Skip to main content

A Paradigm Shift in Reactor Safety with the Molten Salt Fast Reactor

Periodic Reporting for period 3 - SAMOFAR (A Paradigm Shift in Reactor Safety with the Molten Salt Fast Reactor)

Reporting period: 2018-08-01 to 2019-07-31

Nuclear power is an outstanding source for low-carbon electricity production, but some people have concern about safety and long-lived waste production. Scientists and engineers have joined forces to develop a special class of nuclear reactors using a molten salt for both coolant and fuel. In Europe the focus is on the Molten Salt Fast Reactor (MSFR), which can be used either as a breeder reactor with thorium or as a burner reactor to destroy existing plutonium stockpiles.

The ultimate aim of SAMOFAR is to develop nuclear energy based on the Molten Salt Fast Reactor, which is truly safe without long-lived nuclear waste. The scientific content of the project focuses on three sub-objectives. (i) deliver experimental proof of concept of safety features, (ii) provide a complete safety assessment of the reactor and chemical plant, (iii) update the conceptual design with input gathered during the project.
WP1 dealt with the integral safety assessment and overall reactor design. A power plant software simulator was constructed and verified including models for the primary and intermediate fuel salt circuit and the energy conversion system. The MSFR responds quickly to varying demands and adapts its power output without excessive temperature variations. Also, modifying the fuel mass flow rate can control the MSFR. A risk assessment methodology has been developed based on the integrated safety assessment methodology. Postulated initiating events were identified and design modifications to guarantee three lines of defence for each of these events. Modifications to the MSFR design have been made, e.g. on cold plugs and passive decay heat removal.

The main objective of WP2 was to measure safety-related data of fuel salts. An installation was constructed for synthesis of actinide fluorides and experimental studies with molten fluorides. These were used for thermodynamic assessment of the fuel salt important for chemistry control of the salt and nuclear reactor control, like phase equilibrium studies, and measurements of heat capacity and thermal conductivity. Studies were done on the vaporization of fuel salt to evaluate the retention properties. To assess the safety after salt draining, we have studied the interaction of salt with water under gamma irradiation. A new instrument to measure the viscosity of salt was constructed and tested.

In WP3 the major experimental contributions in two large setups (DYNASTY and SWATH) have been achieved. The DYNASTY facility was modified and used to generate experimental data for stability analysis and for validation of numerical codes. The extended DYNASTY loop simulates two connected circuits and was modelled extensively and tested experimentally. SWATH-W and SWATH-S were used to carry out thermal-hydraulics experiments to investigate the performance of numerical models. SWATH-W employs water to study pure hydraulics phenomena. SWATH-S uses a molten salt to perform thermal hydraulics measurements. Phase change phenomena have been studied experimentally and modelled afterwards. Various aspects of cold plug designs have been evaluated numerically and tested experimentally, and a novel design of the cold plug based on the heat transfer balance between an electric heater and gas cooler was demonstrated experimentally.

In WP4 steady state and transient analyses have been performed on the MSFR. Steady-state single physics results were generated and compared Differences were small and attributable to mesh density or modelling differences. Multi-physics code systems were developed and compared using the MSR benchmark and results from the DYNASTY facility. Transient analysis focused on the unprotected loss of fuel flow scenario. Uncertainty quantification included both uncertainties in nuclear data, and the development of novel techniques for the error quantification in multi-physics systems.

The objective of WP5 is to perform a safety assessment and to update the chemical plant design considering chemical and nuclear safety issues. One task focused on zirconium, to determine its activity coefficient, and to design a process to extract it. Activity coefficients were also measured for uranium and iodine. Several studies have been performed to determine the electrochemical and thermodynamic properties of various actinides in the salt needed for experimental studies on the reductive extraction technique envisioned for the MSFR. To this end, ThF4 has been synthesized and purified in large amounts, next to UF4. The scheme of the chemical plant has been updated and thermochemical calculations have been done to provide transfer coefficients. These data have been used to calculate the radionuclide inventory as well as the radioactivity, heat deposition, and shielding requirements at each stage of the processing unit using newly developed software. To improve the corrosion resistance and thermal damage of ni
The work described above contains significant results beyond current knowledge in safety assessment, reactor design, salt data, experimental evaluation, algorithms and modelling, and synthesis of salts and coatings. Progress was also made on risk assessment methods, extension of the salt data base, multi-physics code systems, the experimental setups (DYNASTY and SWATH-W/S) and measurement campaigns, the safety analysis of both reactor and processing plant, etc. Results were published at scientific conferences, journals and other dissemination channels to increase the impact of the project.
Schematic drawing of the MSFR core and emergency draining tank