Molten salt reactors are being discussed as an alternative to the popular pressurized water reactors. Although they have been studied since the 1950s, there has been little development work compared to pressurized water reactors. In a molten salt reactor, liquefied fluoride salts are used as a coolant. Sometimes the uranium fuel is mixed with the coolant.
Molten salt reactors run at higher temperatures for greater efficiency but lower pressures than water cooled reactors. They have passive safety systems that drain out the fuel and coolant if the temperature gets too high. They don't leak radioactive steam. Their wastes are not as radioactive and have shorter half-lives that current water cooled reactors. Molten salt reactors can quickly respond to load changes.
Among the disadvantages of molten salt reactors is the fact that they need to have onsite chemical plants to manage the mixture of salts and to remove fission products. They will require major regulatory design changes for radically different designs. The nickel alloys that hold the molten salt are embrittled by the neutron flux. There is a greater risk of corrosion than in water cooled reactors. Molten salt reactors can be used as breeder reactors to make weapons-grade nuclear material. Some molten salt reactors use fuels that are almost as enriched as weapons grade materials which would be prohibited by current regulations. Neutron bombardment damages graphite moderators.
Researchers at the molten salt reactor and thorium research and develop program at the Shanghai Institute of Applied Physics (Sinap) have formed a partnership with the Australian Nuclear Science and Technology Organization (Ansto) to develop new materials for use in molten salt reactors.
Sinap created a series of samples of nickel (Ni) molybdenum (Mo) alloys that contained different percentages of silicon carbide (SiC) particles which were then taken to Ansto to be characterized. An Ansto representative said, "Structural materials for molten salt reactors must demonstrate strength at high temperatures, be radiation resistant and also withstand corrosion" when explaining the purpose of their research program.
The new NiMO-SiC alloys "possess superior mechanical properties owing to the precipitation, dispersion and solid-solution strengthening of the NiMo matrix". Silicon Carbide particles had been considered for use in such alloys but they had a problem with dislocation at high temperatures. In the new alloys, nickel silicide nano-particle fill the holes between the silicon carbide particles in the matrix. A new powder metallurgy process was used to insure uniform distribution of particles in the alloys. Such dispersion is not possible with standard metallurgical techniques.
"The strength of these alloys stems therefore from the combination of dispersion strengthening by silicon carbide particles, precipitation strengthening by nickel silicide and solid-solution strengthening by molybdenum," the Ansto representative said. "As well as superior high-temperature strength, these newly developed alloys have superior corrosion resistance and radiation damage resistance. The nano-particles present in the microstructure not only provide the obstacles for dislocation motion, but also provide sites/traps for radiation damage effects," he added.
The creation of this new alloy should contribute significantly to the development of molten salt reactors.
