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E. W. MURBACH and W. N. HANSEN' Atomics International, A Division of North American Aviation, Inc., Canoga Park, Calif.
Pyroprocessing Thorium Fuels Arc melting and electrorefining are promising high temperature methods for partial decontamination of thorium-uranium alloy reactor fuels
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P R O G R A M is in progress a t Atomics International to investigate decontamination of irradiated thorium fuels on a gram scale and to evaluate the methods for scale-up operation. When possible, a n attempt is made to derive useful data from the experimental results, such as free energy values and vapor pressures. T h e reprocessing methods have mostly involved high temperature techniques, and research has been directed toward the problem of containing molten thorium. T h e processes do not yield decontamination results comparable to those for solvent extraction, but the product material is suitable for remote refabrication techniques and re-utilization in a reactor. Two methods are considered, based on differences betLveen thorium and fission products-electrorefining, which involves chemical activity, and arc melting, which involves volatility.
Electrorefi ning I n electrorefining, irradiated thorium is dissolved anodically into a molten salt bath along with the more electropositive fission products. T h e more noble fission elements should not dissolve but remain as a n anode sludge. Thorium is deposited a t the cathode, leaving the more electropositive elements in the electrolyte as ions. Thus, electrorefining should yield reasonable decontamination factors. An equation was derived relating the decontamination factor to the standard electrode potentials of the metal ions under consideration : nF D = exp - (E:% -- ETb)
RT
D = amt. of M originally present amt. of M deposited
factors are theoretically possible ; however. these results were calculated for a theoretical cell operating reversibly. I n practice, the cell would not be reversible, and polarization, as well as other effects: undoubtedly occurs. A suitable experimental system, using a nonvolatile bath containing salts of very active metals, which are compatible with the container, was required. A liquid cathode alloy facilitates removal of the product metal with occluded salt from the contaminated bath. Because i t is impractical to operate a cell a t the melting point of thorium, a low melting alloy is required. T h e inert atmosphere furnace \\-as resistance-heated, and the salt baths \\-ere chlorides and fluorides of the alkali and alkali earth metals \vith the corresponding thorium salt. Containers \yere of zirconia, nickel, and graphite, and cathode materials were manganese, iron, and zinc. A dripping cathode has been used in uranium electrorefining by depositing uranium on nickel, forming a low melting alloy which dripped from the cathode ( 2 ) . This method was attempted for thorium electrorefining, using iron and manganese as cathodes. I n one experiment with an iron cathode and a cell temperature of 1000' C., several grams of iron-thorium alloy were produced. However, zinc as a molren cathode has several advantages. I t has a low melting point and therefore the cell can be operated a t temperatures considerably below 1000' C. T h e zinc should be easily separable from thorium because of its low boiling point, and the lowr:r operating temperature allows use of a chloride bath.
Reinol-ccl cs
99.5 99.5 90.0 85.0 79.0
Sr Ru R.E.
Ce Zr
60.0
Most of these results are several orders of magnitude lower than calculated decontamination factors. However, they were not obtained under
Table 1.
Theoretical Decontamination Factors for Separation of Metals from Thorium b y Electrorefining Standard E.M.F.'s for Decontamination Chlorides, AE, Factor, D , Metal Ion V. at 800' C." E:% - E&, T'. n/O 21AE = 10
where n is number of electrons, F , Faraday's constant, R, gas constant, T , temK., E& standarddecomperature in position potential of impurity metal, and ET^, decomposition potential of thorium. With this equation, developed for one bath throughput, theoretical decontamination factors were calculated for a number of metals from thorium (Table I). Excellent decontamination Present address, Brigham Young University, Provo, Utah.
The electrolysis cell is graphite supported inside a Vycor tube heated by a resistance furnace. T h e crucible functions as the anode and the cathode compartment is a recrystallized alumina crucible which contains the molten zinc. T h e cathode lead is protected from the bath by a ceramic sleeve to prevent the deposit of dendritic thorium. Argon purified by passage over hot uranium turnings flows through the Vycor tube. T h e salt bath is 45 mole yc SaC1-45 mole % KC1-10 mole % ZnCI?. Several experiments with unirradiated thorium ivere carried out in this apparatus. After successful operation was proved, an experiment was conducted using irradiated thorium-1 0 \
