Method and apparatus for preventing inadvertent criticality in a nuclear

Abstract

An inadvertent approach to criticality in a nuclear fueled electric power generating unit is detected and an alarm is generated through on-line monitoring of the neutron flux. The difficulties of accurately measuring the low levels of neutron flux in a subcritical reactor are overcome by the use of a microcomputer which continuously generates average flux count rate signals for incremental time periods from thousands of samples taken during each such period and which serially stores the average flux count rate signals for a preselected time interval. At the end of each incremental time period, the microcomputer compares the latest average flux count rate signal with the oldest, and preferably each of the intervening stored values, and if it exceeds any of them by at least a preselected multiplication factor, an alarm is generated. The interval and multiplication factor are chosen such that an alarm is generated early enough in the event to provide adequate time for an automatic system or the operator to take action which prevents the reactor from going critical yet minimizes the likelihood of a spurious response. As applied to a pressurized water reactor system, boron dilution events are detected in time that the chemical and volume control system can be isolated and the valves for the refueling water storage tank can be opened to inject highly borated water into the reactor coolant system to keep the reactor subcritical.