Methods of Redox Control and Measurement in Molten NaCl-CaCl2-UCl3

Abstract

The U.S. Department of Energy is currently supporting the design and planned construction of the Versatile Test Reactor (VTR) capable of performing experiments in a fast neutron field. This will allow for study of the effect of fast neutron irradiation on nuclear fuels, materials, and functional components. The molten salt irradiation experiment will utilize the VTR to study the chemical changes and speciation in molten chloride and/or fluoride salts.In preparation for future experiments involving irradiation of candidate molten salt fuels, the redox potential measurement and control methods in molten NaCl-CaCl2-UCl3 at 500oC were studied. The goal was to identify a metal/metal chloride redox buffer that can be used to control the redox potential, thereby helping to mitigate the corrosion risks posed by irradiation of the uranium salt fuel. An ideal candidate would both adequately reduce the redox potential and possess a low neutron interaction cross section.The first metal/metal chloride redox buffer investigated was Zr/ZrCl4. ZrCl4 has a standard state reduction potential of -2.02 V vs. Cl-/Cl2 at 500° C. The standard state reduction potentials for UCl3 and CrCl2 are -2.41 and -1.55 V, respectively. Zr metal should, thus, oxidize to ZrCl4 and control the redox potential at a level that simultaneously prevents Cr corrosion while leaving UCl3 unreduced from the salt.In order to test Zr metal for redox control, a 6.2-mm diameter Zr rod (99%) was submerged in molten NaCl-CaCl2-UCl3 salt at 600°C. The open circuit potential (OCP) of the salt was measured at a tungsten working electrode vs a Ag/AgCl reference electrode encased in a mullite tube. NiCl2 was added to the salt to simulate the effect of fission on redox potential. After submerging the Zr rod, the measured open circuit potential between the W WE and the RE dropped by -0.63 V over a period of 100 minutes.There is concern as to whether the rate of reaction of insoluble Zr metal with the salt can keep up with the rate of potential change due to the irradiation of the salt. Other redox buffer candidates being considered include uranium, calcium, and sodium. The latter two choices are attractive because of their predicted solubility in the molten salt, supporting homogenous redox control reactions. The problem with the latter three choices is that amounts contacted with the salt will need to be precisely controlled to avoid reduction of UCl3. OCP measurements will also be reported for molten NaCl-CaCl2-UCl3 using U, Na, and Ca as the redox buffer. Conclusions will be drawn regarding the most effective metal to use in the future VTR experiments in order to control the redox potential of the irradiated salt.