Coated particle fuels

Sawa, K. (2007), Advanced Coated Particle Fuel - ZrC Coated Particle Development in JAEA , EUROCOURSE on Coated Particle Fuel, Petten, The Netherlands, 4-7 December. 

Lee, Y-W., et al. (2008), Development of HTGR-coated Particle Fuel Technology in Korea , Nucl. Eng. and Design, 238, 2842-2853. 

The sol-gel process is used to prepare dense, spherical particles of ThOa and (Th,U)02 for sphere-pac and coated-particle fuels. The thoria is dispersed in water from nitrate solutions by slow heating and steam denitration to form a stable sol from which spherical particles are produced. The sol droplets are injected at the top of a tapered glass column containing an upward flow of 2-ethylhexarol (2-EH). The water from the sol particles is slowly extracted by suspension in the 2-EH, and the gelled spheres drop out of the column. Coalescence of the particles is prevented with surfactants in the 2-EH. The sol-gel spheres are dried in steam and Ar at 220°C and sintered in H2 at ISOO C. 

Preliminary results show it should be possible to operate the MHR with a coolant outlet temperature of up to 1 000°C using nuclear-grade graphite fuel blocks, carbon-carbon composite materials for control rods and other internal reactor components, and existing coated-particle fuel technology with silicon carbide (SiC) and pyrolytic carbon coatings. 

The GTHTR300 reactor system has been designed based on the technologies and design codes developed and validated on the test reactor H lT R shown in Figure 9 and with further technical base for high bumup fuel [6] to allow specification of a modified TRISO coated particle fuel to meet commercial system objectives [7]. 

Passive safety features for the MHR include ceramic, coated-particle fuel and an annular graphite core with high heat capacity and low power density. Recently, INL has used the ATHENA thermal hydraulic code to model the response of the MHR during loss-of-flow and loss-of-coolant accidents and has confirmed these passivity safety features work to maintain fuel temperatures well below failure thresholds [8]. 

Gas-cooled reactor coated-particle fuel (1970s)... 

Fuel type. As currently envisioned, the LS-VHTR uses graphite-matrix coated-particle fuel. This fuel can be made into several geometric forms (prismatic blocks, pebble beds, and stringers with fuel pins), each with somewhat different refueling requirements. Refueling experience exists for gas-cooled reactors for each of these fuel geometries. 

As currently envisioned, the LS-VHTR uses a graphite-matrix coated-particle fuel. Three major types (prismatic, pebble bed, and assembly) of fuel can be fabricated. Each has different refueling demands. Section 4 discusses the alternative fuel geometries and the implications for refueling and core design. 

PBRs use the coated-particle fuel in a graphite matrix compacted into pebbles— t5 ically about 6 cm in diameter (Fig. 4.8). Current estimates indicate that pebbles have the lowest fabrication cost of any of the three fuel geometry options. The reactor core is a bed of pebbles. The THTR core is shown in Fig. 4.9. The vertical structures are channels for control rods. 

Simplified thermal-hydraulic analyses, which were benchmaiked against GT-MHR data, indicated that for a fixed core outlet temperature of 1000°C for the coolant, the peak fuel temperature in the AHTR during normal operation will be 110-130°C cooler than for the prismatic helium-cooled NGNP design, and the average fuel temperature at the core outlet will be 30-50°C cooler. This is a direct result of the superior heat transfer properties of the molten salt relative to helium. This is significant because the failure rate of the coated particle fuel increases with increasing temperature. 

Fig. 2.3. Diagram and photographs of high-temperature, TRlSO-coated particle fuel.

Updated neutronics analyses were performed at SNL to better match the reference AHTR geometry and fliel/coolant fractions. As before, the fuel type was TRISO-coated particle fuel with a particle packing fraction of 30% in the fuel compacts. All of the calculations were performed with 1200 K neutron cross-sections, and the thermal scattering function, S(a,P), was also used at 1200 K for the carbon in the fuel compacts and prismatic matrix. 

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... 

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