NETFLUX

In nuclear power plants, the concrete biological shield (CBS) is designed to be very close to the reactor vessel. It is expected to absorb radiation and act as a load-bearing structure. During the lifetime of the plant, CBS is therefore exposed to cumulative high levels of radiation, which can have long-term effects. In particular, this radiation can lead to a reduction in the material and structural mechanical properties. In this context, the project addresses the irradiation damage of concrete structures, with an application to the concrete shielding structure that supports the vessel. It includes an integrated methodology with experiments to obtain new damage parameters and numerical approaches at different scales to produce validated models to evaluate the consequences of irradiation. For the Long Term Operation (LTO) up to and beyond 60 years, the French and Czech operators have to demonstrate the safety of the CBS to the national authorities. To date, this demonstration is based on data obtained under accelerated conditions (high flux) in Material Testing Reactors (MTR): in a few weeks or months, the same damaging fluence is reached as in real low-flux nuclear power plants (NPPs) in 80 years. The main scientific question addressed in this project is to assess whether, for the same total fluence, the levels of damage induced by high flux in MTRs are comparable to those induced by low flux in real structures. In fact, does high flux induce different and higher damage? The project focuses on neutron damage, which is specific to NPPs and less studied than gamma-induced damage that is common in the context of nuclear waste and repositories and induces less structural damage. From an experimental point of view, the NetFlux project will provide a unique set of characterisation data on the effect of neutron irradiation on concrete materials (aggregate, mortar and concrete), largely covering the expected doses at 80 years of operation in NPPs. Flux selected will be different and complementary to those chosen for the current European project ACES (shared with France and the Czech Republic); however, the same materials will be qualified. The irradiations will be performed in the LVR15 reactor and the post-irradiation characterisation in hot cell facilities, both located at CVRez. The accessibility of these specific infrastructures to CVRez is a major advantage for this project. From a modelling and numerical point of view, the NetFlux project aims to develop approaches at both meso and macro scales. Specific numerical models and methods will be implemented at the mesoscale to study the effects of aggregate swelling on the microcracking of the cement paste and paste-aggregate interfaces, and their repercussions on the behavior of concrete. In addition, structural-scale numerical tools based on damage mechanics, considered a reference in France and the Czech Republic for assessing the additional risk of damage in CBS due to irradiation, will be implemented. In the next 10 to 30 years, the results of this study will be used to take decisions on the extension of the life of the current pressurized water reactors in France and the Czech Republic beyond 60 years in a completely independent manner. Finally, it will enable us to make the best choice of materials and design for the future NPPs, which could have an initial lifetime of up to 100 years.