Patent Number: 052951669
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a start-up range neutron monitor system according to the present invention will be described hereunder with reference to the accompanying drawings, in which like reference numerals are added to elements or portions corresponding to those shown in FIGS. 8 to 10 showing a conventional structure. A first embodiment is first described with reference to FIGS. 1 to 3. FIG. 1 shows a start-up range neutron monitor system, which is adapted preferably for a nuclear reactor and is composed of an isolated, i.e. non-earthed, neutron detector 1 disposed inside the reactor, coaxial cables 3 and 4 transferring a neutron detection signal form the neutron detector 1 to a signal processing unit 2a in a monitor 2 disposed in a central control chamber, a preamplifier 5 interposed between these coaxial cables 3 and 4, and a shield cable 10 applied to the coaxial cable 3 as a shield member. Namely, the neutron detector 1 is connected to the preamplifier 5 disposed in a reactor building through the coaxial calbe 3, on the side of the detector, arranged so as to pass a reactor containment vessel, and on the other hand, the preamplifier 5 is connected to the signal procesing unit 2a through the coaxial cable 4 on its output side. These coaxial cables 3 and 4 are composed of cores 3a and 4a and sheaths 3b and 4b for earthing, respectively. It is of course to be noted that the coaxial cables 3 and 4 are composed of a single cable on the way of which the preamplifier is incorporated or two coaxial cables are disposed on the input and output sides of the preamblifier. The preamplifier 5 is composed of an amplifying circuit 5a and a casing 5b forming an earth circuit, and the coaxial cables 3 and 4 have the outer sheaths 3b and 4b which are operably connected to each other through the casing 5b of the preamplifier 5. The shield cable 10 covering the coaxial cable 3 on the side of the detector 1 is formed of a conductive material and is connected to the casing 5b through an earth cable 11. The earth circuit is grounded through the signal processing unit 2a, whereby whole the system is constructed so as to have one point earth structure. According to the start-up range neutron monitor system of this embodiment, electric pulse signals in response to thermal neutrons in the start-up range of the reactor are detected by the neutron detector 1 likely as in the conventional system. Although the detected signal has a weak magnitude, it is amplifed by the preamplifier 5 and subjected to the predetermined signal treatment through the signal processing unit 2a in the monitor 2. FIGS. 2 and 3 are views for the explanatory of the noise shielding functions and effects according to this first embodiment of the present invention. Referring to FIG. 2, floating capacities C.sub.1 to C.sub.3 are present between the coaxial cable 3 on the input side of the preamplifier 5 and the shield cable 10 covering the coaxial cable 3. That is, the floating capacity C.sub.1 exists between the core 3a of the coaxial cable 3 and the shield cable 10, the floating capacity C.sub.2 exists between the core 3a and the sheath 3b and the floating capacity C.sub.3 exists between the sheath 3b and the shield cable 10. Reference numeral 1' denotes a detection signal source for the neutron detector 1. The circuit structure schematically shown in FIG. 2 will be substituted with an equivalent circuit of FIG. 3. In the circuit connected as described above, when the external noise of the noise voltage V.sub.N is transferred to the coaxial cable 3, the external noise is captured by the shield cable 10 and the noise current is earthed through the shield cable 10. Namely, the noise current is directly flown to the earth through a pass in which the earth resistance is most lowered by means of the shield cable 10, whereby the voltage V.sub.12 between input ends P.sub.1 and P.sub.2 of the amplifying circuit 5a caused in response to the noise voltage V.sub.N becomes V.sub.12 =0. According to the shielding structure described above, the external noise hardly affects on the neutron detection signal and the lowering of the S/N ratio caused by the external noise can be hence prevented substantially perfectly. Therefore, there is no need for adding a specific treating circuit to the coaxial cable 3 for the treatment of the external noise and the operational load on the signal treatment cannot be increased, thus easily facilitating the signal treatment. A second embodiment of the start-up range neutron monitor system according to the present invention will be next described hereunder with reference to FIGS. 4 to 6, in which like reference numerals are added to elements or portions corresponding to those of the first embodiment. In the start-up range neutron monitor system of FIG. 4, a ring core 20 formed by forming a magnetic member into a ring shape is newly added, and a portion of the coaxial cable 3 connecting the neutron detector 1 and the preamplifier 5 is wound around the ring core 20. The structure of the monitor system other than this structure of the ring core 20 is substantially the same as the structure of the monitor system of the first embodiment. The noise shielding function and effect according to the second embodiment will be explained hereunder with reference to FIGS. 5 and 6. As described above, in a circuit constituted by the coaxial cable 3 in association with the ring core 20, coils 21a and 21b are equivalently incorporated, and the coils 21a and 21b have inductances L.sub.1 and L.sub.2 of substantially the same value (L.sub.1 =L.sub.2 =L) and have directions of magnetic flux generations reverse to each other with respect to the detection signal source 1'. Namely, FIG. 5 shows an equivalent circuit with rspect to the neutron detection signal in the case where the coil 21a inserted into the core 3a of the coaxial cable 3 has the inductance L.sub.1 and the coil 21b inserted into the outer sheath 3b of the coaxial cable 3 has the inductance L.sub.2. FIG. 6 shows an equivalent circuit with respect to the external noise. In FIGS. 5 and 6, symbols R.sub.C1 and R.sub.C2 represent line impedances of the core 3a and the outer sheath 3b and a symbol R.sub.S represents an input impedance of the preamplifier 5. In this second embodiment, the input impedance R.sub.S of the preamplifier 5 is set to a value sufficiently larger than those of the line impedances R.sub.C1 and R.sub.C2. Referring to the equivalent circuit of FIG. 5, a voltage V.sub.S between both ends of the input impedance R.sub.S in the case of an detection current I.sub.S by means of the detection signal source 1' for the neutron detector 1 is expressed as follows. EQU V.sub.S =I.sub.S .multidot.R.sub.S (2) As can be seen from this equation, the insertion of the coils 21a and 21b does not affect on the detection current I.sub.S. Next, with reference to FIG. 6, showing the state of the external noise invasion, symbols I.sub.n2 and I.sub.n1 represent noise currents passing the core 3a and the sheath 3b, respectively, due to the external noise, and a symbol V.sub.sn represents a noise voltage generated between both ends of the input impedance R.sub.S due to the external noise. In this equivalent circuit, the relationship L.sub.1 =L.sub.2 =L is established and the impedance R.sub.S has a value sufficiently larger than those of the line impedances R.sub.C1 and R.sub.C2. Accordingly, the noise voltage V.sub.sn is approximately determined by an impedance distribution ratio on the side of the sheath and expressed as follows. ##EQU1## In this equation, the line impedance R.sub.C2 is extremely samll and the inductance L is sufficiently larger than the value R.sub.C2 /.omega. , and accordingly, the absolute value of the denominator of the equation (3) becomes larger than 1 and the noise voltage V.sub.sn hence becomes small. Namely, in consderation of circuit theory, as almost all the noise current due to the external noise passes on the side of the outer sheath, the noise voltage V.sub.sn becomes sufficiently smaller than the external noise voltage V.sub.N. Thus, the invasion of the external noise into the signal system can be suppressed, thus improving the S/N ratio. This noise suppressing effect can be more improved as the frequency of the noise becomes higher. According to the second embodiment, substantially the same effects as those attained by the first embodiment can be achieved, thus improving the noise resisting property. Particularly, according to this second embodiment, the coils 21a and 21b having same inductance and reverse magnetic flux directions can be equivalently and easily inserted, respectively, to the core 3a and the core 10. It is of course understood that independent coils can be inserted respectively. In a modified embodiment, a combination of the shield structures of the first and second embodiments may be realized as shown in FIG. 7, in which the like reference numerals are added to elements or portions corresponding to those of FIGS. 1 and 4. According to this modified embodiment, combined effects of those of the first and second embodiments will be attained to thereby further improve the noise resisting property with respect to the input side of the preamplifier. As described above, the structure of the present invention can be applied to other systems each in which the detection system has same one point earth structure, that is, a detection signal source is non-earthed and a signal processing side connected to the detection signal source through a coaxial cable is earthed. According to such structure, the noise resisting property can be remarkably improved even with respect to a weak detection signal.