D.D. Sood

CHEMISTRY OF NUCLEAR FUEL REPROCESSING: CURRENT STATUS

Current status on the chemical aspects of nuclear fuel reprocessing is presented with special emphasis on the Purex process which continues to be the process of choice for the last four decades. Better decontamination from fission products, new methods for uraniumplutonium partitioning and removal of actinides from high active waste are challenging areas in process chemistry. The development work on TRUEX and DIAMEX process for treating high active waste is briefly described. An overview of pyrochemical processes, which are important for Integral Fast Reactor Concept, is presented.

A study of chemical parameters of the internal gelation based sol-gel process for uranium dioxide

Abstract Internal gelation process is one of the important sol-gel routes for the preparation of spherical particles of fuel materials. Successful preparation of defect free fuel particles has been reported only with a narrow range of feed solution compositions. Investigations have been carried out to study the gelation behaviour of solutions containing uranyl nitrate, hexamethylene-tetramine (hexa) and urea with a view to defining the regions of possible interest to the process. A gelation field diagram has been constructed defining regions where a single phase gel can be readily obtained. A number of compositions from this gelation field diagram have been used for the preparation of UO 2 microspheres and it was observed that good spherical particles could be obtained with uranium concentrations ranging from 0.7 to 1.5 molar. The mole ratio (hexa, urea)/uranium for obtaining good particles decreased with increasing uranium concentration.

Synthesis of yttrium aluminium garnet by the glycerol route

Abstract Yttrium aluminium garnet (YAG), represented by the chemical formula Y 3 Al 5 O 12 , was synthesised from the stoichiometric mixture of the metal nitrate solution by boiling with glycerol. For a fixed batch size, the glycerol amount was systematically varied to study its effect on the specific surface area and phase purity of the YAG powder. The quantity of glycerol was optimised to get the desired powder characteristics. Using this technique, single phase YAG powder of high specific surface area was obtained at 1000 °C. Characterisation was done by thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), particle size analysis and by measuring the specific surface area, as well as the carbon content, at various stages, to ascertain the quality of the product.

Determination of thermal properties of Cs2Cr2O7(s, 1) by high temperature Calvet calorimetry

Enthalpy increments have been determined for caesium dichromate in the temperature range 335 to 826 K using a high temperature Calvet micro calorimeter. A solid-solid transition has been observed at (620.5 ± 1.5 K) and the melting temperature was found to be (657.0 ±1.0) K. The corresponding enthalpy values are (15.6 ± 0.2) kJ/mol and (17.0 ± 0.22) kJ/mol. The results thus obtained are utilised for the evaluation of molar heat capacities, standard entropies and free energy functions for Cs2Cr2O7(s, l). The enthalpy increment values were fitted to a polynomial and can be represented by eqs. (1) to (3), respectively: (HTo − Ho298.15)(Cs2Cr2O7, s) (J/mol) = −6.410 × 104 + 1.939 × 102T(K) + 7.441 × 10−2T(K)2 (335 to 620.5 K), (1) (HoT − Ho298.15)(Cs2Cr2O7, s) (J/mol) = 1.313 × 106 − 4.137 × 103T(K) + 3.517T(K)2 (620.5 to 656 K), (2) (HoT − Ho298.15)(Cs2Cr2O7, l) (J/mol) = − 1.218 × 105 +3.890 × 102T(K) (657 to 826 K). (3)

Determination of standard molar Gibbs' energy of formation of Sr3MoO6(s)

An electromotive oxygen concentration cell using calcia-stabilized zirconia (CSZ) electrolyte has been constructed and used to determine ΔfGmo(SrMoO4, s, T). The e.m.f. of cell (II), Pt/SrMoO3(s) + SrMoO4(s)/CSZ/air/Pt, could be represented by E (V) ± 0.006 = 1.515-4.299 × 10−4T (K), while the e.m.f. of cell (III), Pt/SrMoO3(s) + SrMoO4(s)/ CSZ/FexO(s) + Fe(s)/Pt, could be represented by E (V) ± 0.002 = 0.1459 - 5.859 × 10−5T (K). The e.m.f. values were combined with ΔfGmo(T) of SrMoO3(s) and FexO(s) from our earlier studies to get ΔfGmo(SrMoO4, s, T) values as ΔfGmo(SrMoO4, s, T) (kJmol−1) ± 1.2 = −1582.6 + 0.3692T (K) (1046.0 ⩽ T ⩽ 1255.5 K) for cell (II) and ΔfGmo(SrMoO4, s, T) (kJmol−1)± 0.4 = −1583.0 + 0.3699T (K) (1037.5 ⩽ T ⩽ 1278.0 K) for cell (III). From the variation in ΔfGmo(SrMoO4, s, T) (kJmol−1) with temperature and the relevant heat capacity values from the literature, ΔfHmo(298.15 K) of SrMoO4(s) has been obtained as −(1582.9±0.5) and −(1583.3±0.1) kJmol−1 respectively and the average value is −(1583.1±0.6) kJmol−1.

Synthesis of yttrium aluminium garnet by the gel entrapment technique using hexamine

Abstract A novel technique has been developed for the preparation of yttrium aluminium garnet (YAG), represented by the chemical formula Y3Al5O12, by achieving the homogeneous precipitation of the constituent metal hydroxides from the stoichiometric mixture of the metal nitrate solution by the addition of hexamethylene tetramine solution. Aluminium ions form a gel at pH 4, entrapping all the liquid, followed by the precipitation of yttrium ions at a higher pH. In spite of the sequential precipitation, the resultant mass is homogeneous because of the gel entrapment. Using this technique, singlephase YAG powder of high specific surface area (80 m2 g−1) was obtained at 810 °C. Characterisation was done by thermogravimetry, differential thermal analysis, X-ray diffraction and scanning electron microscopy to ascertain the high quality of the product. The technique was subsequently used for the production of highpurity neodymium-doped YAG powder with excellent microhomogeneity on an increased batch size of 1 kg.

Fabrication of UO2 pellets by gel pelletization technique without addition of carbon as pore former

Abstract High density UO 2 pellets for pressurised heavy water reactor fuel were fabricated by a gel pelletization technique from soft UO 2 microspheres prepared by an internal gelation process. Chemical and heat treatment parameters were determined for obtaining soft UO 2 microspheres with optimum properties required for making defect-free, high density, UO 2 pellets. Investigations reveal that a high uranium concentration in the feed broth during the preparation of UO 3 gel particles, conversion to UO 2 via U 3 O 8 , and optimized calcination and reduction temperatures, result in UO 2 microspheres suitable for obtaining good quality green compacts. Green pellets were densified by low temperature sintering in a C0 2 atmosphere at 1200°C to obtain high density UO 2+ x pellets and reduced at 800°C to UO 2.00 in Ar + 8%H 2 Resulting pellets were free from open porosity and black berry structure.

Oxidation and hydrolysis kinetic studies on UN

Abstract The reaction of oxygen and water vapour with UN microspheres containing 0.78 and 10.9 mol% UO2 as impurity was studied under non-isothermal heating conditions in a thermobalance under different partial pressures of oxygen, a fixed pressure of water vapour in argon, and in air. Uranium mononitride was ultimately converted to U3O8, with the formation of UO2 and U2N3 as intermediates. The end product of pyrohydrolysis was UO2. The kinetic parameters were evaluated and the mechanism of the reaction was suggested. Different kinetic models were used to explain the oxidation behaviour of UN.

Phase diagram and thermodynamic calculations of alkali and alkaline earth metal zirconates

Abstract The ternary phase diagrams and partial pressures of various gaseous species over the equilibrium phase fields have been calculated for the MZrO (M = Li, Na, K, Rb, Cs, Sr and Ba) systems by using the SOLGASMIX-PV program, which computes equilibrium composition by direct minimization of the Gibbs energy of a system. The available experimental Gibbs energy data reported in the literature for binary and ternary compounds were used for these calculations. Where no data exist, values were estimated. These ternary phase diagrams are being reported for the first time, except for the lithium system.

Kinetic study of the carbothermic synthesis of uranium monocarbide microspheres

Abstract Uranium monocarbide microspheres were synthesized by carbothermic reduction of porous uranium oxide microspheres with uniformly dispersed carbon black. Kinetics of the reduction was studied under vacuum and flowing inert gas from 1250 to 1550° C. The carbon monoxide gas concentration in the effluent stream during reduction was used to determine the rate of carbide formation. Under vacuum, reduction was found to be controlled by reaction at the reactant-product interface whereas under flowing gas conditions, the diffusion of carbon monoxide gas through the carbide layer was the rate controlling process. The activation energy was 335.1 ± 8.6 and 363.7 ± 7.6 kJ/mol for reduction under vacuum and flowing gas, respectively.