MSRs are liquid-fueled reactors that can be used for production of electricity, actinide burning, production of hydrogen, and production of fissile fuels. Fissile, fertile and fission products are dissolved in a high-temperature molten fluoride salt with a very high boiling point (1400ºC) that is both the reactor fuel and the coolant. The reactor can be built in large sizes with passive safety systems.

The use of a liquid fuel, versus the solid fuels of the other Generation IV concepts, creates potentially unique capabilities that are not achievable with solid-fuel reactors. The unique capabilities include: destruction of long-lived radionuclides without the need to fabricate solid fuels, a wider choice of fuel cycles (once through, waste burning, fissile fuel production) without major changes in the reactor design, limiting the radioactivity in the reactor core (accident source term) by on line removal and solidification of the mobile fission products, and imited excess reactivity requirements in the core due to online fuel management.

However, these capabilities also imply a different set of technical challenges. Two experimental MSRs have established the basic technology for the MSR. The first reactor was the 2.5 MW(t) Aircraft Reactor Experiment that in 1954 demonstrated peak operating temperatures up to 860ºC. This was part of an effort to build a nuclear-powered military aircraft. This was followed in the 1960s by the Molten Salt Reactor Experiment, an 8 MW(t) reactor to demonstrate key features required for a molten salt breeder reactor (MSBR).

Today, the primary interest in the MSR is its potential role in the fuel cycle. Russian and OECD studies have identified the MSR as a potential component of a closed fuel cycle to efficiently burn actinides and thus offer the potential to reduce the long-term radiotoxicity of the wastes produced from production of electricity. The use of liquid fuels avoids some of the technical difficulties (such as fuel fabrication) for burning actinidesespecially the highly radioactive higher actinides. There is a secondary interest in its use for hydrogen production because of the high-temperature capability. The different goals, combined with technical developments since the 1960s, are the basis for renewed interest in MSRs.