Periodic Reporting for period 3 - SAMOSAFER (Severe Accident Modeling and Safety Assessment for Fluid-fuel Energy Reactors)
Periodo di rendicontazione: 2022-10-01 al 2023-12-31
The Molten Salt Reactor (MSR) is a Generation-IV nuclear reactor design expected to excel on safety and sustainability, due to the use of a liquid fuel instead of solid fuel pins, as in regular Light Water Reactors (LWR). The MSR has promising characteristics for the CO2-free energy mix that we need in several decades from now. The MSR constitutes a broad class of reactor types with a neutron spectrum ranging from thermal to fast, that can operate with uranium, or with uranium supplemented with plutonium or thorium, or with thorium supplemented with plutonium. Due to this flexibility it can either operate as a breeder reactor, producing more fuel than it consumes, or as a burner reactor, destroying the current stockpile of plutonium in our legacy waste.
The MSR currently being explored and designed is expected to enter the energy market at large scale in a few decades from now. Meanwhile nuclear knowledge will have evolved and nuclear regulations may have become more strict; therefore the MSR needs to include all innovations possible to comply with the future nuclear regulations. To this end, the SAMOSAFER project has been initiated with the goal to develop simulation models and tools validated with experiments, complemented with the design and demonstration of new safety barriers in MSR to ensure that the MSR can comply with all expected nuclear regulations in 30 years’ time.
The MSR currently being explored and designed is expected to enter the energy market at large scale in a few decades from now. Meanwhile nuclear knowledge will have evolved and nuclear regulations may have become more strict; therefore the MSR needs to include all innovations possible to comply with the future nuclear regulations. To this end, the SAMOSAFER project has been initiated with the goal to develop simulation models and tools validated with experiments, complemented with the design and demonstration of new safety barriers in MSR to ensure that the MSR can comply with all expected nuclear regulations in 30 years’ time.
SAMOSAFER has been subdivided into 6 work packages on 1) safety requirements and risk identification, 2) fuel salt retention, 3) source term distribution and mobility, 4) fuel salt confinement, 5) heat removal and temperature control, 6) reactor operation and safety demonstration, and 7) education and training. The last WP laid the foundation for a new generation of nuclear engineers and scientists. All work packages focused on the Molten Salt Fast Reactor (MSFR) design, which is the EU reactor design promoted within GIF (Generation-IV Initiative).
SAMOSAFER ran from 2019 till 2024. Some specific findings of the project include:
• Transient analysis have been performed on the fluoride-salt and chloride-salt MSFR to investigate, among others, the effects of compressibility of the fuel salt and the damping effect of the Doppler feedback during transients. The compressibility of the fuel salt is a very important phenomenon in fast reactivity induced transients to properly calculate the maximum achievable fuel temperature.
• Risk identification studies for the fuel treatment unit for online fuel salt processing led to an extensive list of PIE’s (Postulated Initiating Events), as well as a PIE Ranking Table as input for a list of recommendations. This study was the follow up of the risk assessment studies for the reactor core performed in SAMOFAR.
• The thermodynamic database for the main MSR fuel and coolant systems has been extended with new experimental data on binary and ternary systems, and expanded to include a number of systems containing corrosion and fission products. Selected systems have been modelled to predict viscosity and density behavior.
• Multiple eutectic compositions were experimentally identified and the thermo-physical properties such as density, melting point, melting enthalpy, evaporation and boiling point were determined. In addition, selected fluoride-based MSR fuel systems were examined for the influence of the fission products on the fuel salt properties.
• The source term in the MSFR has been evaluated and the relevant simulation tools have been benchmarked by inter-code comparisons. Using these tools, the decay heat production and radiotoxicity in various compartments of the reactor system (especially in the fuel treatment unit) have been evaluated. As foreseen, these depend heavily on the removal rates of the various processes (bubbling, fluorination, etc.) which have also been assessed.
• Experimental setups (e.g. Espresso) have been built and numerical studies have been performed to investigate the freezing and (re) melting of fluids against cold walls, important for the safety evaluation of freeze plugs and over-cooling transients.
• Coupled neutronics-CFD codes have been developed and compared to simulate bubble injection and transport, and associated reactor power fluctuations. Using these codes, numerical safety assessment studies have been conducted to assess and reduce the power fluctuations in the core.
• Decay heat removal in the Emergency Draining System (EDS) is very sensitive to the geometry and the material properties of salts and structural materials. It was found that radiative transport plays a prominent role to reduce the maximal temperature and to arrive at realistic designs of the EDS.
• The possibilities and effects of natural circulation of self—heated fluids have been investigated both numerically and experimentally, including sensitivity studies. Both small scale experimental facilities (Rayleigh Bernard cell) as well as large facilities (e-Dynasty and SWATH-S) have been developed and used to these purposes.
• For the control and monitoring of the MSFR, predictive control strategies, and incident detection and classification methodologies have been developed, based on the system behavior modelled in numerical codes.
• Materials studies showed that corrosion can be limited to acceptable levels by controlling the composition and thus the redox potential of the fuel salt.
• A spectral projection method has been developed and used to quantify uncertainties in the safety assessments of the freeze plug design. This method is applicable to other safety components as well.
• The scaling effects and the technological dependence of the codes and methods used for design, safety and operating aspects of the MSR have been evaluated, showing opportunities to reduce the reactor size and develop small modular reactors.
• A summer school and Young MSR conference have been organized, partly with online lectures, and an exploitation workshop to transfer the project results to startups, regulators and other stakeholders. Besides extensive student exchanges among partners, also students and professors participated in the second MSR bootcamp (Berkeley, US).
SAMOSAFER ran from 2019 till 2024. Some specific findings of the project include:
• Transient analysis have been performed on the fluoride-salt and chloride-salt MSFR to investigate, among others, the effects of compressibility of the fuel salt and the damping effect of the Doppler feedback during transients. The compressibility of the fuel salt is a very important phenomenon in fast reactivity induced transients to properly calculate the maximum achievable fuel temperature.
• Risk identification studies for the fuel treatment unit for online fuel salt processing led to an extensive list of PIE’s (Postulated Initiating Events), as well as a PIE Ranking Table as input for a list of recommendations. This study was the follow up of the risk assessment studies for the reactor core performed in SAMOFAR.
• The thermodynamic database for the main MSR fuel and coolant systems has been extended with new experimental data on binary and ternary systems, and expanded to include a number of systems containing corrosion and fission products. Selected systems have been modelled to predict viscosity and density behavior.
• Multiple eutectic compositions were experimentally identified and the thermo-physical properties such as density, melting point, melting enthalpy, evaporation and boiling point were determined. In addition, selected fluoride-based MSR fuel systems were examined for the influence of the fission products on the fuel salt properties.
• The source term in the MSFR has been evaluated and the relevant simulation tools have been benchmarked by inter-code comparisons. Using these tools, the decay heat production and radiotoxicity in various compartments of the reactor system (especially in the fuel treatment unit) have been evaluated. As foreseen, these depend heavily on the removal rates of the various processes (bubbling, fluorination, etc.) which have also been assessed.
• Experimental setups (e.g. Espresso) have been built and numerical studies have been performed to investigate the freezing and (re) melting of fluids against cold walls, important for the safety evaluation of freeze plugs and over-cooling transients.
• Coupled neutronics-CFD codes have been developed and compared to simulate bubble injection and transport, and associated reactor power fluctuations. Using these codes, numerical safety assessment studies have been conducted to assess and reduce the power fluctuations in the core.
• Decay heat removal in the Emergency Draining System (EDS) is very sensitive to the geometry and the material properties of salts and structural materials. It was found that radiative transport plays a prominent role to reduce the maximal temperature and to arrive at realistic designs of the EDS.
• The possibilities and effects of natural circulation of self—heated fluids have been investigated both numerically and experimentally, including sensitivity studies. Both small scale experimental facilities (Rayleigh Bernard cell) as well as large facilities (e-Dynasty and SWATH-S) have been developed and used to these purposes.
• For the control and monitoring of the MSFR, predictive control strategies, and incident detection and classification methodologies have been developed, based on the system behavior modelled in numerical codes.
• Materials studies showed that corrosion can be limited to acceptable levels by controlling the composition and thus the redox potential of the fuel salt.
• A spectral projection method has been developed and used to quantify uncertainties in the safety assessments of the freeze plug design. This method is applicable to other safety components as well.
• The scaling effects and the technological dependence of the codes and methods used for design, safety and operating aspects of the MSR have been evaluated, showing opportunities to reduce the reactor size and develop small modular reactors.
• A summer school and Young MSR conference have been organized, partly with online lectures, and an exploitation workshop to transfer the project results to startups, regulators and other stakeholders. Besides extensive student exchanges among partners, also students and professors participated in the second MSR bootcamp (Berkeley, US).
In SAMOSAFER, specific codes for safety analysis have been developed and validated with experimental results generated in the project, and used to update the MSFR design. A new safety approach has been developed and used for the safety assessment of the MSFR, with emphasis on a complete list of PIE’s, reactivity induced accidents, source term evaluation and distribution, freeze plug design, and temperature control in the reactor. Decay heat transport methods have been extended and the decay heat removal circuit redesigned. Experiments have been performed on the unique SWATH-S, (e-)Dynasty and ESPRESSO facilities. Valuable insights have been gained in the sizing of the fuel treatment unit and the source term distribution. All this work is supported with a whole range of measurements and calculations of thermo-physical data of various fuel salts, made accessible via the JRCMSD database.