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Excerpt from the World Nuclear Association Website on Small and Medium Reactors

        Molten salt reactors

During the 1960s, the USA developed the molten salt breeder reactor concept as the primary back-up option for the fast breeder reactor (cooled by liquid metal) and a small prototype 8 MWt Molten Salt Reactor Experiment (MSRE) operated at Oak Ridge over four years. U-235 fluoride was in molten sodium and zirconium fluorides at 860°C which flowed through a graphite moderator. There is now renewed interest in the concept in Japan, Russia, France and the USA, and one of the six Generation IV designs selected for further development is the molten salt reactor (MSR).  

In the MSR, the fuel is a molten mixture of lithium and beryllium fluoride salts with dissolved enriched uranium, thorium or U-233 fluorides. The core consists of unclad graphite moderator arranged to allow the flow of salt at some 700°C and at low pressure. Heat is transferred to a secondary salt circuit and thence to steamn. It is not a fast neutron reactor, but with some moderation by the graphite is epithermal (intermediate neutron speed). The fission products dissolve in the salt and are removed continuously in an on-line reprocessing loop and replaced with Th-232 or U-238. Actinides remain in the reactor until they fission or are converted to higher actinides which do so. MSRs have a negative temperature coefficient of reactivity, so will shut down as temperature increases beyond design limits.

The following US reactor projects are utilizing fluoride cooled and molten salt cooled reactor (MSR) technology:

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