The neutron-monitoring system is a vital part of both the reactor control system and the plant protection system. The system must have proper grounding and shielding. Some factors involved in determining the proper grounding and shielding methods are

1 The length of the signal conductors between the detectors and amplifiers.

2 Methods used for internal grounding of detectors and amplifiers

3 Methods used for grounding the electrical distribu­tion systems of the building

Several types of neutron sensors are widely used by the nuclear industry (see Chaps 2, 3, and 4) Some variation exists m the manufacture of nuclear-instrument circuits in regard to the method of providing internal signal grounding Some nuclear instruments have the signal ground on the chassis, whereas others have the signal ground insulated from the chassis ground.

The following discussion concerns the grounding sys­tems currently accepted by the nuclear industry in oper­ating power plants

(a) Grounding of Signal and Control Cables. The fol­lowing must be kept m mind when considering the methods to be used for grounding neutron-monitoring systems

1 There is always some potential difference between two points on the earth’s surface

2 Because a cable is connected to a ground bus, it is not necessarily a good ground

3 Ground connections are not always noise-free

Since it is virtually impossible to eliminate noise and induced current m ground connections, the proper proce­dure to follow in wiring practice consists of routing the inevitable currents around the equipment in such a manner that the signal input is not affected

Figure 10 17 shows a typical sensor—preamplifier — count rate-meter combination as often installed, along with some of the sources of error and interference due to potential differences generated by the system at various points Figure 10 18 shows the same system with different grounding connections to eliminate ground loops m sensi­tive circuitry If the ground loops shown in Fig 10 17 are not removed, a “battery” voltage composed of noise is impressed on the opposite ends of the cable shield, thus effectively causing a current in the shield that may add to or modulate the desired signal Ground loops are elimi­nated by isolating the system from ground except at the console This is only one of several noise-rejection ground­ing techniques available Others are electric differential input techniques and the use of balanced lines [see Sec 10-5 6(b)]

(b)

Grounding and Shielding Practice.[26] We made a survey of 10 major nuclear power plants in the United States, including 4 of the largest operating plants, according to MW(e) output in service as of January 1969 The purpose was to collect and compile data on methods used in these plants for grounding and shielding the neutron­monitoring systems. Each had experienced noise problems in the neutron-monitoring channels

Figures 10.19 to 10.21 illustrate the grounding and shielding methods used by the nuclear power plants surveyed. Numerous modifications were made on the equipment after the plants were constructed. Such modifications as adding line filters, radio-frequency (r-f) grounds, and n filters were made to the equipment to suppress noise and interference In a majority of the plants surveyed, a single-point ground system was used either by grounding the system at the nuclear-instrument cabinet or at the neutron detector. Other nuclear power plants provided grounding at both the neutron detector and amplifier cabinet

Figure 10.19 illustrates a grounding method in which the neutron-monitoring-system ground is made at the amplifier cabinet The pulse amplifiers, counting circuits, and other associated circuits are grounded to the building ground. Generally instruments in the cabinet are grounded to the building ground by connecting all instruments to a bus bar in the cabinet The grounding bus bar is connected to the building ground through a grounding cable

The entire neutron-monitoring system from the pulse amplifiers to the neutron sensor is insulated and floated above ground. This method prevents circulating ground currents from causing noise and distortion m the electrical signal being transmitted to the control room.

Figure 10.20 illustrates a method where the single­system ground is made at the sensor or at the preamplifier The signal-cable shielding and ground side of the signals in the amplifier cabinet are all insulated and floated above ground.

Figure 10.21 illustrates a method where multiple- system grounds are used. Grounding takes place at both the amplifier cabinet and the preamplifier The neutron — detector cable may be grounded at the reactor or un­grounded the full distance to the preamplifier

(c) Engineering Data Sheets, Grounding and Shielding.

Engineering data sheets, included as an appendix to this chapter, explain the methods used for grounding the start up channels at 10 major operating nuclear power plants in the United States Included with the system descriptions are comments on operating problems and modifications made to the equipment to prevent noise and interference. These data sheets should be used by both the design and construction engineer in selecting the best method of grounding an instrumentation system. Informa tion is included on how to avoid pitfalls that have been encountered in nuclear power plants and, more important, how to avoid having to make extensive modifications after the plants have been constructed

Grounding and shielding problems in nuclear plants are often associated with the neutron-monitoring start-up channels A number of mechanisms in a nuclear plant generate r-f and low-frequency noise, which finds its way into the neutron-monitoring start-up channels This noise generates signals that must be cut off by a higher discriminator voltage setting, thereby materially reducing channel sensitivity If rate amplifiers and reactor trip circuits are used with the start up channels, inadvertent scrams result where noise increases As the reactor power is increased and current-measuring channels take over from the pulse count-rate channels, the effect of noise is much less important

The following conclusions can be drawn from the information contained in the engineering data sheets

1 A single-point grounding system, grounded to the building ground at the amplifier cabinet with the signal — cable shields and neutron sensors floated above ground, appears to be most widely used by the nuclear industry The entire system is grounded at one point

2 The use of triaxial cable in place of coaxial cable is becoming standard throughout the nuclear industry, thus improving the suppression and rejection of noise and interference in the neutron-monitoring channels

3 In the use of coaxial and triaxial cable, it appears that the required care is not used in the installation of cable connectors and terminations during the construction phase of the plant Rework of cables and connectors is often necessary after plant construction has been completed

4 Eliminating noise at the source is a task that is quite frequently done by operating and maintenance staffs at nuclear plants This task sometimes involves days of tedious work to isolate and eliminate the source of noise Faulty a-c and d-c machinery, relays, switches, motor starters, etc, are sources of noise

5 Techniques, such as the use of line filters, r-f filters, and other electronic means, are being used by some nuclear plants to reject and suppress noise in the neutron­monitoring channels

(d) Noise Filter Design. There are times when a rapid “fix” is needed on the signal input or power input of a piece of instrumentation to eliminate high-frequency noise present on the line Although the use of commercial radio-frequency interference (RFI) and line filters is recom­mended, there are times when a simple 18 dB/octave, low-pass я or T filter can solve a noise problem Figure 10 22 shows design details for networks of this type

When a filter of this type is installed, it should be placed in its own metal box where possible If not possible, at least the input and output leads should be separated Such a filter can be used either as a low-level-signal filter or as a high-level power-line filter if the components are suitably rated These filters may be placed in power input lines to any piece of equipment in a neutron-monitoring system and may also be used m signal input leads to equipment as long as the desired input-signal frequency is below fc These filters are not used in pulse amplifier inputs