Potential impact of accidental releases of hazardous gases on the immediate habitability of a nuclear power plant

Abstract

To protect the public and plant operators, the nuclear power industry demands a continuous evaluation of hazards and safety standards. This dissertation provides a history of toxic chemical accidents and the methodologies to evaluate nuclear power plant toxic chemical hazards. The evaluation of hazards may lead to the implementation of preventative measures, i.e., hardware changes, surveillance and procedural changes; the implementation of mitigative measures; the analysis of post-accident conditions to guide accident response; and guidance for the most effective resource allocation to ensure safety. The evaluation of hazards includes a historical review of toxic chemicals, flammable gas releases, and a compilation of toxic chemicals and flammable gases generally found at or near nuclear power plants. Gases are classified according to their toxicity, weight and flammability to determine which gases are the most hazardous. Biologically, chlorine and ammonia were found to be the most hazardous, while methane gas is the most volatile in terms of flammability. Methodologies to predict the dispersion of gases were also evaluated. Several computer programs used for predicting the dispersion of toxic gases into the atmosphere were evaluated in terms of their accuracy, applicability and usefulness of derived results. Generally, credit is not taken for special nuclear power plant measures to mitigate or prevent a release. The following requirements for nuclear power plant control rooms are not included in this analysis: • Pressurisation of the control room; • Automatic isolation in case of a chemical relapse; • Automatic initiation of filtering with high-grade filters; • Demanding design, purchasing, fabrication and testing requirements to ensure the operability of installed ventilation during the most adverse accident conditions; • Internationally accepted limits are used to determine toxicity and physical characteristics; • Conservative and internationally accepted standards are used to determine dispersion coefficients and receptor location; and • Conservative and international standards are used to determine the release rate and release amount.