Molten Salt Reactor, which employs molten salt mixture as fuel, has many advantages in reactor size and operation compared to conventional nuclear reactor. In developing Molten Salt Reactor, the behavior of fission product in operation should be preliminary evaluated for the correct design of reactor and its associated system including off-gas treatment. In this study, for 100 Mw 46 KCl- 54 UCl3 based Molten Salt Reactor with operating life time of 20 year, the fission product behavior was estimated by thermodynamic modeling employing FactSage 8.2. Total inventory of all fission product were firstly calculated using OpenMC code allowing depletion during neutronic calculation. Then, among all inventory, 46 element species from Uranium to Holmium were chosen and given to the input for equilibrium module of Factsage with its mass. In phase equilibrium calculation, for the correct description of solution phase, KCl-UCl3 solution database based on modified quasichemical model in the quadruplet approximation (ANL/CFCT-21/04) was employed and the coexisting solid phase was assumed to pure state. With the assumption of no oxygen and moisture ingress into reactor system, equilibrium calculation showed that 1% of solid phase and of gas phase were newly formed and, in gas phase, major species were identified : ZrCl4 (47%), Xe (33%), UCl4 (14%), Kr (5%), Ar (1%) and others. This result reveals that off-gas treatment of system should account for the appropriate treatment of ZrCl4 and UCl4 besides treatment of noble gas such as Xe and Kr.

Molten Salt Reactor (MSR) is one of the 4th generation nuclear power systems which is its verified technology in physically and chemically. Among the various salts used for MSR system, the eutectic composition of NaCl-MgCl2 system maintains the liquid state at around 450°C, in the same time, it has high solubility for nuclear fuel chlorides. This characteristic has high advantage for lowering the operating temperature for the MSR, which could reduce the problem of hightemperature corrosion by salt for structural materials significantly. In particular, since MgCl2 has the similar standard reduction potential with nuclear fuel, is used as a surrogate for, many basic researches have been conducted for verifying characteristic of MgCl2. It is well-known that main short-advantage of MgCl2 is hygroscopic properties. MgCl2 changes to MgCl2-xH2O state easily by absorbing moisture in air condition. The hydrated MgCl2 is producing MgOHCl by thermally decomposing at high temperature, the formed MgOHCl corrodes structural materials, even small amount of MgOHCl gives significant damage. Therefore, the purification of MgCl2 has been required for long-term operation of MSR using MgCl2 as a base salt. In this study, the purification of eutectic composition salt for NaCl-MgCl2 has been mainly performed by considering its thermodynamic properties and electrochemical characteristic, and the experimental results have been discussed.

A phosphorylation (phosphate precipitation) technology of metal chlorides is considering as a proper treatment method for recovering the fission products in a spent molten salt. In KAERI’s previous precipitation tests, the powder of lithium phosphate (Li3PO4) as a precipitation agent reacted with metal chlorides in a simulated LiCl-KCl molten salt. The reaction of metal chlorides containing actinides such as uranium and rare earths with lithium phosphate in a molten salt was known as solidliquid reaction. In order to increase the precipitation reaction rate the powder of lithium phosphate dispersed by stirring thoroughly in a molten salt. As one of the recovery methods of the metal phosphates precipitated on the bottom of the molten salt vessel cutting method at the lower part of the salt ingot is considered. On the other hand, a vacuum distillation method of all the molten salt containing the metal phosphates precipitates was proposed as another recovering method. In recent study, a new method for collecting the phosphorylation reaction products into a small recovering vessel was investigated resulting in some test data by using the lithium phosphate ingot in a molten salt containing uranium and three rare earth elements (Nd, Ce, and La). The phosphorylation experiments using lithium phosphate ingots carried out to collect the metal phosphate precipitates and the test result of this new method was feasible. However, the reaction rate of test using lithium phosphate ingot is very slower than that of test using lithium phosphate powder. In this presentation, the precipitation reactor design used for phosphorylation reaction shows that the amount of molten salt transferred to the distillation unit will reduce by collecting all of the metal phosphates that will be generated using lithium phosphate powder into a small recovering vessel.

Molten Salt Reactor, which employs molten salt mixture as fuel, has many advantages in reactor size and operation compared to conventional nuclear reactor. In developing Molten Salt Reactor, Offgas system should be properly designed since the fission products in off-gas accelerates the corrosion in reactor structure materials and deteriorates the purity of liquid fuel. The design of off-gas system therefore requires the preliminary study of the behavior of evolved fission products in off-gas units and the development of off-gas model is crucial in developing such system. In this study, we corrected the off-gas illustrative model proposed by ORNL (Nuclear Engineering and Design, vol 385(15) 111529, 2021) by employing physically consistent concept of capture rate of fission product and holdup. For the application of the corrected off-gas model to Chloride-based 6 MW Molten Salt Reactor, major fission products were firstly determined from OpenMC based neutronics calculation and chain reaction related to the major fission products were defined. Based on these data, the holdup behavior of fission products in off-gas units (decay tank, caustic scrubber, Halide trap, H2O trap and charcoal bad) were investigated.

In this study, molten salt experiments were performed using a multi-purpose molten salt experimental loop to evaluate the corrosion and thermodynamic properties of the molten salt. The multi-purpose molten salt experimental loop is made of 1-inch austenitic 316 stainless steel, and 1/4-inch austenitic 316 stainless steel tubes were welded on the surface of a 1-inch pipe to measure temperatures of molten salt. During the experiment, the molten salt leaked due to corrosion of the welded part of the 1/4-inch tube connected to the 1-inch pipe. Therefore, the cause of corrosion damage of the leaked welded part was analyzed. The effect of NaCl-MgCl2 salt selected as the molten salt on corrosion failure was considered. And based on the operation data of molten salt experiments, the time of occurrence of the issue was estimated. Lastly, the cause of corrosion failure was estimated by comparing and analyzing the pipe shape before and after failure using SEM-EDS.

Molten chloride salts are promising candidates as a coolant for Molten Salt Reactors (MSRs) because of their low cost, high specific heat transfer, and thermal energy storage capacity. The NaCl- MgCl2 eutectic salts have enormous latent heat (430 kJ/kg) and financial advantage over other types of molten chloride salt. Despite the promise of the NaCl-MgCl2 eutectic salt, problems associated with structural material corrosion in the MSR system remain. The hygroscopicity of NaCl-MgCl2 and high MSRs operating temperature accelerate corrosion within structural alloys. Especially, MgCl2 reacts with H2O in the eutectic salt to produce HCl and Cl2, which are known to further exacerbate corrosion by the chlorination of structural materials. Therefore, several studies have worked to purify impurities associated with MgCl2, such as H2O. Thermal salt purification of NaCl-MgCl2 eutectic salt is one method that reduces HCl and Cl2 gas generation. However, MgO and MgOHCl are generated as the byproduct of thermal purification through a reaction between MgCl2 and H2O. The corrosion behavior of MgO within structural alloys after thermal treatment is not well known. This paper demonstrates corrosion behavior within structural alloy after thermal treatment at various temperature profiles of the NaCl-MgCl2 eutectic salt. According to the temperature range, MgCl2·H2O are separated at 100~200°C, and MgOHCl and HCl begin to occur at 240°C or higher. Finally, MgOHCl produces MgO and HCl at 500°C or higher temperatures. After thermal treatments, the H2O, MgOHCl, and MgO content were measured by Thermo Gravimetric Analyzer (TGA) to evaluate significant products causing corrosion. The structural materials were analyzed by the Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) and using the mass change method to observe the type of localized corrosion, the corrosion rate, and the corrosion layer thickness. This study is possible in that it can reduce economic costs by reducing the essential use of expensive, high-purity molten salts because it is related to a substantial financial cost problem considering the amount of molten salt used in industrial sites.

Interests in molten salt reactor (MSR) using a fast spectrum (FS) have been increased not only for having a high power density but for burning the high-level waste generated from nuclear power plants. For developing the FS-MSR technologies, chloride-based fuels are considered due to the advantage of higher solubility of actinides and lanthanides over fluoride-based salts. Despite significant progress in development of MSR technology, the manufacturing technology for production of the fuel is still insufficiently understood. One of the option to prepare the MSR fuel is to use products from pyroprocessing where oxide form of spent nuclear fuel is reduced into metal form and useful elements can be collected via electrochemical methods in molten salt system at high temperature. In order to chlorinate the products into chloride form, previous study used NH4Cl to chlorinate U metal into UCl3 in an airtight reactor. It was found that the U metal was completely chlorinated into chloride forms; however, impurities generated by the reaction of NH4Cl and reactor wall were found in the product. Therefore, in this work, the air tight reactor was re-deigned to avoid the reaction of reactor wall by insertion of Al2O3 crucible inside of the reactor. In addition, the reactor size was increased to produce UCl3 over 100 g. Using the newly designed reactor, U metal chlorination experiments using NH4Cl chlorinating agent were performed to confirm the optimal experimental conditions. The detailed results will be further discussed.