Intermediate-range detectors monitor neutron flux (reactor power) at the intermediate flux level. They also provide indications, alarms, and reactor trip signals. Their range is from the upper part of the source range through the power range (covering a span of eight decades). The design of this instrument is chosen to provide overlap between the source range channels and the partial or full span of the power range instruments. Intermediate-range instrumentation usually consists of two or four channels, each with its own separate detector, cable run, and electronic circuitry. The detectors are usually boron-lined or boron gas-filled compensated ionization or fission chambers. Their accuracy usually does not achieve the accuracy of the power range instrumentation operating in a much narrower range.
The source range instrumentation monitors and indicates the neutron flux level of the reactor core and the rate by which the neutron flux changes during the entire phase of reactor start-up and power operation. The neutron flux is indicated in the percentage of rated power. The rate of change of the neutron population is indicated as the start-up rate (SUR), defined as the number of factors of ten that power changes in one minute. Therefore the units of SUR are powers of ten per minute or decades per minute (dpm). A high start-up rate on either channel may initiate a protective action.
Since the neutrons are electrically neutral particles, they are mainly subject to strong nuclear forces but not electric ones. Therefore, neutrons are not directly ionizing and usually have to be converted into charged particles before they can be detected. Generally, every type of neutron detector must be equipped with a converter (to convert neutron radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous detector, semiconductor detector, etc.).
Ionization chambers are often used as the charged particle detection device. For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n, alpha) reaction can occur. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.
Moreover, isotope boron-10 has a high (n, alpha) reaction cross-section along the entire neutron energy spectrum. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations.
Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in a nuclear reactor. It must be noted that BF3 counters are usually operated in the proportional region.
Intermediate Range – Reactor Safety
As was written, the excore nuclear instrumentation system is considered a safety-related system because it provides inputs to the reactor protection system. The intermediate range neutron flux trip provides the core protection against an uncontrolled RCCA bank rod withdrawal accident from a subcritical condition during start-up. This trip function provides redundant protection to the power range neutron flux – low setpoint. It also provides redundant protection to the source range trip function for boron dilution accidents and controls rod ejection events.
Compensated Ionization Chambers
The ionization chamber, also known as the ion chamber, is an electrical device that detects various types of ionizing radiation. The voltage of the detector is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to produce gas amplification (secondary ionization). Ionization chambers are preferred for high radiation dose rates because they have no “dead time,” a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates.
The compensated ion chamber is utilized in the intermediate range because the current output is proportional to the relatively stable neutron flux, compensating for gamma flux signals. The compensated ion chamber consists of two detectors in one case. The outer chamber is coated inside with boron-10, while the inner chamber is uncoated. The coated chamber is sensitive to gamma rays and neutrons, while the uncoated chamber is sensitive only to gamma rays. By properly connecting the two chambers, the net electrical output from the detector will be the current due to neutrons only.
The voltages between these two sets of electrodes must be balanced to achieve the proper amount of gamma compensation. The consequences of operating with an overcompensated or under compensated chamber are important. If the voltage in the compensation chamber is too high, the detector is overcompensated, and some neutron current, as well as all of the gamma current, is blocked. Indicated power is lower than actual core power. If the compensating voltage is too low, under compensation will occur. At high power, gamma flux is relatively small compared to neutron flux, and the effects of improper compensation may not be noticed. It is extremely important, however, that the chamber be properly compensated during reactor start-up and shutdown.
See also: U.S. Department of Energy, Instrumentation, and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.
Fission Chamber – Wide Range Detectors
Fission chambers are ionization detectors used to detect neutrons. Fission chambers may be used as the intermediate range detectors to monitor neutron flux (reactor power) at the intermediate flux level. They also provide indications, alarms, and reactor trip signals. The design of this instrument is chosen to provide overlap between the source range channels and the full span of the power range instruments.
In general, the ionization chamber, also known as the ion chamber, is an electrical device that detects various types of ionizing radiation. The voltage of the detector is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to produce gas amplification (secondary ionization). Ionization chambers are preferred for high radiation dose rates because they have no “dead time,” a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates. Moreover, in the ionization region, an increase in voltage does not cause a substantial increase in the number of ion pairs collected. The number of ion pairs collected by the electrodes equals the number of ion pairs produced by the incident radiation. It depends on the type and energy of the particles or rays in the incident radiation.
In the case of fission chambers, the chamber is coated with a thin layer of highly enriched uranium-235 to detect neutrons. Neutrons are not directly ionizing and usually have to be converted into charged particles before they can be detected. A thermal neutron will cause an atom of uranium-235 to fission, with the two fission fragments produced having high kinetic energy and causing ionization of the argon gas within the detector. One advantage of using uranium-235 coating rather than boron-10 is that the fission fragments have much higher energy than the alpha particle from a boron reaction. Moreover, the fission fragments resulting from the interaction of neutrons with the coating cause a significantly larger amount of ionization within the fission chamber than the gamma radiation incident on the detector. This results in the neutron-generated charge pulses being significantly larger than the gamma pulses. Pulse size discrimination circuitry can then be used to block out the unwanted gamma pulses. Therefore fission chambers are very sensitive to neutron flux, and this allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining.
Fission chambers are often used as current indicating devices and pulse devices depending on the neutron flux level. In the pulse mode, fission chambers are especially useful due to the very large pulse size difference between neutrons and gamma rays. When power is high in the intermediate range or the power range (i.e., in a high level mixed gamma and neutron flux), fission chambers can be operated in Campbelling mode (also known as “fluctuation mode” or “mean square voltage mode”) to provide reliable and precise neutron related measurements. The Campbelling technique eliminates the gamma contribution because the fission chamber’s dual use is often used in “wide range” channels in nuclear instrumentation systems.