Crud in Solvent Washing Process for Nuclear Fuel Reprocessing

In a solvent washing process for nuclear fuel reprocessing, one of the important problems is a formation of stable emulsions between organic and aqueous phases. These emulsions are called interfacial “crud”. Crud is defined as an emulsion stabilized by finely dispersed solids. These stable emulsions lead to decreased washing efficiency, lower phase separation, disturbance of the interfacial control at the settler of the extractor, and so on. Cruds formed by precipitates of Zr and tributyl phosphate (TBP) degradation products, such as di-n-butyl phosphate (HDBP), mono-n-butyl phosphate (H₂MBP), and phosphoric acid (H₃P0₄) are studied by experiments using a sodium carbonate solution as a washing reagent. Experimental results show that not only pH value of the washing reagent, but also phosphate and zirconium mole ratio (P/Zr) are important in crud formation. Moreover, it is shown that the complex of Zr and HDBP, or Zr and H₂MBP has a significant role in stabilizing emulsions. However, the complex of Zr and H₃P04 is not effective in stabilizing cruds.     

Adsorption of Molybdenum Hexafluoride on Magnesium Difluoride for Uranium Purification in FLUOREX Reprocessing

The nuclear fuel reprocessing method FLUOREX is a hybrid system based on fluoride volatility and using solvent extraction. Spent nuclear fuel is fluorinated, and most of the uranium is recovered as UF₆ gas. UF₆ contains some volatile fission product (FP) fluorides, so we considered their elimination from UF₆ by adsorbing them on fluoride adsorbents. We experimentally examined the adsorption of MoF₆ on MgF₂ adsorbent; MoF₆ is present as a volatile FP fluoride in UF₆ produced by the fluorination of spent nuclear fuel. The adsorption isotherm of MoF₆ adsorption on MgF₂ was obtained at MoF₆ pressures from 10^?4 to 50 kPa. The saturated adsorption amount was 1:3 ± 0:4 mg/m² at MoF₆ pressures from 10^?4 to 1 kPa. At MoF₆ pressure of about 10^?3 kPa, the saturated adsorption amount had no dependence on adsorption temperatures from 398 to 463 K. We deduced that MoF₆ was adsorbed as a monomolecule layer on the MgF₂ surface at MoF₆ pressures from 10^?4 to 1 kPa, and the MoF₆ partial pressure in UF₆ could be decreased below 1 × 10^?4 kPa, which is the specific MoF₆ concentration for the reenrichment process.     

Fundamental Study of the Sulfide Reprocessing Process for Oxide Fuel (I) Study on the Pu,MA and FP Tracer-Doped U3O8

For the recovery of fuel materials from spent nuclear fuel, a novel reprocessing process based on the selective sulfurization of fission products (FP) has been proposed, where FP and minor actinides (MA) are first sulfurized by CS₂ gas, and then, dissolved by a dilute nitric acid solution. Consequently, the fuel elements are recovered as UO₂ and PuO₂. As a basic research of this new concept, the sulfurization and dissolution behaviors of U, Pu, Np, Am, Eu, Cs, and Sr were investigated by γ-ray and α spectrometries in this paper using ²³⁶Pu-, ²³⁷Np-, ²⁴¹Am-, ¹⁵²Eu-, ¹³⁷Cs-, and ⁸⁵Sr-doped U₃O₈ samples. The dependence of the dissolution ratio of each element on the sulfurization temperature was studied and reasonably explained by combining the information of the sulfide phase analysis and the chemical thermodynamics of the dissolution reaction. The sulfurization temperature ranging from 350 to 450°C seems to be promising for the separation of FP and MA from U and Pu, since a clear difference in the dissolution ratio between FP and U was derived by the sulfurization treatment in this temperature range.     

EPR Study of Uranium (V) Species in Photo-and Electrolytic Reduction Processes of UO2 NO3)2-2TBP

Abstract

Electron Paramagnetic Resonance (EPR) and optical spectra of uranium(V) species were observed in both processes of photo- and electrolytic reduction of UO₂(NO₃)₂-2tributylphosphate (TBP) in 80%TBP-n-dodecane solution. The formation of U(V) species was detected by an optical spectrum (λ_max: 770, 970 and 1,420 nm). EPR signal with the value of ff-factor –2.3 and a linewidth of approximately 1,100 Gs was observed during the electrolytic reduction. On the other hand, during the photoreduction the signal with the value of fil-factor –1.94 was observed and there was found a superhyperfine structure with the intensity ratio of 1:2:1, that is caused by the superhyperfine coupling with nuclear spin, I=1/2, of the strongly coordinated ³¹P to the central uranium through oxygen atom. The superhyperfine coupling constant was estimated to be 27 Gs. Moreover, the signal with the value of g-factor –2.00 due to an organic radical was observed. The residue after the thermo-gravimetric analysis of UO₂(NO₃)₂-2TBP was identified as α-UP₂O₇ by the powder X-ray diffraction analysis, indicating the strong coordination of TBP to the central uranium atom.