مجله علوم و فنون هسته ای
سال چهل و چهارم شماره 3 (پیاپی 105، پاییز 1402)

The load control of a nuclear reactor is important due to the nonlinear nature of its dynamics and the dependence of some parameters on the output power. the Proportional-Integral-Derivative controller (PID) is commonly regarded as an easy choice for reliable control. In this research, the relative neutron density in the point kinetics model of a Pressurized Water Reactor (PWR) is controlled by an optimized PID with the meta-heuristic Differential Evolution (DE) algorithm. The Integral of Time-Absolute Error (ITAE) performance index has been used for optimization with this algorithm. The simulation results show that the optimized control system with the DE algorithm has the appropriate efficiency and accuracy in response to power demand.

In a polywell reactor, it is critical to have a stable and energetic virtual cathode that is necessary to accelerate ions and create fusion interactions. Increasing the confinement time of virtual cathode electrons is of critical importance in the performance of polywell reactors. In this paper, COMSOL Multiphysics software was used to perform a three-dimensional numerical simulation in order to investigate the impacts of effective variables of a polywell on the virtual cathode. The findings indicated that the confinement time depends on the distance between the coils, coil radius, coil current, and the kinetic energy of the injected electrons. In addition, by using the simulation results, the dependence of the mentioned parameters on the confinement time is obtained, and then a mathematical model was developed.

In order to modeling the gas behavior inside the rotor of a gas centrifuge, a powerful tool is needed to overcome the computational constraints of gas inside the centrifuge along with the ability to apply all drives at the all flow regimes. Due to the suitability of the DSMC method for all flow regimes formed inside the centrifuge, in this paper, the DSMC method is used to analyze the separation performance of a centrifuge in axisymmetric coordinates. For this purpose, using a multi-scale CFD-MD method, first the momentum accommodation coefficients required to use the Cercignani-Lampis-Lord boundary condition are extracted and then, based on that, the behavior of the gas inside a centrifuge machine is investigated using the DSMC method and the amount of separation power is determined. Based on the comparison of the simulation results with the experimental test results, it was shown that using the proposed hybrid method to determine the amount of momentum accommodation coefficients used in the CLL boundary condition of the DSMC method, can increase the accuracy of determining the amount of separation power of the machine up to 8%.

This research investigates the transfer factor in the soil to plant (as one of the important parameters in safety analysis calculations and determining the amount of radiation of people) of strontium ion (a conventional radionuclide in pollution caused by nuclear accidents) to two plant samples including radish (Raphanus sativus var. sativus) and watercress (Lepidium sativum) as highly consumed plants in the food chain grown in the southern soil of Tehran. In this research, experiments were carried out in pots with a diameter of 15.5 cm and a height of 15.7 cm. These pots contained 2 kg of soil mixed with compost under the natural sunlight conditions of Tehran. In order to investigate the effect of strontium ion concentration on the transfer factor, the desired soil was first mixed with compost at a ratio of 3:1 and, using strontium nitrate salt solution in four different concentrations (50-200-500-1000 mg/kg) was infected. Pots were irrigated weekly with a specific volume of water at two different acidity levels (pH = 5 and 6) to investigate the effect of pH on the transfer factor. Also, the concentration of strontium ions in two tissues of the leaf and root of radish and watercress was done. This was examined to determine the accumulation of strontium ions in each of the tissues and their influence on the transfer factor. The results showed that with the increase in soil strontium concentration, the amount of adsorption of this element in both plants increases and the transfer factor decreases. Also, the plant had more adsorption in the soil with lower pH and a higher transfer factor was reported. The comparison of transfer factor and strontium adsorbed between the two plants also showed that radish adsorbed more strontium than watercress and had more transfer factor. Also, using an experimental method, the distribution coefficient, which is a crucial and practical parameter in pollutant transfer modeling and pollution risk assessment in soil and water resources, was determined by using the transfer factor from soil to plant. In this research, the possibility of decontamination of strontium from the soil by applying radish was investigated. The results showed that this plant could be used as a purifier to purify soils contaminated with strontium.

Carbon-12 ion beams in the energy range of 100-450 MeV/u have excellent conditions for radiation-resistant and deep-seated tumors. At energies above 400 MeV/u, radiation is significantly affected by nuclear fragmentation processes of increasing depth. In this project, using the Geant4 toolkit, three physical models of Binary Intranuclear Cascade (BIC), Lige Intranuclear Cascade (INCL), and Quantum Molecular Dynamics (QMD) for heavy particles are defined in this toolkit. By examining the QMD model available in the Geant4 toolkit, the effects of heavy particles because of fragmentation of the nucleus by calculating the abundance of nuclei produced with an atomic number in the range 5>Z>1 (H, He, Li, Be and B particles) at different depths of the water phantom before and after the Bragg peak and the angular distribution of these particles have been investigated by this model. The particle abundance graph shows that particle production decreases with an increasing atomic number. H and He particles are the most abundant and their range is much broader than the range of primary carbon ions. Also, according to the angular distribution diagram, H and He particles have shown a much wider distribution. This is because they have the highest dose of deposition in the area outside the treatment field. Also, with increasing atomic numbers, the angular distribution decreases.