(331b) Electrorefining of Reduced Spent Nuclear Oxide Fuel at Bench Scale

Spent nuclear fuel from Experimental Breeder Reactor (EBR-II) is being treated at the Idaho National Laboratory (INL) utilizing pyroprocessing technology. At the heart of the pyroprocess is an electrorefining operation that receives the spent metal fuel from EBR-II and effects the separation and recovery of actinides from fission products in the fuel. Extension of this technology to spent nuclear oxide fuels requires a front-end reduction of an oxide fuel to metal prior to electrorefining. Such a reduction technique has been demonstrated at bench scale on spent nuclear oxide fuel with an electrolytic reduction process. A portion of the reduced spent nuclear oxide fuel from the electrolytic reduction process was further subjected to electrorefining operations at bench scale, which is the subject of this report.

A series of six electrolytic reduction runs was performed previously with irradiated oxide fuel in a hot cell at INL. [1] In each of these runs, crushed oxide fuel (nominally 50 g) was loaded into a permeable steel basket and submerged into a molten salt of LiCl – 1 wt% Li2O at 650 ºC. An electric current was applied between the steel basket (cathode) and a submerged platinum wire (anode) in the salt, effecting the reduction of the fuel to metal in the basket while simultaneously forming oxygen gas on the platinum wire surface. Samples of the salt were extracted before and after each reduction run and subjected to chemical and radiochemical analyses. The analytical results quantified the salt soluble fission products (notably, cesium, barium, strontium, and iodine) from the fuel to the salt phase. Fuel from one of the six reduction runs was analyzed, revealing an extent of uranium oxide reduction in excess of 99.7%. Fuel from the other five reduction runs was retained in the original baskets for subsequent electrorefining tests.

In the electrorefining tests, each basket of reduced spent nuclear oxide fuel was submerged into a molten salt of LiCl-KCl eutectic at 500 ºC with a minor fraction of uranium trichloride. An electric current was applied between the reduced fuel basket (anode) and a submerged steel rod (cathode), effecting the dissolution of uranium metal in the fuel basket and its simultaneous deposition (as refined uranium) on the steel cathode. Multiple cathode rods of refined uranium were generated from the complete dissolution of reduced oxide fuel in each basket. Samples of the salt were extracted before and after each electrorefining run, and a refined uranium fuel sample was taken from one of the cathode rods in each run. The salt and fuel samples were subjected to chemical and radiochemical analyses. The analytical results revealed the distribution of fission products within the electrorefining process, as well as the decontamination factors for the refined uranium.

[1] S. D. Herrmann, S. X. Li, M. F. Simpson, S. Phongikaroon, “Electrolytic Reduction of Spent Nuclear Oxide Fuel as Part of an Integral Process to Separate and Recover Actinides from Fission Products,” in publication with Separation Science and Technology (2006).