Patent Number: 041860489
Section: summary

This invention relates generally to nuclear reactors and, more particularly, to an improved neutron flux monitoring system for use with a nuclear reactor. Normal operation of a nuclear reactor, particularly a power reactor, may be divided broadly into three ranges; source or start-up range, intermediate range and power range. In order to indicate the power level in the reactor at all times to permit safe control, the neutron flux in the reactor is monitored. The neutron flux at any point in the reactor is proportional to the fission rate and hence also to the power level. Because of the extreme temperatures and radiation levels in the core, the neutron sensors used for start-up and safety operation are usually placed outside the core. Typical neutron flux measurements in the intermediate range and in the power range are achieved utilizing fission chambers. These fission chambers, which are well known in the art, for practical reasons are placed at specific locations outside but adjacent to the reactor core. It is customary to have an array of detectors both vertical and around the core so that a relatively accurate picture of the flux distribution in the reactor core may be obtained during start-up and power operation of the reactor. The array of fission chambers are also required to provide (usually four) independent and redundant flux measurements for reactor safety purposes. As is known to those skilled in the art, the output of the electronic circuits used with the fission chambers may be logarithmic (used during the rise to power) or linear (used at or near the full power level). Count rate circuits are typically utilized in the low ranges of flux whereas Campbelling type circuitry or d.c. current techniques may be required for the higher flux ranges, where pulse resolution becomes a problem. In the shutdown condition of the reactor, the source level neutron flux is typically at an extremely low level, and because the detectors used for flux monitoring in the higher ranges have such low sensitivity, a separate low level monitoring system has typically been required. In the past, prior art systems in this lower flux range of reactor operation have employed high sensitivity proportional counters feeding locally mounted preamplifiers which deliver signals through coaxial cables to count rate circuits mounted in the control room. Such proportional counters have exhibited significant disadvantages as a result of short operating life and being highly susceptible to gamma background radiation. Moreover, because separate placements, preamplifiers and wiring are required for such a separate system, a significant added cost results. It is an object of the present invention to provide a means for combining the pulse signals from the array of independent fission chamber assemblies placed around the core for power range monitoring and safety purposes to achieve a high system sensitivity to neutrons in the source range and in the shutdown condition without violating their independence. It is a further object of the invention to provide an inexpensive, reliable neutron flux monitoring system capable of measuring the full range of neutron flux in a nuclear reactor, including very low levels of neutron flux.