Fuel type
A description of the total nuclear gas turbine system is given in Crommelin and Van Dam (2000). Here we focus on the most important nuclear aspects.
The fuel load is contained in a ‘cartridge’ consisting of a graphite (nuclear grade) vessel filled with fuel elements to be described further on. This cartridge is placed in a thick walled graphite vessel that is a fixed part of the reactor system; this vessel acts as a neutron reflector enabling the sustaining of the chain reaction. Periodically, once every 3 to 4 years, the cartridge is replaced. A standard 40-ft container can be used, which brings in the fresh fuel and takes away the used cartridge for further treatment. Specialized personnel, managed by a pool-system, should deliver the equipment and the knowledge for this operation. This new approach for nuclear energy production aims for the markets of the non-professional energy producers. So the installation will be designed in such a way that construction, maintenance, refueling, overhaul, repair and general logistic support will be done through the well-proven pool-management system. Non-specialized personnel can perform routine operations.
The heart of the installation is the so-called pebble-bed reactor. This type of nuclear heat source was invented in Germany. The AVR (Arbeitsgemeinschaft Versuchsreaktor) test reactor (15 MWe) was successfully operated and extensively tested during 20 years in Jülich, Germany. The nuclear fuel is based upon the proven high quality German moulded graphite spheres and TRISO (TRIstructural ISOtropic) coated particles (Kugeler and Schulten, 1989).
One of the most important safety features is that radioactive fission products produced during the energy conversion process are confined within the fuel during all operating and accident conditions. Consequently, there will be no release of radioactivity from the fuel particles to the outside world.
This ultimately safe confinement of radioactivity is assured by the design of the fuel elements themselves. The fuel elements consist of graphite balls (also called ‘pebbles’), with a diameter of 6 cm, in which fuel is placed. The fuel particles (Figure 1, bottom right) consist of a uranium-oxide fuel kernel surrounded by four coatings: from inside to outside a porous pyrolytic graphite coating, a high density graphite coating, a silicon carbide coating and an outer pyrolytic high density graphite coating. The diameter of the uranium-oxide kernel is about 1 mm.
The silicon carbide coating acts as an impenetrable containment for the radioactive fission products inside the fuel kernel and the porous coating. The porous coating allows a build-up inside the fuel kernel of fission products, which are partly gaseous. It has in practice been proven that these particles can withstand temperatures of up to 1600°C during an indefinitely long period of time without any release of integrity and consequently without any release of fission products.
Figure 1 – the nuclear fuel (TRISO)
Every pebble contains approximately 10,000 coated particles (the TRISO), the equivalent of 10 g uranium. The large heat transfer surface of these particles and the high thermal conductivity of graphite ensure good heat transfer from fuel to the embedding graphite matrix. This prevents hot spots in the core.
The core, with a volume of approximately 7 m³, produces a thermal power of 20 MW. This low power density (~3 MW/m³), coupled to small particles of fuel, and the good thermal conductivity of graphite, means that fission product release due to the failure of fuel particles occurs at much higher temperatures. Each coated particle can be seen as a miniature pressure vessel that can retain fission products. This ensures, that the fuel element temperature does not exceed 1600°C, even in the event of direct cooling failure. This construction forms the first containment of the radioactive material from the biosphere.
Fuel particles allow some freedom in the uranium enrichment: the enrichment can be chosen in correspondence with the energy production required per fuel cycle; we have assumed 8.6% enrichment in the present study. In our present design the attainable thermal energy extraction per pebble amounts to 0.6 MWdays, which is equivalent to the burning of 1.2 tons of crude oil.
The pebbles can be made oxidation-resistant by coating them with silicon. As a consequence they are fireproof as well as corrosion proof. In fact the graphite matrix and graphite outer layer form the second containment, which is impenetrable for most of the fission products even at a long exposure at a high temperature. The build-up of highly mobile fission products in the helium coolant will be low as was also observed at AVR/Jülich (Schulten et al., 1990).
Use of a gaseous coolant, helium, in a graphite environment is another safety feature. Helium is both chemically and nuclear inert; it does not react with graphite or the metallic core components. Helium cannot become radioactive. In helium no abrupt changes brought about by phase transition are experienced, thus ensuring continuous thermal evacuation throughout operation. The use of graphite (a solid) instead of a liquid (water) as a moderator has the advantage that the moderating behaviour is very constant.