Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
March 21, 1997
NRC INFORMATION NOTICE 97-11: CEMENT EROSION FROM CONTAINMENT SUBFOUNDATIONS
AT NUCLEAR POWER PLANTS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice (IN) to alert addressees to information regarding the possible erosion
of cement from porous concrete subfoundations below the reactor building
basemats at some reactor sites. It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to monitor similar phenomena at their plants. However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.
Background
The containment structure at Millstone Nuclear Power Station Unit 3 (MNPS-3)
has a 3.05-meter [10-foot]-thick reinforced-concrete basemat founded on rock.
Between the foundation rock surface and the underside of the basemat are
several layers of different materials. These layers consist of (1) a 25.4-cm
[10-inch]-thick leveling layer of porous concrete made of coarse aggregates
and Portland cement, (2) a 0.16-cm [1/16th-inch]-thick butyl rubber
waterproofing membrane, (3) a 5.08-cm [2-inch]-thick Portland cement mortar
seal, (4) a second layer of 22.86-cm [9-inch]-thick porous concrete made of
coarse aggregates and calcium aluminate (high-alumina) cement, and (5) a thin
mortar seal (consisting of calcium aluminate cement and sand) on the top of
the upper layer of the porous concrete. In the upper porous concrete layer,
15-cm [6-inch]-diameter porous concrete pipes are installed to collect and
drain ground water which may seep down along the periphery of the containment
wall. The collected water drains into two sumps inside the Engineered Safety
Features (ESF) Building.
The MNPS-3 licensee, Northeast Nuclear Energy Company (NNECO), identified the
issue of cement erosion from the porous concrete drainage system in 1987 upon
examination of the accumulated sludge in the two lower drain sumps in the ESF
Building. The licensee's efforts
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March 21, 1997
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to follow up on and resolve this concern are documented in NRC Inspection
Reports 50-423/94-11 (accession number 9406060281) and 50-423/96-04 (accession
number 9606180450), dated May 27, 1994, and June 6, 1996, respectively. The
main concern is the adequacy of the eroded porous media to transfer the
containment loads to the bedrock. The unexpected erosion of the high-alumina
cement also gave rise to another concern regarding a potential for interaction
between the concrete of the foundation basemat that contains Portland cement
and the high-alumina cement of the subfoundation in the presence of
underground water.
To address these concerns, the licensee performed strength tests on cores
obtained from mockup tests that simulated accelerated degradation of the
porous concrete. The licensee concluded that the load-bearing capacity of the
porous concrete has not been compromised. The licensee is continuing to
investigate the effects of continuous water flow on the time-dependent
degradation of porous concrete. In response to a staff question on settlement
of the containment structure, NNECO has stated that, to date, no detectable
movement of the containment structure has been observed. Furthermore, NNECO
has reported that the loss of cement as a result of erosion from porous
concrete is not significant.
On the basis of a review of all the available information, it appears that
there is no immediate safety concern at MNPS-3 because only an insignificant
amount of cement is estimated to have possibly eroded from the porous concrete
subfoundation since the plant was built in 1975, and because no adverse
consequences of the cement erosion are either predicted or have been observed
at the plant.
Discussion
To address the question of whether there are other nuclear power plant sites
at which these types of conditions could exist, the staff reviewed the updated
Final Safety Analysis Reports (UFSARs) of 24 plants selected on the basis of
(1) the unique practice of the constructor or the architect and engineers
(A&Es) and (2) a sampling of A&Es. The staff found that 12 reactor units had
subfoundation layers of porous concrete: MNPS-3, North Anna Units 1 and 2,
Surry Units 1 and 2, Nine Mile Point Unit 2 (NMP-2), Maine Yankee,
FitzPatrick, Beaver Valley Units 1 and 2, Haddam Neck, and Perry. Of these 12
plants, 3 have surveillance programs for monitoring cement erosion � MNPS-3,
NMP-2, and Maine Yankee. Maine Yankee Inspection Report 50-309/95-08
(accession number 9505240167), dated May 16, 1995, shows that the containment
sump has been under surveillance since 1970 and that no sign of cement slurry
has been found. The NMP-2 licensee has replied verbally that no calcium
aluminate has been found in the sump water sample and that its surveillance
findings will be documented in due course. The staff has obtained the
following information from the licensees of the remaining nine plants
regarding the conditions at their plants:
Virginia Electric & Power Company (VEPCO), the licensee of North Anna
Units 1 and 2 and Surry Units 1 and 2, reported on November 5, 1996,
that Type II, low-alkali, Portland cement (and not calcium aluminate
cement) was used in the porous concrete at North Anna and Surry sites..
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At North Anna Unit 2, water samples taken recently from the basemat sump
and from two domestic water wells at the plant site showed virtually no
aluminum (less than 1 ppm) in the three locations. Insignificant
amounts of calcium and sulfate were noticed at the mat sump. No
detectable settlement of the containment basemat was noticed at North
Anna Units 1 and 2.
At Surry Units 1 and 2, an external subsurface drainage system operates
continuously to control groundwater level. The Unit 1 valve pit area
for the internal drainage system was dry; the Unit 2 area had a few
centimeters of standing water apparently leaking in through a side wall.
A minimum amount of leachate was observed in this area. The settlement
of the containment structures of both units at Surry were found to be
less than 0.32 cm [1/8 inch], which is within the design values (1.27 cm
[1/2 inch]) for settlement.
The Haddam Neck plant (HNP) licensee, Northeast Utilities Services
Company, reported on October 31, 1996, that Type II, low-alkali,
Portland cement (and not calcium aluminate cement) was used in the
porous concrete mix. For the last 10 years at HNP, no slurry was
found in the drainage water from the external containment sump during
the monthly survey done for radiological concerns. Reactor building
settlement is not monitored; however, recent inspections inside the
containment found no evidence of containment settlement and no
indications of degradation of the concrete slab.
The Perry nuclear power plant licensee, Cleveland Electric Illuminating
Company, reported on November 4, 1996, that no cement slurry has been
noted in the drainage and there has been no evidence of pea gravel in
the drainage pipes or manholes. The licensee has, however, reported
that dissolved calcium carbonate is being carried to the drainage pipes
and the sumps, where it is solidifying as a fine particulate which
hardens into hard scale. According to NRC Region III Inspection Report
50-440/96-04 (accession number 9609030241) dated August 21, 1996, the
licensee also discovered an accumulation of mineral deposits that
clogged the pores of the porous concrete pipes and developed an
effective method of clearing the clogged pipes. The licensee plans to
clear the remaining pipes. No containment structure settlement has been
observed at Perry.
The Beaver Valley Power Station (BVPS) Units 1 and 2 licensee, Duquesne
Light Company (DLC), reported on November 7, 1996, that neither of the
two units has experienced drainage from the porous concrete layers below
the basemat. DLC further claims that such drainage is not expected at
BVPS because of the plant's containment design features and the
waterproof membrane that envelops the porous concrete layer, the
containment mat foundation, and the exterior wall up to the plant grade
level. DLC has reported that the total observed settlement of the BVPS .
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March 21, 1997
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containment structures compares favorably with the total predicted
settlement, and that it has not noted any unusual conditions that were
related to the porous concrete layers at BVPS Units 1 and 2.
The James A. FitzPatrick nuclear power plant licensee, New York Power
Authority (NYPA), reported on November 8, 1996, that the porous concrete
subfoundation slab of 15.24-cm [6-inch] (minimum) thickness was
constructed using Type II Portland cement. This slab, laid between the
basemat and the rock, is surrounded by engineered backfill. On top of
the backfill, a 30.48-cm [12-inch]-diameter porous concrete drainage
pipe leading to a sump was laid with its invert level about 1.22 meters
[4 feet] above the top of the porous concrete slab. Any ground water
seeping down along the sides of the reactor building collects in the
sump and is pumped into the drainage system. NYPA has not observed any
cementitious slurry in the water samples being regularly collected from
the drainage sump. NYPA has also reported that the containment
structure has not settled.
On the basis of this information and also on the basis of the staff's
preliminary assessments of MNPS-3 and NMP-2, the staff finds that there is no
immediate generic or plant-specific safety concern related to the porous
concrete subfoundations below the containment basemat at nuclear power plants.
However, the NRC staff is continuing to evaluate the potential long-term
impact of erosion of high-alumina cement at MNPS-3.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate project manager.
signed by M.M. Slosson
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: R. Pichumani, NRR
(301) 415-2734
E-mail: [email protected]
H. Ashar, NRR
(301) 415-2851
E-mail: [email protected]
