SCIENCE
Permanent URI for this collection
Browse
Recent Submissions
1 - 5 of 2036
- ItemDEPLETION ANALYSES OF LOW ENRICHED URANIUM FUELS FOR THE NIGERIA RESEARCH REACTOR-1(2023-03)The use of advanced, accident-tolerant, Low Enriched Uranium (LEU) fuel types is one approach to improving the safety, security and fuel cycle performance of nuclear reactors. In spite of the acceptability of UO2 fuel, uranium-silicide and uranium-molybdenum fuels are being proposed in order to increases the accident tolerance of nuclear reactor cores. Although the as-built fuel of the Nigeria Research Reactor-1 (IRR-1) LEU core is 13% enriched UO2 clad in Zircalloy, previous research have demonstrated that specific uranium-silicide and uranium-molybdenum such as 19.75% enriched U3Si2, U3Si, and U9Mo dispersion LEU fuels clad in aluminum also present comparable neutronic characteristics within recommended safety limit for the NIRR-1 Miniature Neutron Source Reactor (MNSR). However, analysis of the depletion characteristics of the as-built UO2 core as well as the investigated fuel alternates are yet to be conducted as these data are important for optimization of safety, security, fuel management and decommissioning plan a well as conversion of other MNSRs from High Enriched Uranium (HEU) to LEU. Consequently, the SCALE 6.2.3 code system was used to develop new models of the NIRR-1 and perform criticality calculations and depletion analysis for the as-built UO2, U3Si2-Al, U3Si-Al, and U9Mo-Al LEU cores in the present study, using the HEU core as a benchmark. The results showed that the three Dimensional (3D) KENO-VI estimates of the Clean Cold Core Excess Reactivity (CCCER of the core are in good in good agreement with measured data with a bias of less than 4 %. Consequently, the 3D KENO-VI module of the SCALE 6.2.3 code can be used for criticality safety calculations of MNSRs and similar reactors. The neutron flux distribution data indicated a reduction in the magnitude of the average thermal neutron flux in the alternative LEU cores in the range of 7 – 10 % when compared to that of the HEU core. This implies that the thermal power of the U3Si2-Al, U3Si-Al, and U9Mo-Al LEU cores would have to be raised by their corresponding magnitude of percentage flux reduction to match the flux of the HEU core. However, the thermal neutron flux in the as-built LEU-UO2 core is essentially the same with that of the HEU core. Hence, the as-built LEU core will not compromise NIRR-1 utilization for thermal Neutron Activation Analysis. Although the results show that both the HEU and the LEU cores have a burnup of less than 1 % at the End of Cycle of 216 Effective Full Power Days (EFPD), the Uranium-Silicide fuels have a higher burnup when compared to the UO2 fuel. The depletion rates were estimated to be – 0.00120 mk/h, - 0.00124 mk/h, - 0.00123 mk/h and – 0.00105 mk/h for the as-built UO2, U3Si2, U3Si and U-9Mo LEU cores respectively. In all cases, the LEU cores depleted at a slower rate than the HEU core implying an improvement in the fuel economy of the cores compared to the HEU core. The core lifetimes after addition of top Beryllium Shim plates when the reactor is operated continuously for 216 EFPD were estimated to be 38.62, 37.34, 37.53, and 43.78 years for the as-built UO2, U3Si2, U3S i and U-9Mo LEU cores respectively. These estimated core lifetimes are 26% higher than that of the HEU. Consequently, the LEU cores show better fuel cycle performance and demonstrated an advantage of extended core lifetime for utilization in Neutron Activation Analysis. These results are useful in the development of a decommissioning plan for the NIRR-1 and fuel management of accident-tolerant alternative fuels to optimize their performance as promising alternatives to the UO2 fuel.
- ItemDEPLETION ANALYSES OF LOW ENRICHED URANIUM FUELS FOR THE NIGERIA RESEARCH REACTOR-1(2023-05)The use of advanced, accident-tolerant, Low Enriched Uranium (LEU) fuel types is one approach to improving the safety, security and fuel cycle performance of nuclear reactors. In spite of the acceptability of UO2 fuel, uranium-silicide and uranium-molybdenum fuels are being proposed in order to increases the accident tolerance of nuclear reactor cores. Although the as-built fuel of the Nigeria Research Reactor-1 (IRR-1) LEU core is 13% enriched UO2 clad in Zircalloy, previous research have demonstrated that specific uranium-silicide and uranium-molybdenum such as 19.75% enriched U3Si2, U3Si, and U9Mo dispersion LEU fuels clad in aluminum also present comparable neutronic characteristics within recommended safety limit for the NIRR-1 Miniature Neutron Source Reactor (MNSR). However, analysis of the depletion characteristics of the as-built UO2 core as well as the investigated fuel alternates are yet to be conducted as these data are important for optimization of safety, security, fuel management and decommissioning plan a well as conversion of other MNSRs from High Enriched Uranium (HEU) to LEU. Consequently, the SCALE 6.2.3 code system was used to develop new models of the NIRR-1 and perform criticality calculations and depletion analysis for the as-built UO2, U3Si2-Al, U3Si-Al, and U9Mo-Al LEU cores in the present study, using the HEU core as a benchmark. The results showed that the three Dimensional (3D) KENO-VI estimates of the Clean Cold Core Excess Reactivity (CCCER of the core are in good in good agreement with measured data with a bias of less than 4 %. Consequently, the 3D KENO-VI module of the SCALE 6.2.3 code can be used for criticality safety calculations of MNSRs and similar reactors. The neutron flux distribution data indicated a reduction in the magnitude of the average thermal neutron flux in the alternative LEU cores in the range of 7 – 10 % when compared to that of the HEU core. This implies that the thermal power of the U3Si2-Al, U3Si-Al, and U9Mo-Al LEU cores would have to be raised by their corresponding magnitude of percentage flux reduction to match the flux of the HEU core. However, the thermal neutron flux in the as-built LEU-UO2 core is essentially the same with that of the HEU core. Hence, the as-built LEU core will not compromise NIRR-1 utilization for thermal Neutron Activation Analysis. Although the results show that both the HEU and the LEU cores have a burnup of less than 1 % at the End of Cycle of 216 Effective Full Power Days (EFPD), the Uranium-Silicide fuels have a higher burnup when compared to the UO2 fuel. The depletion rates were estimated to be – 0.00120 mk/h, - 0.00124 mk/h, - 0.00123 mk/h and – 0.00105 mk/h for the as-built UO2, U3Si2, U3Si and U-9Mo LEU cores respectively. In all cases, the LEU cores depleted at a slower rate than the HEU core implying an improvement in the fuel economy of the cores compared to the HEU core. The core lifetimes after addition of top Beryllium Shim plates when the reactor is operated continuously for 216 EFPD were estimated to be 38.62, 37.34, 37.53, and 43.78 years for the as-built UO2, U3Si2, U3S i and U-9Mo LEU cores respectively. These estimated core lifetimes are 26% higher than that of the HEU. Consequently, the LEU cores show better fuel cycle performance and demonstrated an advantage of extended core lifetime for utilization in Neutron Activation Analysis. These results are useful in the development of a decommissioning plan for the NIRR-1 and fuel management of accident-tolerant alternative fuels to optimize their performance as promising alternatives to the UO2 fuel.
- ItemANALYSIS OF SET-UP ERRORS IN BREAST CANCER PATIENTS UNDERGOING EXTERNAL BEAM RADIOTHERAPYAT NATIONAL HOSPITAL ABUJA(2023-04)Radiotherapy plays an important role in breast cancer treatment. The main goal of radiationtherapy is to ensure tumor control while avoiding complications to the organs at risks. Uncertainty in the dose deposited in the tumor exists due to a variety of factors which include patient positioning errors. This study assessed and quantified breast cancer patients‟ set-up errors using an electronic portal imaging device and evaluated the dosimetric and biological impact in terms of generalized equivalent uniform dose (gEUD) using predictive models, such as Tumour Control Probability (TCP) and Normal Tissue Complication Probability (NTCP). About 200 breast cancer patients were considered at the Radiotherapy-Oncology department of National Hospital Abuja (NHA). Systematic and random errors were quantified, in addition,three-dimensional treatment planning was performed using CT scan images of the patients. The total prescribed dose was 5000 cGy per 25 fractions for most of the patients. The dosimetric and biological impact of these set-up errors on the target volume and the organ at risk (OARs) coverage were assessed by evaluating the Dose– Volume Histogram (DVH), gEUD, TCP and NTCP. The standard deviations (SDs) of the systematic set-up and random set-up errors were calculated for the Lateral, Anterior-Posterior and Superior-Inferior fields and were found to be 6.6055(4.477), 4.608(3.591), 11.432(8.1748) respectively. Thus, a planning target volume (PTV) margin of 5 mm was defined around the OARs, and around the clinical target volume (CTV). The toxicity of OARs was quantified using gEUD, TCP and NTCP. The data represented that NTCP values for conformal technique was one (1) for the combined lungs at D50, D75, D90 and D98 of the planning target volume (p = 0.05). In addition, NTCP values of the heart were equal to 0.99 at D50, D75, D90 and D98. The TCP outcome shows a negative and not so good correlation with calculated dose to 50%, 75%, 90% and 95% of the target volume (D95%). In conclusion, this research confirmed that more relevant and robust radiobiological parameters should be integrated with more recent dose calculation methods to obtain reliable prediction of organ at risks toxicity and avoid over/under estimating of TCP and NTCP. This is critical if medical decisions have to be based on NTCP estimations in routine practice.
- ItemA STUDY ON THE NIGERIAN NAIRA PER US-DOLLAR EXCHANGE RATE USING ARFIMA-GARCH AND ARFIMA-FIGARCH MODELS(2023-04)Autoregressive Fractionally Integrated Moving Average (ARFIMA) model is widely used in the study of long memory processes but it is not suitable for series exhibiting high periods of volatility. Exchange rate series are characterized by periods of stability followed by periods of instability in volatility which can be modeled by Autoregressive Conditional Heteroskedastic (ARCH) model. A parsimonious generalization of the ARCH model is Generalized ARCH (GARCH), but still, neither ARCH nor GARCH can handle the presence of long memory in volatility. This research investigated the presence of long memory both in mean and volatility of the Nigerian Naira per US-Dollar exchange rate series using the hybrid models of ARFIMA, GARCH and Fractionally Integrated GARCH (FIGARCH) origins. Long memory tests were carried out on fractionally differenced and volatility series. The result of GPH estimator indicated the existence of significant Long Memory in the exchange rate data. Classical ARFIMA model was fitted to the data but the results showed the presence of serial autocorrelation and ARCH effects, signifying the limitations of fitting the ARFIMA model. Hybrid ARFIMA models with conditional variance following GARCH and FIGARCH processes were then respectively fitted to the exchange rate series with much improvement in model fitting. Autocorrelation of residuals and ARCH effects were insignificant showing the adequacy of the fitted hybrid models. At the end of the research, the forecasting performance measures of the fitted ARFIMA-GARCH and ARFIMA-FIGARCH models were determined in terms of RMSE. ARFIMA-GARCH demonstrated a better performance.
- ItemSTRATIGRAPHY AND SEDIMENTOLOGICAL STUDIES OF THE ALBIAN TO MAASTRICHTIAN SEDIMENTS AROUND LAFIA TOWN, LAFIA SHEET 231NW MIDDLE BENUE TROUGH, NIGERIA(2021)The middle Benue Trough is one of the three segments of the larger NE-SW trending Benue Trough. As is the case with the Upper Benue Trough, there does not seem to be any recent work dedicated to the Geology of the Middle Benue Trough. Recent works on the stratigraphy and sedimentological studies of the Albian to Turonian sediments of the middle Benue Trough, covered areas around Keana and Awe respectively (all within sheet 232 NW). This work covered Areas within Lafia (Sheet 231NW). The study area is located southeast of Lafia town. It is accessible by some major roads which includes: Lafia to Makurdi, Doma to Agyaragu and Lafia to Keana. Reconnaissance study was first carried out in the study area on a scale of 1:100000 followed by a detailed field mapping on a scale of 1:50,000. In the course of the field work, the lithostratigraphic successions in the study area was established, sedimentary structures were identified and samples collected for petrographic and sieve analysis. Stratigraphic study in the study area indicates the occurrence of three formations namely; the fluvial Lafia Formation (Campano-Maastrichtian), the shallow marine Awgu Formation (Turonian-Coniacian) and the transitional Awe Formation (Albian-Cenomanian). A total of 12 samples were collected for granulometric analysis, and 11 samples for Petrographic analysis. The Lafia Formation consist of ferrugenised sandstone at the base, clay and clayed loosed sands. Petrographic and sieve analysis revealed that the Ferrugenised sandstone range from fine-medium grain. It is moderately sorted, strongly fine skewed and is a Quartz arenite. More than 60% of the crystals are monocrystalline which is indicative of igneous provenance. The Awe Formation consists of fine-grained white sandstone, with some intercalations of white clay and laminated shale. The sandstones are fine-grained and poorly-moderately sorted which coupled with the bivariate plot that indicates a transitional conditions between shallow marine and fuvial systems. The sandstones of the Awe Formation range from Arkose to Subarkose. The presence of monocrystalline quartz and plagioclase feldspars are all indicators of igneous provenance. The Awgu Formation consists of sandstones that are fine grained, very poorly sorted and strongly fine-skewed. The sandstones of the Awgu Formation are Quartz Arenites. There are ostracodes, echinoderms, and ammonites fossils within the shaley limestone and limestones of the Awgu Formation. This is an indication of a shallow marine environment of deposition. The Sedimentary structures in the study area include; beddings, mudcracks, faults, joints and veins and unconformity. The bedding planes are indications of break in deposition of the sediments while the mudcracks indicates long exposure to a dry warm climate. Other Methods of studies such as aeromagnetic studies, palynological studies and diagenetic studies should be carried out for more geological information of the study area to properly characterise the Middle Benue Trough