مجله علوم و فنون هسته ای
سال چهل و دوم شماره 4 (پیاپی 98، زمستان 1400)

Radiation damage calculation is very important for estimating the lifetime of nuclear power plant instruments. The purpose of the present study is to evaluate the sensitivity of the damage values to the neutron group energy, where the neutron flux rate is maximum. In the present study, we propose three neutron group energy (WIMS, CINDER, and OPENMC) to evaluate the sensitivity of radiation damage calculations. The obtained result from SPECTER code shows that the differences between these results and the standard values (ASTM E-693) for three group energy WIMS, CINDER and OPENMC are respectively 0.25E-04, 0.75E-04, and 1.74E-04 in the ¼ diameter of reactor pressure vessel (RPV). Therefore, the WIMS energy group is the most accurate spectrum compared to other spectrums in this region. Also, these differences are respectively 0.43E-04, 0.52E-04, and 1.86E-04 in the ¾ diameter of RPV. Moreover, the WIMS group energy is the most accurate spectrum in this region. This result shows that increasing the number of neutron energy group cause to reduce the difference between calculation of damage and its standard values. According to the results of calculated damage that induced by WIMS group energy in ¼ diameter of RPV, the values of yield strength at 1, 5, 10, 15, 20, 25, 30, 35 and 40 years are calculated by SPECOMP and SPECTER codes.

The significant defects in the artistic paints are rupture, scratches, and twisting, which are caused by variations in temperature, exposure to moisture, and erosion. Industrial radiography can detect the defect locations, which is a non-destructive test, can be carried out. In radiography testing (RT), the produced radiographs may suffer from some degree of blurriness and low contrast due to the inherent scattering of X-ray, geometric factors such as the size of the beam source, the thickness of the part, and the source to film distance (SFD). The diagnosis of the shapes and sizes of the damages is difficult in some cases. Image processing methods can be used as additional tools for enhancing image quality and more accessible interpretation. In the present study, a Gabor filter technique, based on wavelet analysis with an automatic threshold level, was used to reduce the fogginess of radiographs. The reconstructed images of the proposed algorithm have better contrast, and the defects and signs are more precise than the original image. This algorithm can help the restoration experts for repairing the artistic paintings.

In order to increase the performance of a gas centrifuge used in the uranium enrichment industry, the gas flow field inside it is studied and simulated. In the present work, the full Navier-Stokes equations using the CFD method are used to simulate the gas flow inside the rotor. For the CFD method, a density-based implicit coupling solver was developed in OpenFOAM software, which was used to simulate the gas flow inside the rotor. The separation power was improved in a sample rotor by adjusting feed flow parameters, wall pressure, wall temperature gradient, and scoop drag force. The results show that the process variable had an optimal value in which the separation power is maximum. In order to achieve the maximum separation power of 12.87 kg UF6 SWU/year, the optimum rotor conditions were determined at a feed rate of 90 g/h, wall pressure of 44 torr, the temperature gradient of 25 K, and drag force of 1557 dyne. This study can be considered an important step in improving the performance of centrifuge separation.

The present research aims to measure the physical parameters affecting the differential cross-sections of PIGE reactions in the 45˚R beamline of the Van de Graaff accelerator. The calibration coefficient, the correlation between particle energy and NMR frequency, was determined using the relevant nuclear reactions. The absolute efficiency of the HPGe detector within the energy range of 60 to 10800 keV was obtained using the gamma rays of the standard radioactive sources and the cascade gamma rays due to the proton capture reactions.  Two different techniques determined the solid angle of the charged particle detector. Using the backscattered particles' spectra, the beam current and the number of target nuclei were calculated. Also, the necessity for reducing the laboratory background and identifying the undesired peaks due to neutron-induced reactions was discussed.  Under favorable experimental conditions, the systematic uncertainty for cross-section measurement was estimated to be less than 9%.

Accuracy in the treatment planning of proton therapy depends on the accuracy of the information used to calculate the relative stopping power of tissues in the patient's body. This information is obtained from x-ray computed tomography images using a calibration curve to convert Hansfield numbers to relative stopping power values. Using x-ray computed tomography images leads to errors in estimating the proton range and the proton dose distribution in the treatment plan program. But applying the proton computed tomography eliminates this error and directly calculates the relative stopping power map of the tissues. In the present study, a modern proton computed tomography imaging system was simulated using the Monte Carlo Geant4 toolkit by tracing particle-to-particle trajectory. The purpose of this simulation was the improvement of density resolution of tissue without dose increment. The standard CIRS 062M phantom was irradiated with a 300 MeV proton beam. The energy, position, and direction of particle movement values before and after the phantom were stored in the root file by nuclear detectors. The image matrix phantom was reconstructed as a relative stopping power map using three radon analytical algorithms. The comparison was made regarding dose, density resolution, and RMSE concerning real phantom image data. The proposed algorithm improved the density resolution from 9.1% to 4.3% and RMSE from 26.43% to 6.81% by correcting the angles of the projections at the same dose level.

The gas behavior within a centrifuge machine can be divided into molecular and continuum. The Boltzmann equation is an accurate method for studying the behavior of gas in all areas of a centrifuge. DSMC is one of the methods for solving the Boltzmann equation. In the literature published so far, a presumptive source has been considered the mass source used in the Onsager-Pancake equation in the continuum region. The researchers have presented various presumptive mass sources. In the present work, the feed reaching form to continuum region and its effect on the border between the two areas in the mass source form was calculated using the direct Monte Carlo method. The results were compared with Gunzburger's presumptive mass source. Moreover, the mass source obtained from the presence of hydrogen fluoride light gas in the feed was calculated at the border of two regions as a two-component mixture (ZUF6=0.97, ZHF=0.03), (ZUF6=0.93, ZHF=0.07) and (ZUF6=0.9, ZHF=0.1). The concentration distribution along the rotor was calculated by inserting the obtained flow function into the Onsager-Cohen diffusion equations. Finally, the separation parameters and the separation power were calculated. The results show that the separation factor and the separation power decrease by increasing the light gas amount in the feed entering the machine.

To model a gas behavior within a centrifuge machine, a powerful tool is required similarly. With high-speed cover, all the computational limitations for applying the influential derives such as feed, scoop, baffle effects, etc., formed flow regimes. Therefore, achieving an applicable code or software to simulate the gas behavior within a centrifuge machine to investigate the effect of the derives on the separation in a centrifuge machine, including high-speed rotation, temperature, and mechanical derives, is required. Considering the fact of covering the method based on the molecular DSMC for the all formed flow regimes within the centrifuge machine, in the present work, the method of applying the all influential factors in a separation process of a centrifuge machine in the axial symmetric coordinates on the written code of DSMC and also the dsmcFOAM solver have been studies. The results have been compared with those obtained from the code, including pressure radial changes, axial mass flux, velocity components, and dsmcFOAM solver. A good agreement was found between the results corresponding to the written code and dsmcFOAM solver to simulate a centrifuge machine in the presence of all the parameters.

Due to the unique properties of carbons in depth-dose deposition, this study investigated the carbon therapy of brain tumors using the Geant4 toolkit. To see the effect of phantom material on the dose calculations, three phantoms consisting of realistic brain tissue, soft tissue, and water were considered. The spherical tumor was considered at the center of the brain and the Bragg peak was calculated at the center of the tumor. The dose deposition in depth was different for all three phantoms, so that this difference is about 3 mm for the Bragg peak in the brain phantom and soft tissue phantom, and about 4 mm for the Bragg peak in the soft tissue and water. The depth dose distribution of the secondary particles indicates that the photons deposit most of their dose close to the surface, while for alpha and protons it depends on the Bragg peak depth. Also, the deposited dose in the tumor is more than one hundred times larger than the deposited dose in the brain healthy tissue, and ten thousand times higher than the organs such as the thymus gland and thyroid. The results of this investigation confirmed that more effort should be made to use more realistic phantoms in treatment design.

In ultra-high power laser interaction with plasma, quantum electrodynamics phenomena such as high energy photon emission by electrons, radiation reaction trapping, and anti-particle creation can affect the interaction mechanism. In the present work, the interaction mechanism of the circular and linear laser with an intensity higher than 1023 W/cm2 with rarified plasma in the presence of the radiation reaction force has been investigated using particle in cell simulation. The results indicate that radiation reaction trapping for circular polarization is more effective than the linear one. Also, photons emitted by electrons have a higher density for circular polarization. For both polarizations, at later times of the interaction, considering photon emission and radiation reaction effects lead to the significant decrement of the cut-off energy of electrons. The cut-off energy of the emitted photons for circular polarization is higher than that of linear polarization.

This paper uses a new technique to measure the power and temperature reactivity coefficients. In this method, the core reactivity changes due to the modifications in the power or temperature directly. The values of these coefficients are independent of the worth and positions of the control rods. This method is completely new and no report or article has been reported in this field. Increasing the core power causes negative reactivity due to the temperature feedback, and cooling the core makes positive reactivity. In this method, TRR is critical at constant power in natural cooling mode. After getting a relative equilibrium in inlet and outlet coolant temperature, the fly valve of the reactor is opened, so that the cooling mode of TRR changes from natural to the forced mode. Obviously, by establishing a cooling flow in a short time, all produced heat in the natural mode is removed by forced, then a positive reactivity is inserted. As a result, the positive reactivity increases the reactor power. By measuring the doubling time of the power growing, the value of the inserted reactivity is obtained using inhour equation in each initial power. The mean value of the power reactivity coefficient in TRR is about 1.02 pcm / kW that agrees with both results of the previous and recent simulations.

The 18F radioisotope is produced via the enriched water (H218O) bombardment with a high cyclotron yield. The 18O enrichment value of the water is more than 98% and has been imported from abroad. Hence, only a few percentages of enriched water are used in the first bombardment; the recovered water can be used to produce 18F after removing the chemical and radiochemical impurities. The used water contains organic and inorganic impurities, which should be removed before re-usage. In order to remove organic and inorganic impurities, ultraviolet irradiation and ionic exchange methods were applied in addition to low-temperature condensation in a controlled atmosphere. This study introduced a novel method for purification of used enriched water using chemical and physical methods. The purified enriched water was used for bombardment in cyclotron. Production of 18F radioisotope, and finally the production of 18F-FDG along with the QC test were performed. The qualified results of the 18F-FDG production and its QC tests, confirmed the successful purification of the enriched water.

In the present work, the formation of the wakefield during the interaction of intense laser pulse with a gas medium has been investigated by using PIC simulation code, including the ionization process. The results have been compared with those corresponding to the case of the pre-formed plasma medium. Although in previously published works, the strong launch of forwarding Raman's instability was shown to be as a result of plasma density fluctuations during ionization and the subsequent strong laser pulse modulation, our results indicate that the wakefield amplitude in gas in comparison with plasma considerably depends on the laser pulse shape. For laser pulse with a high slope, the amplitude of the wake electric field is quite the same in gas and plasma mediums. However, as the slope of the laser pulse decreases (soft slope), the wakefield is generated with a larger amplitude in the plasma. A further decrease in the laser pulse slope leads to a larger wake electric field in gas than in a plasma environment.

Due to fascinating theranostic properties of Gold-nanoparticles and Ga-68 radionuclide, 68Ga was grafted on thioglycolic acid (TGA)-functionalized nano host, in order to target the delivery of the radionuclide to targeted organs. In order to have precise evaluation of the behavior of the labeled nano particles in in-vitro and in-vivo experiments, measurement of the radioactivity in tissue with HPGe detector, and precise quality control tests including RTLC, TEM, PET-Scan, were done as well. Biological information of the nano particles shows outstanding changes in excretion mechanism (increasing urinary tract excretion) and complete transfer of nanoparticles to all vital organs of the rodents. Regarding to the outstanding pharmacokinetics properties of these labeled gold nanoparticles such as proper biodistribution, fast excretion, high structural stability, and proper blood circulation, these labeled nanoparticles can be introduced as a potential candidate for diagnostic PET nuclear imaging.

The consumption of fresh fruits and vegetables plays an important role in promoting the general public's health and preventing various diseases. However, these useful products can be a good carrier for transmitting bacteria, parasites, and pathogens to humans. The shelf life of these products is very limited and, after three days of storage under refrigerated conditions, they will lose their consumption and change to a bulk of food waste. Irradiation can be a suitable treatment to ensure safety, increase the shelf-life of fresh vegetables, and create the possibility of exporting them to international markets. In this study, the effects of irradiation doses at 0, 0.25, 0.5, 0.75, and 1 kGy on microbial quality and safety of fresh leek and parsley vegetables preserved at 25 and 4 °C were investigated. This study showed that the samples stored at 25°C due to high microbial load in the first two days were removed from the study cycle. In irradiated samples, until the eighth day, a dose-dependent reduction of the microbial load was observed. Foodborne pathogenic bacteria including E.coli and Staphylococcus aureus were present only in control and 0.25 kGy irradiated samples. Therefore, considering acceptable results in terms of microbial safety and sensory assays, in fresh leafy vegetables processing industry, gamma irradiation at a dose of 0.5 kGy with keeping at 4 °C, can enhance shelf life up to 8 days and decrease wastes of vegetables including leek, parsley.

Rhenium-188 is known as a suitable candidate for the development of therapeutic radiopharmaceuticals. Recently, an alumina-based 188W / 188Re generator has been produced in the country. Owing to that, the main features of a generator will play an important role in the possibility or impossibility of using it in the process of radiopharmaceutical development. In the present study, the generator's main characteristics, including the selection of the appropriate washing solution, the washing curve, and efficiency, were determined. Also, the radionuclide purity, chemical purity, and radiochemical purity of the generator washing product were evaluated. The washing curve of the generator showed that the highest concentration of rhenium-188 activity is present in fractions 2, 3, and 4, which can be used for particular labeling purposes. The yield of the generator and the percentage of tungsten-188 remaining in the solution washed from the generator are 69% and 1.76 × 10-4%, respectively, which is within the acceptable range of European Pharmacopoeia. The result of chemical purity indicates the presence of aluminum ions of less than five ppm in the washing product of the generator. Also, the radiochemical purity of the washing product of the generator was higher than 99% using the ITLC method. In general, the obtained results from the quality control of the 188W / 188Re generator produced in the country are satisfactory. It is hoped that these generators will play a useful role in producing therapeutic radiopharmaceuticals in the country.