Since 2010, operators and regulators have kept a close eye on the effects of the alkali-silica reaction (informally known as ‘concrete cancer’) at NextEra’s Seabrook nuclear power plant. Five years ago the plant won a renewal of its licence to 2050 and recently the NRC’s independent Advisory Committee on Reactor Safeguards (ACRS) checked in on how the site and NRC are monitoring this slow-moving reaction in some parts of the plant’s concrete.
Alkali-silica reaction (ASR) is a reaction driven by aggregates containing certain forms of silica. In an alkaline environment, the silica reacts with hydroxyl ions and a type of gel forms at the interface between the aggregate particles and the cement mortar. The gel expands dramatically as it absorbs water from the surrounding environment. The gel expansion starts off as micro cracking and can later be observed as macro-cracking on the surface. The visual appearance is typically in the form of so-called pattern cracking, with gel staining around the cracks.
The reaction process may take years, but once it is under way, the forces from the expansion cause cracking to appear on the surface. The expansion and cracking of concrete from ASR can potentially impact both the load carrying capacity of the structure, and the load put on that structure.
The American Ceramic Society notes that the concrete used in nuclear power plants is more susceptible to ASR than fine-grained concrete, because it uses high-density aggregates with coarser grains to shield effectively against radiation.
The US has a lot of experience with ASR in other industries, such as transportation infrastructure (eg concrete bridges) and dams, so there is extensive expertise in other bodies such as the Department of Transportation and the Federal Highway and Transportation Authority. However, extensive monitoring and remediation of ASR where it has appeared in other sectors has limited the data points on how it fails, because structures are either remediated or taken out of service prior to failure. What is more, those failures that have been seen are largely in unreinforced concrete.
NextEra confirmed the presence of ASR degradation of concrete in below-grade walls of several Category 1 structures at Seabrook in August 2010. Seabrook had unknowingly used a slow-reactive aggregate in the concrete and as the Nuclear Regulatory Commission (NRC) noted recently, there were ineffective American Society for Testing and Materials (ASTM) Standards at the time of construction. Furthermore, ASR was not expected, so inspections were not looking for it.
Seabrook is the only US nuclear plant where this issue has arisen so far and a review by the NRC staff determined that there were no immediate safety concerns. It justified this conclusion because there remained (and still remains) a margin of safety, the ASR is localised and is growing slowly, and there is ongoing monitoring.
In Seabrook’s ASR timeline:
- 2009 – 2010: Testing confirmed the presence of ASR
- 2013 – 2016: A large-scale test programme was carried out
- 2014 – 2015: NRC identified bulk structural deformation in Seismic Category 1 structures
- March 2019: NRC approved and issued both the license amendment request (LAR) and renewed licence
- Nov 2020: The LAR was challenged by a local advocacy group, but the licensing action was upheld (with modifications) by the Atomic Safety Licensing Board
- Locally-based engineering inspectors now continuously perform ASR-related inspections at Seabrook.
Recently, in September 2024, the independent Advisory Committee on Reactor Safeguards (ACRS) met with the NRC staff for an update on the status of ASR at Seabrook. This followed a previous meeting two years earlier. The meeting discussed recent activities to ensure that the issue remained of ‘no immediate safety concern’.
Inspection and assurance
Cracking was found at the plant in 2009, in preparation for NextEra’s submission of its licence renewal application.
ASTM standard screening tests at the time have been determined to have limited ability to screen very slow-reactive aggregates for ASR. Because of this, NextEra mistakenly assumed that the original cement and aggregate selection would preclude ASR development, and therefore did not initially consider that the observed cracking could be caused from ASR. It initially characterised it as shrinkage cracking. New test standards have since been developed.
NextEra identified several causes for the ASR issue and several reasons for why it was not identified until the renewal review. One cause was that concrete mix for initial construction unknowingly utilized an ASR-susceptible aggregate and the aggressiveness of the groundwater chemistry on Seabrook concrete structures was also identified later.
NextEra concluded that the affected structures were operable, but degraded and non-conforming with licence conditions, because ASR was not initially taken into account in the current licensing basis.
In 2012, the NRC increased oversight to ensure structures remained functional while NextEra developed corrective actions. The company carried out a large-scale test programme at Ferguson Structural Engineering Laboratory at the University of Texas at Austin, using concrete test specimens designed to replicate the reinforced concrete walls at Seabrook. Among other things, this helped determine the best way to measure and track ASR progression in a structure similar to the walls at Seabrook.
The test results were used to develop what were referred to in the ACRS meeting as ‘guard rails’, which included both expansion limits and monitoring techniques, and were incorporated into Seabrook’s current licensing basis via a licence amendment.
This means that as long as Seabrook stays within the identified expansion limits, it can continue to use the original design equations and material properties to determine the capacity of the affected concrete. If it should go outside these limits that does not necessarily mean the structure fails, but it is outside the boundaries of the test programme and Seabrook has to re-examine the structures and demonstrate operability. This varies between structures but there is said to be substantial margin for the expansion rates established for the Texas test programme. A member of the panel commented, “you’re not inspecting up to a design-basis limit. You’re inspecting so that you can raise a flag and start a process.‚
The ACRS meeting heard that on-site NRC staff use the monitoring data to see current expansion levels, along with research and direct inspection that has meant inspectors totalled five weeks of on-site visits between 2022 and 2024. Depending on humidity and temperature, crack sizes open and close slightly. The test programme showed no reduction in capacity up to the limits tested, so the structure can withstand more margin and expansion.
The inspectors have to distinguish between ASR and other types of cracking fi such as freeze-thaw cracking fi and consider whether more than one effect applies.
NRC resident inspectors perform daily on-site oversight of the plant status, reviewing corrective action reports and conducting walk-downs through all the accessible areas of the plant, looking at building deformation and less-obvious signs of ASR.
Since the NRC staff members’ previous meeting with ACRS in 2022, it has conducted five weeks of on-site inspections with a team of NRC inspectors, and were “looking at this stuff on a daily basis.
Action under way
The meeting heard that ASR affects 28 structures at Seabrook (depending on the structure definition, as some are made up of smaller structures).
Six of these are currently outside the licensing basis, ie the original concrete design code equations that have been revised in accordance with a licence amendment that includes ASR loads. Corrective action has to be taken to bring them back into conformance with the licensing basis.
That is likely to involve further analysis for four of the six structures. They are the Service Water Cooling Tower, the Primary Auxiliary Building, the Containment Enclosure Building, and the Residual Heat Removal Vault. An example of the additional analysis required comes from the Service Water Cooling Tower, which has areas with limited access for monitoring the ASR below the water level. That requires a conservative assumption on the ASR expansion level, which increases load demand in the structure. Additional monitoring data to support the underlying assumptions could reduce load demand and bring it back into conformance.
Where reanalysis is not an option, physical modification is required. Two structures fi the Control and Diesel Generator Building and the Emergency Feedwater Pumphouse fi are planned for physical modification.
The NRC staff members said ASR cannot be ‘remediated’ in the sense that ASR is removed and the expansion stops fi that would require total replacement of the area. Instead, it refers to various types of reinforcement, such as through-wall or corner braces, or the addition of thick metal plates to resist vertical expansion in the structure. Monitoring for ASR expansion continues once the remediation is complete.
The ACRS meeting also heard from the C-10 Research and Education Foundation, a group of citizens within a 10-mile (16 km) radius of Seabrook (ie the evacuation pathway emergency planning zone).
The group had sought a rulemaking on ASR in 2014, but it was denied in 2019. The group was concerned that measures were “static and relatively unchangeable because they’re packaged with their licence renewal.
A specialist working with C-10 said the dynamic analysis relied on obsolete models. He was particularly concerned over an assumption that ASR does not reduce shear strength, noting that shear strength was required to manage seismic load. He also said better understanding was needed of internal microcracking caused by ASR and the effect on air tightness, calling for a stronger inspection regime (which is currently on a 15-year basis).
The group also had concerns over violations relative to ASR license amendments and licence conditions. The staff had previously said that the NRC has identified three findings, which NRC described as being “of very low safety Significance,‚ but highlight the fact that “continued focus is warranted.
For example, the Containment Enclosure Building had deformed towards the Emergency Feedwater Pumphouse until they touched and a ‘seismic gap’ was lost.
A separate containment internal structure issue dates back to 2021, discovered during Seabrook’s autumn refuelling outage. The NRC inspectors became aware of cracking and spalling in the reactor cavity pit area of the containment internal structure fi normally inaccessible during operation as a locked high-radiation area. There were visual indications that there was ASR on the other side of the wall. As an interim test, the licensee applied reasonable ASR loads and concluded the structure is still able to withstand the effects of ASR. Later root cause analysis required the development of finite element models. In March 2024, NRC staff were able to go on-site and review the root cause studies as well as the completed structural evaluations.
NextEra wanted to understand the distresses that they were seeing in the containment internal structures, including spalling and cracking in the reactor cavity pit, and also distresses in other areas of the containment internal structure.
The C-10 group asked the NRC to make more data on the issue public, saying that without it “any assessment of ASR’s impact is incomplete and potentially misleading‚. Maintaining expertise on ASR was a concern for both the citizen group and for NRC.
C-10 wanted to know the NRC and NextEra’s plans to expand and preserve their technical understanding of the ASR issue, suggesting that early understanding of the issue had been relatively simplistic and that NRC have very few staff expert in the issue. There had also been changes in the ACRS since its members were last briefed on the issue in 2022.
The NRC staff had also made note of the issue and the need to have people in post “who must be cognizant of the ASR degradation mechanisms present at the site and how the effects manifest throughout the structures‚. They highlighted on-site visits where the team included extra people who had joined the Civil and Structural Engineering Branch and were there to be brought up to speed on ASR, for ‘knowledge transfer’ purposes.
The fact remains that as Seabrook is the only US nuclear reactor to have to manage ASR, retaining institutional knowledge over the long term is a challenge. NRC and the plant operator have to keep a watching brief over their expertise and knowledge with the same care needed to manage the ASR itself.
