The BWR NI system, like the PWR system, has three overlapping ranges as illustrated in Figure 4.

J________ T — ————

Source start-up range

1 I I—

10′ 10" 10′ 10“ 10" 10”

Fig. 4. Typical ranges covered by in-core neutron detectors in a BWR (Knoll, 2000)

The three systems are called source, intermediate, and power range monitors. Unlike the PWR, which uses out-of-core neutron detectors, the neutron detectors are all located in-core. There are also many more detectors used in the BWR NI system than in the PWR system.

The source range monitoring system typically consists of four in-core fission chambers operating in pulse mode. Pulse mode operation provides good discrimination against gamma rays, which is necessary when measuring a relatively low neutron flux in the presence of a high gamma flux. A typical intermediate range monitoring system has eight in-core fission chambers operating in the mean square voltage (MSV) mode. The MSV mode promotes the enhanced neutron to gamma response required to provide a proper measure of neutron flux in the presence of gamma rays for both control and safety requirements. The power range monitoring system typically consists of 144-164 fission ion chambers distributed throughout the core. The fission chambers operate in current mode and are called local power range monitors (LPRM). Current mode operation provides satisfactory neutron response at the high flux levels encountered between 2 and 150% full power. In a typical system, approximately 20 LPRMs are summed to provide input to one of the seven or eight average power range monitoring (APRM) systems. The APRM system provides input for both control and reactor protection systems. In-core flux detectors are used at high power levels (above 10% of full power) because they provide spatial information needed, at high power, to control xenon-induced flux tilts and to achieve the optimum flux distribution for maximum power output. The control system flux detectors are of two types. One type has an inconel emitter and is used for the zone control system. The other type has a vanadium emitter, and is used for the flux mapping system. For power mapping validation, channel temperature differentials are used with measured flows (instrumented channels) or predicted flows (other channels) to determine the estimated channel powers, which are then compared with the powers calculated from the flux mapping readings; this provides an ongoing validation of the accuracy of the flux mapping channel powers.