Air leakage in reactor containments made of concrete. Measurements and modelling

Reactor containments in Sweden mainly consist of large cylinder shaped constructions made of reinforced concrete. There is also a lot of concrete inside the containments in the form of beams, walls and foundations. Inside the concrete cylinder shell a steel liner is embedded which acts as a leak-tight barrier. The main objective of the project was to develop a simulation model that describes transient air pressure distribution in concrete in order to see if the leakage rates obtained from the CILRT (Containment Integrated Leakage Rate Tests) can be explained by the transient air pressurization of concrete pores inside the steel liner. A partial differential equation was derived which describes transient air pressure distribution in concrete pores. The model was validated against experimental results using the computer software Comsol Multiphysics. A simplified finite differences model was also developed in Excel. The permeability and other material properties of relevant concretes were measured. A laboratory test setup was used for verification. At two occasions measurements were done during a pressure test at Ringhals R2 and R4 respectively. Moisture and pressure profiles were measured at various points in the concrete structures. The simulation model shows that that, theoretically, there is significant air fluxes into the concrete structures and that the sum of these fluxes reduces the mass of air inside the containment during a pressure test. Consequently, this mass loss significantly reduces the air pressure during the test. These processes are strongly supported by the pressure measurements at different depths in the concrete. It is quite clear from the measurements that there is a tremendous rise in air pressure also at large depths, even after the pressure is lowered after the test. The simulation models are quite applicable but require detailed knowledge on the geometry and material properties of the concrete and the epoxy coatings at the concrete surfaces. The models could be improved also by including the effect of different moisture contents at different depths, the pressure dependency of the permeability and the dissolution of air into the moisture in the pore system