Prismatic block type

Fuel Coated fuel particle / Prismatic block type... 

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). 

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. 

Refueling differences exist between sodium-cooled reactors and the AHTR. For the AHTR, refueling temperatures are somewhat higher, the fuel geometry is different, the power density of the prismatic-block fuel-type SNF is 1 to 2 orders of magnitude lower, the vapor pressures of the liquid salts are much lower than those of sodium, and the liquid salt is transparent whereas the sodium is opaque. This section provides discussions of design considerations for the LS-VHTR fuel-handling system relative to sodium-cooled fast reactors. 

Pebble bed or prismatic block HTGR type fuel technology, including coated particles... 

CHTR - compact high temperature reactor prismatic block HTGR type fuel and lead bismuth coolant... 

Type of fuel Ceramic coated particles in prismatic blocks, UCO kernels... 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

The fuel pins used in the core are a new type for OCR s. The vast majority of OCR reactors use a TRISO type fuel embedded in a graphite matrix and are thermal neutron spectrum reactors. TRISO fuels are small UC spheres coated with layers of silicon carbide and pyrolitic carbon. While this results in fuels that can be used to extremely high bumup, the uranium density of the fuel is low. A coolant hole is then drilled through the blocks of fuel and these blocks are put in a prismatic array. 

The prismatic type has the coated particles in a graphite matrix forming a fuel rod which is inserted into vertical holes in the moderator blocks. Solid unfueled blocks make up the reflector zone surrounding the active core area. 

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