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Abnormals > AB.NIS-0001 — Nuclear Instrumentation System Malfunction

AB.NIS-0001 — Nuclear Instrumentation System Malfunction

⚠️ DRAFT

AB.NIS-0001 — Nuclear Instrumentation System Malfunction

Purpose

Provides operator actions for responding to malfunctions of the Nuclear Instrumentation System (NIS), including source range (SR), intermediate range (IR), and power range (PR) channels.

Entry Conditions

  • NIS channel failure, erratic indication, or unexpected channel behavior
  • Loss of one or more NIS channels

Key Actions

  • Identify the failed NIS channel and its impact on reactor protection
  • Verify automatic compensatory actions (e.g., automatic SR block/unblock at P-6)
  • Enter applicable TS action statements for inoperable channels
  • Coordinate with I&C for troubleshooting and repair
Exam — 2019 Q30
When a PR NI channel (2N41) fails high, AB.NIS-0001 REQUIRES placing the Rod Bank Selector Switch in MAN prior to placing the channel in tripped condition. Trap: the overpower rod stop block is enabled when a PR channel fails high (stops all automatic and manual rod withdrawal). Candidates may conclude placing rods in Manual is unnecessary — but AB.NIS-0001 explicitly requires it.
Exam — 2023 Q82
When P-6 is LIT, IR NIS channels should indicate approximately 1E-5% power (the SR/IR overlap region). If an IR channel reads 1E-8% power when P-6 is LIT, that channel is inoperable. Per TS 3.3.1.1 bases, the 24-hour action time for one inoperable IR channel takes into account the redundant capability afforded by the operable IR channel and its low probability of failure during this period. Trap: the bases cite low probability of failure of the operable channel, NOT low probability of reactivity events.
Exam — 2020 Q22
AB.NIS-0001 directs removal of a failed IR channel (e.g. N35 failed high) from service IAW S2.OP-SO.RPS-0001, Nuclear Instrumentation Channel Trip / Restoration. For IR channels, SO.RPS-0001 places the LEVEL TRIP switch in bypass (causing OHA E-29 to annunciate). Do not confuse with PR channel removal — PR channels use control power fuse removal and POWER MISMATCH BYPASS switch.
JPM — 2020 Sim-f
PR channel 2N41 fails high. AB.NIS-0001 actions completed (rod control to Manual). The JPM then tests removal of the failed PR channel from service per S2.OP-SO.RPS-0001. PR channel removal requires five switch manipulations at NI Rack No. 81: Detector Current Comparator (upper and lower), Power Mismatch Bypass, Rod Stop Bypass, and Comparator Channel Defeat. OHAs that clear: E-31 (PR OVRPWR ROD STOP) and E-39 (PR CH DEV).
Scenario — 2019 #4
2N41 Power Range channel fails high at 88% power (Event 2). Rods stepping in with no runback in progress — RO places rod control in Manual. OHAs: E-15 PR HI RNG FLUX HI, E-31 PR OVRPWR ROD STOP, E-39 PR CH DEV, E-47 PR NEUT FLUX RATE HI. CRS directs removal from service IAW S2.OP-SO.RPS-0001 — five switch manipulations at NI Rack No. 81: Detector Current Comparator (upper/lower) to PRN41, Power Mismatch Bypass to BYPASS PR N41, Rod Stop Bypass to BYPASS PR N41, Comparator Channel Defeat to N41. CRS enters TSAS 3.3.1.1 Actions 2 and 6 (6 hour LCO). RO withdraws rods to ARO after channel removed.

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