Experimental and Computational Investigations of Candidate Fuel Salt Melt Properties and Corrosion and Irradiation Damage in Nickel for a Molten Chloride Fission System

A novel technology for accelerator-driven subcritical fission is being developed. A proton beam produces spallation and drives fission in a molten salt core. The motivation of the development is its capacity to destroy the transuranic elements in spent nuclear fuel and thereby provide a safe way to close the nuclear fuel cycle. Chloride-based transuranic salts have optimum properties for this purpose, and nickel appears to be attractive as a candidate material for the vessel structure.

The phase diagrams for the NaCl-UCl3 and NaCl-UCl3-CeCl3 systems are investigated, in which Ce was used as a surrogate for Pu. Accurate measurements of the solidus and liquidus phase boundaries of each system are made using differential scanning calorimetry. Significant discrepancies are observed in comparison to previous results.

A remarkable phenomenon is observed in molten salt corrosion at the interface between chloride-based molten salt and pure nickel. Significant grain boundary etching is observed in all corrosion experiments, typical of salt corrosion; furthermore there is evidence of material transport during the corrosion process. Our hypothesis is that surface layer is formed in which Ni is continuously removed from and deposited onto the surface. The re-deposited layer is conformal even to the nano-scale, and appears to fill corroded grain boundaries.

An experiment is conducted in which radiation damage and molten salt corrosion are produced simultaneously on a nickel surface. Proton beam irradiation is used to simulate neutron damage, and the nickel sample is thinned to allow damage deposition characteristic of neutrons at the salt-nickel interface. It was concluded that heat transfer will pose a difficult challenge for using ion-beam radiation as a stimulant to study appreciable radiation damage in molten salt-based systems.