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Systems > Excore NIs

Excore NIs

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Excore NIs

Function

Excore Nuclear Instrumentation provides continuous monitoring of reactor neutron flux from shutdown through full power operation using detectors located outside the reactor vessel. Three overlapping ranges ensure complete coverage: Source Range (shutdown/startup), Intermediate Range (startup), and Power Range (power operation). (UFSAR 7.2.1.2)

Nuclear Instrumentation Ranges

  • Source Range: Two channels, used during shutdown and startup
  • Intermediate Range: Two channels, used during startup
  • Power Range: Four channels, used during power operation (provides input to OT Delta-T, OP Delta-T, high flux trips, and rod control) (UFSAR 7.2.1.2)

Key NIS Permissive Interlocks

These permissives are derived from NIS channels. The master reference for all permissive interlocks is RPS/SSPS.

DesignationDerivationFunctionSource
P-61/2 intermediate range above setpointAllows manual block of source range tripUFSAR T7.2-2
P-83/4 power range below setpointBlocks trip on low flow in a single loopUFSAR T7.2-2
P-102/4 power range above setpointAllows block of power range low setpoint trip; blocks source range tripUFSAR T7.2-2
Exam — 2023 Q72
Subcritical rod withdrawal response: after a short rod withdrawal with the reactor still subcritical, SR counts rapidly increase then stabilize at a new higher value (subcritical multiplication reaches new equilibrium at higher keff). Startup rate (SUR) rapidly increases then gradually decreases to zero. SUR returns to zero because the reactor is still subcritical — a sustained positive SUR only exists at criticality. SR counts do NOT return to the previous value (they stabilize higher) and SUR does NOT remain at a positive value (it decays to zero).
Exam — 2023 Q11
Loss of a vital instrument bus (e.g. 1B) causes associated NI channels to fail low (SR, IR, and PR monitors lose power). The bi-stables trip to SSPS. Above P-6 (approximately 1E-10 amps / ~1% power) the SR trip is blocked. Below P-10 (10% power) the IR high flux trip is active with 1/2 coincidence. At 8% power (above P-6, below P-10): SR trip blocked, PR needs 2/4 (only 1 channel affected), but the IR trip actuates on 1/2 IR bi-stables tripped — reactor trips.
Exam — 2023 Q82
P-6 permissive and IR NIS overlap: when P-6 is LIT, IR channels should indicate approximately 1E-5% power. If an IR channel reads 1E-8% power with P-6 LIT, that channel is not indicating the expected SR/IR overlap and is INOPERABLE. Per TS 3.3.1.1 bases, the 24-hour action time for one inoperable IR channel accounts for the low probability of failure of the operable IR channel during this period. Trap: the bases cite low probability of channel failure, NOT low probability of reactivity events.

PR Channel Trip and Coincidence

Exam — 2019 Q30
When a Power Range NI channel (e.g., 2N41) fails high: AB.NIS-0001 REQUIRES placing the Rod Bank Selector Switch in MAN prior to placing the channel in tripped condition. After 2N41 bistable is tripped per S2.OP-SO.RPS-0001, the coincidence for the PR High Neutron Flux Reactor Trip changes from 2/4 to 1 out of 3 (the tripped bistable counts as a permanent "tripped" input — it is NOT removed from the logic). Trap: candidates may think tripping the bistable removes the channel from the trip logic (reducing to 2/3). In fact, the tripped bistable satisfies one leg of the coincidence, so only 1 more of the remaining 3 channels needs to trip = 1 out of 3.

Tech Spec LCOs

  • TS 3/4.3 — Instrumentation|TS 3/4.3.1 — Reactor Trip System Instrumentation (NIS channels)
Exam — 2022 Q57
P-10 permissive and IR high flux trip: when at least 3/4 PR NIS channels are less than the P-10 setpoint (~9%), P-10 automatically unblocks the 1/2 IR high flux trip (25%) and the 2/4 PR high flux low setpoint trip (25%). With one IR channel failed HIGH (e.g. 2N35), the reactor will automatically trip when power decreases below P-10 during a planned shutdown. P-10 clears on 3/4 PR channels below setpoint, NOT 2/4. Do not confuse the P-10 block logic (2/4 above) with the unblock logic (3/4 below).
Exam — 2022 Q78
TS 3.9.2.2 (refueling source range neutron flux monitors): per TS bases, any combination of NIS source range monitors AND/OR Gamma-Metrics post-accident neutron flux monitors may be used to satisfy the LCO. Two of the four total monitors are required OPERABLE. If one SR channel becomes inoperable (e.g. loss of 2A vital instrument bus), restoring ONE Gamma-Metrics channel to OPERABLE satisfies the requirement and core alterations may recommence.
Exam — 2022 Q82
TS 3.3.1.1 Action 3 for one IR NIS channel inoperable above P-6: reduce RTP below P-6 OR raise RTP above P-10 within 24 hours. P-6 setpoint is 1E-5% RTP. At 1E-4% power, the reactor IS above P-6 and the LCO applies. Reactor power at 1E-4% is above P-6 (1E-5%) — do not incorrectly assess that power is below P-6.
Exam — 2020 Q22
IR channel removal from service per S2.OP-SO.RPS-0001: the procedure places the LEVEL TRIP switch in bypass, which causes OHA E-29 (SR & IR TRIP BYP) to annunciate. IR channels are NOT removed by pulling control power fuses or instrument power fuses — fuse removal is used for PR channels only. There is no POWER MISMATCH BYPASS switch for IR channels (PR channel feature only).
JPM — 2022 RO-A3
Manual QPTR calculation (S2.OP-ST.NIS-0002): for each Power Range detector (N41-N44), upper and lower: divide measured current by REM 100% current value to get detector ratio. Sum ratios, divide by 4 operable detectors to get average. Divide each ratio by average = power tilt. A dropped rod (2D4) causes asymmetric flux in N42 quadrant: N42T = 1.041, N42B = 1.032, both exceeding TS 3.2.4 limit of 1.02 → surveillance UNSAT.
JPM — 2020 Sim-f
PR channel 2N41 removal from service (S2.OP-SO.RPS-0001): at NI Rack No. 81, five switch manipulations in Steps 5.1.5.A-E — DETECTOR CURRENT COMPARATOR upper → DETECTOR CURRENT COMPARATOR lower → POWER MISMATCH BYPASS (defeats input to rod control) → ROD STOP BYPASS (OHA E-31 clears, 2RP4 MANUAL BYPASS CH I lights) → COMPARATOR CHANNEL DEFEAT (OHA E-39 clears). Distinct from IR channel removal which uses LEVEL TRIP switch. RO-only JPM.

Source Range Audible Count Rate

Exam — 2018 Q31
During an automatic RCS makeup in Mode 3 with 2CV179 (PRIMARY WATER FLOW) failed closed, only boric acid flows through the blender → RCS boron concentration rises → negative reactivity → Source Range NI Audible Count Rate indication lowers. If CV179 had failed OPEN, primary water would dilute the RCS → boron concentration lowers → positive reactivity → SR audible count rate would rise.
JPM — 2018 RO-A1
Manual AFD calculation (S2.OP-ST.NIS-0001 Section 5.3) when AFD Monitor alarm is inoperable: at 73% power, AFD Target from REM Table 3 = -1.5, calculated AFD Target = (73/100) x (-1.5) = -1.1 (rounded per P&L 3.2). Lower Limit = -9.0 + (-1.1) = -10.1. Channels III (-10.3) and IV (-10.5) outside the lower target band.
JPM — 2018 RO-A3
Manual QPTR calculation (S2.OP-ST.NIS-0002) with dropped rod 204: N42 quadrant shows highest power tilt — N42T = 1.025, N42B = 1.023. Exceeds TS 3.2.4 QPTR limit of 1.02. Alternate path JPM.

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