In situ borehole heater test at the KAERI Underground Research Tunnel in granite

An in situ borehole heater test was carried out in an underground research tunnel at a shallow depth in granite. During the test, the heater temperature was increased to 90 °C to simulate the thermo-mechanical behavior of crystalline rock under normal underground high-level radioactive waste repository conditions. The air, wall and rock temperatures were measured over a period of about four years. At the end of the test, the heater temperature was increased to 118 °C to simulate abnormal overheating conditions. The peak temperatures at the observation holes located at 0.3 m and 0.6 m from the heater hole were approximately 50 °C and 37 °C, respectively. The temperature measurements allowed observations of the effects of rock joints and heat convection through the tunnel wall on the rock temperature distribution. When the power was shut down, the rock temperatures and stress returned rapidly to the original rock temperature.