مجله علوم و فنون هسته ای
سال سی و پنجم شماره 3 (پیاپی 69، پاییز 1393)

The carbon nanotube membranes which have unique properties can make dramatic changes in adsorption and separation of compounds. Adsorption of pollutants from uranium chemical processing units is one of the most important environmental issues in this industry. Therefore, the adsorption isotherms of UO2, F2, UF6, UF4 and UO2F2 into carbon nanotube were studied by using the Monte Carlo simulation in order to evaluate the performance of carbon nanotubes membranes in the uptake of compounds of uranium chemical processing units. Also in this study, the effects of pressure and diameter of the carbon nanotubes on the adsorption of compounds were investigated. The results indicate that the adsorption of compounds onto the carbon nanotubes is as follows: UO2 > UF4 > UO2F2 > UF6 > F2. In other words, the maximum and minimum adsorption onto the carbon nanotubes are related to UO2 and F2, respectively. The effects of pressure and diameter of the carbon nanotubes on the adsorption of compounds show that the adsorption isotherm of all gases increases with increasing pressure and diameter of the nanotubes and the pressure influence on the adsorption of the compounds depends on the type of component and the carbon nanotubes diameter.

In this research, the effect of the initial concentration of zirconium, nitric acid concentration, volume percentage of organic solvent phase (tributyl phosphate) and the phase ratio on extraction (%) was studied in a batch method and the optimum conditions for continuous operation (Mixer-settler) of these parameters were introduced. In the experimental studies of zirconium extraction in a 6-stage horizontal laboratory mixer-settler, the aqueous solution (feed) was 4 M nitric acid mixed with 4 M HCl containing 60 gr per liter of zirconium. 50%V tributyl phosphate in kerosene, 1000 rpm impeller speed, with the equal phase ratio at ambient temperature and pressure was used for the extraction operations. In this research work, the extraction of zirconium at settler stages within 3 different times has been compared with the theoretical quantities obtained from the McCabe curve. The zirconium extraction (%) at each stage of the mixer-settler was close to the corresponding amount in equilibrium conditions, indicating a very good agreement between theoretical and experimental data.

In this study, the energy and angular distributions of nitrogen ions produced in a plasma focus device, SBUMTPF1, were determined using CR-39 nuclear track detectors and the results were compared with those of the polycarbonate track detectors. For this purpose, by the use of the SRIM software, the ranges of nitrogen ions were calculated in aluminum and subsequently by applying the PVD method, layers of aluminum filters of different thicknesses were deposited on the detectors. By the use of the coated films, the anisotropic distribution of nitrogen ions was investigated by the detectors. The variation of density of the ions was also investigated in different ranges of energy. The study of the SBUMTPF1 angular distribution profiles shows a peak value at an angle of 30˚ with respest to the anode axis. The results found to be consistent with those obtained for the polycarbonate track detectors.

SVHI-Co-60-T irradiation system has been used as a portable one with Co-60 source for onion irradiation. To optimize and for the change of the system application, sufficient knowledge of the absorbed dose of the products is necessary. In this paper, the absorbed dose in the irradiation chamber is calculated for different points using MCNP4C code and then the results are compared with the measurements by the Red, Amber and Clear dosimeters of the Radiation Application Research School, along the central line of the chamber. The values ​​obtained from dosimeters are in good agreement with the code results. The dose distribution in the irradiation chamber has been calculated on a plane with dimensions of 60cm×30cm. The minimum and maximum dose rates and the dose rate at the center of the same dose area of the isodose curve are 0.3kGy/hr, 2.6kGy/hr and 1.25kGy/hr, respectively. Also, the dose rate measured in the central area of the chamber using Red, Amber and Clear dosimeters is 1.37kGy/hr, 1.303kGy/hr and 1.302kGy/hr, respectively. Differences between the calculated dose using MCNP4C code and the measured dose emplaying three types of dosimeters are 8.7%, 3.8% and 3.8%, respectively. After loading, the spacers have been designed in front of new sources with MCNP4C code to have suitable dose with a uniform ratio to change its application from onion to wheat irradiation and also to compare with the measurements.

Neutron external source plays an important role during the start-up of a nuclear reactor. Therefore, the analytical solutions of neutron point kinetics equations in the presence of external source are important in predicting the variation of the neutron population during the start-up of a nuclear reactor. For a constant external source, an analytical solution is worked out, as shown in the previously published articles. Due to fluctuations of the neutron source around a mean value, the source is actually time dependent. Thus, in this research, an alternative analytical solution with one group of delayed neutrons is proposed with the ramp reactivity insertion for the external source with sinusoidal fluctuation during the start-up of the nuclear reactor. The only approximation made in this study is to ignore the second time derivative of the neutron population. The present study is fully in agreement with other studies regarding the limit of very small amplitude fluctuations.

The electron energy distribution function in fuel rod gap of Busheher’s VVER-1000 nuclear reactor was investigated using the Fokker-Planck equation. In this regard, the Fokker-Planck equation was solved by using the Runge-Kutta numerical method, and the non-equilibrium behavior of electrons passing through the gap, as an absorbing medium, was evaluated and analyzed. The programming language C++ was employed to acquire high accuracy. Besides, by utilizing the Monte Carlo GEANT4 code, the fission reaction in the fuel rod was simulated and the energy distribution of electron was calculated. Furthermore, it was found that most of the electrons are essentially thermal in the fuel rod gap and the electrons energy distribution is far from Maxwellian distribution. Also, the krypton effect on the energy distribution of electron was investigated. One of the outstanding points that should be mentioned here is that the krypton has no significant effect on the energy distribution function of electrons in the fuel rod gap.

HPGe detector has been simulated by using MCNP code, and the detector response has been determined. Monte Carlo simulations have been performed for different elemental composition and density as environmental Marinelli samples (z =10-90, ρ = 0.8-2 g/cm3). Likewise, more experiments were carried out in the laboratory scale using five samples with different self-absorption albeit with the same detector described previously. The effects of source elemental composition and density on the self-absorption in gamma-ray spectroscopy of environmental Marinelli samples are discussed. For both cases of experimental and simulated studies a surface plot of the efficiency vs gamma-ray energy and sample density has been calculated for the said detector. By comparing the results upon applying, two methods show that the simulation method has been successful leading to efficiency as a function of both gamma-ray energy and sample density. The same surface can be used to derive an efficiency curve for any sample density within the calibrated range of densities. Also, it is possible to use this three-dimensional curve to reduce the number of calibration standards of different required densities, where it leads to reducing the laboratory operating costs. The results showed that the sample density was more important than the composition in self-absorption problem. In addition, the effects of the sample density on self-absorption in all range of the energy under the study are visible, but the chemical composition effects appear only for the E<400 keV condition.

The minimum detectable activity (MDA) is the activity that a system can detect with a given confidence level and good discrimination against the background radiation. When low levels of activities are counted, the MDA of gamma spectrometer must be comparable to the number of sample counts. In this research, the MDA of AT1315 spectrometer has been experimentally determined and a theoretical correlation between the MDA and experimental parameters such as the sample mass and counting time has been established. Using the correlation, the frequent and cumbersome MDA measurements can be avoided. A quantitative comparison of the theoretical and experimental MDA approaches has been performed.

Given the importance of discovery of radiometric datings methods, especially two radioactive elements uranium and thorium, the radiometric airborne geophysical survey data (produced by the Atomic Energy Organization of Iran) and telemetry methods have been used to explore the radioactive elements. Initially, a high volume of the radiometric processing of digital data, classification and analysis were dealt with, using statistical methods, Abnormalities were separated from the background. With geological and remote sensing surveys conducted in the area, abnormalities in sheet 1:50000 Salmas, Tazeh Shahr survey delivered the result that the study area consists mostly of granite radioactive units in a weight pull into a page or a post-conflict area of newely-formed sediments. Considering the importance of identifying and tracing the radioactive airborne geophysical data layer, combining these maps with the satellite data, the geologically promising areas for radioactive elements were introduced. Then we proceeded to the final assessment of potential radioactive elements in the ground control. Due to the high amount of uranium enriched granitic intrusion in Qushchi and its highly enriched sediments compared with the other areas, to the best of our knowledge, the causative magma masses have been highly radioactive and have been transported to the earth’s crust, albeit with no position for the of formation of uranium reserves.

The accuracy of isotopic compositions measurement is affected by all the ICP-MS instruments because of the phenomenon known as mass bias, where it is defined as difference between the measured and true values in an isotopic analysis. This research has introduced mass bias and also evaluated the effective parameters including concentration of uranium, electrical field behind the second or skimmer cone, detector voltage, plasma power and nebulizer flow. The least mass bias with a relative error -0.3% for the isotopic analysis of uranium results in aqueous media without interference of the matrix with 1.2% RSD of 0.3 mg/L concentration, while the voltages of ion lens 1 and 2 in the ICP-TOF-MS were -700V and -550V, respectively. The instrument was set in the plasma power of 1000 watts, nebulizer flow of 0.78 ml/min, and the detector voltage of -2450V.

Electron Guns are considered as important and useful devices in electron scattering studies. So far, different models of these guns have been designed. In this paper, using the computational code SIMION- 8.0 3D we simulate a low energy electron gun which is appropriate for the electron collision process. This computational code may be used to correct the canonical properties of electrostatic lenses and also to calculate the imposed voltages on the optical components of the electron gun. Moreover, by applying this code the location and diameter of all the apertures (electrodes) and also the tracing of the electron beams through the electrostatic lenses can be determined. The designed electron gun system accelerates and focuses the electrons leaving a thermionic emitter to an interaction domain (target) in a wide range of variable final energy.

Pomegranate is cultivated in many regions of Iran. Due to the pomegranate sensitivity to the Carob moth, it is necessary to develop new cultivars that tolerate the Carob moth (Spectrobates ceratoniae). To induce a genetic variation in the plant, breeding is considered as an essential factor. Mutation breeding is a very effective and beneficial approach for promoting the genetic diversity and developing new varieties of fruit trees and agricultural plants. This study aimed to determine the proper gamma irradiation dose. Accordingly, some cleavage of one year old branches of the pomegranate was used. The survival of bud cuttings in different gamma ray doses was investigated. The gamma ray doses included 12 zero to 120 Gy (0, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120). The experimental design was arranged as a completely randomized one with 12 treatments. The survival rate showed a significant difference at 1% between various doses of gamma-irradiation in comparison to the control samples and the irradiation dose. A liner regression was estimated between the survival rate and gamma ray doses. According to the irradiation dose of LD50, (45 Gy) 36 Gy was determinate as a suitable gamma dose rate during the experiment.