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Project summary


Small modular rector (SMR) technology is a very likely candidate for new built in Europe including Nordic countries. Several designs of SMRs are currently at an advanced level of maturity, ready to be deployed. Passive safety systems are widely used in all SMR designs and are viewed as the key for enhanced safety of SMRs. Safety assessment of the passive systems is not trivial, especially when it is not a malfunction of equipment, but the interactions between physical processes and time evolution of accident scenarios is the cause of the failure. To facilitate licensing process of SMRs in Nordic countries it is important to develop expertise in analysis of passive safety system reliability. In this project we rely on our experience in (i) modeling of relevant thermal-hydraulic phenomena (natural circulation, steam condensation, behavior of large water pools with energy and momentum sources, etc.) and (ii) development of approaches to assessment of systems where both scenarios and phenomena are important contributors to uncertainty (e.g. development of the ROAAM+ approach for assessment of severe accident management effectiveness in Nordic BWRs).


Summary of work at VTT

Performance and modelling capabilities of passive safety systems of SMRs will be studied. The main principles of the passive systems representative to BWRX-300 design will be mapped to understand the main critical functions and phenomena of the systems. The work will concentrate on the performance of the Isolation Condenser System (ICS) and/or the Passive Containment Cooling System (PCCS) and a specific focus is on thermal-hydraulics and steam condensation/evaporation phenomena. Relevant simulation models will be built in the Apros and CFD softwares such as OpenFOAM and Fluent. The CFD models will be built up step- by-step by starting with the simplest approaches continuing to more detailed representations. The models in each step will be validated against available experimental data. Possible large-scale prototypical experiments to be used in the validation are done at the PANDA and PANTHERS facilities. Also other available tests will be investigated in the project. The system model will be developed for the Apros code. The system analyses will be compared with the CFD calculations. Apros and CFD has also a specific coupling tool to connect the simulations and enable the simultaneous simulations. The possibility to apply the coupling tool in the work will be considered.


VTT work in 2024

In 2024, the Apros and CFD models for BWRX-300 representative PCCS heat exchanger will be developed. At first, a simple tube model will be created to familiarise young specialists with the CFD environment. The main focus is on the steam condensation modelling on the heat exchanger tubes located in the containment atmosphere. A suitable approach of steam condensation modelling will be investigated and implemented in the CFD code. After that a whole tube bundle model will be created. The model will be validated against relevant PANDA containment cooler experiment. Also benhmark calculation with the Apros and CFD will be made.


Summary of activities at KTH

KTH will focus on (i) development and validation of modeling approaches relevant to important phenomenology of passive safety systems, (ii) development of approaches to assessment of passive safety system reliability.


One of the important phenomena relevant to SMR passive safety systems is steam condensation in a pool of water. Such pools are often used the main heat sink. Temperature distribution in the pool can significantly affect performance of natural circulation passive safety systems. There is a need for validated mechanistic modeling approaches that can predict realistic pool transients and interactions with the natural circulation system. As it has been demonstrated in NEA Fukushima Benchmarks (BSAF and BSAF-2), contemporary safety analysis codes are not capable to predict thermal stratification and mixing in the pool. The lack of predictive capabilities has led to the initiation of (i) development of mechanistic models, and (ii) experimental campaigns including a series of OECD projects HYMERES, HYMERES-2 and currently ongoing (2021-2025) PANDA project. KTH is a recognized leader in both model development (with more than 40 publications) and in analytical support for design of the validation tests such PANDA and PPOOLEX. In this work, KTH will provide analytical support to the design, pre-test analysis and post-test code validation against OECD/NEA PANDA project tests series P1A6 with steam condensation in the pool. Obtained data will be used for validation and further improvement of the models developed at KTH.


KTH will work on development of approaches to assessment of passive safety system reliability. The approach will build on experience of KTH in analysis of natural circulation systems including instabilities, use of advanced search methods such as genetic algorithm to identification of “failure” conditions in the multidimensional space of accident scenario parameters, comprehensive sensitivity and uncertainty analysis using developed at KTH ROAAM+ approach.


KTH work in 2024

KTH will focus on pre-test analysis and post-test validation against OECD/PANDA test series P1A6. The target of the tests will be clarification of remaining uncertainties in the stratification and mixing of the pool at higher system pressure condition, including near-saturation conditions in the upper part of the pool.


With the second task, KTH will review most recent developments in the area of passive system reliability analysis. Based on the synthesis of the review the knowledge gaps will be identified for further improvement of the methods, specifically applicable to systems such as BWRX-300.


Lead organisation

VTT Technical Research Centre of Finland


Contact person

Ari Silde
Vuorimiehentie 3, Espoo, P.O.Box 1000, FI-02044, Finland.


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Website last modified: 05 July 2024