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[en] The Fluoride-salt-cooled High-temperature Reactor (FHR) is a new nuclear power reactor concept being developed in the U.S. and China. It employs liquid salt as a primary coolant, which was originally developed for a molten salt reactor. The liquid salt utilized is called FLiBe whose melting and boiling temperatures are 459degC and 1433degC, respectively. This new concept reactor has improved safety features because the coated particle fuel pebbles has high temperature margin to retain fission products up to around 1650degC and the high boiling point of the coolant.. The Beyond Design Basis Accident (BDBA) system prevents fuel failure and large scale fission products release in case of a severe accident. The proposed BDBA system insulates the reactor core from the environment to minimize the heat loss during normal operations and utilizes high temperatures in a BDBA to degrade the insulation system to enable decay heat to escape to the environment. This change combined with the high-temperature capability of fuel and coolant enable removal of decay heat and avoidance of high-temperature fuel failure. In the present study, we examine two insulation systems that lose their insulative capabilities at higher temperatures to enable transfer of decay heat to the environment in a BDBA thus maintaining fuel temperatures below fuel failure temperatures. First is the mirror insulation system. This system is composed of multi-layer thin metal sheets. Ideally, these insulation sheets have radiation emissivity (ε) which is zero at 600degC but rapidly increases to one at over 700degC, and then decay heat is released to the earth. Secondly, the firebrick system may be employed with a BDBA salt. Firebrick insulation works because of its low thermal conductivity and voids in the firebrick. A BDBA salt can be included in the silo that melts at high temperatures. The salt penetrates the firebrick, fills the voids, and increases the thermal conductivity while absorbing decay heat. (author)
[en] Highlights: • Evaluation of decay heat removal systems of FHR during thermal-hydraulic transient was performed. • Heat removal performances of FHR systems are investigated. • FHR’s decay heat removal system shows sufficient heat removal capability to avoid catastrophic accidents. - Abstract: The Fluoride-salt-cooled High-temperature Reactor (FHR) is one of the Generation IV nuclear reactor concepts being investigated mainly in the U.S. and China. Liquid salt with high boiling point of over 1400 °C is used as a coolant and its baseline fuel is the graphite-matrix coated-particle fuel developed for high-temperature gas-cooled reactors. To establish advanced safety systems in FHR, passive decay heat removal system which utilizes fluid’s natural circulation is considered. Three systems are proposed: Direct Reactor Air Cooling System (DRACS), Reactor Vessel Air Cooling System (RVACS), and Silo Cooling System (SCS). Although these three safety systems were previously developed for other types of reactors, their heat removal performances need to be evaluated for the FHR plant. The current study provides evaluation of decay heat removal systems of FHR during thermal-hydraulic transient. In addition, FHR’s grace period to start the reactor cooling was determined under the condition where no operable heat removal systems are available. The current study shows that FHR’s decay heat removal systems possess sufficient heat removal capability to avoid catastrophic accidents. The construction of Fluoride-salt-cooled High-temperature Test Reactor (FHTR) is currently being planned and its development is a key step to realize the commercialization in near future. This study provides performance evaluation of decay heat systems for FHTR under thermal-hydraulic transient, and also suggests the adequate operation procedure suitable for FHTR to avoid a severe accident.
[en] Highlights: • Molten metal spreading experiment was conducted. • Molten metal spreading area and thickness data was obtained for molten copper. • Key parameters affecting the spreading area and thickness were identified. - Abstract: In this paper, experimental investigation of the molten metal spreading behavior that was carried out at Hokkaido University using high frequency inductive heater is presented to address the fundamental behavior of the molten metal spreading and deposition behaviors on dry flat plate. Molten copper was utilized as a test sample, and dataset was obtained for the falling molten metal on dry stainless-steel plate at various elevations, nozzle sizes and initial temperatures. During the spreading transient, high-speed thermo-camera was utilized to measure the molten metal’s surface temperature. Immediately after the solidification, solidified molten metal’s spread area and deposition thickness were measured. Based on the database obtained, dimensional analysis was conducted to identify the key parameters responsible for the molten metal spreading. From the obtained database, new experimental correlation was developed which is capable of predicting the spreading area at reasonable accuracy. Present analysis provides characteristic information of molten metal spreading and deposition behaviors which will be useful for the corium relocation problem in severe accident analysis.
[en] In this paper, experimental investigation of the molten metal jet's colliding and spreading behaviors on the flat steel surface covered with water layer was carried out. High-frequency induction heating system was utilized to produce the molten metal sample and it was released to the wet surface from a fixed elevation. As the molten metal collides against the surface, it rapidly goes through solidification while spreading on the wet surface. High-speed thermo-camera was utilized to measure the molten metal's surface temperature during the spreading transient. Once the molten metal completely solidifies, molten metal's spread area and thickness were measured. From the obtained database, a dimensional analysis was conducted to investigate the key parameters responsible for the molten metal spreading on the wet surface. Based on the key non-dimensional parameters identified in the current analysis, the new empirical correlation was proposed. Its predictive capability was found to be 18.9% in mean absolute relative deviation.