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16.4 BWR instrumentation 225

16.2.6 Actuator status sensors
As demonstrated in the Three Mile Island - II accident, it is not enough to rely on
signals sent for actuators to operate. Actuators, such as valves, can fail to operate
in response to signals from the control system or an operator. An independent mea-
surement of actuator status is needed to confirm operation.

16.3 PWR instrumentation
Fig. 16.14 shows the major sensors and their placement in a typical pressurized water
reactor power plant. Instrumentation in both primary and secondary sides of the plant
are shown.
Specific instrumentation channels include the following:

• In-core and ex-vessel neutron detectors. In-core sensors are on a drive system that
moves the sensors to desired core locations to provide neutron flux distribution
(mapping) information. Ex-vessel neutron detectors are placed in instrument
wells in the biological shield surrounding the reactor vessel. Typically, there are
eight detector assemblies (long ionization chambers). The nuclear
instrumentation monitors reactor power from source range to intermediate range,
to 120% of full power [6].
• N-16 detectors are located on hot leg piping and are used to measure both reactor
power and primary coolant flow rate (the latter has an accuracy of 1.5%).
• Core-exit thermocouples located above the core, typically at about 45 locations.
• Sensors for digital rod position indication.
• Hot leg and cold leg RTDs (resistance temperature detectors) needed for both
control and safety system functions.
• Pressurizer pressure and water level.
• Primary coolant flow rate.
• Steam generator feedwater flow rate, steam flow rate, water level.
Redundancies are included in all these measurements.
Fig. 16.15 shows in-core instrumentation in a typical PWR [6].
Fig. 16.15 shows flux detector thimble guide tubes (for flux mapping) introduced
from the vessel bottom and core-exit thermocouples that are inserted from the top of
the reactor vessel.
Major balance of plant instrumentation includes steam flow rate and temperature
at different turbine stages, condenser pressure, feedwater temperature, and feed
pump conditions.

16.4 BWR instrumentation
Instrumentation for a typical BWR appear in Fig. 16.16.