Incores
Function
Incore instrumentation systems provide core temperature and flux measurements for post-accident monitoring and core power distribution mapping. Includes Core Exit Thermocouples (CETs), the incore flux mapping system, and the Subcooling Margin Monitor. (UFSAR 7.6, 7.7)
Core Exit Thermocouples (CETs)
CETs measure coolant temperature at the exit of fuel assemblies. Used for:
- Post-accident core cooling assessment (inadequate core cooling monitoring)
- Input to the Subcooling Margin Monitor
- Core power distribution verification
Exam — 2023 Q57
In-Core Temperature Monitoring (CET) System: reference junction box is located outside containment. Ambient temperature changes are compensated by a temperature compensator circuit — CET readings are NOT affected by reference junction box temperature changes. In-Core system design upper range is 2300°F.
Incore Flux Mapping System
Moveable incore detectors are inserted into thimble tubes to map the core power distribution. Used for:
- Verification of power distribution limits (FQ, F-delta-H)
- Calibration of excore nuclear instruments
Subcooling Margin Monitor
Calculates RCS subcooling margin from CET temperatures and RCS pressure. Used in EOPs to assess core cooling adequacy.
Exam — 2020 Q58
Subcooling Margin Monitor (CETPS) inputs: representative CET temperature, RCS wide range pressure, containment pressure, and containment radiation level. PZR pressure is NOT used — PZR pressure instrumentation is narrow range (reading no lower than 1700 psig). RCS hot leg temperatures are NOT used — representative CET temperature is the correct temperature input.
Adverse Containment Mode
The SMM switches to ADVERSE mode when containment conditions are degraded. Either condition alone triggers adverse mode:
- Containment pressure > 4 psig
- Containment radiation (R44A/B) > 1E05 R/HR
Exam — 2020 Q26
SMM adverse containment logic: either containment pressure > 4 psig OR R44A/B > 1E05 R/HR triggers ADVERSE mode (only one condition required). When containment pressure lowers to < 3 psig, the SMM automatically resets from a pressure-driven adverse signal. However, the SMM does NOT automatically reset when radiation levels lower below adverse threshold — manual action is required to reset from a radiation-driven adverse condition. Common trap: >1E06 R/HR is the integrated dose number used in procedures, NOT the adverse containment threshold (which is >1E05 R/HR).
Connections
- Related systems: Excore NIs, RCS, RPS/SSPS, Containment, Radiation Monitoring
- Related EOPs: EOP-CFST-1 — Critical Safety Function Status Trees
- Related exam questions: 2023 Q57, 2020 Q26, 2020 Q58
- Related exams: 2023 NRC Written Exam, 2020 NRC Written Exam