School of Pure and Applied Sciences
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Item Analysis of the Impact of Cross-sectional Data Discrepancies on the Effectiveness of Radiation Shielding Design Using Monte Carlo Codes(International Journal of Physics, 2019) Kebwaro, Jeremiah MonariThe effect of cross-sectional data discrepancies on the effectiveness of radiation shielding design has been investigated in this paper. The MCNP code with cross-sections from ENDF/B-V and ENDF/B-VII has been used to determine the gamma ray dose equivalent, H*(10), behind a lead glass shield enclosing a slow neutron source. It is observed that the radiative capture gamma ray dose behind the shield is higher when ENDF/B-V cross-sections are used compared to that produced by ENDF/B-VII cross-sections. The discrepancy is due to absence of energetic primary gamma rays when ENDF/B-VII is used. The results show that shielding design using ENDF/B-VII cross-sections could underestimate the shield by a fair margin and compromise safety. It is therefore necessary to consider more than one release of ENDF/B when using lead glass for shielding slow neutron capture gamma rays. The discrepancies need to be addressed in the next releases.Item Reproducibility of (n,γ) gamma ray spectrum in Pb under different ENDF/B releases(Elsevier, 2016-02) Kebwaro, Jeremiah MonariRadiative capture reactions are of interest in shielding design and other fundamental research. In this study the reproducibility of (n,γ) reactions in Pb when cross-section data from different ENDF/B releases are used in the Monte-Carlo code, MCNP, was investigated. Pb was selected for this study because it is widely used in shielding applications where capture reactions are likely to occur. Four different neutron spectra were declared as source in the MCNP model which consisted of a simple spherical geometry. The gamma ray spectra due to the capture reactions were recorded at 10 cm from the center of the sphere. The results reveal that the gamma ray spectrum produced by ENDF/B-V is in reasonable agreement with that produced when ENDF/B-VI.6 is used. However the spectrum produced by ENDF/B-VII does not reveal any primary gamma rays in the higher energy region (E > 3 MeV). It is further observed that the intensities of the capture gamma rays produced when various releases are used differ by a some margin showing that the results are not reproducible. The generated spectra also vary with the spectrum of the source neutrons. The discrepancies observed among various ENDF/B releases could raise concerns to end users and need to be addressed properly during benchmarking calculations before the next release. The evaluation from ENDF to ACE format that is supplied with MCNP should also be examined because errors might have arisen during the evaluation.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 New Measurement of Antineutrino Oscillation with the Full Detector Configuration at Daya Bay(PHYSICAL REVIEW LETTERS, 2015-05) Kebwaro, Jeremiah MonariWe report a new measurement of electron antineutrino disappearance using the fully-constructed Daya Bay Reactor Neutrino Experiment. The final two of eight antineutrino detectors were installed in the summer of 2012. Including the 404 days of data collected from October 2012 to November 2013 resulted in a total exposure of 6.9×105 GWth-ton-days, a 3.6 times increase over our previous results. Improvements in energy calibration limited variations between detectors to 0.2%. Removal of six 241Am-13C radioactive calibration sources reduced the background by a factor of two for the detectors in the experimental hall furthest from the reactors. Direct prediction of the antineutrino signal in the far detectors based on the measurements in the near detectors explicitly minimized the dependence of the measurement on models of reactor antineutrino emission. The uncertainties in our estimates of sin2 2θ13 and |∆m2 ee| were halved as a result of these improvements.Analysisoftherelativeantineutrinoratesandenergyspectrabetweendetectorsgave sin2 2θ13 = 0.084±0.005and |∆m2 ee| = (2.42±0.11)×10−3 eV2 in the three-neutrino framework.Item Search for a Light Sterile Neutrino at Daya Bay(PHYSICAL REVIEW LETTERS, 2014-07) Kebwaro, Jeremiah MonariA search for light sterile neutrino mixing was performed with the first 217 days of data from the Daya Bay ReactorAntineutrinoExperiment. Theexperiment’suniqueconfigurationofmultiplebaselinesfromsix2.9GWth nuclear reactors to six antineutrino detectors deployed in two near (effective baselines 512 m and 561 m) and one far (1579 m) underground experimental halls makes it possible to test for oscillations to a fourth (sterile) neutrino in the 10−3 eV2 < |∆m241| < 0.3 eV2 range. The relative spectral distortion due to electron antineutrinodisappearancewasfoundtobeconsistentwiththatofthethree-flavoroscillationmodel. Thederivedlimits on sin2 2θ14 cover the 10−3 eV2 .|∆m2 41|. 0.1 eV2 region, which was largely unexplored.Item Independent Measurement of θ13 via Neutron Capture on Hydrogen at Daya Bay(Physical Review, 2014) Kebwaro, Jeremiah MonariA new measurement of the θ13 mixing angle has been obtained at the Daya Bay Reactor Neutrino Experiment via the detection of inverse beta decays tagged by neutron capture on hydrogen. The antineutrino events for hydrogen capture are distinct from those for gadolinium capture with largely different systematic uncertainties, allowing a determination independent of the gadolinium-capture result and an improvement on the precision of θ13 measurement. With a 217-day antineutrino data set obtained with six antineutrino detectors and from six 2.9 GWth reactors, the rate deficit observed at the far hall is interpreted as sin2 2θ13 =0.083 ± 0.018 in the three-flavor oscillation model. When combined with the gadolinium-capture result from Daya Bay, we obtain sin2 2θ13=0.089 ± 0.008 as the final result for the six-antineutrino-detector configuration of the Daya Bay experiment.Item Radiometric assessment of natural radioactivity levels around Mrima Hill, Kenya(International Journal of the Physical Sciences, 2011-07) Kebwaro, Jeremiah Monari; Rathore, I.V.S.; Hashim, N.O.; Mustapha, A.O.Mrima Hill, located in the South coast of Kenya is known for high natural background radiation, due to the presence of radiogenic heavy minerals such as monazites and carbonatites. The activity concentration of 238U, 232Th and 40K in soil samples from the hill have been determined by gamma ray spectrometry using NaI(Tl) detector and decomposition of measured gamma-spectra. As a measure of radiation hazard to the public, gamma radiation dose rates were also estimated. The average activity concentrations of 238U, 232Th and 40K were 207.0±11.3, 500.7±20.0 and 805.4±20.0 Bqkg-1, respectively. The mean absorbed dose rate in air is 440.7±16.8 nGyh-1 while the estimated annual average effective dose rate is 1.11±0.01 mSvy-1. The absorbed dose rate due to gamma radiation from naturally occurring radioactive materials is above the global average value of 60 nGyh-1 (UNSCEAR, 2000)