Nuclear reactors are powered by nuclear fission. During a typical fission event, a slow-moving neutron is captured by the nucleus of an atom of fissile material (material with nuclear properties making it capable of splitting by fission to release energy). These atoms are typically of very high-mass and when they split roughly in two when they capture a thermal neutron. These neutrons have energies similar to the energies of the vibrating atoms they're scattering through. Each fission event generally results in the release of two or three high-energy neutrons, which scatter in the reactor's moderator until they become thermal neutrons. On average, in stable operation, one neutron from a fission event is absorbed and causes another fission event.
The HT3R's core is especially designed to run at particularly high temperatures. As the neutrons scatter in the moderator, their speed is determined by the vibrational speed of the moderator material they scatter in, which is determined by the temperature of the moderator. If the temperatures of the HT3R's core should ever rise beyond normal operating temperatures, the typical speeds of the neutrons sustaining the fission reaction increase. As we know from quantum mechanics, the size of the neutrons shrinks at these speeds, causing the neutrons to miss the fissile nuclei which would ordinarily capture them before they scattered out of the core and into the surrounding radiation shielding walls. As the fraction of the absorbed neutrons decreases, the reactions decrease and less energy is released. This removes both human error and mechanical malfunction from controlling the safe regulation of the reactor core's power even in the worst imaginable scenarios. Because of its advanced design, the HT3R's core's safety is ultimately determined by the very physics by which it operates and not by the intervention of operators and control rods. Control rods and highly-trained operators will, of course, remain a fundamental part of the basic reactor control during normal operation.