As an additional method for shutdown, a separate, passively cooled container below the reactor can be included. For designs with the fuel in the salt, the salt thermally expands immediately with power excursions, as opposed to conventional reactors in which the negative coefficient of reactivity is delayed since the heat from the fuel must be transferred to the moderator. Safety concepts rely on a negative temperature coefficient of reactivity and a large allowable temperature rise to limit reactivity excursions. MSRs are often planned as breeding reactors with a closed fuel cycle-as opposed to the once-through fuel currently used in U.S. In this respect an MSR is more similar to a liquid metal cooled reactor than to a conventional light water cooled reactor. Operating pressures can be much lower, and the temperatures higher. MSRs, especially those with fuel in the molten salt, differ considerably from conventional reactors. MSRs offer multiple advantages over conventional nuclear power plants, although for historical reasons they have not been deployed. 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 in the reactor core. MSR's can also be refueled while operating (essentially online- nuclear reprocessing) while conventional reactors must be shut down for refueling (Pressure tube heavy water reactors like the CANDU or the Atucha-class PHWRs, and British-built Gas-cooled Reactors ( Magnox, AGR) being notable exceptions).Ī further key characteristic of MSRs is operating temperatures of around 700 ☌ (1,292 ☏), significantly higher than traditional LWRs at around 300 ☌ (572 ☏), providing greater electricity-generation efficiency, the possibility of grid-storage facilities, economical hydrogen production, and process-heat opportunities. Another advantage of MSRs is that the gaseous fission products ( Xe and Kr) do not have much solubility in the fuel salt, and can be safely captured as they bubble out of the molten fuel, rather than increasing the pressure inside the fuel tubes over the life of the fuel, as happens in conventional solid-fueled reactors. They operate at or close to atmospheric pressure, rather than the 75-150 times atmospheric pressure of a typical light-water reactor (LWR), hence reducing the need for large, expensive reactor pressure vessels used in LWRs. This eliminates the risk of hydrogen explosions (as in the Fukushima nuclear disaster). In addition, the use of molten salt coolant prevents the evolution of hydrogen gas possible in water cooled reactors. In most designs, the fuel mixture is designed to drain from the core to a containment vessel in emergency scenarios, where it will solidify in fuel drain tanks. MSRs are considered safer than conventional reactors because they operate with fuel already in a molten state. Increased research into Generation IV reactor designs began to renew interest in the technology, with multiple nations having projects, and as of September 2021, China is on the verge of starting its TMSR-LF1 thorium MSR. The 1950s Aircraft Reactor Experiment was primarily motivated by the compact size that the technique offers, while the 1960s Molten-Salt Reactor Experiment aimed to prove the concept of a nuclear power plant which implements a thorium fuel cycle in a breeder reactor.
Only two MSRs have ever operated, both research reactors in the United States. 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 molten salt mixture.
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