TRISO-coated particle fuel elements

TRISO-coated particle fuel arrangement in hexagonal fuel elements. 

GT-MHR spent fuel elements, with their TRISO-coated particle fuel, achieve these qualities to a much greater degree than other waste forms, including spent zircalloy-clad fuel rods irradiated in the LWR. GT-MHR spent whole elements are an excellent waste form for permanent disposal. 

The VHTR has two typical reactor configurations, namely the pebble bed type and the prismatic block type. Although the shape of the fuel element for two configurations are different, the technical basis for both configuration is same, such as the TRISO-coated particle fuel in the graphite matrix, foil ceramic (graphite) core structure, helium coolant, and low power density, in order to achieve high outlet temperature and the retention of fission production inside the coated particle under normal operation condition and accident condition. The VHTR can support alternative fuel cycles such as U—Pu, Pu, mixed oxide (MOX), and U—thorium (Th). 

An IHTR is a pebble-bed molten salt-cooled reactor. Pebbles consist of TRISO-coated particle fuel, and the coolant is driven through natural circulation. The reactor core is a long right circular cylinder with an annular core that consists of fuel pebbles and molten salt coolant. Fig. 15.13 shows a schematic of a 600-MWth IHTR. There are graphite neutron reflectors in the center and on the top, bottom, and outside of this fuel annulus. Vertical bores in the central and outer reflectors are provided for the reactivity control elements. R D activities being pursued include a molten salt natural circulation loop, as shown in Fig. 15.14, which has been set up to perform thermal... 

Spherical fuel elements with TRISO coated particles are used, which have proven capability of fission product retention under 1 bOO C in accidents. 

The FBNR uses TRISO particles within SiC-coated spherical fuel elements in an up-flow coolant stream, which, if interrupted, allows the particles to relocate into a subcritical, well-cooled configuration. 

The comparable low fractional release of the fuel elements with TRISO-coated particles is caused by a corresponding lower U-contamination of the graphite matrix during fuel manufacturing, and therefore the long time behaviour of the fuel elements with TRISO particles is even more convenient. 

Two options are being considered for FBNR fuel elements. One is the zircaloy-cladded uranium dioxide spherical fuel pellet, and the other is spherical fuel element made of TRISO type coated particles. The enrichment for the U02/zircaloy option is about 3%, and for the coated particle option it is about 8%. Light water acts as both coolant and moderator. In a coated particle option, graphite also contributes to the neutron moderation. The module size will depend on fuel type and on the enrichment allowed. For example, the core of a reactor with U02/zircaloy fuel may have a diameter of 25 cm. The core tube may need to have larger diameter when coated particle fuel is used. 

ACACIA is also based on certain PBMR technologies, e.g. TRISO type coated particle fuel and spherical fuel elements, and the HTR-Module technologies, e.g. control rods. 

TRISO Coated fuel particles (left) are formed into fuel rods (center) and inserted into graphite fuel elements (right). 

Fuel The fuel and fuel element design is derived from that of the MHGR, and illustrated in Figure 3. The TRISO fuel is protected from the lead by the graphite fuel element structure. However, there is no chemical reaction between molten lead and the silica-carbide coating of the TRISO fuel particles, and the solubility of silica-carbide in lead is negligible. 

The MFEs are coated particles similar to TRISO fuel with the outer diameters of about 2 mm. They consist of 1.5-1.64 mm diameter uranium dioxide spherical kernels coated with 3 ceramic layers. The inner layer, called a buffer layer, is made of 0.09 mm thick porous pyrolythic graphite (PyC) with the density of 1 g/cm, providing space for gaseous fission products. The second layer is made of 0.02 mm thick dense (1.8 g/cm ) PyC, and the outer layer is 0.07-0.1 mm thick corrosion resistant silicon carbide (SiC). The fourth, outer PyC layer is assumed to be absent. SiC protection layers, manufactured by chemical vapour deposition (CVD) method, create resistance of graphite components against water and steam at high temperatures. Small fuel elements are able to confine fission products indefinitely at temperatures below 1600°C. 

The 6 cm spherical fuel elements of HTR-10 are made of TRISO type coated particles (CP) and graphite matrix. One CP consists of a UO2 kernel with a diameter of 0.5 mm, which is successively coated with layers of low density pyrolytical carbon, inner high density isotropic pyrolytical carbon, silicon carbide and outer high density isotropic pyrolytical carbon, with thicknesses of respectively 90, 40, 35 and 40 pm. About 8,300 coated particles are dispersed in the graphite matrix, which is 5 cm in diameter, to form the fuel zone of a fuel... 

Both reactors have in total produced about 1 Million of spent fuel elements during their operating time. The typical fuel element is a tennis-ball sized sphere from graphite, containing up to twenty thousand pinhead-sized fuel particles containing oxide or carbide fuel each. The particles are surrounded by a high-porosity buffer layer to limit the internal pressure from swelling and gas production, and coated with a hi -density pyrocarbon layer (BISO) or with a combination of two pyrocarbon layers with a silicon carbide layer in between (TRISO) to retain radionuclides (see FIG. 1). 

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