Molten salt reactor: Encyclopedia II - Molten salt reactor - The liquid salt very high temperature reactor

Molten salt reactor - The liquid salt very high temperature reactor

Research is currently picking up again for reactors that utilize molten salts for working fluids. Both the traditional molten salt reactor and the Very High Temperature Reactor (VHTR) have been picked as potential designs to be studied under the Generation Four Initiative (GEN-IV). A version of the VHTR currently being studied is the Liquid Salt Very High Temperature Reactor (LS-VHTR). It is essentially a standard VHTR design that uses liquid salt as a coolant instead of helium. It relies on "TRISO" fuel dispersed in graphite. The fuel graphite would be in the form of graphite rods that would be inserted in hexagonal moderating graphite blocks. The molten salt would pass through holes drilled in the graphite blocks. The LS-VHTR has many attractive features, including: the ability to work at very high temperatures (the boiling point of most molten salts being considered are >1400°C), low pressure cooling that can be used to more easily match hydrogen production facility conditions (most thermo chemical cycles require temperatures in excess of 750°C), better electric conversion efficiency than a helium cooled VHTR operating at similar conditions, passive safety systems, and better retention of fission-products in case an accident occurred.

Technological issues

Much of the current research on this class of reactor designs is focused on small compact heat exchangers. Since the working fluid in the designs which rely on dispersed fuel in the molten salt will be undergoing fission, it would be extremely radioactive. Because of the high amounts of radioactivity, all tubes containing the primary, fuel bearing, molten salt must be shielded, adding to the expense of construction. 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, especially since at 850°C thermo chemical production of hydrogen becomes possible. Materials for this temperature range have not yet been found, though carbon composites or carbides might be feasible.

Adapted from the Wikipedia article "The liquid salt very high temperature reactor", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki