The stabilization of molten corium is recognised as essential if a safe and stable state is to be reached following a severe accident. Among the possible options, In-Vessel Melt Retention (IVMR) appears as an attractive solution that would minimize the risks of containment failure (less Hydrogen produced, no corium-concrete interaction), if it can be proved to be feasible. The strategy is already adopted for the VVER 440 type 213 based on thorough research work for the Finnish Loviisa NPP and Hungarian Paks NPP. It is also included in the design of some new Gen.III reactors like AP-1000, APR 1400 and Chinese CPR-1000. It has also been studied in the past for other reactor concepts like KERENA (BWR) or VVER-640. Current approaches for reactors with relatively small power, such as VVER 440 or AP600, use conservative assumptions. However, for higher power reactors (around 1000 MWe), it is necessary to evaluate the IVMR strategy with best-estimate methods in order to address the uncertainties associated with the involved phenomena. Additional R&D is needed to ensure and demonstrate adequate safety margins, including identification of efficient technical solutions for the external cooling of the vessel and performing best-estimate evaluation of relevant scenarios. Among other provisions, the possibility of cooling the corium inside the vessel by direct injection of water into the degraded core, may be considered because it is likely to remove a significant part of the residual power. The goal of the project is an analysis of the applicability and technical feasibility of the IVMR strategy to high power reactors, both for existing ones (e.g. VVER 1000 type 320 units) as well as for future reactors of different types (PWR or BWR). The main outcomes of the project will be elevant assumptions and scenarios to estimate the maximum heat load on the vessel wall, improved numerical tools for the analysis of IVMR issues and a harmonized methodology on the IVMR.

The EASI-SMR project intends to address the safety issues related to the LW-SMR in order to provide advances that should support implementation of such technologies as soon as possible. The EASI SMR project activities are aimed at ensuring that these reactors will be designed, constructed, commissioned and operated in the safest possible way and in accordance with existing regulations. The consortium was carefully chosen so that the research entities can provide the necessary research teams and support facilities across the European Continent and beyond. EASI-SMR will address the safety issues associated with major LW-SMR innovations: • Passive systems • Soluble Boron-free cores • Co-generation and hybridation • Additive manufacturing to improve compactness of Nuclear Steam Supply System • Multi-units operation The work aims to provide insights for European LW-SMR projects, in particular: • NUWARD SMR, a French design of a reactor generating 170 MW of electricity production. • LDR-50, a Finnish design of a district heating reactor of 50 MW EASI-SMR is closely linked with NUGENIA TA6 and the European SMR pre-Partnership’s WS5.

ELSMOR (towards European Lisencing of Small MOdular Reactors) aims to create methods and tools for the European stakeholders to assess and verify the safety of light water small modular reactors (LW-SMR) that would be deployed in Europe. ELSMOR advances the understanding and technological solutions pertaining to light water SMRs on several fronts: • Collection, analysis, and dissemination of the information on the potential and challenges of Small Modular Reactors to various stakeholders, including the public, decision makers and regulators. • Development of the high level methods to assess the safety of LW-SMRs • Improvement of the European experimental research infrastructure to assist in the evaluation of the novel safety features of the future LW-SMRs. • Improvement of the European nuclear safety analysis codes to demonstrate the capability to assess the safety of the future LW-SMRs Establishing education and training in the field of innovative nuclear reactors for young professionals is also emphasized. The ELSMOR project is built upon the expertise of the consortium that consists of technical support organizations, technical research centres, industrial partners, and universities with the long experience in European nuclear safety analysis and the development and implementation of innovative nuclear technologies. The industrial partners include utilities, small medium sized enterprises as well as the consortium currently developing the French LW-SMR (F-SMR design). The developers of European safety analysis tools and other computer codes use their well-established paths for exploitation of the improved and validated simulation tools. The licencing approaches and methods would be expected to be directly utilized by SMR designers like the French consortium. The outcomes should make the licensing process more fluid and comprehensive; this should also be true from the regulator point of view.

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.