Patent Number: 052951669
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a start-up range neutron monitor (SRNM) system in a nuclear power plant particularly for suppressing contamination of external noise. A nuclear power plant includes a reactor building in which is installed a reactor containment vessle in which a reactor is disposed. A structure of a known start-up range neutron monitor system in a nuclear power plant is shown in FIG. 8, and the known SRNM system of FIG. 8 comprises a neutron detector 1 arranged in a reactor, coaxial cables 3 and 4 for transferring signals detected by the neutron detector 1 to a signal processing unit 2a disposed inside a monitor 2 arranged in a central control chamber, and a preamplifier 5 disposed between these cables 3 and 4. Namely, the neutron detector 1 is operably connected to the preamplifier 5 in the reactor building through the coaxial cable 3 connecting the neutron detector 1 and the preamplifier 5 by penetrating inside the reactor containment vessel and the coaxial cable 4 connects the preamplifier 5 and the signal processing unit 2a. These coaxial cables 3 and 4 are composed of cores 3a and 4a and outer sheaths 3b and 4b for earthing, respectively. An earth circuit has one point earth structure earthed through the signal processing unit 2a. In the known start-up range neutron monitor system of FIG. 8, electric pulse signals in response to thermal neutrons in the start-up range in the reactor are detected. The thus detected signal has a weak magnitude, so that the detected amplified by the preamplifier 5 and then treated with by the signal processing unit 2a of the monitor 2. However, since the known SRNM system has a structure in which, as described above, the neutron detector 1 and the preamplifier 5 are connected through the coaxial cable 3, when the external noise is transferred to the coaxial cable 3, an S/N (signal/noise) ratio of the weak signal is extremely lowered by the external noise, thus being inconvenient. This problem will be explained in detail with reference to FIGS. 9 and 10. Supposing that the external noise is invaded into the coaxial cable 3 on the input side of the preamplifier 5 and a noise current I.sub.N is caused by the external noise in the outer sheath 3b, a circuit in such case will be modeled as that shown in FIG. 9 and an equivalent circuit is shown in FIG. 10, in which reference numeral 1' denotes a detection signal source by means of the neutron detector 1. Referring to FIGS. 9 and 10, the start-up range neutron monitor system has, as a whole, one point earth structure in the central control chamber, and the neutron detector 1 has an isolated, i.e. non-earthed, structure. For this reason, when an impedance of the coaxial cable 3 and the circuit is supposed to R.sub.C, a noise voltage V.sub.12, caused between both poles P.sub.1 and P.sub.2 of the preamplifier 5 is represented as EQU V.sub.12 =I.sub.N .multidot.R.sub.C ( 1) That is, even if the noise current I.sub.N be weak, the noise voltage V.sub.12 becomes R.sub.C times of the current I.sub.N, so that the S/N ratio of the detected weak signal of the neutron detector 1 is lowered, thus requiring a complicated signal processing circuit of the monitor 2 disposed on the output side of the preamplifier 5 and an increased load for calculation of the signal processing, thus imparting adverse influence on the signal treatment. SUMMARY OF THE INVENTION An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art and to provide a start-up range neutron monitor system capable of easily processing a neutron detection signal by improving a noise resisting property on the input side of a preamplifier disposed, for example, in a reactor building in a nuclear power plant. This and other object can be achieved according to the present invention by providing, in one aspect, a start-up range neutron monitor system for monitoring neutrons generated from a neutron source, comprising: a neutron detector disposed in a non-earthed state and adapted to detect neutrons generated from the neutron source; PA1 a coaxial cable for externally transmitting a detection signal from the neutron detector; PA1 a preamplifier incorporated on a way of the coaxial cable for amplifiying the detection signal; PA1 a signal processing unit operably connected to the preamplifier through the coaxial cable to process the detection signal amplified by the preamplifier, the coaxial cable being composed of a first cable portion connecting the neutron detector and the preamplifier on an input side of the preamplifier and a second cable portion connecting the preamplifier and the signal processing unit on an output side of the preamplifier; and PA1 a cable shield disposed so as to cover the first cable portion of the coaxial cable, PA1 wherein an earth side circuit on the signal processing unit is earthed and the cable shield is connected to an earth side circuit of the preamplifier to thereby constitute the entire system as one point earth structure. PA1 a neutron detector disposed in a non-earthed state and adapted to detect neutrons generated from the neutron source; PA1 a coaxial cable for externally transmitting a detection signal from the neutron detector, the coaxial cable being composed of a core and an outer sheath surrounding the core; PA1 a preamplifier incorporated on a way of the coaxial cable for amplifiying the detection signal; PA1 a signal processing unit operably connected to the preamplifier through the coaxial cable to process the detection signal amplified by the preamplifier, the coaxial cable being composed of a first cable portion connecting the neutron detector and the preamplifier on an input side of the preamplifier and a second cable portion connecting the preamplifier and the signal processing unit on an output side of the preamplifier; and PA1 a coil assembly including first and second coils incorporated in the core and outer sheath of the first cable portion of the coaxial cable respectively, the first and second coils having same inductance and being arranged so as to generate magnetic fluxes in directions reverse to each other. The coaxial cable is composed of a core and an outer sheath surrounding the core. The cable shield is composed of a shield cable arranged to coaxially surround the coaxial cable. The preamplifier is composed of an amplifier circuit and a casing constituting the earth side circuit of the preamplifier, the shield cable being connected to the casing. The start-up range neutron monitor system is disposed in a nuclear power plant including a reactor building, a reactor containment vessel disposed in the reactor building and a reactor disposed in the reactor containment vessel, wherein the neutron detector is disposed in the reactor, the preamplifer is disposed in the reactor building and the first cable portion of the coaxial cable penetrates the reactor containment vessel. According to this aspect of the start-up range neutron monitor system of the present invention, when an external noise is invaded into the first cable portion of the coaxial cable on the input side of the preamplifier, a noise current in the outer sheath of the coaxial cable is earthed through the most outside cable shield. That is, the noise current is directly earthed not through the outer sheath of the coaxial cable and a voltage generated to the input end of the preamplifier becomes substantially zero, so that the external noise hardly affects on the signal processing unit for the neutron detection signal. In another aspect of the present invention, there is provided a start-up range neutron monitor system for monitoring neutrons generated from a neutron source, comprising: The coil assembly comprises a ring core and a portion of the first cable portion which is wound up around the ring core. The start-up range neutron monitor system is disposed in a neuclear power plant including a reactor building, a reactor containment vessel disposed in the reactor building and a reactor disposed in the reactor containment vessel, wherein the neutron detector is disposed in the reactor, the preamplifer is disposed in the reactor building and the first cable portion of the coaxial cable penetrating the reactor containment vessel. According to this aspect of the start-up range neutron monitor system of the present invention, the inductances of the coils inserted into the core and the outer sheath of the first portion of the coaxial cable on the input side of the preamplifer are the same with each other and these coils have the magnetic flux generating directions reverse to each other. Accordingly, the insertion of the coils do not affects on the neutron detection signal by making larger an input impedance on the side of the preamplifier than those of the core and the outer sheath of the coaxial cable. Moreover, interferance of the noise current due to the external noise to the detection signal can be reduced by setting the inductance of the coil to a value larger than a value obtained by dividing the impedance of the outer sheath by an angular frequency.