a self - actuating reactor shutdown system incorporating a thermionic switched electromagnetic latch arrangement which is responsive to reactor neutron flux changes and to reactor coolant temperature changes . the system is self - actuating in that the sensing thermionic device acts directly to release the control rod without reference or signal from the main reactor plant protective and control systems . to be responsive to both temperature and neutron flux effects , two detectors are used , one responsive to reactor coolant temperatures , and the other responsive to reactor neutron flux increase . the detectors are incorporated into a thermionic diode connected electrically with an electromagnetic mechanism which under normal reactor operating conditions holds the the control rod in its ready position . upon reaching either a specified temperature or neutron flux , the thermionic diode functions to short - circuit the electromagnetic mechanism causing same to lose its holding power and release the control rod , which drops into the reactor core region under gravitational force .