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Nuclear Instrumentation System (NIS)

NIS measures neutron flux across roughly ten decades, from shutdown source levels to ~120 % rated thermal power. Three overlapping detector ranges (source, intermediate, power) feed both the reactor protection system and the operator indications used in E-0, FR-S.1, and shutdown-margin verification.

Function

Provides flux indication from shutdown to full power, with overlap between adjacent ranges so the operator never loses a flux reading during reactor startup or shutdown. Source-range counts feed the boron-dilution alarm and the FR-S.2 dilution-detection criterion. Power-range channels feed the high-flux trip, the overpower-ΔT trip, and rod-deviation surveillance.

Components

flowchart LR
    SR_DET["Source-Range Detectors<br/>(BF3 proportional,<br/>2 channels)"]
    IR_DET["Intermediate-Range<br/>(CIC compensated ion,<br/>2 channels)"]
    PR_DET["Power-Range<br/>(uncompensated ion,<br/>4 channels — N41-N44)"]
    SR_DRAW["Source Range<br/>«SR-N31» «SR-N32»<br/>(10⁻¹ to 10⁵ cps)"]
    IR_DRAW["Intermediate Range<br/>«IR-N35» «IR-N36»<br/>(10⁻¹¹ to 10⁻³ amps)"]
    PR_DRAW["Power Range<br/>«PR-N41»…«PR-N44»<br/>(0-120 % RTP)"]
    RPS["Reactor Protection System<br/>(trip logic)"]
    BD_ALARM["Boron-Dilution<br/>Alarm «BD-ALARM»"]
    OP_IND["Operator Indication<br/>(MCB strip charts)"]
    OVERLAP["Range overlap:<br/>SR/IR ~10² cps<br/>IR/PR ~10⁻⁴ amps"]

    SR_DET --> SR_DRAW
    IR_DET --> IR_DRAW
    PR_DET --> PR_DRAW
    SR_DRAW & IR_DRAW & PR_DRAW --> RPS
    SR_DRAW --> BD_ALARM
    SR_DRAW & IR_DRAW & PR_DRAW --> OP_IND
    SR_DRAW -. overlap .- IR_DRAW
    IR_DRAW -. overlap .- PR_DRAW

    classDef det stroke:#856404,stroke-width:2px
    classDef draw stroke:#1f618d,stroke-width:2px
    classDef logic stroke:#943126,stroke-width:2px
    class SR_DET,IR_DET,PR_DET det
    class SR_DRAW,IR_DRAW,PR_DRAW draw
    class RPS,BD_ALARM logic
  • Source-range detectors — two BF3 proportional counters; located ex-core; sensitive in shutdown and approach-to-critical regime; output as counts per second.
  • Intermediate-range detectors — two compensated ion chambers (CIC); cover from ~10⁻¹¹ to ~10⁻³ amps DC current; overlap source range at the low end and power range at the high end.
  • Power-range detectors — four uncompensated ion chambers, top and bottom sections (axial flux shape); channels N41–N44; feed the high-flux trip, overpower-ΔT, and axial-flux-difference (AFD) monitoring.
  • Detector wells — ex-core, in the bioshield annulus around the reactor vessel; positions chosen for flux response and access.
  • Drawer electronics — signal processing, range-overlap logic, trip bistables; located in instrument racks; drive RPS coincidence.

Instrumentation

  • Source-range channels: «SR-N31», «SR-N32»
  • Intermediate-range channels: «IR-N35», «IR-N36»
  • Power-range channels: «PR-N41», «PR-N42», «PR-N43», «PR-N44»
  • Boron-dilution alarm: «BD-ALARM»
  • Axial flux difference: «AFD»
  • Wide-range flux indication (composite): «WR-FLUX»

Setpoints

Parameter Value Source
Source-range high flux trip (block above P-6) configurable; nominal 10⁵ cps Vogtle Tech Spec 3.3.1
Intermediate-range high flux trip (block above P-10) 25% RTP equivalent Vogtle Tech Spec 3.3.1
Power-range neutron flux — high trip 109% RTP Vogtle Tech Spec 3.3.1
Power-range neutron flux — high rate trip 5% RTP per second Vogtle Tech Spec 3.3.1
Overpower-ΔT trip per Tech Spec equation Vogtle Tech Spec 3.3.1
Boron-dilution alarm source-range count rate ×2 over baseline Vogtle UFSAR §15.4.6
AFD operating band ±5% from target Vogtle Tech Spec 3.2.3

Normal alignment

  • All ten channels in service; no bypassed channels
  • Source-range trip blocked above P-6 (intermediate-range threshold)
  • Intermediate-range trip blocked above P-10 (power-range threshold)
  • Power-range trips active
  • AFD within Tech Spec band

Failure modes

  • Single-channel inoperability — 2-of-4 logic for power range tolerates one bad channel; Tech Spec LCO entered (typically 72 h)
  • Detector saturation in source range during shutdown — gamma background can lift counts above source-range baseline; consult reactor engineering for interpretation
  • Loss of source-range indication during dilution — FR-S.2 cannot rely solely on source-range; boron sample is the verification
  • Stuck power-range channel — drives rod-deviation alarm; surveillance per Tech Spec; bypass requires SS authorization
  • AFD outside band — penalty entered into core operating limits; may require power reduction per Tech Spec

References

  • Vogtle UFSAR §7.2 (Reactor Trip System)
  • Vogtle UFSAR §7.7.1.7 (NIS — nuclear instrumentation)
  • Vogtle Tech Spec 3.3.1 (RPS Instrumentation)
  • Vogtle Tech Spec 3.2.3 (Axial Flux Difference)
  • NRC Westinghouse-style Technology Systems Manual, Chapter 6 (NIS)