Department of Physical Sciences
Permanent URI for this collectionhttp://localhost:4000/handle/20.500.12092/1963
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Item Simulation of (16)O (n, p) (16)N reaction rate and nitrogen-16 inventory in a high performance light water reactor with one pass core(Applied Radiation and Isotopes; Elsevier, 2014-12) Kebwaro, Jeremiah Monari; Zhao, Yaolin; He, ChaohuiThe rate of activation of the isotope (16)O to (16)N in a typical HPLWR one pass concept was calculated using MCNP code. A mathematical model was used to track the inventory of the radioisotope (16)N in a unit mass of coolant traversing the system. The water leaving the moderator channels has the highest activity in the circuit, but due to interaction with fresh coolant at the lower plenum, the activity is downscaled. The calculated core exit activity is higher than values reported in literature for commercial boiling water reactors.Item Investigation of photoneutron and capture gamma-ray production in Pb and W under irradiation from 16N decay radiation(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms; Elsevier, 2015-09) Kebwaro, Jeremiah Monari; Zhao, Yaolin; He, ChaohuiLead and tungsten are potential alternative materials for shielding reactor ex-core components with high 16N activity when available space limits application of concrete. Since the two materials are vulnerable to photonuclear reactions, the nature and intensity of the secondary radiation resulting from (γ,n) and (n,γ) reactions when 16N decay radiation interact with these materials need to be well known for effective shielding design. In this study the MCNP code was used to calculate the photoneutron and capture gamma-ray spectra in the two materials when irradiated by 16N decay radiation. It was observed that some of the photoneutrons generated in the two materials lie in the low-energy range which is considered optimum for (n,γ) reactions. Lead is more transparent to the photoneutrons when compared to tungsten. The calculations also revealed that the bremsstrahlung generated by the beta spectrum was not sufficient to trigger any additional photoneutrons. Both energetic and less energetic capture gamma-rays are observed when photoneutrons interact with nuclei of the two materials. Depending on the strength of the 16N source term, the secondary radiation could affect the effectiveness of the shield and need to be considered during design.Item Design and optimization of HPLWR high pressure Turbine gamma ray shield(Nuclear Engineering and Design; Elsevier, 2015-04) Kebwaro, Jeremiah Monari; Zhao, Yaolin; He, ChaohuiThis work proposes the optimum gamma ray shield thickness around the HPLWR high pressure turbine for different occupancy periods in the turbine building. Monte Carlo method was employed in the design process and only radioactive nitrogen-16 was considered as the source of radiation. Five grades of concrete (ordinary, magnetite, heavy magnetite, steel magnetite and barite) were used as shielding materials. The isotope source term in the high pressure turbine was estimated by modeling the HPLWR three pass core in MCNP and tracking the inventory using a simple algorithm. The high pressure turbine was thereafter modeled in MCNP with a concrete shield arrayed in layers around it. The surface flux tally and ICRP74 dose conversion coefficients were employed to estimate the dose profile across the shield. For some shielding materials, exponential functions were fitted on the calculated data to extrapolate dose values beyond the model thickness. The optimum shield thickness was determined by comparing the calculated dose profiles with dose limit proposals in the IAEA standard (NS-G-1.13) on radiation protection considerations during nuclear power plant design. It was observed that with a 120 cm thick heavy concrete shield, the turbine building would be safe for most occupancy periods. However for ordinary concrete the shield would require some extension to guarantee safety. For very long occupancy (more than 10 person hours per week), magnetite shield may also require slight extension. It can therefore be concluded that the shield thickness recommended for BWR turbines (which operate on a direct cycle like HPLWR) could be sufficient for HPLWR if high density concretes are used.Item Evaluation of candidate materials for SCWR turbine and balance of plant shielding(Progress in Nuclear Energy; ScienceDirect, 2015-03) Kebwaro, Jeremiah Monari; Zhao, Yaolin; He, ChaohuiSince the coolant leaving the SCWR core contains an assortment of radioisotopes, it is necessary to identify appropriate materials for shielding ex-core components. Photon attenuation characteristics, photo-neutron production capacity and cost effectiveness of candidate materials were investigated in this study. WinXcom computer code was used for attenuation studies while other properties were surveyed in literature. High Z materials (lead and tungsten) show excellent gamma attenuation properties however they are expensive and could be vulnerable to photo-neutron production if used for shielding turbines or the pressure vessel exit steam-line. Barite concrete which is a moderate attenuator could also be susceptible to photo-neutron production if used for shielding components with high Nitrogen-16 activity. Heavy concretes with iron aggregates on the other hand show fair attenuation and are not susceptible to photo-neutron production in the energy range of photons released from reactor water. In terms of cost, concretes are cheaper and easy to fabricate compared to high Z materials especially when a shield of intricate shape is required. Depending on the available space for the shield, heavy concretes would be the most appropriate materials for shielding the SCWR turbine and balance of plant. However in case of space limitation, their attenuation capacity can be enhanced by introducing high Z materials in reasonable proportions.Item Evaluation of permutite for removal of radiocobalt from nuclear wastewater(Journal of Radioanal Nucl Chem, 2014-07) Zhao, Xianghai; Luo, Yong; He, Chaohui; Zong, Pengfei; Zhang, Kai; Kebwaro, Jeremiah Monari; Li, Kui; Fu, Baofeng; Zhao, YaolinA synthetic zeolite, commercially available as permutite was tested for removing radiocobalt from aque- ous solutions by sorption. The sorption process was studied as a function of contact time, pH, ionic strength and solid content by batch technique. The results revealed that the process is dependent on the pH and ionic strength under ambient conditions. The kinetic sorption was fitted well by a pseudo-second-order rate equation. The thermodynamic parameters (DH0, DS0, DG0) suggested that sorption behavior of Co(II) is a spontaneous and endothermic pro- cess. The study shows that permutite has excellent potential for disposal of wastewaters containing radiocobalt.