Thesis (MSc (Physics))--Stellenbosch University, 2010. / ENGLISH ABSTRACT: This dissertation reports on the impact of a set of selected nuclear fuel management parameters on reactor operations of the PBMR core. This is achieved by performing an assessment of the impact of nuclear fuel management parameter variations on the most important safety and economics issues for the PBMR core. These include the maximum fuel temperature at steady state and during Depressurized Loss of Forced Cooling (DLOFC) accident conditions. The reactivity worth of the Reactor Control System (RCS which determines the shutdown capability of the reactor core and the average discharge burn-up of fuel are also established. The fuel management parameters considered in this study include different enrichment levels, heavy metal loadings and fuel sphere circulation regimes. The impact and importance of these parameters on plant safety and economics is assessed. The dissertation will report the effects on the standard core physics parameters such as power peaking, multiplication factor, burn-up (safety and economics) and derive the benefits and drawbacks from the results. Based upon the findings from this study, and also experimental data, an optimum fuel management scheme is proposed for the PBMR core. / AFRIKAANSE OPSOMMING: Hierdie verhandeling beskryf die uitwerking van ‘n gekose stel kernbrandstofparameters op die bedryf van die PBMR reaktor. Die impak wat variasies in kernbrandstofparameters op belangrike veiligheids- en ekonomiese oorwegings het, is tydens hierdie studie ondersoek. Van die belangrikste oorwegings is die maksimum brandstoftemperatuur tydens normale, konstante bedryf, asook gedurende ‘n “Depressurized Loss of Forced Cooling (DLOFC)” insident waar alle verkoeling gestaak word. Ander belangrike fasette wat ondersoek is, is die reaktiwiteitwaarde van die beheerstelsel (RCS), wat die aanleg se vermoë om veilig af te sluit bepaal, asook die totale kernverbruik van die brandstof. Die kernbrandstofparameters wat in ag geneem is, sluit die brandstofverryking, swaarmetaalinhoud en die aantal brandstofsirkulasies deur die reaktorhart in. Die belangrikheid en impak van elk van hierdie parameters is ondersoek en word in die verhandeling beskryf . Daar word verslag gelewer oor die voor- en nadele, asook die uitwerking van hierdie variasies op standaard reaktorfisika-parameters soos drywingspieke in die brandstof, neutronvermenigvuldigingsfaktore en kernverbuik van die brandstof, vanaf ‘n veiligheids- en ekonomiese oogpunt. Gebaseer op die gevolgtrekkings van hierdie studie, tesame met eksperimentele data, word ‘n optimale kernbrandstofbestuurprogram voorgestel.

The increasing demand for energy and the effect on climate change are some of the big drivers in support of the nuclear renaissance. A great amount of energy is spent on studies to determine the contribution of nuclear power to the future energy supply. Many countries are investing in generation III and IV reactors such as the Westinghouse AP1000 because of its passive cooling system, which makes it attractive for its safety. The pebble bed high temperature gas cooled reactors are designed to be intrinsically safe, which is one of the main drivers for developing these reactors. A pebble bed reactor is a high temperature reactor which is helium–cooled and graphitemoderated using spherical fuel elements that contain triple–coated isotropic fuel particles (TRISO). The success of its intrinsic safety lies in the design of the fuel elements that remain intact at very high temperatures. When temperatures significantly higher than 1600 °C are reached during accidents, the fuel elements with their inherent safety features may be challenged. A pebble bed reactor has an online fuelling concept, where fuel is circulated through the core. The fuel is loaded at the top of the core and through gravity, moves down to the bottom where it is unloaded to either be discarded or to be re–circulated. This is determined by the burnup measuring system. By circulating the fuel spheres more than once through the reactor a flattened axial power profile with lower power peaking and therefore lower maximum fuel temperatures can be achieved. This is an attractive approach to increase the core performance by lowering the important fuel operating parameters. However, the circulation has an economic impact, as it increases the design requirements on the burnup measuring system (faster measuring times and increased circulation). By adopting a multi–pass recycling scheme of the pebble fuel elements it is shown that the axial power peaking can be reduced The primary objective for this study is the investigation of the influences on the core design with regards to the number of fuel passes. The general behaviour of the two concepts, multi–pass refuelling and a once–through circulation, are to be evaluated with regards to flux and power and the maximum fuel temperature profiles. The relative effects of the HTR–Modul with its cylindrical core design and the PBMR 400 MW with its annular core design are also compared to verify the differences and trends as well as the influences of the control rods on core behaviour. This is important as it has a direct impact on the safety of the plant (that the fuel temperatures need to remain under 1600 °C in normal and accident conditions). The work is required at an early stage of reactor design since it influences design decisions needed on the fuel handling system design and defuel chute decay time, and has a direct impact on the fuel burnup–level qualification. The analysis showed that in most cases the increase in number of fuel passes not only flattens the power profile, but improves the overall results. The improvement in results decreases exponentially and from ten passes the advantage of having more passes becomes insignificant. The effect of the flattened power profile is more visible on the PBMR 400 MW than on the HTR–Modul. The 15–pass HTR–Modul design is at its limit with regards to the measuring time of a single burnup measuring system. However, by having less passes through the core, e.g. tenpasses, more time will be available for burnup measurement. The PBMR 400 MW has three defuel chutes allowing longer decay time which improves measurement accuracy, and, as a result could benefit from more than six passes without increasing the fuel handling system costs. The secondary objective of performing a sensitivity analysis on the control rod insertion positions and the effect of higher fuel enrichment has also been achieved. Control rod efficiency is improved when increasing the excess reactivity by means of control rod insertion. However, this is done at lower discharge burnup and shut down margins. Higher enrichment causes an increase in power peaking and more fuel–passes will be required to maintain the peaking and temperature margins than before. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.

The increasing demand for energy and the effect on climate change are some of the big drivers in support of the nuclear renaissance. A great amount of energy is spent on studies to determine the contribution of nuclear power to the future energy supply. Many countries are investing in generation III and IV reactors such as the Westinghouse AP1000 because of its passive cooling system, which makes it attractive for its safety. The pebble bed high temperature gas cooled reactors are designed to be intrinsically safe, which is one of the main drivers for developing these reactors. A pebble bed reactor is a high temperature reactor which is helium–cooled and graphitemoderated using spherical fuel elements that contain triple–coated isotropic fuel particles (TRISO). The success of its intrinsic safety lies in the design of the fuel elements that remain intact at very high temperatures. When temperatures significantly higher than 1600 °C are reached during accidents, the fuel elements with their inherent safety features may be challenged. A pebble bed reactor has an online fuelling concept, where fuel is circulated through the core. The fuel is loaded at the top of the core and through gravity, moves down to the bottom where it is unloaded to either be discarded or to be re–circulated. This is determined by the burnup measuring system. By circulating the fuel spheres more than once through the reactor a flattened axial power profile with lower power peaking and therefore lower maximum fuel temperatures can be achieved. This is an attractive approach to increase the core performance by lowering the important fuel operating parameters. However, the circulation has an economic impact, as it increases the design requirements on the burnup measuring system (faster measuring times and increased circulation). By adopting a multi–pass recycling scheme of the pebble fuel elements it is shown that the axial power peaking can be reduced The primary objective for this study is the investigation of the influences on the core design with regards to the number of fuel passes. The general behaviour of the two concepts, multi–pass refuelling and a once–through circulation, are to be evaluated with regards to flux and power and the maximum fuel temperature profiles. The relative effects of the HTR–Modul with its cylindrical core design and the PBMR 400 MW with its annular core design are also compared to verify the differences and trends as well as the influences of the control rods on core behaviour. This is important as it has a direct impact on the safety of the plant (that the fuel temperatures need to remain under 1600 °C in normal and accident conditions). The work is required at an early stage of reactor design since it influences design decisions needed on the fuel handling system design and defuel chute decay time, and has a direct impact on the fuel burnup–level qualification. The analysis showed that in most cases the increase in number of fuel passes not only flattens the power profile, but improves the overall results. The improvement in results decreases exponentially and from ten passes the advantage of having more passes becomes insignificant. The effect of the flattened power profile is more visible on the PBMR 400 MW than on the HTR–Modul. The 15–pass HTR–Modul design is at its limit with regards to the measuring time of a single burnup measuring system. However, by having less passes through the core, e.g. tenpasses, more time will be available for burnup measurement. The PBMR 400 MW has three defuel chutes allowing longer decay time which improves measurement accuracy, and, as a result could benefit from more than six passes without increasing the fuel handling system costs. The secondary objective of performing a sensitivity analysis on the control rod insertion positions and the effect of higher fuel enrichment has also been achieved. Control rod efficiency is improved when increasing the excess reactivity by means of control rod insertion. However, this is done at lower discharge burnup and shut down margins. Higher enrichment causes an increase in power peaking and more fuel–passes will be required to maintain the peaking and temperature margins than before. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.