The detectors and cables discussed in this section operate with electronic amplifiers and ratemeters as diown in Fig 2.49.

Fu; 2.49 Tjpieal pulse counter channel

The pulses due to neutrons at the detector are unall and are usually amplified by a head amplifier mounted near the detector, though the distance can be up to 80 m in AGRs. The output of the head amplifier is led to the main pulse amplifier, pulse height discriminator and ratemeter. This produces a IX’ signal which drives indicators and chart recorders and operates alarm and trip units. The complete chain ь show n in Fig 2,49,

The neutrons produce larger pulses at the detector [han rays and so the effects of 9 rays can be reduced by counting pulses only above a certain lev­el, using a pulse height discriminator. The gain of the pulse amplifier is adjusted to achieve the best discrimination.

With large у ray levels the number of pulses due to 7 rays is large and since they can become super­imposed, their height can add up to a level compara­ble to that due to neutrons. This effect is known as ‘gamma pile-up’ and is reduced by modifying the pulse shape. For example, with a typical AGR sys­tem integrate and differentiate time constants of 40 ns are employed in a wide bandwidth system, typically up to 20 MHz.

In order to cover the low power levels and give good overlap with the logarithmic DC channel, a wide range is required and it is convenient to have a logarithmic scale. Typically the usable range is Ю to 106 pulses/s. This characteristic is obtained by a log­arithmic diode pump circuit, which produces a DC voltage proportional to the logarithm of the pulse rate. This DC voltage is amplified to provide an output to drive indicators, chart recorders and operate alarm and trip units.

In addition the DC output is fed into a differ­entiating circuit and, in a similar fashion to that em­ployed in the ion chamber period meter discussed in the next section, provides a doubling time signal which drives indicators, control and alarm signals. Further details are given in Table 2.6.

A typical pulse counting channel measures pulse rates from fission or BFj chambers in the range of 10 pps to 106 pps. In the Heysham 2 AGR power station, a P8 fission chamber feeds pulses through about 20 m of mineral insulated cable to a head amplifier. The output of the head amplifier at 50 ohms impedance is fed through about 100 m of flexi­ble superscreened coaxial cable to the main pulse amplifier.

The channel measures pulse rate on a 5-decade logarithmic scale and provides outputs to operate a front panel meter and external trip and recording equipment. The log output voltage is differentiated internally to provide an output which is proportional to the doubling time of the reactor power. A front panel meter indicates doubling time (DT) and DT outputs are provided to operate external equipment.

The head amplifier is housed in a sealed metal can and the remainder of the channel is in modular form mounted in a 483 mm chassis. Trip circuits are pro­vided in an external unit.