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What is the purpose of the Thermal Margin Monitor?


  • Provides TM/LP trip setpoint to the RPS
  • Provides the VHPT signal to the RPS.
  • Provides an ASI alarm prior to exceeding Technical Specification limits.
  • Provides an annunciated alarm if any of the following conditions exists
  1. DT-Nuclear Power Deviation
  2. TMM Calculator Trouble
  3. TC Alarm


What is the purpose of the VHPT and TM/LP Trip?

VHPT and TM/LP Trip

The TMM provides a means of protecting fuel at all power levels without excessive restrictions on full power operations.


Why do we have a Thermal Margin/Low Pressure (TM/LP) Trip?

Ensures fuel protection during slow reactivity addition transients starting at low reactor power

Allows Steam Generator tube plugging while maintaining 100% power. 

The use of Partial Shielded Assemblies (PSAs) to reduce irradiation of the reactor vessel has introduced distortions to flux shape as well as reduced thermal margins.


What is the function of the Variable High Power Trip?

Variable High Power Trip (VHPT)

  • Functions to trip the reactor when the greater of Nuclear Power or Delta-T power exceeds a preset High Power Trip value.
  • Provide capability to limit transients quickly beginning at low power levels, without operator action.
    • Used to have to credit operator actions for slow reactivity insertion events (i.e. dilution)


What is the function of the Axial Shape Index Alarm (ASI)?

Axial Shape Index Alarm (ASI)

  • ASI is a concern at and above apprx 25%
  • Alerts operator that core axial flux shape is approaching the limits assumed in the safety analysis
  • To assure the core axial flux shape is maintained within the boundaries of the safety analysis
  • This alarm will aid the operator in limiting a local power rise caused by axial power shifts.  Therefore, any potential accident or transient starts from within the axial shape assumed in this safety analysis


What is the definition of ASI?

Power in lower core - power in upper core

devided by total core power

via the excores

ASI is computed for each quadrant

ASI corrects for variations in power at a constant DNB


What are the plant saftey limits?

DNB > 1.17

  • Overheating of the fuel cladding is prevented by restricting fuel operation to within the nucleate boiling regime
  • Normally 2.0 at Plant

LHR < 21 kw/ft

  • Overheating of the fuel is prevented by maintaining the steady state, peak Linear Heat Rate (LHR) below the level at which fuel centerline melting occurs

Pressure < 2750

  • Protects pressur boundary of PCS.


Describe the four areas of the boiling curve.

  1. Single-phase convection - Heat is transferred by single-phase convection until the heated surface exceeds the saturation temperature by a small amount
  2. Nucleate Boiling Region - Vapor bubbles form at the heat transfer surface but condense in the cold liquid, so that no net generation of vapor is realized.  
  3. Partial Film Boiling - Bubbles become so numerous that they begin to coalesce and clump near the heating surface.  A portion of the heating surface gets blanketed with vapor.  The vapor blankets act as heat insulators.
  4. Film Boiling - If attempts are made to attain large heat fluxes with film boiling, as high as those possible with nucleate boiling, for example, the temperature of the heated surface may become so high as to result in damage to the material being heated, namely, the fuel cladding.  


What happens at DNB?

A rapid rise in DT with no rise in heat transfer.


What is Critical Heat Flux?

heat flux at which DNB occurs.


What is the largest impact the operator has on DNBR?

The largest is Axial Shape Index via control rod insertion.

Insertion pushes flux down in the core.

A fully inserted or fully withdrawn control rod causes insignificant changes in the axial flux; however, a partially inserted control rod will cause considerable axial flux distortion.


How does a downpower impact DNBR?

Causes power to shift to the top of the core due to the change in THOT

THOT will be decreasing by a greater amount than TCOLD; therefore, the effect on the temperature drop will try to push power to the top of the core. = Negative ASI


How do Boron concentration changes impact DNBR?

A 1 ppm rise in soluble boron will have greater negative reactivity worth in the bottom of the core than at the top of the core where the density of water is less.

Flux will shift to the upper region of the core thereby creating a more negative ASI.

A dilution of the soluble boron will result in flux shifting toward the bottom of the core thereby creating a more positive ASI


How do Xenon oscillations impact DNBR?

Xenon oscillations are more prominent at end of cycle

These oscillations occur with a period of about 28 hours

Generally such oscillations are convergent BOL and get more divergent near EOL



What are the inputs to the Thermal Margin Monitor?



NI Power Range channels (upper, lower, and total detector outputs)


What are the TMM power supplies?

Power Supply Y-10, Y-20, Y-30 and Y-40 for channels A. B, C, and D, respectively

Lithium battery for 64K RAM (Memory) to maintain the variable constants

Second battery system is a set of three NICAD batteries used for the real time internal clock.


What are the outputs of the TMM?

TMM Outputs

  1. Nuclear Power/VHPT setpoint and Pre-trip alarm
  2. TM/LP Pre-trip alarm setpoint
  3. TM/LP setpoint and VHP trips to RPS channels A, B, C, and D
  4. TM/LP and VHP trip alarms and ASI alarm
  5. TM/LP Setpoint Low Alarms
  6. Multifunction alarms on EK-06D


What is the function of the ASI Function Generator?

Outputs the relationship between ASI via incores vs ASI via excores.

Utilizes Shape Annealing Factor

Accounts for flux leakage from upper to lower and vice versa



What is the function of the Local Power Density Function?

LPD Function Generator

The Local Power Density Function computes the maximum allowable ASIs (negative and positive) for the existing power level (QR2).

Outside of these bounds is an unanalyzed condtion.

Generates two outputs (YP and YN) to each comparator


What is the purpose of the LPD Functio Block?

Disables the ASI alarm. (Setpoint 15% ascending and 14.5% descending power)


What are the TM/LP Trip Inputs for Tcold and the setpoints?

Measures Tc1 and Tc2. Takes the higher of the two is the Tc signal.

  • TC provides input to the PVAR setpoint calculation
  • TC also provides an input to calculate DT Power (B)

If at any time TC > TC MAX or TC < TC MIN  - get alarm

  • TC MAX  = 544°F
  • TC MIN = 525°F


How is THOT Ave Generated and what is it used for?

TMM takes TH1 and TH2 measured and the average of the 2 generates a TH-AVE signal to determine the DT Power (B) signal.


How is the DT Power (B) signal developed?

DT Power (B) signal

  • Inputs from TC and TH-AVE used to generate DT power.
  • Bias term set in the TMM adjusts for heat balance


B = KaDT + KbDT(Tc) = KtDT2 + t d/dt(aDT+Tc) + Bias

Ka, Db, Kt are constants

t = 14

a = -0.03


How is DT developed in the TMM?

Delta T is 

∆T is the maximum TCOLD signal TC subtracted from the average hot leg temperature


What is DT Power (B) signal used for?

DT Power (B) signal is used for:

  • To determine TCAL (currently has no effect on TCAL since it is multiplied by a constant, KC, which is currently set at 0.0.)
  • B is also sent to determine the Power Peaking Function, which is used to determine QR1 for the TM/LP setpoint calculation.
  • DT Power (B) signal is also sent to the VHPT and ASI alarm calculation and to indications on C-02 and C‑27.


What is Pvar and how is it calculated?


  • It is the low pressure trip limit


  • PVAR = LambdaQDNB + BetaTC + g
  • QDNB is a function representing axial and radial power peaking effects

Of the factors which determine PVAR, only TC and QDNB are affected by plant operations


How does Tcold impact Pvar?

Tcold impact Pvar

As the inlet temperature of the reactor coolant rises (assume power and pressure constant) the coolant will approach saturation conditions and, therefore, approach DNB in the theoretical "hot channel"

To protect against reaching DNB conditions PVAR rises as TC rises (TM/LP trip occurs at higher pressure).


How do QDNB changes impact Pvar?

QDNB changes impact Pvar

  • QDNB is the "penalty" in the PVAR formula that causes the trip setpoint to rise due to an abnormal axial shape index (ASI)


What is the Axial Function Generator, Qa?

Axial Function Generator, Qa

For a given ASI, Yi, the axial function will calculate QA, which will be used as a factor in computing the penalty factor QDNB.

A negative ASI will produce a greater DNB penalty into the PVAR equation than a positive ASI.

  • The minimum value of QA (QA = 1.0) occurs at a positive ASI of + 0.200.


What is the Power Peaking Function (QR1) genrator?

Power Peaking Function (QR1)

  • Input is Q1, which is the higher of either DT power (B) or nuclear power.
    • The largest values of QR1 occur at high power and the value doesn't reach unity (1.0) until power equals 100% power
  • Can be thought of as the "power portion" of the DNB penalty.
    • A given ASI becomes more restrictive as power rises



At less than 100% power, what is the value of QR1 based upon?

QR1, the Power Peaking Function

Based upon hot leg saturation concerns

If the hot leg becomes saturated the DT power calculation will be inaccurate since it does not account for the latent heat of vaporization which is encountered at saturated conditions.  Instead the DT power calculation is based upon sensible heat only.  This hot leg saturation limitation is the basis for the QR1 function for power levels less than 100%.


How is the TM/LP Trip Calculation made?

TM/LP Trip Calculation

Pvar is compared to Pmin and the larger is chosen.

Pmin is 1760 psia

If TM/LP setpoint drops to 1720 psia, a circuit failure alarm is generated.




Power, Q, is the higher of nuclear power or DT power, B.

QPTR is the Q pre-trip alarm level

QTR is the Q trip level.

  • QTR is constrained between QTR MIN and QTR MAX.

QTR MIN = 30%

QTR MAX = 106.5%

QTR is limited to current reactor power plus 15% when the reset button is pushed

QPTR is limited to 13.5% above reactor power


How does the VHPT change with power?

On power de-escalations QTR follows Reactor Power, Q.  The trip is always 15% above the Reactor Power level on power reductions until it reaches a QTR MIN, 30%.

On power escalations QTR follows Reactor Power, Q, to a maximum of QTR MAX = 106.5%.  However, the trip must be manually reset prior to reaching the pre-trip setpoint (Q + 13.5%) to avoid a reactor trip.  The new trip setpoint is 15% above Q at the time the trip was reset

If the trip reset is pushed at or below 15% reactor power, the trip setting stays at 30%.


How do reset the VHPT setpoint?

Normally accomplished by pushing the VHPT reset button (1 for each of the 4 TMM channels) on C-02

Can also be reset on the TMM channel drawer while viewing the ALARMS screen and pushing the button for VHPT RESET

TMM will only recognize reset once each 10 seconds


How does boron concentration impact Thermal Margin Monitor input and output signals?

Boron Impact

Can impact measured ASI (Ye), and PCS temperatures (TC and TH).

  • Adding boron to the PCS shifts ASI in the negative direction
  • Dilution shifts ASI positive
  • Boration is used to reduce TAVE and thus TC and TH
  • Dilution is used to raise temperatures.

Possible impacts include ASI alarm, TM/LP becomming more/less limiting.

Minimal/no impact to VHPT


How does PCS Tave impact TMM outputs?

PCS Tave  TMM Output Impacts

ASI Alarm

  • Minimal impact at constant power (small TAVE changes to keep within 3°F of TREF)
  • ASI shifts more positive as power and TAVE are raised from 0 – 100%.


  • Temperature is directly proportional to PVAR


  • At constant power, TAVE changes do not impact DT Power and thus the VHPT setpoint.



How does Rod Motion impact TMM outputs?

Rod Motion Impact on TMM Outputs

ASI alarm

  • Rods in moves ASI in the ‘+’ direction and rods out moves ASI in the ‘–‘ direction
  • Most limiting in the negative direction (rods out)


  • Rods in, PVAR goes down with TC
  • Rods out, PVAR goes up with TC
  • ASI impacts QDNB and thus PVAR
  • ASI portion is more predominant. 


  • impacts VHPT only to the extent that reactor power is changed.



How does Main feedwater temperature impact TMM outputs?

Main feedwater temperature impact to TMM

With no operator action, a drop in main feedwater temperature will cause a drop in TC and a rise in PCS DT.  Also, actual power measured (Q and DT power) will rise to produce the power needed to heat the colder feedwater

ASI alarm – minor effects for normal range of feedwater temperature changes.


  • Minimal impact for normal changes
  • A feedwater temperature drop causes TC to drop 1°F and DT to increase by 1°F
    • The rise in QDNB offsets the drop in TC to cause PVAR to rise by ~ 26 psi


What are the TMM Modes of operation?

TMM Modes of Operation


  • Used only by I&C
  • Channel is inoperable while in TEST


  • All channel functions operable and safety functions and calculations of the system are performed


  • Normal safety functions of the TMM are disabled


What are the Admin Limits associated with changing TMM constants?

Changing Constants Admin Limits

With the sole exception of TMM Constant #6, TMM Constants shall not be changed by Operations Personnel except under both the following conditions:

  • Direct guidance from I&C Engineer or Reactor Engineer
  • Knowledge and consent of Shift Manager


Why is the CLR Key pushed six times prior to selecting Data Modify mode?


Press CLR six times.

In NORMAL mode, there is a buffer

The keypad will function to enter new constants into a storage buffer. The new constants will not replace the old constants until the TMM is taken to the DATA MODIFY mode.


What is the precaution with faulty TM/LP Trip Units?

TM/LP Trip Units

With a TMM drawer in the NORMAL mode, it has been demonstrated that it is possible for the TMM microprocessor to malfunction when the "ADJUSTABLE PARAMETERS" screen is displayed in such a manner that the TMM will not process a trip signal. If any other TMMs are inoperable, then the "ADJUSTABLE PARAMETERS" screen shall not be displayed with its TMM in the NORMAL mode unless the inoperable TMMs Variable High Power and TM/LP RPS Trip Units are tripped.


How doess ASI Calibration impact TM/LP and ASI alarm setpoints if not done correctly?

If any individual channel ASI is not calibrated with the total core Axial Offset as measured by the incore detectors

  • The TM/LP Trip and the ASI alarm setpoints associated with that channel shall be declared inoperable.


When do we not manually reset VHPT setpoints?

The four Variable High Power Trip Setpoints shall NOT be manually reset during plant transients.


What are the criteria VHPT reset during power escalation?

DWO must be complete

Prior to Reactor Power escalation above 28.5% and prior to reaching the pre-trip setpoint (less than or equal to 13.5% Delta power) during power escalation

Must be stable plant conditions

  1. Reactor power has NOT changed more than 1/2% for 10 minutes following a transient (N/A for controlled power escalation in accordance with GOP-5, “Power Escalation in Mode 1."
  2. All Control Rods are withdrawn above PDIL
  3. Pressurizer pressure is between 2010 psia and 2100 psia, is controlled, and is approaching normal operating pressure of 2060 psia.
  4. TAVE is within 3°F of TREF and is controlled
  5. Pressurizer level is being controlled between 40% and 60% and is approaching the normal programmed level
  6. Both Steam Generator levels are between 60% and 70%.



When do you calibrate TMM Delta T power?

Calibration of TMM Delta T Power

IF the heat balance differs by more than ± 1% from indicated DT Power on any TMM "PRIMARY" screen, OR calibration of DT Power Indication Channels is desired, then go to SOP-35, Section 7.2.5, "Calibrating DT Power Indication Channels."

IF the heat balance differs by more than ± 2% from the indicated DT Power on any TMM "PRIMARY" screen, THEN PERFORM SOP-35, Attachment 1, "Heat Balance Verification."


What is the immediate action of a TMM Error Code?

TMM Error Code

Immediately contact I&C to report error


What happns on TMM Loss of Power?

Loss of Preferred AC Bus Y-10, Y-20, Y-30, Y-40

  • If any of the 4 power supplies is lost, the affected TMM channel will de-energize and the affected RPS channel will trip (both powered from the same preferred AC bus).

If just TMM power is lost

  • Associated TM/LP trip setpoint fails low, and the VHPT, and ASI alarm will trip.


How do we protect against static discharge?

Proper technique is to discharge yourself on a control panel frame or metal object connected to the control panel

A good practice is to hold the panel with one hand while operating the equipment with the other