Materials, Instrumentation, and Control
Materials degradation
Austenitic stainless steels (304/316) and nickel alloys (Inconel 600/690 and their welds) are used for internals and steam generator tubing; Alloy 600/182/82 has driven primary-water stress corrosion cracking, motivating Alloy 690/52/152 replacements. Sensitization (chromium carbide precipitation at grain boundaries) enables intergranular SCC. The RPV is low-alloy steel with stainless cladding; irradiation embrittlement raises RT_NDT and lowers upper-shelf energy, evaluated under 10 CFR 50 Appendix G and 50.61 for pressurized thermal shock. Zircaloy cladding corrodes and picks up hydrogen, leading to hydride embrittlement; flow-accelerated corrosion attacks carbon steel piping. Control rods: Ag-In-Cd or B₄C.
Instrumentation and control
Ex-core neutron detectors are chosen by range: source range proportional counters, intermediate range compensated ion chambers, power range uncompensated ion chambers. The reactor protection system is redundant and independent with coincidence logic (commonly 2-out-of-4), designed to IEEE 603 principles. Trip setpoints include allowances for calibration uncertainty and must be below the analytical safety limit.
Traps
- Assigning the wrong detector type to a range.
- Confusing startup rate (decades per minute) with reactor period (seconds).
- Treating a trip setpoint as equal to the safety analysis limit.
Worked example
Startup rate in decades per minute ≈ 26.06 / period(s). A 26 s period is about 1 DPM; a 2.6 s period is about 10 DPM - a fast, easily recognized transient.