In many applications it is essential to know the rate of change of power. This rate normally increases or decreases exponentially with time. The time constant for this change is referred to as the period. A period of five seconds means that the value changes by a factor of e (2.718) in five seconds. Figure 34 shows a basic period meter circuit.

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Figure 34 Period Meter Circuit

Placing the signal through an RC circuit causes a voltage that is proportional to the reciprocal of the period. If the current output from the ionization chamber is constant, no current flows through resistor R, and the output voltage is zero. This corresponds to an infinite period. As the ion chamber output current changes, there is a voltage transient across capacitor C, and current flows through resistor R. The more rapid the transient, the greater the voltage drop across resistor R, and the shorter the period.

Rate information is displayed on a meter in decades per minute, and since it is used by the operator to monitor the rate of change of power during startup, it is termed startup rate. Startup rate (SUR) equates to reactor period using Equation 6-10.

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where

The reactor operator adjusts control rods so that an upper limit, such as 1 DPM, is not exceeded. This allows an orderly increase in reactor power.

Summary

The source range uses a proportional counter. The intermediate range uses a compensated ion chamber. The power range uses an uncompensated ion chamber. Terms used to describe the electrical circuits are summarized below.

Circuit Terminology Summary

Signal-to-noise ratio is the ratio of the electrical output signal to the electrical noise generated.

A discriminator selects the minimum pulse height to be counted.

Analog is a mechanism in which data is represented by continuously variable physical quantities.

Logarithm is the exponent that indicates the power to which a number is raised to produce a given number.

Reactor period is that amount of time required for neutron flux to change by a factor of e.

Decades per minute is the rate at which neutron flux is changing by a power of 10 in each minute.

Scalar is a measurement or quantity which is capable of being represented on a scale.

Startup rate is the rate at which neutron flux is changing measured in decades per minute.