Understanding the damage of reactor pressure vessels (RPV) and internal steels caused by neutron irradiation is fundamental for a safe and reliable operation of a nuclear power plant during its originally planned service time and beyond. However, the rules for assessing the degradation of RPV material due to neutron irradiation were established more than 30 years ago. Although these rules have evolved ever since, they still rely on empirical or semi-empirical approaches, mainly due to a limited understanding of neutron irradiation damage.
R&D on the assessment of the service life of nuclear plants is a multidisciplinary enterprise. Not only does it require knowledge of material ageing, it also necessitates a sound understanding of factors such as loading conditions, reactor water chemistry, as well as the influence of these aspects on plant safety.
Besides, it is highly difficult to measure irradiation evolutions, as it is caused by a variety of causes (such as chemical composition, manufacturing history, temperature, neutron flux and fluence, time of exposure, etc.). It is also very challenging to get material irradiated in relevant conditions and to manage the characterisation of irradiated materials in dedicated facilities.
Hence, one of the key pre-conditions to improve our understanding on neutron irradiation damage is to select a set of laboratories and combine their experimental techniques used for analysing a set of irradiated materials in well-defined conditions. This multi-disciplinary task can only be carried out in a collaborative way between laboratories from different countries, each of them providing their individual expertise. Several European R&D projects – among them LONGLIFE and PERFORM 60 – did already focus on the integrity assessment of the components used today, according to collected data. As a matter of fact, their results – a common database and a numerical modelling platform – have been of relevance towards achieving a common understanding on irradiation induced degradation and consequently on lifetime assessment methods and practices for nuclear power plants throughout Europe. SOTERIA takes up these projects’ achievements and moves a step forward by adopting an innovative two-dimensional research approach (see Approach).
Building on the achievements made by the previous European projects LONGLIFE and PERFROM 60, SOTERIA takes a step further by integrating progressively more complex parameters from material studies, such as microstructure defects and heterogeneities.
Towards that objective, SOTERIA will adopt a two-dimensional approach, which brings together two complementary research communities: experimentalists and numericists.
Even if the coupling of experiments and simulation is a basic concept in science, the strength of SOTERIA lies in the performance of experiments at the same relevant scales as numerical modelling (from nano- to micro-scale). As a matter of fact, this procedure allows to better assessing the outcome of this two-dimensional approach illustrated below:
SOTERIA multi-scale approach from nano- to macro scales with a strong link between the developed modelling tools and experimental characterisation techniques
The experiments to be carried out within SOTERIA involve carefully selected materials, which are relevant for existing European reactors provided by the industrial project partners, and best suited for a systematic study of the various parameters explained above. Additional materials, such as representative model alloys, and irradiations – including ion irradiations, each time they provide added value for the understanding of specific issues – are also taken into account.
Simulation-oriented experiments aiming at validating models at different scales will therefore be set up, using the following equipment and materials:
an irradiation platform with simultaneous double or triple ion beams, when necessary, and possible in-situ control of environmental parameters (temperature, mechanical stress) in order to facilitate detailed examination and testing, and hence facilitate the understanding of the behavior of neutron-irradiated materials
hot cells with a large set of characterisation tools – including advanced ones (like APT) – for neutron-irradiated materials,
proton-irradiated materials in order to simulate neutron irradiation phenomena.
Modelling tools will be parameterised using representative in-pile and out of pile experimental data. The most effective tools and models developed during the project will be integrated into a platform. As research data resulting from SOTERIA is meant to be used for the development of safe and economical long-term operation of existing nuclear plants, the specifically developed industrial version of this modelling platform will be validated by the SOTERIA End-User Group (see Collaboration), thus reaching a Technology Readiness Level of 5 (TRL5).
The End-User Group defines key industrial issues and validates the modelling tools developed with the support of experimental data.