ML12339A274

Concrete Summary 4-12

ML12339A274
Person / Time
Site:	Seabrook
Issue date:	09/13/2012
From:	C-10 Research & Education Foundation, Union of Concerned Scientists
To:	NRC Region 1
References
Download: ML12339A274 (3)

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CONCRETE DEGRADATION AT THE SEABROOK NUCLEAR POWER PLANT In 2010, NextEra Energy applied to the Nuclear Regulatory Commission (NRC) for a 20-year renewal of the operating license for the Seabrook nuclear power station in New Hampshire. Its current license was issued in 1990 and does not expire until 2030, but the license renewal being done now would extend operation until 2050. Analysis of the plant must therefore identify and understand aging issues well enough to predict its behavior for the next 40 years.

This demanding task was made more difficult by the recent discovery of the degradation of concrete in several of the reactors safety related structures. The concrete degradation may also imply that corrosion of steel reinforcements hidden within the thick concrete walls has been occurring for years. Without understanding the scope and especially the cause of these processes, no credible basis exists for assessing the current structural status of the plant, or predicting what it will be in the future.

The NRC inspectors found that groundwater infiltration at the plant had caused widespread cracking of the concrete, due to a process called alkali-silica reaction (ASR) that had been going on for years. These findings were consistent with those of earlier inspections by NextEra personnel. i The NextEra inspections found groundwater infiltration at the plant, including pooling of water; evidence of moisture permeating the concrete walls; cracking and spalling (flaking of the surface) of concrete; and corroded steel supports, piping, base plates, and anchor bolts.

Alkali-silica reaction (ASR)

Alkali-silica reaction can occur when certain forms of silica in the bulk material in concrete (such as crushed rock and sand) react in the presence of water with chemicals like sodium or potassium, which are commonly found in the cement paste. This reaction produces a gel that forms in the pores of the concrete and then expands, causing stress and cracking in the concrete.

As this process continues over time, those cracks can join together to form larger fissures in the cement and undermine the structural integrity of the concrete. ASR damage in concrete has been known for 70 years.

Current studies do not appear to have determined the origin of the ASR occurring at Seabrook. ASR could result from a combination of two factors, both of which involve exposure to moisture:

1. ASR may be intrinsic to the concrete if the materials in the original concrete mixture have a composition that allows them to react with each other when exposed to moisture from the environment;

2. Chemicals in the environment can change the chemistry in the concrete pores, resulting in ASR; these changes may be caused by exposure to water with a high salt content.

It is important for the NRC to understand the origin of the ASR. If it is due to the composition of the concrete mixture used in building the plant then it could be a widespread problem throughout the plant structure.

It is also important to recognize that the fine pore structure of concrete will draw water from any soil it is in contact with even if the soil is not saturated with water. This fact must be taken into account in designing mitigation measures.

In addition to causing degradation of the concrete itself, ASR can be associated with corrosion of steel reinforcement bars and other steel structures embedded in the concrete. If the water in the concrete causing the ASR contains chloride or sulfate, both of which have been detected in the groundwater around Seabrook, those chemicals can remove the usual protection the concrete environment provides embedded steel and render the steel susceptible to corrosion. Both ASR and corrosion of embedded steel can decrease the durability and strength of reinforced concrete.

Surface cracking of the concrete is just the external manifestation of ASR. By the time it is observed, ASR and possibly steel corrosion have typically been occurring for years within the concrete. Still unknown is whether ASR is occurring elsewhere in the plant but has not yet become apparent at the surface of those structures.

As a result, one cannot make assumptions about the structural state of the concrete, or the effect of aging over the next 40 years, without thorough assessment and testing to determine the extent of ASR throughout the plant and throughout the bulk of the concrete walls and floors, and the level of corrosion of steel reinforcement throughout the plant.

Unanswered Questions Independent expert analysis, conducted by Paul Brown, Professor of Ceramic Science and Engineering at Penn State University, of the existing reports on concrete degradation at Seabrook identified the following key questions that require additional information or further study:

1. Is ASR inherent to the concrete used at Seabrook and induced merely by exposure to water regardless of its source, or is ASR induced by chemicals in the ground water? If it is inherent, how widespread is this problem throughout the concrete in the plant?

2. What, quantitatively, is the extent to which ASR is occurring, both with respect to locations throughout the plant and with respect to the position within the thickness of the concrete walls themselves?

3. What basis is there for predicting whether or not ASR has occurred or will occur at locations not yet identified?

4. Is the ASR accompanied by other deleterious reactions, especially corrosion of embedded steel and sulfate attack, and how severe and widespread are those effects throughout the plants structure?

5. What basis is there for predicting the rate of corrosion of the steel reinforcement in the concrete that is currently exposed to infiltrating ground water, or that is likely to be exposed in the future? What effect will the corrosion have on the structure at Seabrook?

6. What, quantitatively, is the extent to which ASR has reduced mechanical properties of the concrete compared to control samples of concrete obtained from non-damaged locations? What effect will the damaged areas have on the structure?

7. What basis is there for predicting the future loss in important mechanical properties of concrete, such as compressive and splitting tensile strengths, at locations where ASR has been identified?

8. What basis is there for predicting the plant's ability to maintain structural integrity and safety-related functions in a seismic event given the extent of the ASR damage and steel corrosion in plant structures?

9. What mitigation/remediation strategies are available and what is the basis for establishing the probability of success of those strategies?

Next Steps The above questions must be answered before the structural state of the Seabrook plant can be determined. Without that information any claims about the structural integrity of the plant are not credible.

Professor Paul Brown, the independent concrete expert who reviewed the reports on Seabrook identified the following information needed to clarify the implications of existing studies, and suggested these additional tests:

1. Documents from the plant state that core samples show ASR from moderate to severe, but do not give the criteria used, or say where the ASR occurred. Studies must determine if the ASR is localized or occurring uniformly throughout the walls. As part of this, studies should characterize the materials in the concrete that are reacting and determine if they were used throughout the concrete in the plant.

2. Moisture migrating through the concrete walls can cause deposits of calcium carbonate and ASR gel on the inner surface of the walls. Studies should be done to determine the composition of these deposits. The presence of chloride suggests chloride-induced corrosion of steel embedded in the concrete. The presence of sulfates also suggests corrosion of the steel and of surface deterioration of the concrete, which means that applying waterproof layers to the walls may not be possible.

3. Analyses should determine the extent of chloride penetration in the bulk concrete. The presence of chloride not only allows steel corrosion, but if it was carried in as sodium chloride can establish the mechanism for ASR and give information about how exposure to water in the environment affects deterioration.

4. More complete analyses of the ground-water chemistry should be carried out. To determine the mechanism of ASR, it is important to determine if ASR is limited to regions where the ground water has high sodium content.

5. Greater detail is needed about the 87 locations noted in the existing reports where spalling of the walls is significant; information about environmental conditions these areas are exposed to is especially important.

This information should include whether floors as well as walls are deteriorating. If the floors do not show degradation, tests should be conducted to understand why not.

6. Because concrete can extract moisture even from soil that is not saturated, dewatering the soil with sumps and drains may not be an effective method of reducing moisture in concrete. The dewatering plan at Seabrook must be explained more fully to understand its purpose and possible effects.

7. While tests have been conducted on compressive and splitting tensile strengths of concrete structures at Seabrook, more detail is needed about the various testing procedures to understand the results and implications of those tests. For example, while existing reports claim that the modulus of elasticity in concrete at Seabrook decreased by 47%, more information is required to understand this result and its implications.

8. Tests should be conducted to compare compressive and splitting tensile strength of sample concrete cores to cores taken from areas where ASR is not occurring, not to companion cylinders that were made at the time of construction from the same concrete.

April, 2012 i

NextEra Energy Seabrook- NRC License Renewal Inspection Report 05000443/2011007. United States Nuclear Regulatory Commission.

May 23, 2011. http://pbadupws.nrc.gov/docs/ML1113/ML111360432.pdf (pg 17)