A Brief Description of Molten Salt Reactors (MSRs)
A simplified flow diagram of the primary and secondary salt circulating systems of molten salt reactors (MSRs) is shown in Fig. 1. The reactor core is formed from an array of graphite bars with channels between them for the circulating salt. The fluoride molten salt fuel mixture circulates through the reactor core, the primary heat exchanger and the pump, and returns to the plenum at the bottom of the reactor vessel after an on-line (or fast) fuel reprocessing. In the heat exchanger the heat form the primary salt is transferred to a sodium fluoroborate intermediate coolant. This intermediate coolant salt in turn is circulated through a steam generator (second heat exchanger) to produce steam for the generation of electricity.
The MSR can also be realized as a sub-critical accelerator driven system which is currently investigated.
The Brief History of Molten Salt Reactors and Motivations of R & D
Molten salt reactors (MSR) were first proposed by ORNL in the later 1940’s attempt to design a nuclear-powered aircraft. The attraction of molten fluoride salts for that program was the great stability of the salts, both to high temperature and to radiation, because they have high solubility for uranium, are among the most stable of chemical compounds, have very low vapor pressure even, have reasonably good heat transfer properties, are not damaged by radiation, do not react violently with air or water, and are inert to some common structural metals. The Aircraft Reactor Experiment, a small reactor using a circulating molten fluoride fuel salt, was built at ORNL to study the nuclear stability of the circulating fuel systems. The molten salt fuel was a mixture of NaF, ZrF4, and UF4, the moderator was BeO, and all the piping was Inconel. In 1954 the MSR was operated successfully for 9 days at steady-state outlet temperature ranging up to ca. 860°C and at powers up to 2.5 MW(th).
That molten salt reactors might be attractive for civilian power applications was recognized from the beginning of the nuclear powered aircraft by those who took part in this program, and a group at ORNL began work in 1957. A further molten salt reactor experiment (MSRE) was built and operated successfully at a power level of 7.3 MW(th) until end of 1969. Molten salt reactor experiment ran for long period of time and was very successful. This experiment demonstrated well that the Molten Salt Reactors was extremely flexible. The MSBR operated initially on U-235. the U-235 was later removed and replaced with U-233 with no change in design. Six hundred grams of plutonium were added to the MSRE, during its operation, in addition to the plutonium that was bred from the uranium in its initial operation with U-235. Thus, the MSRE operated on the three major fissile material with no safety incidents.
Last years important R & D efforts were placed worldwide to find the ways to reduce the long term radio nuclide inventory resulting from the nuclear power generation. The molten salt reactors offer the potential for burning actinides and some long-lived isotopes, other than plutonium and uranium. The advantage of burning these isotopes in this type of reactors is that molten salt reactors can be charged on-line with spent fuel and no fuel(-pin) re-fabrication is needed Because of this, currently molten salt reactors for transmutation are investigated within the 5th Framework Programme MOST of European Union.