A High Temperature Gas-cooled Reactor (HTGR) has several features different from conventional light water reactors such as inherent safety characteristics, high thermal efficiency and high economy. On the other hand, one of disadvantages of the HTGR with a prismatic core is to require rather long-term and expensive refueling, resulting in relatively long maintenance period and high cost. To solve the disadvantage, the present study challenges the core design of a small-sized reactor for long refueling interval by increasing core size, fuel loading and fuel burn up compared with the High Temperature engineering Test Reactor (HTTR). The preliminary burn-up calculation suggested that approximately 6 years of long refueling interval was found to be reasonably achieved. A refueling interval longer than 6 years may be possible by decreasing further power density, subsequently larger core size with operational reactor power of 120MWt, but this idea was not taken by the requirement of the reactor that the core size shall be accommodated reasonably in the core with double size of the HTTR at maximum.Copyright © 2011 by ASME