[en] For the purpose of computing criticality parameters of the solutions with uranium and plutonium, the atomic number density was formulated and programmed in the computer. The amount of solvent in the solution was calculated from solute concentration and density of the solution. The numerical expression of the solution density was based on the literature data for aqueous nitrate solution, and on theoretical consideration for 30% TBP-n. dodecane solution. The calculated values of solution density were discussed compared with that in ARH-600, the criticality handbook in the United States. (author)

[en] For the purpose of criticality experiments of the nitrate solution systems composed of uranium and/or plutonium, the designs of Criticality Safety Experimental Facility(CSEF) are under-going in JAERI. In this report, by using the developed calculational code for the atomic number density of the nitrate solution of U/Pu mixture, the ratio H/Fissile has been calculated as a function of fuel concentration and acidity. Critical parameters such as infinite multiplication factor, effective multiplication factor, and the critical diameter of infinite cylinder have been evaluated with Monte Carlo code KENO-IV of JACS system. The dependence of multiplication factors on the formula for the atomic number density is described by comparing the calculated results with our formula and those with ARH-600 formula. Based on our formula, the effects of acid molality and Plutonium valence state on the criticality of nitrate solution have been discussed. (author)

[en] Build up of Pu in a co-decontamination process of fuel reprocessing due to shift of process parameters from normal conditions and its effect on nuclear criticality of the system were investigated with computer codes. Being based on the flow sheet of a Purex reprocessing plant, the calculation shows that Pu accumulates mainly in the aqueous phase of extraction stages under the conditions in which concentration of U increases in the organic phase, i.e. a decrease of the extractant flow rate or the TBP concentration, an increase of feed flow rate or U concentration, or a decrease of scrub acidity. Relating the maximum Pu concentration and the time required for maximum build up to the change of the flow sheet conditions, an effective neutron multiplication factor k_eff of the system involving Pu build up was calculated for the first mixer-settler bank of a model plant. The analysis revealed that the extractor would become critical in approximately 45 h at earliest provided that shift of the parameters stayed within a specified range during operation. Required performance of the process control instruments, safety margin in concentration limit, and other operation safety considerations necessary for the prevention of Pu build up in the extraction bank are discussed. (author)

[en] In the Purex solvent extraction process, change of process conditions, such as a decrease of solvent flowrate, causes an accumulation of Pu(IV) in a mixer settler bank. When the concentration of Pu(IV) exceeds a certain concentration limit in the organic phase, a heavy organic phase, the third phase, forms. These phenomena, the accumulation and the third phase formation, remarkably affect concentration profile of Pu, liquid-liquid interfacial level of mixer settlers and composition of the solvent. In the present study, some parameters which influence the criticality, k_eff, were examined. Effects of matrix solvent, concentration profile of Pu, interfacial levels and TBP content in the organic solvent on the k_eff value were examined by the Monte Carlo Code MULTI-KENO with MGCL 26 and 137 groups of constants library made from ENDF/B-IV. The result of the analysis showed that the differences of k_eff value between the organic and the aqueous solutions were as small as approx. 1%, and the accumulation of Pu in a central part of system caused higher K_eff value, and TBP content higher than 30% in the organic solvent showed lower k_eff value than that of 30% TBP solvent. (author)

[en] Build-up of Pu in a partitioning process of Purex fuel reprocessing at the abnormal conditions and its effect on nuclear criticality of the system were investigated with computer codes. Being based on the flow sheet of a model mixer settler process, the calculation shows that a considerable increase of Pu concentration occurs solely in the aqueous phase of the 1BX bank under the condition that flow rate of Pu strip solution decreases. The decrease of Pu strip flow causes not only a decrease of the total aqueous flow which consequently concentrates Pu(III) in the aqueous phase, but also an increase of acidity of the aqueous and the organic phases which changes the kinetics of the redox reactions of Pu with U(IV) and nitrous acid. Kinetic data of the reoxidation of Pu(III) with nitrous acid were critically evaluated to be used in calculation. The analysis revealed that the model 1BX bank would become critical in approximately 80 h if Pu strip flow would cease. Requirements to assure criticality safety of the partitioning process, e.g. prevention and detection of the cessation of Pu strip flow, detection methods of abnormal distribution profile of Pu or U, were discussed. (author)

[en] We prepared 150kgU of 10%²³⁵U uranium solution for the critical assemblies (STACY, TRACY) by dissolution of mixture 1.5%²³⁵U uranium dioxide pellets and 12%²³⁵U uranium dioxide pellets with the fuel treatment system of NUCEF. In order to find optimum operation conditions for dissolution, we carried out preliminary experiment using each one pellet and characteristic experiment using dissolver. In this report, results of these experiments and operation were described. As a result of these experiments, we obtained following operation conditions; initial nitric acid 7M, temperature 80degC, operation time 8h. Under these conditions, we dissolved UO₂ of over 99% satisfactorily and prepared 150kgU of fuel solution. (author)

[en] The mock-up test program for dissolution of plutonium dioxide was planned to determine the preparation method of plutonium nitrate solution for criticality experiments at the Nuclear Fuel Cycle Safety Engineering Research Facility (NUCEF). The program consists of dissolution of plutonium dioxide by electrolytic oxidation method, valence adjustment and extractive purification of plutonium from ²⁴¹Am and silver. This report describes the design conditions, specifications and performance of the experimental apparatus and the glove box. (author)

[en] Extraction experiments were carried out to study characteristics of the third phase formation in the 30 % TBP-n-dodecane-U(IV)-HNO₃ system. The third phase is formed by mixing 10 ml of aqueous solution and 10 ml of TBP-n-dodecane. Volumes and concentrations of U(IV) and HNO₃ of the aqueous, light organic and the third phases were measured after an equilibrium. Concentrations of TBP, n-dodecane and H₂O in two organic phases were also measured. The numerical equations which calculate the volume and the concentrations of U(IV) and HNO₃ not only for the third phase but also for the light organic phase were deduced by the regression analysis method as functions of concentrations of U(IV) and HNO₃ in the initial aqueous solution. The results of present study are as follows: 1) U(IV) concentration and volume of the third phase increased from 70 to 190 g/l and from 1.5 to 3.5 ml, respectively, with increasing in U(IV) concentration of the initial aqueous solution, from 40 to 130 g/l. 2) There were good correlation between U(IV) concentration of the initial aqueous solution and U(IV) concentration of the third phase, and between volume of the third phase and U(IV) concentration of the initial aqueous solution. 3) The TBP concentrations of the third phase (50 to 80 %) were correlated to U(IV) (40 to 130 g/l) and HNO₃ concentrations (1.5 to 6 mol/l) of the initial aqueous solution. 4) The concentrations of U(IV) and HNO₃ in the equilibrium aqueous phase were equal to the values calculated with an equation for distribution ratio without taking into account the third phase formation. (author)

[en] Calculations of the criticality parameters have been performed for solution fuels such as uranyl nitrate, plutonium nitrate and their mixture by use of a newly proposed formula (SST formula) of the solution density. By comparison of the calculated infinitive multiplication factor and critical buckling using SST formula with those by the present formula based on Maimoni's and Burger's, the effect of the density formula for nitrate solution on the criticality calculation were studied. (author)

[en] Accumulation of the actinides in the extraction stage of the Purex process was investigated with mathematical models in terms of the U(VI)-loading effect, i.e. decrease of distribution ratios with increase of concentration of U(VI) in the organic phase. Time excursion of the accumulation profiles of U, Pu, Np and HNO₂ under several flowsheet conditions was estimated for a mixer settler process with a simulation code. Process experiments using U(VI) and U(IV) were carried out with mixer settlers to verify the simulation results. The concentration profiles of U(IV) and U(VI) agreed fairly well between the experimental and the calculated. Behavior of the peaking position of U(IV) build-up, however, did not always agree well. The causes of the discrepancy were discussed in terms of mass balance of U(VI). Fluctuation of the volume ratio of the two phases in the extractor during operation, particularly during the transient, affects significantly the total mass of U(VI) present in the extractor and hence the concentration profile of the element accumulated. The role of the third phase of Pu (IV) to criticality safety was mentioned. (author)