مجله علوم و فنون هسته ای
سال سی و پنجم شماره 1 (پیاپی 67، بهار 1393)

CaF2:Dy,Tm nanoparticles have been synthesized by the hydrothermal method. The cubic lattice structure has been confirmed by the X-ray diffraction (XRD) pattern and an average size of 37 nm is obtained using Deby-Schere`s formula. The shape and size of particles are also observed by scanning electron microscopy (SEM). The thermoluminescence (TL) glow curve of the produced nanoparticles contains three overlapping glow peaks at approximately 407, 440 and 485 K. The TL phosphor characteristics has been studied for different Dy and Tm concentrations and the maximum sensitivity has been obtained at 3 mol% and 0.5 mol% of dysprosium and thulium impurities, respectively. The linear TL dose response over the absorbed dose rate of 10000 Gy, is recommend to be applied as a good candidate for high dose dosimetry.

During the past few years, real time gamma and neutron dosimeters have been developed and silicon diodes have frequently been used in these dosimeters. Silicon diodes are sensitive to gamma, and their neutron sensitivity is ensured with a converter layer on their front surfaces. In this study, a special converter detector design is optimized for dosimetry and determination of the neutron and gamma dose equivalent. Data analysis based on the pulse height is used for measuring the neutron and gamma dose equivalent. Neutron and gamma dose equivalent are determined in the energy range of 1-12MeV and 0.3-6MeV, respectively. The lower limit of dosimetry for gamma and neutron are 0.015μSv and 10μSv, respectively. In the 241Am-Be source radiation, a good agreement has been observed between the calculated and experimental measurements and the errors corresponding to gamma and neutron are less than 15% and 18%, respectively.

The Humphrey spiral is one of the mineral concentration techniques based on gravity separation. In various mineral processes, the characteristics of concentration performance are indicated by the grade and recovery factors. These factors depend on appropriate selection of process parameters. In this work, The L9 orthogonal array is used in the design of an experiment based on multi-objective optimization method to achieve the maximum concentration grade and recovery. The input process parameters that have been considered are the feed size, feed rate, and feed solids. The results show that the Humphrey spiral can be considered as a suitable technique for the preconcentration of thorium. Accordingly, upon considering the parameters which are affecting the performance of the Humphrey spiral, the grade and recovery of thorium increase to 49 and 5 percent, respectively.

The effect of heavy metals biosorption process parameters from the Esfahan Uranium Conversion Facility (UCF) wastewater by Cystoseira indica biomass was determined. The sorption capacities of heavy metals (U, Ni, and Cu) were studied by untreated biomass, biomass treated by formaldehyde and biomass treated by calcium solution and was concluded that the biosorption performance of biomass treated by calcium solution was better than the other adsorbents. Furthermore, the pH of 5 and equilibrium time of 120 min were obtained as the optimum conditions. Also, the kinetic data were well fitted with the pseudo-second-order kinetic model and the chemical reaction step was recognized to be the controlled- rate step. On the other hand, the results showed that the biosorption capacity decreased by increasing the biomass concentration. Also, the XRF analysis showed that the other heavy metals in wastewater were adsorbed by the Ca-pretreated brown algae. The FTIR analysis of adsorbent before and after the contact with wastewater showed that the functional groups of hydroxyl, carboxyl and amin had the most important role in the heavy metal sorption. Also, the BET analysis showed that the chemical adsorption of heavy metals changed the cell wall structure of biomass.

To study microbial oxidation of ferrous ions through the uranium bioleaching process, experiments were carried out in the internal loop air-lift reactor by Acidithiobacillus ferrooxidans. The microbial oxidation kinetics was evaluated with the Monod correlation and modified models for the substrate and product inhibition. The maximum recovery of uranium in the biological and control tests were 97.1% and 21%, respectively. Evaluation of the experimental data with the mentioned models showed that the modified model for the substrate inhibition gave a good fitting for all aeration rates. The R2-values were found to be 0.98, 0.97, 0.94 and 0.94 for the air superficial velocity of 0.0065, 0.0085, 0.01 and 0.015 m/s, respectively.

Replacement of four individual, time-consuming and expensive ASTM standard methods for determination of Mo, As, Th and K in samples having uranium matrix with one new proficient method is considered. The main advantages of this method are overcoming the mentioned shortages and preserving the precision and accuracy of the analysis results for the desired technical specification of materials and nuclear industries. In the new method, the interfering uranium matrix was removed by extraction with a mixture of tri (2- ethylhexyl) phosphate (TEHP): n-Heptane. The elements were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) simultaneously. The effective parameters on the extraction including: pH, the remained uranium and temperature were studied. The dynamic range of calibration curve related to the element type was linear (R2>0.999) at 10-500 μg/g U. The method detection limits (MDL) of 0.51, 0.66, 1.1 and 0.55 μg/g U were obtained for Mo, As, Th and K, respectively. The obtained quantity of En for all elements was less than 0.26. The analytical precision and bias of current method at two different concentration levels were determined and compared with the available standard methods denoting an evidence of the method verification. The decreased analysis of time duration from ~24 hr to <2 hr and the remarkably reduced instrumental and material charges are some of the joined industrial and applied properties of the present method.

A magnetic biosorbent composed of nanoparticles of magnetite covered with sugarcane bagasse and denominated magnetic baggas was prepared. The magnetic composite was used to remove U(VI) ions from aqueous solutions. The magnetite was synthetized by simultaneous precipitation by adding a solution of NaOH to the aqueous solution containing Fe2+ and Fe3+. The magnetic bagasse presented superparamagnetic properties; that is, it showed a high magnetization of saturation without hysteresis. The magnetic bagasse was characterized by XRD and SEM techniques. Nitrogen adsorption/desorption analysis on magnetic bagasse showed a nanostructure with an average particle size of 34 nanometers, with a specific surface area of 102.3 m2 g-1, and average pore diameter of 6.23 nm. Its adsorption performance was evaluated by determining the adsorption capacity of U(VI) ions by means of batch method. The results indicated that the biosorption capacity was significantly affected by the pH solution, biosorbent dosage, contact time, and initial uranium concentration. The uranium binding by the biomass test was rapid, and achieved 92% of the sorption efficiency within 20 min. The optimum biosorption (97.4%) was observed at pH 4.0, biosorbent dosage of 5 mg/L, initial uranium concentration of 50 mg L-1 within 90 minutes. The maximum adsorption capacity of the magnetic bagass for U(VI) ions was at biosorbent dosage of 1 g L-1 and obtained to be 32.04 mg g-1. The kinetic data were fitted well to a pseudo-second-order rate equation (R2=0.9996). The adsorption process conformed the Langmuir and Dubinin-Radushkevitch (D-R) adsorption isotherm models. Gibbs free energy (DG°) and enthalpy change (DH°) indicated that the reaction had been spontaneous and exothermic in nature at the studied temperatures. In the desorption studies, 94.5% of  adsorbed U(VI) was recoverd with hydrochloric acid as an eluent.

Numerical solution of governing thermal-hydraulic equations in the core of a pebble bed modular reactor (PBMR) is investigated using the porous media approach. By considering that there is a high temperature helium gas in the core, the NJOY code is used to generate cross sections at these temperatures. Then, the heat flux in the core is obtained in the axial and radial directions by the MCNP code and is consequently used in the computational fluid dynamics (CFD) simulation as a semi- sine and an algebraic function. The major characteristics of the flow field have been identified, whereby the thermal–hydraulic parameters such as temperature and pressure profiles have been specified and compared with the other available data. The results of the MCNP4C and CFX.12 in comparison with the other codes confirmed the present calculation to be used in this type of reactor. Other results that obtained with the use other codes and softwares prove that the inclusion of the compressibility is quite reasonable, where it leads to a slight difference between the measured temperature, pressure and velocity and the actual ones, where it enables us to improve the Darcy-Weisbach equation in this type of reactor.

There are two criteria named primary and secondary standards, to calibrate vacuum pressure gauges. The most important primary standards are: "mercury manometer vacuum standard", "volume expansion vacuum standard" and "orifice flow vacuum standard", where they are used in low vacuum, medium vacuum and high vacuum ranges, respectively. In this paper we have tried to explain all the methods of standardizing vacuum pressure gauges and their scientific bases. We have also described briefly the importance and the usage of the methods which are applied in nuclear researches.

The influence of gamma ray radiation on ion-exchange capacity of sodium-clinoptilolite was studied. The absorber sample was subjected to gamma irradiation with doses of 70 to 2000 kGy. The chromium (VI) adsorption capacity of the irradiated sample was determined by the atomic adsorption spectrometry technique. The chromium (VI) adsorption was performed in the concentration range of 80 up to 1800 PPm and with a pH of 7. The results showed that the maximal radiation dose used in this research (2000 kGy) did not affect the adsorption capacity of the studied sample.

Gamma spectroscopy was performed on Marinelli beaker standards containing soil with different densities of ρ = 1 to 1.5 gr cm-3 upon using a HPGe detector. The spectra were analyzed by a specific software and then the efficiency vs. energy was calculated for each sample. The calculated efficiencies for different densities were compared with the efficiency of the reference sample, and the self-absorption correction factors were calculated for soil samples in the range of 59-1408 keV energy. The obtained correction factors were applied successfully for the calculation of the activity of an unknown sample.

An experimental study for the convection heat transfer coefficient in a region of nanofluid containing Al2O3 oxide nanoparticles of 20 nanometer diameter in water as a base fluid through circular annular tube in the cosine thermal flux boundary condition was carried out. The primary purpose of this investigation was accomplished on the surface temperature of the heat source (inner pipe) determined at the maximum upper than the middle of pipe for the whole entry temperatures and surface temperature for nanofluid which was less than that of the base fluid. However, the nanofluid did not have any effect on the location point on the maximum temperature surface. Then, the convection heat transfer coefficient and Nusselt number were scrutinized showing that both of them increase by increasing of the volume fraction and Reynolds number. The maximum value of the heat transfer coefficient of nanofluid belongs to the volume fraction of 1.5% and the Reynolds number near 2100 which is 19%, compared to that of the base-fluid. The effect of entrance temperature and pressure of nanofluid on the heat transfer coefficient was also studied. The experimental data have shown that by increasing the entrance temperature, the heat transfer coefficient improves but the pressure has a negligible effect on heat transfer. The results demonstrated that the relative pressure drop of nanofluid increased remarkably by increasing the volume fraction. Furtheremore, we observed that by decreasing the Reynolds number the pressure drop increased because of more sediment of nanoparticle at lower velocities.