Graphite-Coated Particle Fuel Elements

Limitations on the fuel-element surface temperature can practically be eliminated if the fuel element can be made entirely of ceramic materials. The ceramic material must serve both as a structural and a containment material to control the release of fission products. By dispersing the fuel in the ceramic material, it is also possible to limit radiation damage to 

Thirty-six pin AGR fuel assembly (a) cutaway, (b) end view (courtesy of United Kingdom Atomic Energy Authority). 

This type of fuel element has the advantage that the structural parts of the element are unaffected by fission recoil damage, and, hence, the fuel burnup is not limited by impairment of the structural integrity of the fuel element. Furthermore, since these fuel elements consist only of graphite and the fuel materials themselves, both of which can withstand very high temperatures, a large improvement in the maximum allowable fuel surface temperature becomes possible—1000°C (1800°F) or higher. 

Finally, there is no metallic cladding material to absorb neutrons para-sitically. 

The core of the Peach Bottom HTGR contains 804 fuel elements (57), having an active fuel region about 7.5 ft (230 cm) in length with 24-in. 

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Experiments on simple pyrolytic-graphite-coated particles in the GAIL-III B fuel element showed that Xe having a half-life of 100 hr, for example, was held up sufficiently long for about 99 % of the decay to occur within the fuel element, even after an exposure corresponding to 3 years with an 80% load factor in the Peach Bottom reactor (41). The release fraction for this isotope was, then, about 10 at the end of the fuel exposure life. Release fractions for Kr , Kr , and Kr , for example, having half-lives in the neighborhood of 1 hr, were about 10 at the end of the fuel exposure. The measurements showed that the release fractions increased approximately linearly with exposure. 

On the other hand, the USA adopts a block type fuel element in HTGRs. In this fuel element, coated particle fuel and graphite powder are mixed and sintered, then shaped into thin rods, and loaded and sealed into holes in a hexagonal column-shaped graphite block. The experimental... 

Similar to other nuclear installations, the defence-in-depth concept incorporated in the MARS provides for multiple barriers to radioactivity release from the fuel and for measures to maintain the integrity of these barriers. Such a barrier structure largely leans upon the known properties of the fuel (spherical fuel elements with coated particles), i.e., the retention of a large amount of radionuclides in a ceramic fuel kernel and the prevention of radionuclide release to the coolant by the fuel particle coatings. The graphite matrix of fuel elements that has an ability to absorb certain radionuclides facilitates a reduction of radioactivity release to the coolant. A two-circuit plant scheme provides an additional barrier to radioactivity release to the environment. 

A two zone spherical fuel element of 60 mm diameter consists of a graphite matrix with fuel in the form of coated particles and a 5 mm thick cladding of dense graphite. The average fuel element density is -1700 kg/m, which is less than the density of a molten salt coolant at the operating temperature ... 

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. 

The reactor core is comprised of hexagonal graphite fuel elements containing coated fuel particles in compacts loaded into holes drilled in the graphite blocks, with additional holes in the blocks for coolant flow. The fuel elements and coated particle fuel are illustrated in... 

Type of fuel UO2 enriched up to 8.77% in coated particles dispersed in graphite matrix, spherical fuel elements of 6 cm diameter. 

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... 

Figure 8. Spherical AVR fuel element consisting of dispersion of coated fuel particles in a carbonaceous matrix contained within a machined graphite shell. The sphere is 6 cm in diameter. (After Ref. 5.)...

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

The thorium-containing fuels of present interest are only those of the kind used in HTGR and HWR in the future fuel from MSR-like (or other) transmutation devices may become inqmrtant. In the HTGR fuel elements the fertile Th02 and fissile U02 (or U02) particles are coated differently and embedded in a graphite matrix. 

There are multiple barriers to the release of fission products from an HTGR core the fuel kernel, the particle coatings, the fuel rod matrix, and the fuel element graphite. The effectiveness of the individual barriers to fission product release may depend upon a number of factors Including the chemistry and half-llfes of the various fission products, temperature, and Irradiation effects. These barriers are described briefly below. 

The models and material property data for predicting fission metal release from fuel particles and fuel elements are described in Ref. 4. The transport of fission metals through the kernel, coatings, fuel rod matrix, and fuel element graphite is modeled as a transient diffusion process in the TRAFIC code (Section 4.2.5,2.2.1.2). The sorption isotherms which are used in the calculation of the rate of evaporation of volatile metals from graphite surfaces account for an increase in graphite sorptivity with increasing neutron fluence. 

HTR-10 uses spherical fuel elements with ceramic coated particles, graphite as the core structure material and helium as the coolant The fuel elements are charged from the core top and removed from the core bottom via a discharge tube with multi-pass recycling Fig 4 shows the cross section and primary circuit of the HTR-10 The primary system consists of a reactor pressure vessel and IHX-SG vessel are arranged in... 

The HTR-10 test reactor uses spherical fuel elements which are made completely of ceramic materials. Uranium dioxide as nuclear fuel is in the form of coated particles which are dispersed in the graphite matrix of the fiiel elements. Graphite serves as neutron moderator... 

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