Beryllium Moderated Reactors - Molten-salt Cooled Reactors

Molten-salt Cooled Reactors

Molten-salt-fueled reactors are quite different from molten-salt-cooled solid-fuel reactors, called simply "Molten Salt Reactor System" in the Generation IV proposal, also called MSCR, which is also the acronym for the Molten Salt Converter Reactor design. These reactors were additionally referred to as Advanced High Temperature Reactors (AHTRs), but since about 2010 the preferred DoE designation is Fluoride High-temperature Reactors (FHRs).

The FHR concept cannot reprocess fuel easily and has fuel rods that need to be fabricated and validated, delaying deployment by up to twenty years from project inception. However, since it uses fabricated fuel, reactor manufacturers can still profit by selling fuel assemblies.

The FHR retains the safety and cost advantages of a low-pressure, high-temperature coolant, also shared by liquid metal cooled reactors. Notably, there is no steam in the core to cause an explosion, and no large, expensive steel pressure vessel. Since it can operate at high temperatures, the conversion of the heat to electricity can also use an efficient, lightweight Brayton cycle gas turbine.

Much of the current research on FHRs is focused on small compact heat exchangers. By using smaller heat exchangers, less molten salt needs to be used and therefore significant cost savings could be achieved.

Molten salts can be highly corrosive, more so as temperatures rise. For the primary cooling loop of the MSR, a material is needed that can withstand corrosion at high temperatures and intense radiation. Experiments show that Hastelloy-N and similar alloys are quite suited to the tasks at operating temperatures up to about 700 °C. However, long-term experience with a production scale reactor has yet to be gained. Higher operating temperatures would be desirable, but at 850 °C thermo chemical production of hydrogen becomes possible, which creates serious engineering difficulties. Materials for this temperature range have not been validated, though carbon composites, molybdenum alloys (e.g. TZM), carbides, and refractory metal based or ODS alloys might be feasible.