Climatic conditions inside nuclear reactor containments - Monitoring campaign

Författare

Mikael Oxfall

Summary, in English

The reactor containment is one of the most important buildings at a nuclear power plant. In case of an accident the containment wall is the final protection against radioactive leakage to the surrounding environment. A reactor containment contains different types of concrete structures. They can be grouped as external structures constituting a barrier between the inside of the containment and the outside environment, and internal structures e.g. to sectionalize the internal space of the containment and the biological shield. Concrete structures in reactor containments are e.g. used as load bearing structures, protection of the steel liner from e.g. corrosion and to adsorb radioactive nuclides. During operation the climatic conditions inside the reactor containment can be harsh, with temperatures over 50 °C and low relative humidities (RH). Knowledge about the condition in the containment and in the concrete structures is thereby important in order to evaluate the condition of the reactor containment and possible changes over time.

The objective of this dissertation is to present a measurement setup suitable for long term measurements in concrete and investigate how the climatic conditions inside the reactor containments affect the concrete structures and vice versa. The project is divided into three parts in which the first part contains a material study of concrete from one nuclear reactor containment. The results from the material study showed that no clear variation of moisture transport properties over the depth of a structure could bee observed. The findings will be of great value in the continuation of the PhD project which will follow this Licentiate project, in which a model to predict future and past moisture conditions in the concrete will be developed.

In the second part an accuracy evaluation of a measurement setup is presented. The setup was used to measure the RH and temperature onsite. In the accuracy evaluation the effects of both leakage through the setup and temperature were evaluated. The results showed that measured RH was influenced by the temperature changes as expected. Furthermore, the results indicated leakage through the setup. The leakage may have influenced the measurements of the devices placed closed to the surface of the structure. However, it is concluded that the setup is suitable for long term measurements. An supplementary accuracy evaluation will be presented in the upcoming doctoral thesis.

The third part presents result from three monitoring campaigns conducted at different nuclear reactor containments in Sweden. The monitoring campaigns were conducted during one operational year. The campaigns included measurements in different zones of ambient RH and temperature and measurements, on different depth, inside the concrete.

The results showed that measurements on shallow depth were critical but if properly installed there were no indication of leakage. In all zones a clear moisture profile was observed which indicates that the concrete within the reactor containment are still drying after about 30 years of exposure to high temperatures and low RH.

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