Design study of block-pin type fuel for experimental VHTR (EVHTR) core

In MARK-II core designs of EVHTR, the minimum Reynolds number in the core is about 2600 not satisfactory for the thermal performance. Different cores and fuels have been investigated to improve the thermal characteristics. The following were revealed: (1) Reynolds number rises to 5,000 with hollow fuel rods, if high control-rod reactivity worth in the core is allowed. (2) Reynolds number rises to 15,000 with annular fuel rods, if laminar flow in the outer cooling channel is allowed. (3) Reynolds number rises to 10,000 with semi-pin fuel, if higher temperature of the fuel block is allowed. Higher Reynolds number of the EVHTR core can thus be attained under some undesirable conditions. In the MARK-III core design, therefore the compromise of low control-rod worth, non-laminar flow and lower block temperature, following MARK-I and MARK-II is retained, but the minimum Reynolds number is increased to 3,580 because of the reduction in number of fuel rods (from 1,251 in MARK-II to 813 in MARK-III). (auth.)