This invention relates to the detection of conditions in a nuclear-fueled electric power-generating unit, and more particularly, to methods and apparatus for estimating the effective neutron multiplication factor in a nuclear reactor.
The power level of a nuclear reactor is generally divided into three ranges: the source or start-up range, the intermediate range, and the power range. The power level of the reactor is continuously monitored to assure safe operation. Such monitoring is typically conducted by means of neutron detectors placed outside and inside the reactor core for measuring the neutron flux of the reactor. Since the neutron flux at any point in the reactor is proportional to the fission rate, the neutron flux is also proportional to the power level.
Fission and ionization chambers have been used to measure flux in the intermediate and power range of a reactor. Such fission and ionization chambers are capable of operation at all normal power levels, however, they are generally not sensitive enough to accurately detect low level neutron flux emitted in the source range. Thus, separate low level source range detectors are typically used to monitor neutron flux when the power level of the reactor is in the source range.
U.S. Pat. No. 4,588,547 discloses a method and apparatus for determining the nearness to criticality of a nuclear reactor. That invention takes advantage of the fact that when the reactor is subcritical, the neutron flux generated by an artificial neutron source, and the direct progeny by fission, is higher than that generated by neutrons from natural neutron sources in the reactor fuel and progeny of those neutrons. However, that patent does not address the approach to criticality when a reactor approaches criticality due to withdrawal of control rods.
During the approach to reactor criticality, the signals from the source range detectors are typically used to determine whether the reactor is critical or will achieve criticality before the scheduled or planned core conditions are achieved. Assemblies of control rods in the form of control banks, are used to regulate reactor activity through controlled absorption of the neutrons released in the fission process. When a reactor is to be made critical by withdrawal of the control banks, which is the typical method used for all reactor startups following the initial startup in each operating cycle, changes in control bank position cause changes in the magnitude of the source range detector signals which are not entirely indicative of core reactivity changes. This behavior makes it difficult for the reactor operator to use the source range detector information properly. Ideally the reactor operator would like to be able to not only determine whether the reactor is currently critical, or is likely to be critical before the planned critical conditions are achieved, but how close to critical the core actually is. In order to accurately determine how close the reactor is to critical, a means of using the source range detector signal information that does not rely only on the magnitude of the signal change from one control bank configuration to another is required.