Patent Number: 044951442
Section: summary

BACKGROUND OF THE INVENTION The present invention generally pertains to fission chamber detector systems for monitoring neutron flux in a nuclear reactor, and is particularly directed to increasing the monitored range, improving alignment of processed signals derived from different portions of the monitored range, and providing high neutron signal sensitivity in a hostile environment. A fission chamber detector is a type of neutron detector that is preferred for use in neutron flux monitoring systems because it has been proven to have a longer life and to be more reliable than other types of neutron detectors. In a typical prior art fission chamber detector system for monitoring neutron flux in a nuclear reactor, a number of fission chambers are located inside a biological shield that surrounds the reactor core. Neutron signals produced in response to the detection of neutrons are transferred over conductors, such as coaxial cables to a preamplifier unit located inside a containment vessel for the reactor. The preamplifier unit amplifies the neutron signals for enabling further transmission via coaxial cables. In prior art systems, the preamplifier units are located within the containment vessel for the nuclear reactor because in such prior art systems, the quality of the neutron signals would be so much diminished by electrical noise, attenuation, and signal reflection if the preamplifier units were located more than one hundred feet (thirty meters) from the neutron chambers that the sensitivity of the system would be impaired. The location of the preamplifier units within the containment vessel makes the preamplifier units susceptible to being rendered inoperable in the event that they are subjected to a hostile environment such as exists when the reactor suffers a loss of coolant accident. In the event of such an accident, the environment within the containment vessel is severely changed. Steam, boric acid, caustic sprays and other contaminants that are adverse to electrical circuits permeate the air, and the temperature and the air pressure increase to such an extent that preamplifier units in conventional containers would not withstand the increased temperature and would be damaged by such contaminants as penetrated the container under the conditions of increased pressure. Also the radiation level would increase to make the preamplifier units inoperative from the radiation damage. Yet, it is particularly important that neutron flux within the biological shield be monitored during and following a loss-of-coolant accident. This would require preamplifier units located within the containment vessel to be shielded from high radiation and temperature and to have containers that can withstand high pressure and be impermeable to contaminants. It is impractical and very expensive to meet this requirement. It is desirable to monitor neutron flux over an extra wide range of up to twelve decades. However, most prior art fission chamber detector systems for monitoring neutron flux have a useful range of only ten decades. A decade is the range from 10.sup.n to 10.sup.n+1. In some prior art systems for monitoring neutron flux, the range has been extended to twelve decades by using proportional counters in combination with fission chamber detectors. However, proportional counters have a relatively short lifetime and their performance is rapidly degraded by the presence of gamma rays and the high temperatures surrounding the reactor core. In processing the amplified signals to provide indications of reactor power and the rate of change of reactor power, the system utilizes pulse counting for the lower portion of the monitored range and a mean square voltage processing technique for the upper portion of the monitored range. In prior art systems difficulties have been encountered in aligning the pulse count signals with the mean square voltage signals. Heretofore, it has not been possible to obtain an accurate alignment with a simple calibration system, and it has been necessary to use a nuclear reactor in adjusting the alignment. A particularly difficult problem in signal alignment that arises in prior art monitoring systems, such as described in U.S. Pat. No. 3,579,127 to Thomas, is the presence of spurious transients in processed signals indicating the rate of change of reactor power. These transients are caused during the transitions between the pulse count signals and the mean square voltage signals. To minimize this problem, prior art monitoring systems make use of complex bias and alignment circuitry and require expensive time consuming alignment procedures. SUMMARY OF THE INVENTION The present invention is a fission chamber detector system for monitoring neutron flux in a nuclear reactor over an extra wide range, with high sensitivity in a hostile environment. The coaxial cables that are used for conducting neutron signal pulses from the fission chamber detectors to a preamplifier and signal conditioning unit are uniquely constructed to enable the preamplifier and signal conditioning unit to be located outside of the containment vessel without significantly diminishing the quality of the neutron signals. Each of these coaxial cables includes a center conductor; a coaxial high temperature insulating layer closely covering the center conductor; a coaxial dielectric layer closely covering the insulating layer, the dielectric layer being resistant to damage caused by nuclear radiation; a coaxial conductive solid sheath layer closely covering the dielectric layer; and a coaxial outer insulating layer, said outer layer being resistant to nuclear radiation damage. Preferably, the sheath layer is copper tubing. The solid sheath is tightly sealed to a coaxial connector to protect the respective interiors of the cable and connector from potentially destructive contaminants under high pressure. To reduce attenuation and to increase noise immunity in the coaxial cable, the center conductor and the solid sheath layer have low resistance. Signal reflection in the cable is reduced by terminating each coaxial cable at the preamplifier unit by an impedance that matches the characteristic impedance of the coaxial cable. With the preamplifier and signal conditioning unit located outside of the containment vessel, transmission of the amplified and conditioned signals from the preamplifier and signal conditioning unit can be accomplished by twisted shield pairs rather than by more expensive coaxial cables as is done in prior art systems. This is possible because of the nature of the signal conditioning that is accomplished in the preamplifier and signal conditioning unit of this invention. Although the coaxial cable is constructed to withstand the adverse effects of a hostile environment, such as would occur in the event of a loss-of-coolant accident, a double barrier against such adverse effects is provided by using a metal hose, junction boxes and a container for the fission chambers to cover the coaxial cables and fission chambers within the containment vessel. This barrier protects the cable and connectors from potentially dangerous contaminants under high pressure and further shields out electromagnetic radiation to reduce electrical noise. In another aspect of the present invention, power indication signals obtained by a pulse counting technique for a lower reactor power range and power indication signals obtained by a mean square voltage processing technique for a higher overlapping reactor power range are accurately aligned for providing indications by a single display device. In this aspect of the invention, amplified neutron signal pulses from a fission chamber detector are (1) separately conditioned by a threshold detector and processed by a countrate circuit to provide a first power signal that is representative of power in the lower reactor power range; and (2) conditioned to provide a conditioned signal that is proportional to the square root of reactor power and then processed by a mean square voltage circuit to provide a second power signal that is representative of power in the higher reactor power range. The region of overlap of the higher and lower power ranges is determined by circuit noise, which defines the lower limit of the higher power range and by countrate loss at the power level at which the pulses counted by the countrate circuit occur at such frequency as to become indistinguishable, which defines the upper limit of the lower power range. A voltage-controlled switch is coupled to the countrate circuit and to the mean square voltage circuit for passing the first power signal onto a first output line when the second power signal is less than a predetermined voltage that is representative of a reactor power level below the power level at which the counted pulses in the first pulsed signal become indistinguishable, and for passing the second power signal onto the first output line when the second power signal level is equal to or exceeds that predetermined voltage. To obtain indications of the rate of change of reactor power, the first and second power signals are differentiated to provide respective first and second rate-of-change signals. A slave switch is connected to the differentiators and coupled to the voltage-controlled switch for passing the first rate-of-change signal onto a second output line when the voltage-controlled switch passes the first power signal onto the first output line, and for passing the second rate-of-change signal onto the second output line when the voltage-controlled switch passes the second power signal onto the first output line. The first and second rate-of-change signals are accurately aligned without incurring spurious transients. A power signal and a rate-of-change-of-reactor-power signal for a low range of reactor power that is applicable during reactor start-up are derived from very sensitive threshold detectors in the preamplifier and signal conditioning unit and by a separate countrate circuit. These are used with the aligned power signals and rate-of-change-of-reactor-power signals derived as described above to enable neutron flux to be monitored over a range of up to twelve decades. In still another aspect of the present invention, the preamplifier and signal conditioning unit includes at least one preamplifier having an input stage that includes a semiconductor switching device having a gate terminal, an input terminal coupled to the coaxial cable for receiving neutron signal pulses from the fission chamber, and an output terminal coupled to an amplifier stage for providing the received neutron signal pulses to an amplifier stage in the preamplifier when the switching device is rendered conductive; and a control circuit connected to the switching device and having a control terminal for rendering the switching device conductive when a first predetermined voltage is applied to the control terminal and for inhibiting conduction by the switching device when a different second predetermined voltage is applied to the control terminal. The control circuit includes a conduction path from the control terminal to the amplifier stage. In accordance with this aspect of the invention, a test signal generator is connected to the control terminal of the input stage for providing a test signal having a voltage level equal to or exceeding the second predetermined voltage for thereby inhibiting the semiconductor switching device from conducting and for providing the test signal to the amplifier stage for testing the operation of the monitoring system. Test signals can thereby be inserted for calibrating the monitoring system without the use of an electro-mechanical switch in the preamplifier and signal conditioning unit. Additional features of the present invention are described in the description of the preferred embodiment.