HTR FUEL | FUEL AND FUEL CYCLE
Solid waste
Uranium recovery
U3O8
production
Kernal
TRISO coating
Fuel element production
Fuel spheres or compacts
Left, figure 2: Structure of the NUKEM HTR TRISO fuel production plant
Waste-water treatment Graphite
Matrix graphite material
Effluent
core meltdown is practically impossible as the generated heat will intrinsically be able to passively dissipate into the environment even without an active helium cooling circuit. This is supported by the small core power density (compared to a PWR) and the large heat capacity and temperature stability of the graphite-based core itself. HTR concepts are thus suitable to be combined with the
technical advantages of the SMR, modularity, the potential for standardisation, increased design integration and reduced size. The availability of high-quality TRISO fuel is key to all HTRs. The HTR-typical high outlet temperatures can also be utilized for chemical or other industrial processes that have used fossil fuel generated heat so far.
HTR Fuel The key component of each HTR is its tightly-specified fuel, which allows it to operate at full performance. Starting in the early 1960s, research and development of HTRs and their associated fuel was carried out in Europe and the USA. In Europe work was concentrated in the UK and Germany. The German HTR programme was initiated in the early 1960s as part of a civil nuclear development programme. Within this programme, NUKEM, for example, was focused on the design of fuel elements, fuel specifications, the development of the fuel manufacturing processes and the actual production of HTR fuel. During the 1970s and 1980s NUKEM’s 100% subsidiary HOBEG (Hochtemperaturreaktor- Brennelement GmbH) manufactured and supplied more than 250,000 spherical fuel elements for the AVR experimental nuclear power plant at Jülich and more than 1,000,000 fuel elements for the Thorium High Temperature Reactor (THTR-300) at Hamm-Uentrop in Germany. Based on a highly systematic approach and the development of special quality control procedures for the production processes, fuel quality was continuously investigated and quality standards were established. Consequently, the highest level of HTR fuel quality with regard to minimum fission product release was achieved at this time – and still represents today’s quality standards. The German experimental AVR (construction began in
1961) was the origin of succeeding pebble bed HTRs like the German THTR-300 (construction started in 1971), Chinese experimental reactor HTR-10 (construction began in 1995), its power producing predecessor HTR-PM (construction start
2012, 250 MWt per unit) and the South African PBMR (never constructed for financial reasons, 400 MWt). PBMR and the HTR-PM are examples of HT-SMRs and both use spherical fuel elements which are based on the HOBEG/NUKEM pebble manufacturing process. As opposed to the ‘German-origin’ pebble bed reactors
there is another concept based on cylindrical fuel compacts originating from the United Kingdom experimental Dragon Reactor (construction began in 1960, 20 MWt). Fuel compacts are arranged in a prismatic fuel assembly – usually a hexagonal graphite block with rod-shaped openings that are filled with the cylindrical fuel compacts. HTR fuel in the form of a cylindrical compact or a
spherical pebble consists of many small uranium kernels of about 0.5 mm in diameter. Uranium can either be in the form of pure uranium dioxide or uranium oxycarbide (UCO), which is a mixture of uranium dioxide with a certain fraction of uranium carbide.
While the German Thorium-High-Temperature-Reactor
(THTR-300) utilised highly enriched uranium (HEU of 93 %) with added thorium, nowadays only uranium with lower enrichment levels is used due to the risk of proliferation. High-assay low enriched uranium (HALEU) is established as the term to describe uranium with enrichments ranging from 5% to 20%, which are usually used for modern advanced reactors, including HTRs. Each uranium oxide or carbide kernel is coated with
several layers of pyrolytic carbon (PyC) as well as a durable silicon carbide (SiC) layer. While the inner PyC layer is porous and capable of absorbing gaseous fission products, the dense outer PyC layers forms a barrier against fission product release. The SiC layer improves the mechanical strength of this barrier and thus the retention capacity for certain fission products. The proven German TRISO spherical fuel, based on the NUKEM design, has demonstrated the best fission product release rate, particular at high temperatures. The enriched uranium TRISO particles were contained in a moulded graphite sphere. A NUKEM fuel sphere consists of approximately 9 g of uranium (about 15,000 TRISO-coated kernels) and has a diameter of 60 mm – the total mass of a fuel sphere is 210 g. In more recent projects NUKEM also developed cylindrical compact fuel based on the same TRISO fuel kernels. A cylindrical compact has a typical
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