SAMOSAFER builds on the results of the SAMOFAR project

In our approach we will demonstrate the strength of each barrier between the fuel salt in the core and the environment and where possible make suggestions for improvements. We will follow the radionuclide inventory from the ‘inside’ (fuel salt) to the ‘outside’ in all possible scenarios with the aim to prevent and mitigate severe accidents. In our analyses, we will include the whole reactor system including the reactor core, FTU and EDS. Besides a Work Package (WP) on Project Management (WP8), the proposal contains seven scientific parts:

WP1 Safety requirements and Risk identification

WP1 lays the foundation for the safety approaches applied in the work packages 2-6. It will compare the EU nuclear safety standards with the MSR safety case and extrapolate these to future requirements, identify the risks, and the postulated initiating events for the FTU, like was done for the core and the EDS in the SAMOFAR project [24]. This WP will also explore and give guidelines for the safety approaches to be taken for the fuel salt retention in WP2, the source term redistribution in WP3, the radionuclide confinement in WP4, the decay heat removal in WP5, and the reactivity control in WP6. Furthermore, a global overview of integral experiments will be made to provide benchmark cases and to identify the needs for integral experiments. This WP is carried out with a strong involvement of the nuclear industry and TSO.

WP1 steers the research performed in WP2 to WP6 by identifying current safety requirements, nuclear risks, and improvements, and by exploring and anticipating future requirements.

WP2 Fuel salt retention

The first barrier to radionuclide release in severe accidents is the fuel salt itself. The philosophy in the MSR safety case is to control the salt conditions such that the FPs divide in two classes: the ones that can easily be removed from the fuel salt and the ones that will be strongly bound to the salt. The aim of this WP is to provide tools and data necessary to control the fuel salt behavior in the MSR and to assess the influence of fission products and corrosion products on the fuel salt properties. To this end, we will extend our current code systems (DG-FLOW and OpenFoam) developed in SAMOFAR with a new thermochemistry module, and perform extensive experimental validation and modeling. We will perform Post Irradiation Examination (PIE) on the SALIENT-01 samples irradiated in the HFR in Petten to study fuel salt behavior (gaseous FP production, FP speciation, FP relocation, particle size distribution of noble metals, etc) of the irradiated fuel salt. Other properties will be measured as well, complemented with Molecular Dynamics (MD) simulations.

In summary: WP2 models and measures the retention of fission products in the fuel salt, and the effect of fission products and corrosion products on the physico-chemical properties of the fuel salt to reduce the source term in case of severe accidents.

WP3 Source term distribution and mobility

In WP3 we evaluate the nuclide inventory through the whole reactor including the FTU. The equilibrium nuclide concentrations in the core depend on the treatment scheme, therefore a new coupled code system simulating fuel burnup and chemical extraction methods will be developed for a complete and accurate safety assessment. Simulation tools will be used to assess the extraction efficiencies of the gaseous FPs and micro-particles (noble metals) via helium bubbling. The latter will be done at a macroscopic multi-phase level and will be validated using the EXPRESS facility. The chemical extraction processes in the FTU will be evaluated experimentally and assessed with the new simulation codes.

In conclusion: WP3 models and quantifies nuclide extraction processes and quantifies the resulting source term and its chemical form in the reactor core and in the FTU.

WP4 Fuel salt confinement

Essential for resistance against severe accidents is the ability of fuel salt to flow freely to redistribute the decay heat and to bring the reactor in a safe condition under abnormal operating conditions. Because heat conduction in the fuel salt is poor compared to heat convection (high Prandtl number), the fuel salt may easily solidify against cold walls or in cold regions. In WP4 we will develop cutting edge simulation tools to evaluate melting and solidification phenomena of the fuel salt. These will subsequently be used to optimize the drain system (EDS) and other barriers including the freeze plugs and valves. The software will be validated using SWATH-S and ESPRESSO.

WP4 develops and validates simulation models and tools to assess the redistribution of the fuel salt in the reactor system. The flow paths of the fuel salt to the EDS will be optimized to enable the safe storage of the fuel salt without overheating.

WP5 Heat removal and temperature control

In this WP we develop simulation models and tools for the safe removal of decay heat from the fuel salt in the reactor core and in the EDS. Because the fuel salt is partially transparent to infrared radiation, radiation heat transfer is much more important for decay heat removal than in solid fuel reactors. On the other hand natural convection is less effective, due to the internal heat production in the fuel salt, which leads to smaller density differences. This WP studies both the natural convection phenomena, the effects of turbulence on heat transfer, and the radiation heat transfer mechanisms including experimental validation using SWATH-S and DYNASTY. The resulting models will be used to optimize the fuel circuit and the EDS with regard to decay heat removal.

WP5 develops and validates simulation models and tools for the safe removal of decay heat from the EDS to the environment and to optimize the design of the EDS to avoid severe accidents due to overheating.

WP6 Reactor operation and control, and safety demonstration

In WP6 the results of our project are integrated by final application of our simulation models and tools to new safety barriers. First the MSFR operational states will be defined as well as the emergency operating procedures. Subsequently advanced monitoring and predictive control strategies will be investigated to prevent severe accidents. By control of the Redox potential of the fuel salt, corrosion can be reduced, increasing in the long term the mechanical strenght of the fuel circuit. The barrier designs from WP3-5 will be incorporated to show their effectiveness with regard to risk reduction and severe accident prevention including uncertainty quantification. The effects of scaling laws on the occurrence and effects of severe accidents will be investigated to find generic relations for the design of MSRs. This knowledge will be transferable to other reactor designs as well.

WP6 will demonstrate the effectiveness of the validated simulation models and tools and the effectiveness of the barriers to reduce the risks and to prevent and mitigate severe accidents in the MSFR.

WP7 Education & Training and Dissemination & Exploitation

This WP focuses on the Education and Training of PhD students and postdoctoral researchers in the project and beyond, and on the activities to maximize dissemination and exploitation. We will organize a summer school and a Young-MSR conference for students, trainees, postdocs, scientists and engineers, and address an even wider community via webinars and other means such as a software simulator. To widen the skillset of our students, we will support and stimulate the exchange of students and trainees. To build a software user community for our simulation tools, we will organize an Exploitation workshop with hands on lessons using the software. The exploitation of our results will be targeted on strategic stakeholders needed for the further development of the MSR. These stakeholders include national labs, TSO’s, industry, SME’s, policy makers, etc. The AB members will be consulted to enlarge our impact and exploitation.

WP7 transfers our knowledge to a new generation of scientists and engineers and to our stakeholder community. A software user community will be established to increase the dissemination and exploitation of our simulation tools and our impact.