Wayne Boyes

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Neutronics characteristics of a 165 MWth Xe-100 reactor

Nuclear Engineering and Design, 2020

The Xe-100 is a high temperature, helium-cooled, graphite moderated, pebble bed reactor (HTGR) fe... more The Xe-100 is a high temperature, helium-cooled, graphite moderated, pebble bed reactor (HTGR) featuring a 6times through, multi-pass (MEDUL = MEhrfachDUrchLauf) fuelling scheme. It is designed for delivering heat in the form of superheated steam at 565°C and 16.5 MPa. This steam can be used to generate electricity, provide process heat for petrochemical application, or for cogeneration applications, such as the treatment of contaminated water resources, whilst simultaneously generating electricity. The design is characterized as a Generation IV reactor, with a core that cannot melt and featuring safety characteristics that will eliminate the need at any time to evacuate or displace the public, or that will cause unallowable contamination of the land. The technology is based on proven technology that has demonstrated these safety claims both experimentally and in commercial deployment. The foundation of proof ensures high levels of design and technical readiness. In the overview presented below a parametric comparison is offered of a 165 MW th design against that of Xenergy's 200 MW th reference Xe-100 design. The excess reactivity in both cases is shown to be limited by the continuous fuelling regime, whilst the core geometry and selected core power density enables passive heat removal. In this way the desired intrinsic safety design features of a typical GEN IV design are guaranteed. It is shown that even in the event of a depressurized loss of forced coolant (DLOFC) design basis event, no significant amount of radiological material will be released. A larger margin of tolerance is afforded in the 165 MW th design case, as expected. The coupled core neutronic and thermo-fluid dynamics design for the Xe-100 is performed with the VSOP-A and MGT systems of codes. For the equilibrium core, VSOP results demonstrate that the spherical fuel powers (maximum 3.0 kW) and operational temperatures (< 1000°C) fall well within the envelope of the design criteria. Adequate reactivity control and long-term, cold shutdown are provided by two separate, independently actuated systems, while the overall negative reactivity temperature coefficient is illustrated over the total operational range.

$Fig. 9. Isotopic depletion and HM utilization in the Xe-100 core. Pebbles emerging from the core after only one pass contain a high Pu-239 fraction of the total Pu content. After 6 passes, the plutonium vector is comparable to the spent fuel content of other thermal reactors.$

$Fig. 13. Core volume fractions at indicated temperatures during a DLOFC as- sumed at plant end of life. achieved at around 17 h into the event. The average fuel temperature is also shown and a core volume vs fuel temperature representation given in Fig. 13 to help explain the difference.$

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A neutronic study to reduce the costs of pebble bed reactors by varying fuel compositions

Pebble Bed Reactor (PBR) fuel is expensive at around 17% of the total cost of electrical power pr... more 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

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