Climatic conditions inside nuclear reactor containments: Evaluation of moisture condition in the concrete within reactor containments and interaction with the ambient compartments

Abstract: Safety is the top priority at a nuclear facility. The nuclear power plants are designed to prevent radioactive leakage to the surroundings, both during normal operation as well as in case of a severe accident. One of the most important structures in a nuclear power plant, with regard to safety, is thus the reactor containment wall. The containment wall is the last main barrier to prevent radioactive leakage, and it is designed to limit and control internal hazards if all other barriers fail. In order to understand and identify the potential deviation of the barrier, the effects of changes in the materials and how these changes occur and propagate have to be understood.
The work presented in this thesis concerns the moisture condition within nuclear reactor containment inner walls in addition to other concrete structures within the containments. The study aims to describe earlier, ongoing and future moisture contributions, and redistribution of moisture within and from the concrete structures within the containments.
An in situ measurement setup for long term monitoring of relative humidity and temperature in concrete was designed and installed in four reactor containments. The setup was used to monitor the actual conditions within the containments and in the concrete structures, and how they change over time. The measurements showed that all containments within the study complied with the regulated conditions with regard to temperature. The stable humidity in the air within the containments indicated that the dehumidification apparatus at the sites worked as anticipated, and that the measured conditions can be considered as "as-designed conditions", even though there is no regulation regarding permissible humidity. The results from the monitoring campaigns were further used to validate a model which was designed to describe the ongoing drying and moisture redistribution in the concrete structures.
The measurements and simulations done in this study show that the concrete structures within the reactor containment are still drying after approximately 30 years of operation, and will continue to dry and contribute with moisture to the ambient compartment for the remaining part of the service life for the reactors. The simulations presents that 35–45 % of the initial evaporable water had dried out, until this study, and that the amount for 60 years of operation is 45–55 %. The main drying has already occurred, and the moisture contribution to the ambient compartments will continue to decrease, thus contributing less moisture to the air in the containment in the future.