مجله علوم و فنون هسته ای
سال سی‌ام شماره 3 (پیاپی 49، پاییز 1388)

In this paper two geometries for pulsed neutron counter structure have been introduced and to increase the activation counter efficiency, plastic scintillation along with silver foils was used. Cubic and cylindrical geometries for activation counter cell were modeled using MCNP4C code. In respect of absorpstion reaction rate in silver, the number of silver foils and the length of the counter were optimized. The optimum length of 14 centimeters had been proposed for counter cell and because of the economic aspects, the optimum number of silver foils for cubic and cylindrical geometries are 20 and 10, respectively. The optimum data were used to construct a cubic counter and the neutron yield of SBUPF1 plasma focus device was measured by this counter. Experimental results show that about 3.71Î107 neutrons are produced per pulse.

In uranium yellow cake production, for the extraction and purification of uranium, primarily the uranium ore is grounded, sieved and then leached with (sulphuric acid). The ion-exchange has been identified as a proper method in high perfect recovery of uranium from the acidic solution where uranium concenteration is less than 500ppm. This method is operated in two ways: 1 (fixed bed 2) moving bed. Moving bed system has some advantages over the fixed bed such as higher mass transfer during ion exchange process. In this study, the effects of various parameters of multiple compartment ion exchange in recovery of uranium are investigated. After performing different experiments in batch system, the optimum pH of feed solution was found to be equal to 1.5 and the residence time between the resin and feed solution was 3 hours. Also, the results of fluidized bed continious system include the optimum flow rate of 22lit/hr and the residence time of 7 minutes (the height of column was 180cm). To increase the efficiency in height limit, the recycle system is used.

Ideal magnetohydrodynamic (MHD) equilibrium is vulnerable to numerous unstabilizing mechanisms. Instabilities introduce distortions to the plasma magnetic surfaces and their boundaries, their driving force being the radial gradient of plasma toroidal current density. For certain modal numbers, internal kink modes may develop, and their study is feasible according to the energy principle, in which the change in total potential energy is evaluated due to the disturbance. In this paper, we present a new analysis of MHD equilibrium and stability, and apply it to Damavand Tokamak which has a large aspect ratio. For this purpose, we combine the perturbation and Green's function methods to solve the equilibrium configuration problem. At this stage, plasma profiles are found explicitly in terms of Bessel functions, and we present a simple expression for estimation of total toroidal plasma current. Then the rest of plasma profiles, including poloidal magnetic flux, safety factor, and toroidal current density, are obtained and plotted. In the next step, we turn to the stability calculations and characterize the stable and unstable ideal MHD modes of Damavand plasma.

In this study, the effects of γ-radiation on microbial load and efficient compounds derived from caraway seeds were investigated. Caraway seeds were irradiated at dosages of 10 and 25 kGy by a Co60 source. Then the efficient compounds were extracted and the composition and microbial load of caraway extracts were evaluated before and after γ-irradiation. The results showed that while decreasing the microbial content of caraway seeds, among the 22 oil fractions separated by GC-MS, only three components were markedly affected by gamma irradiation. Pre-treatment of caraway seeds with 10 and 25 kGy radiation also results in a significant (p<0.05) increase in flavonoids content of hydro alcoholic extract. These data may suggest that irradiation of caraway seeds without unfavorable changes on their effective components results in significant decrease in microbial content of this product.

The aim of this research is to specify a method by which large reactors can be described by point model, and point kinetic equations can be applied. By this method the in-core reactivity can be measured and the reactor size and dimensions do not have any effect on the kinetic equations and the measured reactivity. The effective multiplication factor of a reactor (k) is a value that is a function of time (not a function of location in the core). By solving the reactor modeling equations through which the dimensions of the reactor are considered, different values are obtained for k. If these values differ very much with each other, the spatial effects should be considered in measuring reactivity. These effects affect the needed time for showing the correct reactivity by reactimeter and eliminating these effects reduces aferesaid time. In this study it is shown that in large reactors if at any time we consider the in-core neutron flux equal to the maximum flux at that time, the reactor can be considered as a point. In this research the other schemes which specify the reactor reactivity are also studied, compared with the maximum flux scheme, and it is concluded that the maximum flux scheme is the most precise and applicable method that can be used to approximate a reactor as a point.

In this study, the production of labeled vitamin B12 with one strain of actinomycet in a novel synthetic medium by batch fermentation method was investigated. After releasing vitamin B12 from microbial cells, the solution containing cobalamin analogues was fixed on amberlite XAD-4 exchanger and eluted by various solutions. Vitamin B12 in the collected fractions were measured by HPLC method and also in order to confirm the existence of cobalamin compounds in the samples, TLC technique was used. In the optimal conditions, production of vitamin B12 by this microorganism was 1456µg/l. The best recoveries of vitamin B12 from the column were obtained when the column was eluted by ethanol followed by acetone. The Rf values for cyanocobalamin and methylcobalamin, produced by this microorganism were the same as the standard values of Rf. The desorption of labeled vitamine B12 was carried out by different eluants, including acetone, ethanol (absolute), ethanol 25% and deionized water. The activity of labeled vitamin B12 was 8.55, 6.90, 0.79, 5.75 Bq/ml, respectively. Therefore, the total specific activity of 21.99Bq/ml was obtained for labeled vitamin B12.

In an attempt to find a natural and inexpensive method to control the stored-product pests, the effect of combination of gamma radiation and essential oil from Perovskia atriplicifolia (Benth) on the adults of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) was investigated. Experiments were carried out at 27±1°C and 65±5% R. H. under dark condition. Two experimental regimes were designed: 1) irradiation of fumigated adults 2)fumigation of irradiated adults. In combination experiments, mortality was assessed 14 days after the first treatment. The results showed significant synergistic effect of gamma radiation with essential oil on adults of T. castaneum (1-7 days old). Gamma irradiation at dosage 100Gy, alone caused 12.5% mortality on T. castaneum but when these irradiated insects were treated by 7.66 µl/l air of P. atriplicifolia oil 7 days after irradiation, (caused 6.25% mortality alone) mortality percent reached 32.5%. These results provide the basis for successful use of gamma radiation in the presence of the essential oil for management of T. castaneum.

CaSO4:Dy thermoluminesconce dosimeter (TLD) is one of the best materials for environmental and personal dosimetry due to its high sensitivity and high dose saturation. In this investigation, the dose response of CaSO4:Dy with three concentrations, 0.11, 0.15 and 0.2 mol% of dysprosium, to 60Co gamma rays doses up to 3.7Gy were studied. The effect of heating program on glow curve was also studied. The glow peaks of CaSO4:Dy were seen at 100oC and 167oC. The pellets have high sensitivity and in the case of sample with 0.2 mol% of dysprosium, the relative sensitivity is 12. The dose response is found to be linear up to 3.7Gy and the fading was approximately about 36% in 10 days.

In this work, using simulation in the working conditions for the radiation workers the gamma and high energy X-rays’- absorbed dose in the eye cornea, thyroid and testicles were measured by the RANDO phantom, TLD 100H (Li:Mg:P:Cu) dosimeters and Harshaw 4500 reader. The measurements were performed on pre-determined parts in both normal and emergency conditions. The result of this research indicates that the lower absorbed dose in various body parts of radiation workers in an ordinary working condition and in permissible areas is lower than the allowable absorbed  dose. Moreover, despite the fact that the irradiation level for the whole body and different body parts are 10 times more in restricted and prohibited areas, the absorbed dose is yet measured to be less than the allowable dose. However, incidences such as human error in discharging radioactive samples from the reactor pool can cause the whole body irradiation of a radiation worker to exceed the allowable dose of  20 mSv per year.

Onion (Allium cepa L.) is considered as an important vegetable that a considerable amount of the product is lost in the storage process. The loss prevention and reducing methods are related to the production stages and post harvest conditions. One of the effective methods after harvesting is irradiation of bulbs by ionizing radiations as gamma ray. This research was conducted for 2 years in Seed and Plant Improvement Institute for surveying of different ionizing electron doses on four cultivars composed Ghermeze Azar Shahr, Sefide Kashan, Dorcheh Isfahan and Sefide Qom. The research was performed in split plot design with three replications that cultivar and radiation doses were the main and sub factors, respectively. Irradiated bulbs were stored for four months in ambient conditions (10-15ºC and 60-70% RH) and the storage traits composed sprouting and rotting percentage, weight loss percentage, dry matter percentage and tissue firmness (kg/cm2) recorded in 40, 80 and 120 days intervals. The results showed that cultivars had significant difference in all traits. Sefid Qom and Ghermeze Azar Shahr cultivars had the least sprouting percentage and weight loss percentage in all periods. Dry matter percentage and tissue firmness in Sefid Qom was higher than other cultivars. The irradiation caused significant improvement in traits specially 120 days after storage. The reciprocal effects of experimental factors were not significant in traits exceptly in sprouting and rotting percentage after 80 days storing and weight loss after 120 days storing.