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Safe long term operation of light water reactors based on improved understanding of radiation effects in nuclear structural materials

Periodic Reporting for period 3 - SOTERIA (Safe long term operation of light water reactors based on improved understanding of radiation effects in nuclear structural materials)

Reporting period: 2018-09-01 to 2019-08-31

SOTERIA was initiated to take advantage of the knowledge accumulated during the previous, related EU-funded projects (PERFECT and PERFORM60) and represents an attempt to investigate the time and dose dependent evolutions taking place in actual, critical nuclear components, made of complex industrial materials. The safe prolonged lifetime of these components can be accurately evaluated indeed, provided the material ageing causes are quantitatively evaluated and understood, including the inherent mechanical response variabilities. SOTERIA project specifically attempts at evaluating this variability issue and analyse/predict it based on corresponding material ageing mechanisms, evaluated on quantitative and deterministic grounds. The overall aim of SOTERIA is to help the operators and regulators to improve the understanding, and hence the prediction of ageing phenomena occurring in reactor pressure vessels (RPVs) and internals, in order to ensure safe long-term operation (LTO) of the existing European NPPs fleet.
The work performed (WP2) confirms that the investigated neutron flux effects has a negligible impact on RPV and internal steels, in terms of yield stress evolution. The influence of specimen size, specimen cutting location, orientation and irradiation embrittlement on the fracture toughness scattering, the distribution of the fracture (cleavage) initiation size and the reference temperature T0 has been investigated (WP3). The effect of additional uncertainties in RPV surveillance data have been examined, with a view to quantify their impact on the total uncertainty (and scatter) and to improve the use of surveillance data, from an end-user point of view. WP4 has delivered significant, original (rare…), high quality results, in an attempt to develop and underpin fracture models required for management of civil PWR baffle bolt cracking, including advanced characterisation on a post-stress corrosion cracking test specimens, IASCC testing on proton-irradiated material and machined/cold-worked material, including post-test characterisation, TEM investigations on specimens exposed to varying chemical environments, crack initiation tests on neutron-irradiated O-ring test along with visual inspection of the specimens post-test, studies on He effect on grain boundary crack initiation and Cr diffusion experiments within representative PWR temperature range.
Efforts on the modelling of irradiation microstructure were upheld (WP5); including improved description of interaction between point defects, solute, dislocation and grain boundaries, improved description of sink strengths and account of precipitates in rate theory. These developments allow for successful description microstructure formation dynamics, in a set of realistic materials (both RPV and internals) through OKMC-based or rate theory modelling. The results have improved the level of understanding of microstructure evolution and important physical properties (mechanical, chemical, etc…) on radiation hardening and IASCC: in particular, interaction and diffusion of point defects and grain boundaries, depending on the grain boundary types considered. Modelling of oxide kinetics for austenitic model alloy focused on the role of cold working on Cr depletion and highlighted necessary further developments. The developed models has been assessed by the SOTERIA End-User Group throughout the project duration, taking into account industrial reference cases. Recommendations included (1) the delivery of different versions of the platform to the End-User Group, (2) the training of the end users on the particular version, (3) the feedback from the end-users and the consideration of this feedback in the further development of the models.
Thanks to the work done during the SOTERIA project, the understanding of radiation effects in nuclear structural materials, which strongly affects a reactor’s longevity, has been deepened. The achievements of SOTERIA shall have a significant and lasting impact on several levels: SOTERIA is expected to considerably contribute to the continuous safe operation of the existing fleet of nuclear reactors in Europe and thus help to (1) ensure the energy production needs (2) reduce CO² emissions to half the levels reached in 2005, (3) strengthen the nuclear workforce in order to meet future demands, by investing in education and training. By providing new information and guidelines regarding the long-term operation of nuclear power plants to the relevant expert committees, SOTERIA will contribute to the development of national policies and decisions regarding the choice of future energy sources. Nuclear power plants operators, having the responsibility of demonstrating the safety basis for long-term operation, in particular as regards materials ageing, will benefit from the project’s findings on how to improve nuclear safety. SOTERIA also optimises the industrial impact of its results by fully integrating the end users in its research approach. The results from SOTERIA are expected to have significant impact on the scientific community, by addressing questions linked to condensed matter physics, corrosion, metallurgy, modelling, as well as the development and improvement of modelling approaches and tools from nano- to microscale, whose improvement can be useful for a large set of application in materials physics and chemistry. Through innovative methods and tools developed and the integration of knowledge carried out in SOTERIA, the project results will allow utilities to increase the reliability of safety measurements in nuclear power plants. SOTERIA will also allow to identify the materials properties most affected by irradiation, temperature or corrosion issues, and indicate how to adapt the procedures used to assess safety criteria. Increasing the safety of long-term operation will prevent accidents. SOTERIA knowledge is made available through a user-friendly computational platform, addressing the complexity and variability of nuclear materials in critical NPP components. SOTERIA influence will endure for many years beyond its completion, through the innovative, cost-effective and informative surveillance methodologies developed.
SOTERIA Work Breakdown Structure