The overall ambition of SOTERIA is to allow nuclear power plant operators to move from a reactive to a proactive management of material ageing in nuclear plants, and of their operation.
Observation of ageing will no longer be the only way to adapt maintenance and follow-up procedures, but prediction will make it possible to establish appropriate surveillance. The operators will have new tools to decide on interventions. This also represents a major economic advantage, as it will be possible to better plan the suspension of the plant activity for maintenance.
Although the tools developed in the previous, related European projects LONGLIFE and PERFORM 60 (see Collaboration) allowed a better understanding of ageing phenomena in nuclear power plants, they were mainly used as a support for observation and not for prediction.
By building its approach on the main challenges faced by the industry, SOTERIA will be able to bring these existing tools a step forward in order to predict the ageing mechanisms in nuclear power plants. This progress beyond the state-of-the-art will represent a major breakthrough in the ageing management of nuclear power plants and it will help answering essential questions related to the long-term operation of nuclear plants. All in all, the progress done in SOTERIA is situated on three different levels:

1. Progress beyond the state of the art in irradiation embrittlement studies

SOTERIA will obtain proper experimental data to solve important key issues needed for safe reactor pressure vessels (RPV) and internal steels’ long-term operation by addressing flux effects, late irradiation effects, uncertainties in RPV irradiation surveillance, prediction, validation of embrittlement trend curves and internal steels swelling.
Due to the broad application of various microstructural techniques (such as APT, SANS, and TEM) to investigate specific irradiation effects in representative reactor pressure vessels and internals, the accuracy and reliability of those methods will be further improved and developed for ready-to-use end-user applications.
The outcome of the experimental work will result in significantly improved correlations of microstructural data and mechanical properties, supported by multiscale modelling, more reliable prediction and interpretation of RPV and internals irradiation embrittlement and – at least for RPV steels – an innovative application of microstructural techniques in monitoring long-term operation behaviour.

2. Progress beyond the state-of-the-art in stress corrosion cracking studies

Recent work1,2 on irradiated stainless steel at low to medium doses has emphasised the potential role of the localisation and deformation in intergranular cracking initiation of irradiated stainless steel as a key feature in irradiation-assisted stress corrosion cracking damage.
However, experimental evidence from the PERFORM 60 project shows a lack of correlation between channel deformation modes and intergranular cracking in inert environments in highly irradiated austenitic stainless steels3. Therefore, it could be necessary to consider that new failure modes may have a role and be relevant regarding long-term operation concerns.
SOTERIA will push research further in order to predict the grain boundary cracking susceptibility, taking into account crystallography and neighbouring grains based on normal stress field description.

3. Progress beyond the state-of-the-art in modelling

Previous and current modelling efforts regarding the microstructure, the plasticity and the fracture toughness of reactor pressure vessels and internal steels do lack comparison with experiments based on industrial in-service material.
Hence, the main innovation in the modelling tools developed in SOTERIA will be the improvement of the existing modelling platform and the development of a specific industrial version of this platform to be applied on industrial cases provided by the SOTERIA End-Users Group. These specific tools based on the platform will be able to answer industrial questions on each specific scale (simulation of the microstructure versus the irradiation, estimation of the toughness, etc.).
These improvements will allow modelling of industrial cases, for example with realistic loadings on reactor pressure vessels and internals and not only on the model alloy. Another major improvement will be the possibility for the user to choose the type of input to be used – direct experimental data or data coming from other SOTERIA models – in order to obtain a given output. This will facilitate the use of the SOTERIA results by non-specialists of a given module.

The illustration below shows the progress beyond the state-of-the-art (SOA) to be made by SOTERIA based on the achievements of the two related EU projects mentioned above:

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