Instrumentation and Control
Domain 6 tests measurement, logic, and setpoints. The exam expects you to know how a signal becomes a trip and why redundancy is not the same as independence.
Core rules
- Neutron detectors cover different ranges: source-range detectors (fission chambers or BF3) for startup, intermediate-range compensated ion chambers for linear power, and power-range ion chambers for operational power and trips.
- Temperature sensors include RTDs for slower, accurate process temperatures and thermocouples for faster, wider-range measurements. Installation depth, bypass flow, and response time affect the reading.
- Trip logic often uses 2-out-of-4 voting: one failed detector does not trip the plant, but two agreeing channels do. Redundancy must be independent; a shared power supply or common calibration error can defeat it.
- Setpoints include instrument uncertainty. A trip setpoint is not always the same as the analytical limit; calibration and surveillance verify the as-found condition.
Worked example
A power-range trip has four channels and a 2-out-of-4 logic. One channel fails low, one fails high, and two remain valid. A true high-power condition causes the two valid channels to trip; the failed-high channel also trips, so the logic still protects. But if both valid channels share a clogged sensing line, the apparent agreement is false. That is a common-cause failure, not redundancy.
Traps
Assuming a 2-out-of-4 logic tolerates two failures in the same direction. Calibrating all channels from one reference without checking independence. Treating a sensor's local display as the trip signal. Ignoring response time when a fast transient is involved.