Analysis of Chemical Behavior of Nitrogen Oxide Formed by Thermal Decomposition of FP Nitrates in Accident of Evaporation to Dryness by Boiling of Reprocessed High-Level Liquid Waste

Abstract

　An accident of evaporation to dryness by boiling of high-level liquid waste (HLLW) is postulated as one of the severe accidents caused by the loss of cooling function at a fuel reprocessing plant. In this case, volatile radioactive materials, such as ruthenium (Ru), are released from the tanks with mixed vapor of water and nitric-acid into the atmosphere. In addition, nitrogen oxides are also released, formed by the thermal decomposition of metal nitrates of fission products (FP) in HLLW. It has been observed experimentally that nitrogen oxide strongly affects the transport behavior of Ru under the anticipated atmospheric conditions in cells and/or compartments of the facility building. Chemical reactions of nitrogen oxide with water and nitric acid are also recognized as complex phenomena that undergo simultaneously in the vapor and liquid phases. An analysis method has been developed by coupling two types of computer codes to simulate not only thermohydraulic behavior but also chemical reactions in the flow paths of carrier gases for quantitative estimation of the amount of Ru released to the environment. A simulation study has also been carried out with a typical facility building to demonstrate the feasibility of the developed simulation method.