
88-25 Minimum Edge Distance for 5/16/88 All holders of OLs
Expansion Anchor Bolts or CPs for nuclear
power reactors.
88-24 Failures of Air-Operated 5/13/88 All holders of OLs
Valves Affecting Safety- or CPs for nuclear
Related Systems power reactors.
88-23 Potential for Gas Binding 5/12/88 All holders of OLs
of High-Pressure Safety or CPs for PWRs.
Injection Pumps During a
Loss-of-Coolant Accident
88-22 Disposal of Sludge from 5/12/88 All holders of OLs
Onsite Sewage Treatment or CPs for nuclear
Facilities at Nuclear power reactors.
Power Stations
88-21 Inadvertent Criticality 5/9/88 All holders of OLs
Events at Oskarshamn or CPs for nuclear
and at U.S. Nuclear power reactors.
Power Plants
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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