m.h. mallah

This paper aims to produce uranium with 20% enrichment for research reactor fuel directly from natural uranium with waste xw≤0.3% through a cascade. This purpose has utilized five strategies, including single-optimized tapered, square, 2-section squared-off, 3-section squared-off, and 4-section squared-off cascades. To design and optimize the cascades, five software codes, "STC-PSOA," "SQC-PSOA," "2SQC-PSOA," "3SQC-PSOA," and "4SQC-PSOA," have been developed, with the PSO optimization algorithm employed for optimization. In the first strategy, it is not possible to achieve uranium with a 20% enrichment level using one arrangement and one step. The maximum product enrichment level attainable is 3.73%. However, in the other four strategies, uranium with a 20% enrichment level can be produced in two separation steps with a single arrangement. Notably, when employing the strategy of using 4-section squared-off cascades, the product production is 11.1%, 3.1%, and 1.0% higher compared to square, 2-section, and 3-section squared-off cascades, respectively.

In the first year of fuel loading of nuclear power reactors, uranium enriched at different levels is used (in the type of VVER-1000 reactor, the levels of enrichment are 4.0, 3.6, 3.3, 0.3, 2.4, 2.2, and 1.3%). In this work, for the production of enriched uranium in the first cycle, two parallel methods have been used, one using square cascades and the other using symmetrical tapered cascades. Computational codes have been developed for the design and optimization of the cascades. The meta-heuristic PSO method has been used for optimization. The results show that: 1. if square cascades are used, it is possible to directly produce all uranium enriched with different levels; 2. if tapered cascades are used, it is possible to produce all uranium indirectly, and uranium enrichment at different levels is only possible with a mixing unit in the enrichment facility; 3. The total number of centrifuges or cascades used to produce enriched uranium shows a reduction of about 22–39% compared to the square cascades. 4. If gas centrifuges with high separation capacities are used in square and tapered cascades, the total number of gas centrifuges required for the production of enriched uranium will be close.

Among the noble gases, Xenon isotopes are the most widely used. So far, no specific strategy has been published for separating all Xenon stable isotopes with the highest number of stable isotopes. In this research, a computational code, SQCAS, is prepared to determine the target isotope separation strategy in light or heavy current. The prepared code investigates the separation of the natural isotopes in proportion to the feed concentration by changing the parameters of the square cascade and modeling it at each step. All stable isotopes of natural xenon are separated using a square cascade in 32 steps for the certain feed (200kg). The proposed square cascade has 20 stages. In each stage, 10 centrifuges were used. Also, the separation factor for the unit mass difference in this research equals 1.2. Using this strategy, the enrichment of nine isotopes increases from 0.9393, 0.0009, 0.01917, 0.2644, 0.408, 0.2118, 0.2689, 0.1044, and 0.087 to 98.72%, 91.3%, 92.79%, 92.63%, 83.77%, 90.12%, 91.64%, 95.68%, and 99.04%, respectively. The results show that the highest recovery is related to the end isotopes, and the lowest recovery is related to Xe-135 isotope.

In the enrichment industry, it is very important to study the transport coefficients such as thermal conductivity, viscosity and diffusion coefficients of isotope gases such as xenon, tellurium hexafluoride and uranium hexafluoride. In general, there are two perspectives for studying the behavior of gas: the macroscopic and the microscopic perspective. The macroscopic model considers the behavior of a gas as continuous and the microscopic model, considers the gas as separate particles and for each particle, a position and velocity at a specific time. In this paper, the thermal conductivity, viscosity and diffusion coefficients of single-component, binary mixture, multicomponent and isotopic gases are investigated using the microscopic properties of gases. Then the values of these coefficients are determined for all isotopes of xenon, tellurium hexafluoride and uranium hexafluoride. The COT POD software has also been prepared by these relationships extracted for transport coefficients. Due to the lack of experimental results for isotopic validation, the results of microscopic relationships obtained from the software are compared with the experimental results of a mixture of different gases including helium and neon and then neon, argon and krypton multi-component systems. Results have a good agreement with each other.

In the present work, the performance of a square cascade in the separation phase of tellurium stable isotopes is studied. In this cascade, the selection of an appropriate feed stage, the first stage cut, and the cut of the cascade are the effective parameters. The results showed that in the various separation factors, selection of the middle feed stage leads to appropriate results. Also, if the cut of the first stage is selected in such a way that the cuts of stripping section are obtained the same, it is one of the cut-offs for which the two grouping parameters of components will have the highest possible value. Based on the selection of this cut, one of the possible strategies available through using the code-named SQCASSIM for the separation of tellurium stable isotopes to the enrichment of more than 90% is presented.

In this study, recovery of uranium from leach liquor using solvent extraction technique in a horizontal pulsed sieve plate column was experimentally investigated. Extraction of uranium from an aqueous sulfate solution with 0.25M concentration acid, by using TOA solvent, diluted in kerosene and n-decanol modifier of 5%, 90% and 5% v/v respectively was studied and the influences of operation parameters including the phase flow rates along with pulsation intensity on the dispersed phase holdup, drop size and drop size distribution, as well as extraction efficiency, were evaluated. Increasing pulsation intensity along with the dispersed phase flow rate and decreasing the continuous phase flow rate caused to increase the extraction efficiency. Results showed that the horizontal pulsed sieve plate column has high efficiency for extraction of the uranium from leach liquor. Moreover, a comparative evaluation of available correlations proposed for the prediction of mean drop size and hold up in such columns have been conducted. New correlations for estimating mean drop size and hold up in a horizontal pulsed sieve-plate column were presented, which provided AARE of 7.7% and 8.2%, respectively.

In the present research, we addressed the investigation parameters of the two proposed cascade models for separation of multi-component mixtures (Matched-X and Pseudo binary type 1 cascades) to separate Te-123 isotope. Besides, we compared their performance by changing parameters k, k1, and k2. The results showed that for a cascade with 10 stages and feed stage 5 and feed flow rate 100 g.s-1, the minimum total interstage flow of the matched-X cascade occurs in the condition k = 7, k1 = 1 and k2 = 3, with the amount of 1881.133 g.s-1. In this case, the concentration of Te-123 in the product stream of the cascade is equal to 0.0349 while the maximum product concentration of Te-123 occurs at k = 1, k1 = 1 and k2 = 2 with the amount of 0.0389. In this cascade, the total inter stage flow is equal to 2159.914 g.s-1. Also, the results represented that in the Pseudo binary type 1 cascade, the minimum total interstage flow and maximum product concentration of Te-123 occur at the same time, in the condition k = 1, k1 = 1 and k2 = 2 with the amounts of 2159.914 and 0.0389 g.s-1, respectively.