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Overcoated Particles
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Same over coated particle with overcoat broken off |
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Fuel compacts are used as the energy source in the Very High Temperature Reactor (VHTR). The VHTR is comprised of a graphite block core with cylindrical fuel compacts inserted into the graphite blocks. The fuel compact-loaded blocks are then arranged in such a way to create an annular core. Coolant gas helium is then blown from the bottom of the core upwards and reaches an outlet gas temperature of ~950°C. This hot helium can then be used to do work, such as spin a turbine for electricity. The current use of the VHTR is the energy provider to a hydrogen production facility so that the hydrogen can be produced without emitting green house gases. The combined complex of a VHTR and hydrogen production facility is called the Next Generation Nuclear Plant (NGNP). The NGNP is one of the Generation IV DOE reactor concepts.
Fuel compacts are made via an overcoating and compacting process. The first step in the overcoating process in the formation of matrix, which is a combination of natural graphite, synthetic graphite, and thermosetting resin. The matrix, which is in powder form, is then used to overcoat TRISO particles. TRISO particles contain a uranium bearing kernel surrounded by layers of low density carbon, pyrolytic carbon, and silicon carbide. The TRISO particles are less than 1 mm in diameter. Overcoating of these particles takes place in a rotating vessel, where matrix and atomized methanol is added to the particles in set quantities. The overcoating process is continued until a layer of overcoat is built up around each TRISO particle. The thickness of the overcoat layer is ~0.2 mm.
Following overcoating, the particles are poured into a steel die and pressed into a cylinder. The overcoat layer acts as a buffer to the TRISO particles and flows into the void space between particles. Once the compact is ejected from the die it is fired in a tube furnace to 950°C under flowing helium. A second heat treatment step is performed, taking the compacts to 1800°C while under vacuum. After the second firing the compacts are complete and ready to be inserted into the graphite blocks that comprise the VHTR core.
For further information on this subject, please view the attached PowerPoint presentation. |
