As nuclear power plants continue to implement best practices to reduce the total radioactivity in plant effluents, other radionuclides that were not previously significant fractions of the effluent streams will need to be quantified and reported. Carbon-14 can become a principal radionuclide for the gaseous effluent pathway as the concentrations of other radionuclides decrease. This report documents the potential opportunities for further understanding the impact of nuclear power plant operations on Carbon-14 generation, chemical form and release.

Background

Due to improvements in nuclear power plant effluent management, the types and concentrations of most radionuclides released through gaseous effluent pathways have decreased. As a result of this decrease, Carbon-14 can become a principal radionuclide for the gaseous effluent pathway. In the latest revision of Regulatory Guide 1.21, Measuring, Evaluating, and Reporting Radioactive Material in Liquid and Gaseous Effluents and Solid Waste, the United States Nuclear Regulatory Commission (US NRC) recommended that utilities evaluate whether Carbon-14 is a principal gaseous effluent radionuclide at their nuclear power plants, and if so, report the amount of Carbon-14 release. In 2010, the Electric Power Research Institute (EPRI) developed a methodology for the estimation of Carbon-14 in nuclear power plant gaseous effluents based on site-specific neutron flux information (EPRI Report 1021106) to support the industry in reporting precise Carbon-14 concentrations and doses to the public. During this 2010 project, EPRI identified potential opportunities for further understanding the impact of nuclear power plant operations on Carbon-14 generation, chemical forms, and release.

Objective

To understand the impact of nuclear power plant operations on Carbon-14 generation, chemical forms, and release.

Approach

The project team conducted a detailed evaluation of how Carbon-14 is generated within the boiling water reactor (BWR) and pressurized water reactor (PWR) nuclear power plant cores, and the impact of the reactor coolant on the original chemical forms of the Carbon-14 generated. The project team then investigated how the Carbon-14 was transported from the core to the rest of the plants’ systems and components. The research focused on the gaseous waste management system and how the different designs, components and operation of this system may impact the chemical form and release of Carbon-14. Carbon-14 measurement data from United States nuclear power plants and experiences from international light water reactors and heavy water reactors were explored to understand the generation, chemical form, and release of Carbon-14 in different operation modes and power plant types. The project team also explored the potential for organic Carbon-14 to convert into inorganic Carbon-14 in the atmosphere to understand how this may impact dose pathway calculations.

Results

The results of this work confirm the results of EPRI Report 1021106 with respect to the generation, chemical forms, and release of Carbon-14 from nuclear power plants, and provide a more detailed investigation of the nuclear power plant impacts on these parameters. Carbon-14 that is produced in the coolant at BWRs and PWRs is primarily released via gaseous release pathways, typically in the form of carbon dioxide at BWRs, and methane at PWRs. However, at PWRs, a small fraction of Carbon-14 can be released in the carbon dioxide form during outages if reactor coolant system (RCS) chemistry becomes oxidizing as a result of peroxide addition. This information can be used to refine calculations related to Carbon-14 chemical forms during outage and non-outage years. No significant effect of passing gaseous waste streams through charcoal beds on the total release or the form of the release at PWRs or BWRs is expected. Limited data also indicate that passing the gaseous wastes through recombiners at PWRs also does not result in oxidation of the organic Carbon-14 compounds to carbon dioxide. However, additional data will be needed to verify this observation.

The design and operation of Canada Deuterium Uranium Reactor (CANDU) nuclear power plants indicate that more Carbon-14 is produced in the pressurized heavy water reactors than in BWRs and PWRs. However, gaseous releases are of similar magnitude since most of the Carbon-14 is produced in the calandria of the CANDU where it is in the form of bicarbonate, and can be effectively removed by the anion exchange resin. This is a unique capability of the CANDU due to its design and operating chemistry.

With respect to conversion of organic Carbon-14 to inorganic Carbon-14, methane is a relatively stable chemical species and there should be essentially no conversion of Carbon-14 methane to carbon dioxide between the nuclear reactor gas phase release points and the site boundary.

Application, Value and Use

EPRI conducts research and development on nuclear power plant effluents to support industry best practices in minimizing and managing the impact of permitted radioactive releases to the community and the environment. As nuclear power plants continue to implement best practices to reduce the total radioactivity in plant effluents, other radionuclides that were not previously significant fractions of the effluent streams will need to be quantified and reported. Additionally, as stakeholders become increasingly concerned about environmental protection, more in-depth and precise knowledge of the potential impacts of nuclear power plant operations on the environment will be necessary. EPRI conducts research and development activities to provide the industry with best practices for accurately estimating the source term, transport, and release of Carbon-14 and other radionuclides from nuclear power plants. These research and development efforts will support the nuclear power industry in effectively communicating with stakeholders about nuclear power plant effluents.