A neutronic study to reduce the costs of pebble bed reactors by varying fuel compositions
Boyes, Wayne Arthur
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Pebble Bed Reactor (PBR) fuel is expensive at around 17% of the total cost of electrical power produced. This if for the Steenkampskraal Thorium Limited (STL) financial model for a single 100 MWth reactor with a cylindrical core, Once Through Then Out (OTTO) fuelling cycle with a thermal efficiency of 40%, due to a steam temperature and pressure of (540˚C, 15MPa), with 33 MWel being sent to the grid. The fuel costs are modelled based on a capacity of 250 000 fuel spheres per annum fuel cost model courtesy of STL. A larger fuel plant would reduce the fuel sphere costs but would require a larger fleet of PBRs. If the fuel costs per unit of electrical energy produced ($/kWh) can be reduced it would make the pebble bed reactor a more feasible option and could make them comparable in terms of costs to other types of nuclear technologies. The two major components of the cost of producing the fuel spheres are broken down into the manufacturing costs (kernel casting costs, kernel coating costs, graphite matrix costs and the fuel sphere production costs) and the nuclear material costs (SWU’s, enrichment, HM loading amount and raw material costs). The cost of the manufacturing dominates over the cost of the nuclear materials. Apart from reducing fuel sphere manufacturing costs, the other important way to reduce the total fuel cost is to increase the total amount of heat energy generated from each fuel sphere, i.e. increasing the cumulative energy per fuel sphere. This can be done by increasing the burn-up of the nuclear fuel and by increasing the heavy metal (HM) loading per fuel sphere. This was attempted by varying the enrichment and HM loading for LEU and for a mixture of thorium (Th) and LEU (ThLEU), as well as a mixture of Th and Highly Enriched Uranium (HEU). The neutron physics and thermo-hydraulic performance of this core were simulated using the VSOP 99/11 suite of codes. The aim with the addition of Th was to improve the neutron economy as the U-233 which is bred from the Th-232 is the fissile nuclear fuel with the best neutron economy in thermal nuclear reactors. The HEU-based fuels showed the best performance. However, since the use of HEU is contentious in most countries, due to its high nuclear weapons proliferation risk, the performance of the HEU fuels will be excluded from this summary. As was expected, the results showed that the burn-up of the nuclear fuel increased sharply and monotonously with increasing enrichment. Therefore, the LEU with the A neutronic study to reduce the fuel costs of pebble bed reactors by varying fuel compositions highest allowable enrichment, namely 20 wt%, produced the highest burn-up and therefore the highest cumulative energy per fuel sphere and therefore the lowest total fuel cost per unit of electrical energy produced. Adding Th to LEU fuel spheres in order to obtain a ThLEU fuel sphere by definition reduces the enrichment of the mixture and increases the HM loading. This decrease in enrichment decreased the burn-up to such an extent that the ThLEU fuel spheres always produced lower cumulative energies per fuel sphere than the pure 20 wt% LEU fuel spheres from which they were derived. From the very low HM loading of 5 g HM per fuel sphere, the burnup first increased with increasing HM loading until it peaked at 7 g HM for LEU and at 10 g HM for ThLEU, where after it decreased with increasing HM loading. The reason for the poor burn-up at very low HM loadings was excessive neutron leakage from the core. The reason for decreasing burnups at very high HM loadings was that the decreasing distance between fuel kernels resulted in under moderation, which decreases the neutron economy and is thus known to reduce burn-up. Since, for a given burn-up, cumulative energy per fuel sphere is directly proportional to HM loading, increasing the HM loading above the values of maximum burn-up initially resulted in increased cumulative energies, where after it peaked and declined as increasing HM loading sharply reduced the burn-up by increasing the problem of under moderation. The maximum cumulative energy was achieved at 16 g HM loading for LEU and at 16 g HM loading for ThLEU, however the lowest Levilised Unit Energy Costs (LUEC) were achieved at 12 g HM for LEU and 12 g HM for ThLEU for all the enrichments. The general trend which is observed is that as the enrichment and HM content increases, the cumulative amount of heat energy per fuel sphere increases and the LUEC decreases. However, there is a point at where increasing the HM loading to an even greater extent actually increases the LUEC and there is an optimum for each fuel type. Generally, 12 g HM actually provided a lower LUEC than 16 g HM for each fuel type. The lowest total fuel costs were achieved for the following fuel compositions for a single First of a Kind (FOAK) reactor per site, as opposed to a multi-pack (many reactors on one site). This explains why the resulting costs were high. A neutronic study to reduce the fuel costs of pebble bed reactors by varying fuel compositions LEU at 20 wt% for 12 g HM would provide the lowest Levilised Unit Energy Cost (LUEC) of 117 US$/MWh, closely followed by LEU at 10 g HM (LUEC = 117 US$/MWh) and LEU at 16 g HM (LUEC = 118 US$/MWh). Lower enrichments such as 15 wt% ThLEU have the lowest LUEC at 12 g HM (LUEC = 119 US$/MWh), followed by ThLEU at 16 g HM (LUEC = 120 US$/MWh) and ThLEU at 10 g HM (LUEC = 120 US$/MWh). LEU at 15 wt% would have a minimum at 12 g HM (LUEC = 121 US$/MWh). At 10 wt% enrichment it can be seen that at higher enrichments ThLEU has a lower LUEC compare to LEU at the same enrichment. ThLEU being lowest at 16 g HM (LUEC = 125 US$/MWh) followed by 12 g HM (LUEC = 126 US$/MWh) while LEU at 12 g HM is lowest (LUEC = 131 US$/MWh). Increasing the enrichment unfortunately substantially increased the maximum fuel temperature during Depressurised Loss of Forced Coolant (DLOFC) accidents. The reason was that by increasing the enrichment, the sharpness in the peaks of the axial power density profiles, which were observed near the top of the fuel core, increased. This increased power hotspot near the top of the core also resulted in a hotspot in the decay heat and thus in the maximum temperature profiles during DLOFC accidents. This increase in DLOFC temperatures with increasing enrichment means that there is a limit to the extent that increasing enrichments can be used to reduce fuel costs. On the other hand, increasing the HM loading slightly reduced the maximum DLOFC temperatures. The reason was that increasing HM loading reduced the sharpness of the axial power density peaks and thus, by the logic explained above, reduced the maximum DLOFC temperatures slightly. Although it was not investigated in this study, it is well-known that increasing HM loading increases the risk of the reactivity increases and, therefore, power increases during water ingress accidents. This means that although increasing HM loading in many cases decreased the total fuel cost and decreased the maximum DLOFC temperature slightly, there is again a limit to which this technique can be utilized safely. Near the optimum point, a large increase in HM loading also often produced only a small decrease in fuel cost and DLOFC temperature. In such cases it is recommended A neutronic study to reduce the fuel costs of pebble bed reactors by varying fuel compositions that the lower HM loading be selected, as this will probably produce a relatively large reduction in the safety risk regarding water ingress, at the cost of only a small increase in fuel cost. The addition of thorium did in fact increase the maximum DLOFC fuel temperature. However, the ThLEU fuels at lower enrichments did not exceed the maximum DLOFC temperature limit. The fuel plant cost model that was used assumed a production rate of 250 000 fuel spheres/annum (147.26 $/FS, 10 wt% 10 g HM LEU). However, increasing this to to 800 000 fuel spheres/annum would reduce the fuel costs drastically to 91.05 $/FS. However, a large enough pebble bed fleet would be needed to warrant such a large fuel plant. An example of a fuels that would be suitable for this reactor is LEU at 10wt% at 12 g HM (LUEC = 131 US$/MWh), which would provide a low LUEC and would adhere to the maximum DLOFC temperature limit. However, lower HM loadings are advisable, such as 7 g HM loading, due to problems associated with water-ingress at high HM loadings. This fuel configuration would produce a LUEC of 138.49 US$/MWh. Correspondingly, an advisable fuel configuration for ThLEU would be 10wt% and 12 g HM. The reason for advising a higher HM loading for ThLEU is that ThLEU can be inherently safe at a higher HM loading in a water ingress scenario. The resulting LUEC being 126 US$/MWh. This study was a multi-criteria decision/optimization analysis which kept the geometry, control rod positions (situated in the top reflector) and the amount of fuel passes (OTTO) constant. The fuel types were varied (LEU, ThLEU & ThHEU) as well as the HM loadings, enrichments and mixtures of the various material fractions within the fuel spheres. The fuel plant cost models were adapted for the various fuel and enrichments. A comparison was done to see how the various fuels behaved and how much cumulative energy each fuel type could provide while carefully observing how the fuel costs changed in the fuel plant cost model. The safety case for each fuel type in terms of maximum fuel temperature during a DLOFC and the reactivity spike due to water-ingress was discussed. However, it was not calculated. A neutronic study to reduce the fuel costs of pebble bed reactors by varying fuel compositions Safety is the number one priority in nuclear engineering and cost comes second. Therefore, the final fuel choices which are suggested to a utility are the safest fuels in the study and secondly the most cost effective. The study is suited to reactor cores of various geometries and loading schemes (OTTO & MODUL) due to the fact that the general trends are the same, that is the more heavy metal and enrichment, the higher the cumulative amount of heat energy etc. The HTR cost figures are generic and not specific to a certain country.
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