The present paper proposes a long-term reactivity control concept of high-temperature gas-cooled reactors (HTGRs) using 3He mixed with 4He as a coolant, which can control excess reactivity and suppress power peaking without control rods. The Japan Atomic Energy Agency has proposed GTHTR300 for the demonstration of a HTGR. In this reactor design, especially the core neutronics, control rods cannot be inserted deeply enough to control excess reactivity because control rod insertion causes unacceptable power peaking. Furthermore, adding burnable poison also increases power peaking. Therefore, extending the cycle period between refueling is difficult.

Against this background, the present paper proposes a concept for controlling excess reactivity without causing power distribution distortion using a 3He mixed coolant. This 3He mixed coolant adds negative reactivity. Specifically, around 10 000 pcm, negative reactivity is given by 500 ppm of 3He. Furthermore, this 3He mixed coolant decreases power peaking by the absorption of thermal neutrons in the thermal neutron–rich region. Based on the burnup calculation, the added 500 ppm of 3He suppressed the excess reactivity for more than 1 year.

From the viewpoint of economics, the cost of the 3He mixed coolant stays four times in the case of 500 ppm. This cost is not extremely expensive; moreover, this cost is not the dominant factor in the commercial reactor system. Therefore, this concept could potentially be applied from technical and economic viewpoints.