Degrading concrete at a New Hampshire nuclear plant has prompted the Nuclear Regulatory Commission to alert all of its facilities and applicants to the same risk elsewhere.
The Seabrook Station Nuclear Power Plant, which is applying for license renewal, has been working to address concrete cracking since discovering the problem in its control building in August 2010.
|Patterned cracking like this can be indicative of ASR-induced degradation. The Nuclear Regulatory Commission is distributing the photo as a generic example not taken from the nuclear industry.|
The problem is known as Alkali-Silica Reaction-induced (ASR) concrete degradation, a slow chemical degradation process that occurs when alkalis—usually from cement—react with certain types of silica in the aggregate when moisture is present.
The reaction produces an alkali-silica gel that can absorb water and expand to cause micro-cracking of the concrete. Excessive expansion of the gel can lead to significant cracking.
‘Substantial Reduction’ in Strength
NRC inspectors have confirmed the degradation at Seabrook but say the facility’s structural integrity has not been affected.
While continuing to address the problem, however, the NRC has taken the additional step of issuing a general notice about the issue.
Examination and testing of below-grade core samples at Seabrook have revealed “substantial reduction in compressive strength compared to test cylinders cast during construction and a modulus of elasticity substantially lower than the expected value,” NRC reported Nov. 18 in an Information Notice.
ASR is just one type of alkali-aggregate reaction that can degrade concrete structures. Visual indicators of the problem include “craze,” “map” or “patterned” cracking and the presence of alkali-silica gel. But the problem requires confirmation by optical microscopy performed as part of petrographic examination of concrete core samples.
Older Testing Standards Cited
ASTM has issued a variety of standards for testing concrete aggregate during construction, to verify that only non-reactive aggregates are present.
However, ASTM now cautions that two of those standards—ASTM C227 and ASTM C289—“may not accurately predict aggregate reactivity when dealing with late- or slow-expanding aggregates containing strained quartz or microcrystalline quartz,” according to the NRC.
Matthew Trump / Creative Commons
|The Seabrook Station Nuclear Plant is the first to confront ASR-induced concrete degradation in its licensing renewal process.|
“Therefore, licensees that tested using ASTM C227 and ASTM C289 could have concrete that is susceptible to ASR-induced degradation.”
ASTM C289 sets a “Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method)”; ASTM C227 sets a “Standard Test Method for Potential Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method).”
The Seabrook Station performed concrete testing during construction with ASTM C289 and ASTM C295, which were the accepted standards at the time.
Water Membrane Damage
“The licensee believes that the waterproof membrane was damaged during original installation or backfill activities causing water intrusion that resulted in the ASR problems,” the NRC said. “Water intrusion was exacerbated by the fact that dewatering channels were abandoned.”
Once ASR occurs, and because it degrades various properties of concrete at different rates, “relationships between compressive strength and tensile or shear strength and assumptions about modulus of elasticity that were used in the original design of affected structures may no longer hold true,” the NRC said.
The “suggestions” in the NRC notice are not requirements and require “no specific action,” NRC said.
However, the agency advises preventing ASR by:
• Selecting non-reactive materials during initial construction;
• Controlling water infiltration by protecting waterproof membranes or by adding dewatering channels; and
• Visually inspecting for ASR-type cracking and for the presence of alkali-silica gel in areas where concrete is exposed to moisture.
Structural monitoring should always evaluate the results of periodic assessments to determine the extent and rate of any degradation, the NRC says.
Corrosion, Water Infiltration Noted
At Seabrook, NRC inspectors noted the deterioration in their report.
“The walk-down inspections discovered the following plant material conditions (at Seabrook Station); (a) large amount of groundwater infiltration, (b) large amount of calcium carbonate deposits, (c) corroded steel supports, base plates and piping, (d) corroded anchor bolts, (e) pooling of water and (f) cracking and spalling of concrete,” the report states.
“The inspection further noted that the below-grade, exterior walls in the Control Building B Electrical Tunnel ... have random cracking and for several years have been saturated by groundwater infiltration.
“The severity of the cracking and groundwater infiltration varies from location to location,” the report continues. "A comparison of the 2010 concrete compression test results to the 1979 concrete compression test results indicated a 21.7 percent reduction in the compressive strength of the concrete.”
Renewal Process Delayed
Seabrook operator NextEra Energy Resources has applied to extend the plant’s operating license to 2050, although the current license will not expire until 2030. Seabrook is the first plant to address ASR-induced concrete degradation as part of license renewal.
However, the NRC findings have pushed back the preliminary Safety Evaluation Report for the license extension by nearly a year, to May 2012, and the final report until December 2012, according to seacoastonline.com.
Meanwhile, NRC Branch Chief Arthur Burritt said at a meeting June 8 that he was not confident in NextEra’s ability to control the ASR problem.
“This is the first time this has come to our attention,” Burritt said, according to seacoast online. "Most plants are not affected. It's because of the concrete aggregates used at Seabrook that we have this problem. We have not been able to draw any conclusions about the ability of the utility to maintain these buildings.”
NextEra spokesman Alan Griffith said the plant still met or exceeded all design specifications. He said NextEra had hired third-party ASR experts to help develop repair strategies.