method and apparatus for determining the approach to criticita of a nuclear reactor

摘要

The approach to criticality of a nuclear reactor provided with an artificial neutron source is monitored by two neutron detectors, one which measures the neutron flux in a localized region around the artificial neutron source, and the other located simlarly with respect to the core geometry but removed from the localized region. When the reactor is subcritical, the artificial source dominates and the outputs of the two detectors are noticeably different. As the reactor approaches criticality, the critical mode distribution of neutrons, which is symmetrical with respect to the two detectors, becomes dominant and the detector outputs approach each other in magnitude. The ratio of the output signals therefore, provides an indication of the reactor reactivity state. Either linear or logarithmic ratio signals can be generated, and if desired, either can be compared with an appropriately scaled reference signal representative of a selected reactivity state, such as a multiplication factor of 0.99, with the result of the comparison used to control a "near-criticality/not near-criticality" logic output.