[en] Purpose: To improve the earthquake resistance of a fuel graphite block forming a reactor core for an HTGR type reactor. Constitution: The confronting surfaces of both a control block and a reference block are tapered in a reactor core structure having a plurality of reference fuel blocks for surrounding a control block as a structural unit, and a projection is provided on the outer periphery of the end of the control block. As a result, the respective blocks can be smoothly telescoped in and out in the fuel handling region with a gap formed therebetween, and the gaps are partly made to reduce the contact area between the fuel blocks at an earthquake time so as to improve the earthquake resistance of the reactor core structure. (Kamimura, M.)