Thermo-hydro-chemo-mechanical modeling of inner containments of nuclear reactor buildings in prestressed concrete

In the context of life extension and safety demonstration of nuclear reactor buildings following the severe accidents of Chernobyl (in Ukraine, 1986) and Fukushima (in Japan, 2011), the “Confinement Assessment of a vessel during an Accident” (MACENA) project supported by the French National Research Agency (ANR) has been initiated since 2013. The experimental campaigns and modelling works of this project are based on an experimental containment vessel mock-up at scale 1/3 named VeRCoRs which is constructed and monitored by Electricity of France (EDF). The main issues for long-term operation of nuclear power plants that have more than 50 years old consist of ageing mechanisms in concrete and prestressing losses, as the internal concrete wall is prestressed with steel wires. These phenomena influence the cracking occurrence that can be significant if the “Loss-of-coolant accident” (LOCA) induced conditions are maintained for several weeks. The material characteristics of nuclear containment building can be modified by the loading prior to the LOCA, leading possibly to a reduction of tensile strength (early cracking, pre-operational pressurization test). The loadings during the LOCA i.e. increase of temperature and vapour relative pressure can then propagate the cracks. Thus, the material properties depend on the solicitation history since the concrete pouring In that context, this PhD thesis introduces a computational strategy adopted to consider the damage accumulation using a single mechanical model from the early age until the LOCA. Considering the effect of chemo-physical of the concrete on the response of this mechanical model along the life of the structure requires the preliminary prediction of the evolution of chemical, thermal and hydric state from casting to later ages, including the LOCA. Several multiphysics models are studied, improved or tested in this thesis work: the first model is devoted to predicting the hydration degree, temperature, water content and porosity of concrete at an early age; the second allows to follow the evolution of temperature and water saturation for later ages (ageing period and effects of the LOCA conditions on the thermal and hydric state of concrete). The mechanical model uses the results of the two previous ones to estimate the delayed deformations of concrete, the relaxation of prestressing wires, and the risk of cracking. This methodology was developed to avoid the loss of information relative to material state throughout the life of the structure. The main advantages of the methodology are the possibility to consider automatically the accumulation of damage until the LOCA on the one hand and on the other hand the improvements made to the material model to be used in the LOCA conditions, which improve the modelling quality, especially during the early age.