![]() This reduces the needed containment to 300 years versus the tens of thousands of years needed by light-water reactor spent fuel. Discharged wastes are mostly fission products with shorter half-lives. Close fuel cycles reduce environmental impacts: chemical separation turns long-lived actinides into reactor fuel. MSRs enable cheaper closed nuclear fuel cycles, because they can operate with slow neutrons.However, most MSR designs place radioactive fluid in direct contact with pumps and heat exchangers. A low-pressure MSR does not require an expensive, steel core containment vessel, piping, and safety equipment.The molten salt coolant is not damaged by neutron bombardment, though the reactor vessel is. Fluoride salts dissolve poorly in water, and do not form burnable hydrogen. In some designs, the fuel and the coolant are a single fluid, so a loss of coolant carries the fuel with it. Passive decay heat removal is achieved in MSRs.MSRs offer many potential advantages over light water reactors: The temperatures of some designs are high enough to produce process heat, which led them to be included on the GEN-IV roadmap. Fuel drains into the container during malfunctions or maintenance, which stops the reaction. An additional method is to place a separate, passively cooled container below the reactor. In conventional reactors the negative reactivity is delayed since the heat from the fuel must be transferred to the moderator. For designs with the fuel in the salt, the salt thermally expands immediately with power excursions. MSRs exploit a negative temperature coefficient of reactivity and a large allowable temperature rise to prevent criticality accidents. ![]() MSR designs are often breeding reactors with a closed fuel cycle-as opposed to the once-through fuel currently used in conventional nuclear power generators. In this respect an MSR is more similar to a liquid metal cooled reactor than to a conventional light water cooled reactor. MSRs, especially those with fuel in the molten salt, offer lower operating pressures, and higher temperatures. ![]() Relevant design challenges include the corrosivity of hot salts and the changing chemical composition of the salt as it is transmuted by the neutron flux. This increases electricity-generation efficiency and process-heat opportunities. MSR operating temperatures are around 700 ☌ (1,292 ☏), significantly higher than traditional LWRs at around 300 ☌ (572 ☏). MSR's can be refueled while operating (essentially online- nuclear reprocessing) while conventional reactors shut down for refueling (notable exceptions include pressure tube heavy water reactors like the CANDU or the Atucha-class PHWRs, and British-built Gas-cooled Reactors such as Magnox, AGR). The gaseous fission products ( Xe and Kr) have little solubility in the fuel salt, and can be safely captured as they bubble out of the fuel, rather than increasing the pressure inside the fuel tubes, as happens in conventional reactors. This reduces the need and cost for reactor pressure vessels. They operate at or close to atmospheric pressure, rather than the 75-150 times atmospheric pressure of a typical light-water reactor (LWR). This eliminates the risk of hydrogen explosions (as in the Fukushima nuclear disaster). In addition, hydrogen evolution does not occur. The fuel mixture is designed to drain without pumping from the core to a containment vessel in emergency scenarios, where it solidifies, quenching the reaction. MSRs eliminate the nuclear meltdown scenario present in water-cooled reactors, because the fuel mixture is kept in a molten state. As of May 2023, China had not announced the ignition of its TMSR-LF1 thorium unit following its scheduled date of February 2023. Increased research into Generation IV reactor designs renewed interest in the 21st century. The 1950s Aircraft Reactor Experiment (ARE) was primarily motivated by the technology's compact size, while the 1960s Molten-Salt Reactor Experiment (MSRE) aimed to demonstrate a nuclear power plant using a thorium fuel cycle in a breeder reactor. Two research MSRs operated in the United States in the mid-20th century. A molten salt reactor ( MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a mixture of molten salt with a fissionable material. ![]()
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