The pebble bed type High Temperature Gas-cooled Reactor (HTGR) is among the interesting nuclear reactor designs in terms of safety and flexibility for cogeneration applications. In addition, the strong inherent safety characteristics of the pebble bed reactor (PBR) which is based on natural mechanisms improve the simplicity of the PBR design, in particular for the Once-Through-Then-Out (OTTO) cycle PBR design. One of the important challenges of the OTTO cycle PBR design, and nuclear reactor design in general, is improving the nuclear fuel utilization which is shown by attaining a higher burnup value. This study performed a preliminary neutronic design study of a 200 MWt OTTO cycle PBR with high burnup while fulfilling the safety criteria of the PBR design.The safety criteria of the design was represented by the per-fuel-pebble maximum power generation of 4.5 kW/pebble. The maximum burnup value was also limited by the tested maximum burnup value which maintained the integrity of the pebble fuel. Parametric surveys were performed to obtain the optimized parameters used in this study, which are the fuel enrichment, per-pebble heavy metal (HM) loading, and the average axial speed of the fuel. An optimum design with burnup value of 131.1 MWd/Kg-HM was achieved in this study which is much higher compare to the burnup of the reference design HTR-MODUL and a previously proposed OTTO-cycle PBR design. This optimum design uses 17% U-235 enrichment with 4 g HM-loading per fuel pebble.
The research related to thermal management has been significantly inreased, especially for NPP safety. The use of passive cooling systems both during the accident and operation become reliable in the advanced reactor safety systems. Therefore it should be enhanced through experimental studies to investigate heat transfer phenomenon of the heat decay in transient cooling condition. An investigation has been performed through experiment using an NC-Queen apparatus constructed with rectangular loop. Piping were consisting of tubes of SS316L with diameter, length, and width of 3/4 inch, 2.7 m, and 0.5 m respectively. The height between heater and cooler was 1.4 m. The experiment used initial water temperature at 70 o C, 80 o C, and 90 o C in heater area. Transient temperature was used as experimental data to calculate water mass flow rate. The results showed that the temperature in heater area and cooler area were decreasing of about 90.6% and 95.7% at initial temperatur of 80 o C, and of about 71.1% and 59.4% at initial temperature of 70 o C. Those results were at higher initial temperature of 90 o C compared with the initial temperature of 90 o C. The average of water mass flow rate increased 81.03% from initial temperatur of 70 o C. It was shown that the averages of removed heat in every second from water due to heat loss and cooler, were 3.51 watts, 5.06 watts and 6.85 watts respectively. The initial condition of heat stored in the water was quite different, but to the cooler heat removal capacity and heat loss was almost the same.
Graphite is used as the moderator, fuel barrier material, and core structure in High Temperature Reactors (HTRs). However, despite its good thermal and mechanical properties below the radiation and high temperatures, it cannot avoid corrosion as a consequence of an accident of water/air ingress. Degradation of graphite as a main HTR material and the formation of dangerous CO gas is a serious problem in HTR safety. One of the several steps that can be adopted to avoid or prevent the corrosion of graphite by the water/air ingress is the application of a thin layer of silicon carbide (SiC) on the surface of the fuel element. This study investigates the effect of applying SiC coating on the fuel surfaces of pebble-bed HTR in water ingress accident from the reactivity points of view. A series of reactivity calculations were done with the Monte Carlo transport code MCNPX and continuous energy nuclear data library ENDF/B-VII at temperature of 1 200 K. Three options of UO2, PuO2, and ThO2/UO2 fuel kernel were considered to obtain the inter comparison of the core reactivity of pebble-bed HTR in conditions of water/air ingress accident. The calculation results indicated that the UO2-fueled pebble-bed HTR reactivity was slightly reduced and relatively more decreased when the thickness of the SiC coating increased. The reactivity characteristic of ThO2/UO2-fueled pebble-bed HTR showed a similar trend to that of UO2, but did not show reactivity peak caused by water ingress. In contrast with UO2- and ThO2-fueled pebble-bed HTR, although the reactivity of PuO2-fueled pebble-bed HTR was the lowest, its characteristics showed a very high reactivity peak (0.33 Δk/k) and this introduction of positive reactivity is difficult to control. SiC coating on the surface of the plutonium fuel pebble has no significant impact. From the comparison between reactivity characteristics of uranium, thorium and plutonium cores with 0.10 cm thick SiC coating, it can be concluded that the effect of SiC coating on core reactivity in water ingress accident is more dominant for the pebble-bed HTR fuelled with thorium than those with uranium and plutonium fuels.
Fuel management of once-through-then-out (OTTO) cycle Pebble Bed Reactors (PBR) was studied, in particular the start-up condition of the core before it achieves the equilibrium condition. Optimum and simple fuel management performance in the start-up condition is important for the practical deployment of PBR. There is no option for fuel re-insertion in an OTTO cycle PBR, hence nuclear fuel utilization is an important factor not only in the equilibrium condition but also in the start-up condition. The purpose of the study was to find an optimum procedure to improve burnup performance of fuel management in startup of the OTTO cycle PBR. Initial Heavy Metal (HM) loading in the core, power density, and multiplication factors were the main parameters investigated. The target of the analysis was a small sized 10MW PBR. A newly developed code system for OTTO cycle PBR was used. The code system is capable of performing neutron transport and depletion calculations of the OTTO cycle PBR covering whole of its fuel management scheme from initial loading to equilibrium condition. In this study, fuel composition in the start-up condition was limited to the same composition (a single enrichment) as the fuel in equilibrium condition for simplicity of the whole fuel management. The equilibrium condition of the PBR was investigated first. Based on the equilibrium condition two start-up fuel management schemes, a mixed and top-bottom scheme, were investigated. It was found that mixed scheme is better compare to top bottom scheme in achieving efficient HM-loading. Mixed scheme also gave a lower maximum power density. For the chosen target equilibrium design with 10wt% enrichment and 12g-HM/pebble the minimum initial HM-loading using mixed and top-bottom scheme was 97.1 kg and 161.9 kg, respectively. While the maximum power density at that minimum initial loading was 4.5 W/cm 3 and 4.9 W/cm 3 , respectively.
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