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NRC OKs Cracking Measures at Seabrook

Friday, August 16, 2013

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A new inspection report on concrete degradation at the Seabrook Nuclear Power Plant shows that appropriate measures have adequately addressed the cracks' root causes and potential impact, federal regulators announced.

The Nuclear Regulatory Commission released its second follow-up Inspection Report for the New Hampshire power plant on Aug. 9, saying that plant operator NextEra Seabrook LLC had appropriately assessed and determined that its crack-riddled structures remain operable.

At the same time, the NRC announced that all 11 action items assigned to NextEra Seabrook LLC had been satisfactorily completed and have been closed.

Uncertainty and an Alert

The concrete problem dates to 2009 and eventually prompted a general alert by NRC concerning the potential for similar issues at other nuclear plants.

While preparing to apply for Seabrook's 20-year operating license extension (from 2030 to 2050), NextEra noticed pattern cracking when concrete core samples were removed from below-grade structures that had been in contact with groundwater for several decades.

Seabrook Nuclear Power Plant
NRC

Not understanding the effects of groundwater on ASR in below-ground concrete structures is a root cause of the concrete issues at the Seabrook plant, the NRC said.

The problem was confirmed in 2010 as a type of concrete degradation known as an Alkali-Silica Reaction (ASR)—a chemical process so slow that Seabrook is the first nuclear plant in the United States to exhibit it in on-site concrete structures.

Although the NRC has maintained that the problem does not threaten the structure's integrity, the issue  prompted the NRC to issue a general alert in December 2011 to all of its facilities and applicants regarding the risk of the same problem elsewhere.

Action Items

The NRC's original Confirmatory Action Letter, issued May 16, 2012, included 11 action items for NextEra to complete by certain deadlines throughout 2012 and 2013. In July 2012, NRC staff formed a working group to provide oversight of NextEra's steps to resolve the ASR issues, as well as provide coordination of inspections and other associated evaluations and assessment activities.

According to the recent Inspection Report, "The review and closure of each [Confirmatory Action Letter] CAL item signifies the NRC's satisfactory assessment of NextEra's commitments and planned corrective actions to address the ASR non-conforming condition at Seabrook Station."

Reviews to Continue

However, the report warns, "The completion of the CAL follow-up inspections does not represent the completion of NRC review and oversight of NextEra's actions to address the ASR issue."

The NRC still expects a number of ongoing activities from NextEra to address the issue, including an evaluation of structural performance based on a large-scale testing program of beam specimens representative of Seabrook reinforced concrete structures, scheduled to be completed by 2015.

About ASR

ASR-induced concrete degradation, a slow 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.

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. The problem requires confirmation by optical microscopy performed as part of petrographic examination of concrete core samples.

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.

Items Completed

Based on the NRC inspection team's first follow-up Inspection Report, dated Dec. 3, 2012, five of the action items were reviewed and closed; a sixth item required NextEra to take additional actions.

alkali silica reaction
NRC

Seabrook was the first on-site concrete structure to exhibit the slow-moving ASR degradation in the U.S. nuclear industry. The problem can cause patterned cracking and the presence of alkali-silica gel.

The completed items from the first inspection included:

  • Revise the prompt operability determination for an electical tunnel exterior wall;
  • Revise the prompt operability determination for the containment enclosure building, the emergency feedwater pump house, the residual heat removal equipment vault and the diesel generator building;
  • Submit the evaluation, "Impact of ASR on Concrete Structures and Attachments";
  • Complete short-term aggregate expansion testing (ASTM C 1260 Mortar Bar Expansion Test); and
  • Perform the initial six-month interval crack measurements and crack indexing at 20 locations in areas that exhibit the highest crack indices (to be performed every six months until a reliable trend of ASR progression is established).

The Aug. 9, 2013, follow-up Inspection Report closed the remaining six items:

  • Submit the root cause for the organizational causes associated with the occurrence of ASR at Seabrook Station and related corrective actions (initially reviewed and left open in the Dec. 3, 2012, report);
  • Submit the corrective action plan for the continued assessment of ASR in concrete structures at Seabrook Station, including development of remedial actions to mitigate the affects of ASR, where warranted;
  • Complete long-term aggregate expansion testing, ASTM C 1293 Concrete Prism Test (deleted on a technical basis);
  • Submit the technical details of the testing planned at the contracted research and development facility;
  • Update the Maintenance Rule Structures Monitoring Program to include monitoring requirements for selected locations in areas that exhibit ASR; and
  • Complete anchor test program.

Monitoring 'Inadequacies' Cited

When NextEra first submitted its root cause evaluation, the NRC said that it "did not clearly describe the performance and organizational factors that contributed to inadequacies in the Structures Monitoring Program (SMP) and the failure of the Seabrook staff to have identified ASR degradation of reinforced concrete structures sooner."

University of Texas-Austin
Ferguson Structural Engineering Laboratory

NextEra is working with a laboratory at the University of Texas at Austin to conduct large-scale testing in an attempt to correlate it to structures at Seabrook. The 45,000-square-foot laboratory is one of the largest structural research facilities in the world.

A revised root cause evaluation identified two causes:

  • (1) "the ASR developed because the concrete mix design unknowingly utilized a coarse aggregate that would, in the long-term, contribute to ASR," and
  • (2) "based on the long-standing organizational belief that ASR was not a credible failure mode due to the concrete mix design, dispositions for condition reports involving groundwater intrusion or concrete degradation, along with the structures health monitoring program, did not consider the possibility of ASR development."

NextEra said a contributing cause was that it did not prioritize groundwater elimination or mitigation, resulting in more concrete area exposed to moisture.

Action Plan, Testing Program

In June 2012, NextEra submitted a corrective action plan to NRC, outlining major elements of diagnosis, evaluation, prognosis and mitigation of affected structures. Since then, the company has started to implement the initial phases, allowing it to more clearly refine the plan focus future actions. NextEra provided an updated corrective action plan, dated May 1, 2013, to document these changes.

Destructive testing of ASR-affected test specimens is underway at the Ferguson Structural Engineering Laboratory at the University of Texas at Austin, as part of NextEra's proposed large-scale specimen testing program.

The purpose of the test program is to provide data and information to establish the current and future implications of ASR on reinforced concrete structures at Seabrook. FSEL is preparing test specimens that represent the plant's structures, and NextEra plans to take additional core samples from the Seabrook structures to better correlate the test results using petrography and mechanical testing.

As of Thursday (Aug. 15), NextEra had not submitted a response to the recent inspection. All publicly available documents on the NRC's oversight at the nuclear plant, including operator responses, updated plans and all inspection reports, can be found here.

   

Tagged categories: Concrete; Concrete defects; Inspection; Nuclear Power Plants

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