PASTELS aims to significantly increase the knowledge within Europe of innovative passive systems, namely SACOs and CWCs, and the ability of several European system and CFD computational codes to be able to accurately model key phenomena such as natural circulation loops and condensation. This is very challenging due to their very specific properties, i.e. small driving forces working against high resistive forces which are specific to the concept of these technologies. Given the growing use of the SACO and CWC technologies in non-European NPPs, it is essential, especially with the foreseen future use of Small Medium Reactors (SMR) that the European nuclear community is able to adapt its current numerical tools to this promising technology. Extensive experimental testing (SET, CET and integral experiments) with representative operating conditions on semi-industrial full scale test facilities (PKL facility [DE] and PASI facility [FI]) will provide essential data to support the improvement of the numerical activities. Existing data from PERSEO and HERO-2 facilities will also be used. The numerical and experimental activities will be conducted in an integrated step-by-step approach. PASTELS will investigate improvements to models, novel methodologies for the coupling of system and CFD codes working at different scales. Additionally, important knowledge on the behaviour of the SACO and CWC will be captured through the observation of their behaviour during the test campaigns. Different and similar computational codes will be used by the partners in order to be able to benchmark and compare the different results obtained, understand the causes and propose strategies to improve them. All project results will feed into extensive methodology guidelines and a roadmap to achieving licensing and implementation of these innovative passive system technologies in future European NPPs.

The safety of a nuclear reactor relies heavily on modelling. In the last two decades, making use of the increased computer power available, advanced multi-physics solvers have been developed to reduce the level of conservatism when simulating pressurized water reactors. These tools rely on a first-principles based approach and produce solutions with a much finer spatial resolution. However they are seldom used in practice due to, amongst other things, the lack of dedicated experimental data for validation, especially when it comes to their improved spatial resolution. The EVEREST project intends to address this issue by quantifying the impact of using advanced MP models for the modelling of a VVER reactor ("usefulness"); by demonstrating the accuracy of their results, especially the improved resolution through the production of dedicated experimental data (“trustworthiness”); and by promoting them to key groups of the nuclear engineering community (students, utilities, regulators). The consortium is built around the necessary research facilities and expertise from all the required actors of the nuclear industry, both within Europe and outside. In terms of impact, the project will produce scientific knowledge towards producing electricity using a climate-neutral energy system, in a safe and efficient way. The advanced models will provide more accurate and detailed information about the current safety margins in nuclear reactors enabling more informed decisions on setting the regulatory limits; and resulting in an improvement of the plant safety as a whole. A better understanding of the safety margins for a nuclear power plant could also allow power uprate. The same approach can be envisioned for research reactors and allow more users to carry out research activities. Finally, the EVEREST project will have a long-term impact on knowledge preservation through the organization of a summer school and trainings as well as the funding of mobility grants.

In the EU, most of the nuclear power plants (NPPs) are currently in the second half of their designed lifetime, making lifetime extension an important aspect for the EU countries. One of the most limiting safety assessments for long term operation (LTO) is the reactor pressure vessel (RPV) integrity assessment for pressurized thermal shock (PTS). The goal is to demonstrate the safety margin against fast fracture initiation or RPV failure. To verify safe operation of existing NPPs going through LTO upgrades, advanced methods and improvements are necessary. In the EU, currently used PTS analyses are based on deterministic assessment and conservative boundary conditions. This type of PTS analyses is reaching its limits in demonstrating the safety for NPPs facing LTO and need to be enhanced. However, inherent safety margins exist and several LTO improvements and advanced methods are intended to increase the safety margins of PTS analysis. Additionally, the quantification of safety margins in terms of risk of RPV failure by advanced probabilistic assessments becomes more important. The main objectives of this project are establishing of state-of-the-art for LTO improvements having an impact on PTS analysis: NPP improvements (hardware, software, procedures), development of advanced deterministic and probabilistic PTS assessment method including thermal hydraulic (TH) uncertainty analyses, quantification of safety margins for LTO improvements and development of best-practice guidance. After establishing the LTO improvements, TH calculations will be performed including also uncertainty quantification relevant to PTS assessment. Benchmark calculations for both deterministic and probabilistic RPV integrity assessment will be performed with the goal to establish the impact of LTO improvements and TH uncertainties on the overall RPV integrity margins.

The SOCRATES project addresses critical gaps in our understanding of the liquid source term during severe nuclear accidents and offers innovative solutions to mitigate and monitor the release of radionuclides into the environment. The project contributes to the mid-to-long-term management of nuclear power plants after a severe accident by enhancing safety, environmental protection, safe waste management and public well-being. The main contributions of the topical project are: - Enhanced understanding of liquid source term - New computer models for the liquid source term phenomena - Innovative absorbent materials to effectively trap key radionuclide species, particularly Cs and Sr - Miniature size radiochemical laboratory for radionuclides - Education and training for nuclear safety - Recommendations for long-term operations, waste management and severe accident management strategies. The management of possible leakages of contaminated water, which may happen more frequently due to aging of reactor components and joints, will gain new remedies from SOCRATES results to tackle and mitigate the contaminants inside the plant. The project's research on the liquid source term directly impacts a majority of existing and new nuclear reactors, encompassing diverse reactor designs and technologies. Recommendations based on SOCRATES results will support nuclear community on international scale by giving guidelines how to manage liquid source term. Developed computer models for the analysis of liquid source term phenomena will benefit industry, safety authorities and research community in the safety assessments of NPPs. The models developed in SOCRATES will be implemented in severe accident analysis codes, such as AC2 and ASTEC. When the design of new sorbent materials for radionuclides will be performed with computer simulations, it will enhance the digitalization of safety developments, and enable considerations of multiple sorbent composition variations with low cost.