The high-temperature engineering test reactor has been designed whose outlet gas temperature is 950°C. That is the highest temperature in the world for a block-type high-temperature gas-cooled reactor. The power distribution in the core was optimized by changing the uranium enrichment to maintain the fuel temperature at less than the limit (1600°C). Deviation from the optimized distribution due to the burnup of fissile materials was avoided by flattening time-dependent changes in local reactivities. Flattening was achieved by optimizing the specifications of the burnable poisons. Control rod destruction of the optimized power distribution was avoided by limiting the depth of insertion. The insertion depth of the control rods is limited by reducing the excess reactivity of the whole core by the burnable poisons to the minimum value necessary for operations.