Monte Carlo simulation of a research reactor with nominal power of 7 MW to design new control safety rods

The Monte Carlo simulation has been established for a research reactor with nominal power of 7 MW. A detailed model of the reactor core was employed including standard and control fuel elements, reflectors, irradiation channels, control rods, reactor pool and thermal column. The following physical parameters of reactor core were calculated for the present LEU core: core reactivity (ρ), control rod (CR) worth, thermal and epithermal neutron flux distributions, shutdown margin and delayed neutron fraction. Reduction of unfavorable effects of blockage probability of control safety rod (CSR)s in their interiors because of not enough space in their sites, and lack of suitable capabilities to fabricate very thin plates for CSR cladding, is the main aim of the present study. Making the absorber rod thinner and CSR cladding thicker by introducing a better blackness absorbing material and a new stainless steel alloy, respectively, are two studied ways to reduce the effects of mentioned problems.     

Two safety coefficients for a typical 13 × 13 annular fuel assembly

Recently, annular fuel rods are proposed for both PWRs and BWRs to achieve more average power density and therefore many neutronic as well as thermal-hydraulics calculations have been made to find more performance in the future reactors. Also, some safety margins are studied for the proposed uranium-nitride (and/or other fissile materials) annular pins. Our aim herein is to study two important safety coefficient of the annular fuel core. These are “prompt reactivity” and “power coefficients”, where all investigations are made using MCNP-5 code. Also, a thermal resistance model for annular fuel heat transferring were given and then we have calculated its (a case) thermal resistance numerically.     

Burnup analysis and in-core fuel management study of the 3 MW TRIGA MARK II research reactor

The principal objective of this study is to formulate an effective optimal fuel management strategy for the TRIGA MARK II research reactor at AERE, Savar. The core management study has been performed by utilizing four basic types of information calculated for the reactor: criticality, power peaking, neutron flux and burnup calculation. This paper presents the results of the burnup calculations for TRIGA LEU fuel elements. The fuel element burnup for approximately 20 years of operation was calculated using the TRIGAP compute code. The calculation is performed in one-dimensional radial geometry in TRIGAP. Inter-comparison of TRIGAP results with other two calculations performed by MVP-BURN and MCNP4C-ORIGEN2.1 show very good agreement. Reshuffling at 20,000 MWh step provides the highest core lifetime of the reactor, which is 64,500 MWh. Besides, the study gives valuable insight into the behaviour of the reactor and will ensure better utilization and operation of the reactor in future.     

单细胞模型微观剂量学的Monte Carlo模拟研究

研究电离辐射在生物体内的能量沉积及相应的生物效应时,有必要研究单个细胞或部分细胞中能量吸收的统计特性。本工作利用蒙特卡罗软件Geant4,开发了计算单细胞微观剂量学程序,依据微观剂量学理论,对α释放子211At在单细胞模型中的微剂量学特征进行了研究。分析了不同大小细胞中单事件比能与存活分数的特点,并对真实细胞模型中微剂量学特征进行了分析。通过与简单数学模型结果对比,分析了细胞不同材料及形状对模拟结果的影响。本工作建立的体元单细胞模型为单细胞微剂量学研究提供了工具,同时也很易于扩展到真实组织模型。     

Criticality qualification of a new Monte Carlo code for reactor core analysis

In order to accurately simulate Accelerator Driven Systems (ADS), the utilization of at least two computational tools is necessary (the thermal–hydraulic problem is not considered in the frame of this work), namely: (a) A High Energy Physics (HEP) code system dealing with the “Accelerator part” of the installation, i.e. the computation of the spectrum, intensity and spatial distribution of the neutrons source created by (p, n) reactions of a proton beam on a target and (b) a neutronics code system, handling the “Reactor part” of the installation, i.e. criticality calculations, neutron transport, fuel burn-up and fission products evolution. In the present work, a single computational tool, aiming to analyze an ADS in its integrity and also able to perform core analysis for a conventional fission reactor, is proposed. The code is based on the well qualified HEP code GEANT (version 3), transformed to perform criticality calculations. The performance of the code is tested against two qualified neutronics code systems, the diffusion/transport SCALE-CITATION code system and the Monte Carlo TRIPOLI code, in the case of a research reactor core analysis. A satisfactory agreement was exhibited by the three codes.     

Assessment of RELAP5 point kinetic model against reactivity insertion transient in the IAEA 10 MW MTR research reactor

The current study emphasizes an aspect related to the assessment of a model embedded in a computer code. The study concerns more particularly the point neutron kinetics model of the RELAP5/Mod3 code which is worldwide used. The model is assessed against positive reactivity insertion transient taking into account calculations involving thermal-hydraulic feedback as well as transients with no feedback effects. It was concluded that the RELAP5 point kinetics model provides unphysical power evolution trends due most probably to a bug during the programming process.     

A Monte Carlo computer code for evaluating energy loss of 10 keV to 10 MeV ions in amorphous silicon materials

The basic concepts of a computer simulation code for determining the energy loss of ions in the 10 keV to 10 MeV energy range in amorphous silicon materials were presented and discussed. Data obtained were found in good agreement with those obtained by using a SRIM programme. Electronic and nuclear energy losses were evaluated. Variation of the energy loss as a function of the incident ion energy were studied. This new computer code is a good tool for evaluating stopping powers of various materials for light and heavy ions.     

Measurement of dose rate profile and spectra through a cylindrical duct vis-à-vis Monte Carlo simulation studies for optimisation of reactor shield design

In the design of a nuclear reactor, penetrations are provided in the top shield to carry out some essential operations. Radiation streaming is envisaged through such penetrations. To avoid radiation streaming, complementary shielding is provided. Optimisation of complementary shielding is carried out by performing calculations using MCNP code. Uncertainties in the calculations are taken care of by incorporating a safety factor. The assumption of the safety factor, while designing the reactor shielding, has been validated by undertaking experimental measurements on a similar geometry vis-à-vis the computed values obtained using MCNP code. The results of the present work agree with the safety factor of two assumed during the shield design. The details of gamma spectral measurements carried out with high purity germanium detector to understand the pattern of the scattered spectrum are also presented.     

Film growth by polyatomic C2H5 bombarding a diamond (1 0 0) surfaces: Molecular dynamics study

The deposition of polyatomic C₂H₅⁺ ions is studied using classical molecular dynamics simulations with a new improved Brenner potentials developed by Brenner. The simulation results show that when the incident energy is less than 65 eV, the deposition coefficient of H is larger than that of C atoms. When the incident energy is larger than 65 eV, the deposition of H is less than that of C atoms. With increasing incident energy, a transition from Csp3-rich to Csp2-rich in the grown films is found.     

Imaging performance evaluation in depth-of-interaction PET with a new method of sinogram generation:A Monte Carlo simulation study

In conventional PET systems,the parallax error degrades image resolution and causes image distortion.To remedy this,a PET ring diameter has to be much larger than the required size of field of view(FOV),and therefore the cost goes up.Measurement of depth-of-interaction(DOI)information is effective to reduce the parallax error and improve the image quality.This study is aimed at developing a practical method to incorporate DOI information in PET sinogram generation and image reconstruction processes and evaluate its efficacy through Monte Carlo simulation.An animal PET system with 30-mm long LSO crystals and 2-mm DOI measurement accuracy was simulated and list-mode PET data were collected.A sinogram generation method was proposed to bin each coincidence event to the correct LOR location according to both incident crystal indices and DOI positions of the two annihilation photons.The sinograms were reconstructed with an iterative OSMAPEM(ordered subset maximum a posteriori expectation maximization)algorithm.Two phantoms(a rod source phantom and a Derenzo phantom)were simulated,and the benefits of DOI were investigated in terms of reconstructed source diameter(FWHM)and source positioning accuracy.The results demonstrate that the proposed method works well to incorporate DOI information in data processing,which not only overcomes the image distortion problem but also significantly improves image resolution and resolution uniformity and results in satisfactory image quality.     

Study of a new cusp field for an 18 GHz ECR ion source

A feasibility study was performed to generate new sufficient mirror cusp magnetic field (CMF) by using the coils of the existing room temperature traditional 18 GHz electron cyclotron resonance ion source (ECRIS) at RIKEN. The CMF configuration was chosen because it contains plasma superbly and no multipole magnet is needed to make the contained plasma quiescent with no magneto-hydrodynamic (MHD) instability and to make the system cost-effective. The least magnetic field, 13 kG is achieved at the interior wall of the plasma chamber including the point cusps (PC) on the central axis and the ring cusp (RC) on the mid-plane. The mirror ratio calculation and electron simulation were done in the computed CMF. It was found to contain the electrons for longer time than in traditional field. It is proposed that a powerful CMF ECRIS can be constructed, which is capable of producing intense highly charged ion (HCI) beam for light and heavy elements.     

Excitation functions for some W, Ta and Hf radionuclides obtained by deuteron irradiation of Ta up to 40 MeV

Experimental excitation functions for deuteron induced reactions up to 40 MeV on mono-isotopic Ta (¹⁸¹Ta) were measured with the activation method using a stacked foil irradiation technique. From high resolution gamma spectrometry and X-ray analysis cross-section data for the production of ¹⁸¹W, ^{177,178g,180g,182m+g}Ta, and ^179m2,180mHf were determined. Comparison with the scarce earlier published data are presented and results for values predicted by different theoretical codes, adapted for more reliable calculations for d-induced reactions, are included. Thick target yields for ^{182m+g,180g,178g}Ta and ¹⁸¹W were calculated from a fit to our experimental excitation curves. Using dose conversion factors and irradiation scenarios, possible occupational doses to maintenance or scientific personnel around high power accelerators where Ta based structural elements (collimators, beam stoppers, shielding) are present could be derived.     

Experimental study of the fracture kinetics of a tubular 16MnNiMo5 steel specimen under biaxial loading at 900 and 1000 °C. Application to the rupture of a vessel bottom head during a core meltdown accident in a pressurized water reactor

This study proposes an experimental analysis of the creep, crack initiation and crack propagation phases in a 16MnNiMo5 steel subjected to thermomechanical loading representing a core meltdown accident in a pressurized water reactor involving the transfer of a molten corium bath to the bottom head. The experimental setup enabled a biaxial mechanical loading (internal pressure + tension) to be applied to a tubular specimen at 900 and 1000 °C. In addition to the usual temperature, load, displacement and pressure measurements, the specimen was observed by two high-speed numerical cameras and an infrared camera. The crack's initiation and propagation conditions and the depressurization law were inferred from these measurements. At such temperatures, creep induces very large strains prior to the occurrence of the cracks which, in the worst-case scenario, can propagate at velocities as high as several meters per second. The design of the experiment enabled us to study the influence of the temperature (magnitude and hoop distribution), of the toughness of the steel (two grades were studied) and of the volume of pressurized gas. The results show that creep and crack propagation are highly dependent on temperature, and also that crack initiation and propagation are highly dependent on the degree of heterogeneity which is responsible for the localized initiation of the crack.     

Benchmarking of evaluated nuclear data for uranium by a 14.8 MeV neutron leakage spectra experiment with slab sample

An experimental system for benchmarking of evaluated nuclear data by a 14.8 MeV neutron leakage spectra measurement with slab sample has been setup at China Institute of Atomic Energy (CIAE). The first measurement of the neutron leakage spectra from a 10 × 10 × 5 cm pure uranium slab sample at 45° and 135° was reported. The experimental results were compared with the calculated ones by MCNP-4C simulation, using the evaluated data of uranium from the CENDL3.0, ENDF/B-VII and JENDL3.3 libraries. This work shows that the apparatus and the data processing procedures work well. And more experiments will be done in the future for benchmarking of the evaluated data, especially for the CENDL data.     

Desorption of gold nanoclusters from gold nanodispersed targets by 200 keV Au5 polyatomic ions in the elastic stopping mode: Experiment and molecular-dynamics simulation

Au nanoislet targets (∅ 2-60 nm) were bombarded by 200 keV polyatomic ions (40 keV/atom), which deposit their energy mainly in the nuclear stopping mode: ∑(dE/dx)_n = 30 keV/nm and ∑(dE/dx)_e = 2 keV/nm. The matter desorbed in the form of nanoclusters was registered by TEM. The total transfer of matter was determined by neutron-activation analysis. The total yield of the ejected gold reached high values of up to 2.6 × 10⁴ atoms per Au₅ ion. The major part (2 × 10⁴ atoms per ion Au₅) of the emission is in the form of nanoclusters. The results are compared with the data of similar experiments with 1 MeV Au₅ (200 keV/atom) and other projectiles. The analysis of the experimental data and the comparison to molecular-dynamics simulation results of the desorption process show that the desorption of Au nanoislets is induced by their melting, build-up of pressure and thermal expansion.     

Microstructures and mechanical properties of Fe-14Cr-3W-Ti-Y2O3 steel with 1 wt.% Cu addition fabricated by a new method

Effect of 1 wt.% copper addition on microstructures and mechanical properties of Fe-14Cr-3W-0.4Ti-0.4Y₂O₃ (wt.%) alloy has been investigated. Mechanically alloyed powders and pre-alloyed powders were blended and consolidated by hot extrusion. A bimodal grain structure with large grains (10-20 μm) and nanometer grains was formed. Through aging treatment, ε-Cu phase with face cubic centered lattice structure precipitated from the supersaturated solid solution. After aging for 6 h, the microhardness reached a peak value of HV326, which was attributed to precipitation of copper-rich phase. The alloy exhibited a high strength due to the strengthening of both copper-rich precipitates and Y-Ti-O nanoclusters, and an excellent ductility due to its bimodal structure.