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Safety Analysis of SMR with PAssive Mitigation strategies - Severe Accident

Periodic Reporting for period 1 - SASPAM-SA (Safety Analysis of SMR with PAssive Mitigation strategies - Severe Accident)

Période du rapport: 2022-10-01 au 2024-03-31

Integral Pressurized Water Reactor (iPWR) are ready to be licensed as new builds because they start from the well-proven and established large Light Water Reactor (LWR) technology, incorporate their operational plant experience/feedback, and include moderate evolutionary design modifications to increase the inherent safety of the plant. However, despite the reinforcement of the first three levels of the Defence-in-Depth (DiD), e.g. with the adoption of passive safety systems, a sound demonstration of iPWR ability to address Severe Accidents (SA) should be carried out (DiD levels 4-5). Therefore, some scenarios that could lead to SAs need to be postulated and deterministically studied. Therefore, the systematic analyses of the applicability and transfer of the current available SA experimental database (developed for current large-LWR) for iPWR safety assessment studies, and the analyses of current codes capabilities to simulate SA phenomena in iPWR are novel topics of current high interests.

In this framework, the key objective of the SASPAM-SA project is to investigate the applicability and transfer of the operating large-LWR reactor knowledge and know-how to the near-term deployment of iPWR, in the view of SA and Emergency Planning Zone (EPZ) European licensing analyses needs.

Key Outcomes of SASPAM-SA :
- To be supportive for the iPWR licensing process by bringing up key elements of the safety demonstration needed;
- To speed up the licensing and siting process of iPWRs in Europe.

The achievement of the overall elements is assured by a consistent and coherent work programme, reflected in the technical Work Packages (WP) structure:
WP1- Coordination;
WP2- Input deck development and hypothetical SA scenarios assessment (SCENARIOS);
WP3 - Applicability and Transfer of the Existing SA experimental database for iPWR Assessment (EXP);
WP4 - Assessment of code capabilities to simulate and evaluate corium retention in iPWRs (IVMR);
WP5 - Assessment of the code capabilities to simulate IPWR containment and characterize mitigation measures efficiency (CONT);
WP6 - Characterization of iPWR EPZ (EPZ);
WP7 - Communication, dissemination and exploitation (DISSE).

In order to maximize the knowledge transferability and impacts of the project two generic design-concepts, characterized by different evolutionary innovations in comparison with larger operating reactor, have been selected for the analyses: a) IPWR characterized by a submerged containment and electric power of about 60 MWe, called design 1; b) IPWR characterized by the use of several passive systems, a dry containment and an electric power of about 300 MWe, called design 2.These two generic reactor concepts include the main iPWR design features, considered in the most promising designs ready to go on the European market, allowing to assess in a wider way the capability of codes (SA and CFD) to simulate the phenomena typical of iPWR. It is not the project’s objective to assess the generic reactor designs selected but, based on the project findings, allow a more general statement on the code’s applicability to currently favored designs under postulated SA conditions.
SASPAM-SA has made significant progress in advancing the safety analysis practice for iPWRs.

In WP2, input-decks for design 1 and 2 have been developped for the target SA and CFD codes.WP2 identified DBA and BDBA scenarios to evaluate the capability of state-of-the-art codes to simulate the main features of iPWRs. The SA scenarios have been identified, and the code (SA and CFD) capability to predict degradation phenomena has been tested. The conditions in the containment and in the vessel that characterize iPWR scenarios have been identified, and a dedicated database has been fully developed to be used in WP3 to assess the applicability of existing experimental data to iPWRs. Across all simulations of the postulated SA scenarios and using all SA codes, no lower head failure was observed in WP2 analyses.

In WP3, a methodology was developed to evaluate the applicability of the existing experimental data for iPWRs. The developed methodology is used to evaluate existing experimental data in order to be able to use those data in iPWR safety analysis.
The methodology was used to evaluate experimental data of relevance to iPWRs, e.g. natural circulation, core degradation, containment, including hydrogen risk, and source term.

WP4 current conservative analyses, carried out with a 0D model developed within the project, support WP2 findings indicating that the maximum heat flux can be managed by pool boiling, and the residual vessel thickness ensures mechanical resistance; therefore, as expected, the first results show that in-vessel retention strategy appears feasible, with a good safety margin. Some specific features of IVMR in iPWRs appear: as example one remarkable feature of the two designs studied is the quite large amount of power lost by radiative heat transfer to the top structures of the vessel. However, detailed data and complete calculations would be necessary to go further. It will be the objective of the next steps of the WP4.

WP5, related to containment analysis, is set to begin; the partners continued their work on the Design-1 and Design-2 input-decks to allow for fission product behaviour and source term considerations.

WP6 is assessing the EPZ for iPWRs by analyzing SA scenarios and their radiological impacts. Initial findings suggest iPWRs may have lower radiological impacts and require smaller EPZs compared to large NPPs, facilitating their siting near populated areas.
• Identification of plausible SA scenarios for iPWR to be used for further analyses:
- Help TSOs and regulators to select the plausible scenarios to be investigated in a licensing process;
- Industries can use the scenarios to improve the mitigation strategies of iPWR in case of a SA.
• Evaluation of the capability of SA and CFD codes to reproduce the relevant phenomena of iPWR;
• Understanding of DBA, BDBA, and SA sequences for iPWR. This will constitute the ground for further development of Severe Accidents Management Guidelines (SAMG) to prevent accidents or mitigate their consequences avoiding radiological releases;
• Assessment and consolidation of state-of-art safety analyses methodologies applied to iPWRs and the training of the codes’ users in their application;
• Improving and extending the code users experience by building their expertise for SA in iPWR;
• Training new code users among the youngest generations;
• Improve available code guidelines and best practises;
• Build the know-how for the analysis of IVMR phenomena and to identify specific IVR features and near-term code development needs.
• Build the know-how for the demonstration of the ultimate confinement function of the iPWR containment in SA conditions;
• Applicability of tools and methods for EPZ iPWR analysis.
• Build the European background useful to develop SAMG to strengthen the safety of iPWR built in Europe.
Heat removal distribution in Design 1, Design 2 and HPR.
ANSYS CFX (left, iPWR Design-1) and containmentFOAM (right, iPWR Design-2) models mesh
In-vessel hydrogen production during the SA-1 in the generic design 1
Key Objectives and Key Outcomes of SASPAM-SA
Proposed methodology for evaluation of the applicability of existing experimental data to iPWRs
Pressure in the RPV and containment during DBA in the generic design 2
Generic design 2 reactor concept layout
ACKNOWLEDGMENT/DISCLAIMER
SASPAM-SA project diagram and WP interconnections
Physical barrier for the confinement of radiaoactive material- Water cooled reactor in operation
0D model heat transfer visual schematization
Generic design 1 reactor concept layout
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