Review of Radiation-Induced Concrete Degradation and Potential Implications for Structures Exposed to High Long-Term Radiation Levels in Nuclear Power Plants (NUREG/CR-7280, ANL/EVS-20/8)

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Publication Information

Manuscript Completed: February 2020
Date Published: July 2021

Prepared by:
B. Biwer¹
D. Ma¹
Y. Xi²
Y. Jing²

Environmental Science Division¹
Argonne National Laboratory
Lemont, IL 60439

Department of Civil, Environmental²
  and Architectural Engineering,
University of Colorado
Boulder, CO 80309-0428

M.Sircar
Technical Lead, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

The Expanded Material Degradation Assessment Report, NUREG/CR-7153 Vol. 4, Aging of Concrete and Civil Structures, identifies issues that are low-knowledge but high significance for concrete and concrete degradation related to the long-term operation of nuclear power plant (NPP) structures. Irradiation of reactor pressure vessel (RPV) concrete support structures emerged as the highest research priority, mainly because there was insufficient data to increase existing knowledge about the effects of irradiation on concrete mechanical properties. U.S. Nuclear Regulatory Commission (NRC) staff is conducting research activities to investigate this topic (see SECY-14-0016, January 31, 2014). This report reviews the state of knowledge related to radiation-induced degradation, estimated radiation levels, a limited survey of reactor support structures, and important design criteria. Collectively, this information provides a general framework for understanding the effects of irradiation on RPV concrete support structures.

This evaluation reviewed recently completed and ongoing license renewal-related research on the characterization of the degradation of irradiated concrete in NPP structures near the RPVs. A proper evaluation requires an understanding of (1) the effect of radiation on concrete (i.e., what degradation modes exist and which are the most important in a light-water reactor [LWR] environment), (2) the radiation levels concrete structures experience in the near-RPV environment, and (3) the functions those concrete structures must maintain over the course of an 80-year reactor lifetime.

The highest estimated neutron fluence level at the outer face of the RPV wall was found to be greater than 1 × 10¹⁹ n/cm² (E > 0.1 MeV) for all pressurized-water reactors (PWRs) and, with one exception, less than 1 × 10¹⁹ n/cm² for all boiling-water reactors (BWRs). These estimates are near or at the core mid-plane. The neutron fluence levels will rapidly decrease above the top and below the bottom of the core. There are indications that streaming effects could increase the fluence levels near RPV supports in these areas, but are not higher than the core mid-plane values.

Neutron fluence levels above 1 × 10¹⁹ n/cm² (E > 0.1 MeV) at the typical temperatures in LWR cavities (below 100°C) can significantly degrade concrete’s physical and mechanical properties. The onset of noticeable degradation can appear at fluence levels above 1 × 10¹⁸ n/cm². However, there is insufficient evidence to change the currently adopted value of 1 × 10¹⁹ n/cm² as a damage threshold value for regulatory purposes. The contributing factors and degradation mechanisms are not well understood. There is strong, but not conclusive, experimental evidence that the primary effect is related to the disordering effect of neutrons on aggregate mineral structures, especially those with a covalent bond structure such as that found in siliceous aggregates with the quartz structure. Other issues are that aggregates are a mix of different mineral types that vary from plant to plant. The differential expansion of minerals within the aggregate may cause cracking.

Gamma dose levels above 1 × 10⁸ Gy may result in the degradation of concrete properties, but there is no data directly related to isolated gamma irradiation of concrete in an air environment. The primary impact of gamma irradiation on concrete is water loss from the cement paste due to heating and radiolysis, which results in some shrinkage of the cement paste. The loss of water results in more open pore space within the cement paste, but this effect may be partially counteracted by gamma-induced carbonation of portlandite to calcite, where the calcite occupies slightly more volume than portlandite.

The synergistic effects of neutron and gamma radiation on concrete are not known at this time. Most of the reviewed neutron irradiation data were obtained using a nuclear reactor as a source which also produced gamma radiation. However, many tests did not track the corresponding gamma dose or qualify the neutron spectrum, and the relative values of neutron irradiation and gamma dose in the tests were not necessarily the same as those under commercial LWR operating conditions. In addition, little neutron- or gamma-only data under LWR conditions is available for comparison.

In addition, the extrapolation of experimental sample degradation to actual scale operating conditions still requires study. Radiation-induced volume expansion of the aggregate is considered to be one of the main contributors to the degradation. The aggregate expansion under structural confinement will create internal stress fields which may extend much further inside the bioshield, beyond the irradiated depth.

Other issues that remain to be investigated further are radiation rate effects and the lack of data on the effects of irradiation at the interfaces of steel and concrete, such as at anchorages and reinforcements.

The reactors that are more susceptible to concrete bioshield-support degradation are those in which the reactor supports sit on the concrete bioshield close to, and directly exposed to, the core radiation. The PWRs that rest on support skirts, neutron shield tanks, or pedestal (metal column) supports may not have any significant long-term concrete irradiation degradation issues because the distance from the core to the base of the reactor cavity is far enough to protect them and/or shielding is provided by a neutron shield tank. However, the effect of irradiation on concrete support structures near the core mid-plane requires evaluation, and the effect of irradiation on steel structures and components continues to be considered along with its impacts on concrete support structures.

This review indicates that all operating PWRs have the potential to generate neutron fluence levels in the reactor cavity that could result in concrete degradation before 80 years of operation. However, the extent of any potential degradation of concrete RPV supports cannot be quantified in a general manner because plant-specific, detailed design information of the RPV supports is necessary, knowledge of the radiation levels at plant-specific support locations is largely unknown, and the effect and extent of nuclear irradiation on the concrete supports in an LWR operating environment is not well understood and subject to uncertainty.

Although BWRs are expected to experience lower radiation levels than PWRs and are not likely to experience issues related to concrete irradiation-induced degradation, certain aspects of a given design may need to be addressed. Furthermore, there are NPPs that are operating under off-normal conditions in some cases and that are monitored as part of their aging management plans; these off-normal conditions could impact concrete irradiation degradation mechanisms. Thus, the impact of nuclear radiation on critical concrete support structures should be considered on a case-by-case basis as part of a Subsequent License Renewal application.