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Explain the purpose of the Excore Nuclear Instrumentation System

Excore Nuclear Instrumentation

  • Eight channels of instrumentation – 2 Source Range (SR), 2 Wide Range (WR), and 4 Power Range (PR)
  • Monitors neutron flux from the source range through 125% power.
  • Provide signals used to process a reactor trip if any core safety limit is approached
    • Start-up Rate (SUR) directly to Reactor Protection System (RPS)
    • Variable High Power Trip (VHPT) via Thermal Margin Monitor (TMM) to RPS
    • Thermal Margin/Low Pressure (TM/LP) via TMM to RPS



Explain the purpose of the Incore Nuclear Instrumentation System

Incore Instrumentation

  • Provide measured data that may be used in evaluating the neutron flux distribution in the reactor core.
  • The incore instrumentation also contains thermocouples, 16 of which are post-LOCA environmentally qualified.
  • Data may be used to evaluate thermal margins and to estimate local fuel burnup.
    • Incore detectors can measure local neutron flux density
  • Used to calibrate excores with respect to Axial Shape Index (ASI)/Linear Heat Rate (LHR).


What is the makeup of the incore instrumentation?

  • Self-powered Rhodium-103 detectors
  • 36 strings of 5 detectors at different elevations in the string, per string for a total of 180 detectors
  • Thermocouples at the top of each string to measure core exit temperature.
  • 16 are EEQ qualified and displayed on C-11A front (4 per quadrant)


What principle provides for the function of the excores?

Neutron leakage outside the core to the reactor cavity is a linear function of the neutron flux density in the core and thus reactor power.

Fission Chambers measure this leakage.


What is the construction of a fission chamber?

  • Dual concentric aluminum cylinders
    • Cylinders oppositely charged
  • Neutron-sensitive coating (3.5 g of uranium oxide per chamber) on surfaces exposed to gas.
    • suffers less than 1% burn up during a 40 year life
    • no measurable loss of sensitivity over lifetime
  • Gas-filled between sensors (90% argon and 10% nitrogen)


How does a fission chamber operate?

  • Reaction:  n + U-235 = high energy fission fragments
  • Fission fragments ionize the gas and cause DC current flow/pulses.
    • Average current pulse of 2 uA.
    • 1 pulse per fission, normally
  • Above 106 CPS (5 X 10-3% PWR) you get errors.
    • Pulse pile-up occurs, pulses start to overlap
    • Use Campbelling circuit to account for this effect.
  • Pulse height (amplitude) discrimination is used at low pulse rates


What factors impact excore instrumentation accuracy?

  • Shielding from reactor vessel wall, cavity concrete, detector wells and detector materials
  • Low leakage core
  • Core voiding
  • Cold Leg Temperature Shadowing
  • Rod Shadowing
  • Fuel burnout
  • Boron concentration
  • Reactor poisons


What is a low leakage core design and how does it impact the Excores?

Low leakage core

  • Burned assemblies placed at periphery
  • Assemblies without active fuel pins

Recalibrate Excore NIs to account for core loading.


What is Cold Leg Temperature Shadowing and how does it impact the Excores?

Cold Leg Temperature Shadowing

Cooler water is denser, the denser water reflects more neutrons back into the core and the ex-cores will read a lower power than that which actually exists.

  • Typical values are .5% power/°F.  
  • If Tc were to lower 10°F, power would read too low by 5%.

Effects are compensated by periodic calibration of the Ex-cores to match calorimetric power. 

During start-up, target Tave restrictions minimize changing Tcold, thus reducing cold leg temperature effects.


What is Rod Shadowing and its impact to Excore instrumentation?

Rod Shadowing

  • Detectors only see neutron leakage from the core periphery due to short neutron migration length.
  • Anything that causes the peripheral fuel assemblies to indicate a power level not representative of the core average power will cause an erroneous power level to be seen by the Ex-cores
    • Rod insertion has such an effect, as radial power distribution is altered.
  • Effects of Rod Shadowing are compensated by operating with all rods out at full power and periodic calibration of the Excores to match calorimetric power.


How does fuel burnout impact Excore instrumentation?

Fuel Burnout

  • Causes the neutron flux density to shift to the outside of the core as fuel in the center of the core burns out. 
  • Reactor power will read higher than actual.
    • Must be adjusted for as the core ages.


What range does source range nuclear excore instrumentation cover?

How does it relate to wide range.

Source Range

  • 6 decades from 0.1 to 105 cps
  • Approximately 10-10 to 10-4 % power

When SR is ~ 3 cps, WR should read ~ 1x10-7 %


What does wide range excore nuclear instrumentation cover?

How does it relate to source range?

Wide Range

  • Covers 10 1/3 decades from 10-8 to 200 % power.

When SR is ~ 3 cps, WR should read ~ 1x10-7 %


What are the power supplies for NI-1/3A and NI-2/4A?

NI-1 and NI-3 - Y-30

NI-2 and NI-4 - Y-40


How does gamma discrimination work in the source range detectors?

SR Discriminator A7

Discrimination against alpha, gamma and electronic noise

  • Provides a uniform, conditioned pulse for each neutron pulse that exceeds the threshold voltage (bias)
  • Since neutron pulses are larger than gamma pulses, the bias is set to pass only neutron pulses.

Neutron count rate is proportional to power, while gamma count is proportional to power history


How does gamma discrimination work in the wide range detectors?

WR Discriminator and Bandpass A8

Two modes of operation – pulse mode and Campbelling mode

Pulse Mode

  • Current pulses are recorded as discrete events per unit time up to 106 cps.
  • pulse pile-up above 106 cps

Campbelling Mode

  • The mean square variation of the detector output current is proportional to the neutron count rate (which is proportional to flux).
  • Further refined by AC signal squared is proportional to power
    • Example: if gamma pulse is 1/10 of a neutron pulse, & we square this ratio, the gamma contribution to power becomes 1% of the over all signal rather than 10%.


What is the Campbelling Mode Step Change?

Campbelling Mode Step Change

SR/WR monitors will indicate a step change of about 0.04% when indicated power reaches 0.05% to 0.10% indicated power range due to the switching from the pulse mode to the Campbelling mode


What is the Optical Isolator?

Optical Isolator

  • Allows two signal processors to operate from a single amplifier
    • Prevents failure of one from causing failure of the other
  • Outputs are sent to C-150A for display and to the Signal Processor in C-06
  • HS-0102C selects display to Control Room or C-150A (AHSP)
  • Powered from Y-30 breaker 12 or from C-150 power PS-0110A (selected by HS-0102C)


WR Channel NI-03 supplies what RPS channels? NI-04?

NI-03 supplies A and C

NI-04 supplies B and D



What are the Power Range Safety Channels within the Excore monitoring system?

Power Range Safety Channels

Four channels

  • Measure flux linearly over the range of 0% - 125% power.
  • Provide signal to the TMM, and meters, recorders and auxiliary contacts

Primary safety function is to provide instantaneous nuclear power signal to the Variable High Power Trip function within the TMM


How are the Power Range Detectors arranged?

Detectors are located in four detectors wells spaced about 90° apart.

  • One detector for each quadrant of the core enables us to detect any radial flux imbalance. (Quadrant Power Tilt)
  • Detectors 2 sections allows for monitoring neutron flux across full height of core and for comparing top and bottom half neutron flux.


How do Power Range Detectors operate?

Power Range detector is a 2 section Ion Chamber design

  • Active neutron sensitive sections are 68.75 inches long.
  • Lined with boron-10
    • Reaction: n + B-10 → B-11* → Li-7- + α
  • High voltage (680 - 700 VDC) placed between electrode and canister
  • Alpha particle and the lithium are both charged particles, cause additional ionization in the detector

Not compensated for gamma radiation since the gamma is proportional to reactor power. 500 neutrons to 1 gamma at full power.




What are the power supplies for the power range detectors?

NI-5 – Preferred AC Bus Y10

NI-6 – Preferred AC Bus Y20

NI-7 – Preferred AC Bus Y30

NI-8 – Preferred AC Bus Y40


What voltage does a power range detector panel unit trip at?

<= 675 VDC (EK-06-C8)


How does the response time of incore detectors compare to excores?

Incore detectors are self-powered rhodium detectors. 

Can take nearly 4 minutes to see a power change.


What is Thermionic emission and its impact?

Thermionic emission

  • At high temperatures: >600F
  • Incore detector emitter receives a false high signal as electrons are driven away from it. Especially if voiding is occuring via accident condtions.


What is the function of the linear amplifiers in the power range drawers?

Linear Amplifiers

Signal from each detector section (top and bottom) is amplified by an independent amplifier.

Output of each amplifier sent to:

  • PPC via an auxiliary output
  • Front panel (C-06) meter indicating 0‑125% power
  • TMM for calculation of Axial Offset (ASI)
  • Summer
  • Comparator/Averager


How does the control rod drop detection circuit function?

Receives signal output from Summer. Summed output of upper/lower amplifiers.

Compares the immediate signal level with a time delayed (8 seconds) signal level

8% signal drop in under 8 seconds activates a relay that provides a dropped rod alarm.



How does the 15% Bistable Trip Unit function?

Receives output signal from Summer.

Wide Range Channel High Power Rate Trip (SUR)

  • Enabled at less than 15% full power but >10-4% power
  • Disabled above 15% full power

Loss of Load Trip

  • Enabled above 15% power
  • Disabled below 15% power

TMM Alarm Generation

  • Enables ASI alarm via TMM above 15% power
  • Disables ASI alarm below 14.5% power

Generator Coastdown Trip

  • Enabled above 15% on both NI-5/6
  • Disabled below 15% on both NI-5/6


What is the function of the Comparator/Averager?

Receives Summer Output from each Power Range Channel

  • Averages signals from 4 PR channels
  • Compares channel vs average
  • Indicates Quadrant Power Tilt Ratio

Channel Deviation Alarm at 5% and 10%

  • Annunciates on C-06 EK-06-03/04 Rack C
  • Also illuminates a 5% or 10% light on the front of the NI drawer



What indication are on C-150A, Auxiliary Hot Shutdown Panel?

SR/WR Signal Processor – same as on C-06

HS-0102C selects where NI-1/3 readout

  • In Control Room position power supply is Y-30
  • In C-150A position selects C-150A PS-0110A as the power supply.


What is subcritical multiplication?

Subcritical Multiplication

Keff < 1

  • Control Rods withdrawn, SR CPS will rise and a positive SUR is observed. 
  • Rod withdrawal is stopped, SR CPS will level off and SUR return to zero
  • AS Keff approaches 1.0, the length of time for SR CPS and SUR to level off becomes longer.
  • Reactor is critical with SR indication raising steadily; constant SUR with no rod motion


When Source Range indicates 3.0 cps, what should WR indicate?

10-7% power


When is POAH?

WR at 0.1 to 0.5% power


What provides DNB protection?

The following protect the reactor from reaching conditions that could lead to DNB

  • Core Power – Variable High Power Trip (VHPT)
  • Axial Shape Index – ASI TS LCO
  • Fuel Assembly Power – Incore power monitoring (LHR and Total Radial Peaking Factor)
  • Thermal Margin/Low Pressure (TM/LP) - The function of the TM/LP trip is to protect against slow heat-up and depressurization transient events.


If SPI/Host and Excores are not available to monitor LHR, what do you do?

Given plant conditions and neither the SPI/Host System nor the Excores are available to monitor LHR, determine the actions required.

PO-3, manually read the incores. Same as ELAP