An irradiation capsule for high-temperature fuel irradiations has been developed to permit constant temperature control over a range of about ±35% of design power. Control is achieved by the variation in thermal conductivity of a binary gas mixture in a control annulus located between the test specimen and the capsule coolant. For the binary mixture, helium, which is a high-thermal conductivity gas, and a gas of lower conductivity, such as neon, nitrogen, or argon, may be used. The control method is unaffected by time or radiation damage. In-pile operation of capsules using this method of control has demonstrated that the desired temperature may be controlled to within ±25°F automatically, and probably more closely if manually controlled. The automatic control system also protects the capsule from temperature overshoot during a fast reactor recovery following a scram.