Core Protection Calculators (CPC)

Question 1

What is the power supply for the Core Protection Calculators?

Answer 1

PN

Class 1E Instrument AC Power (PN) Auxiliary Protection Cabinets each have their own independent Class 1E, 120 VAC power sources which are backed-up with an alternate Class 1E power source.

Question 2

What is the Sum of 51in regards to CPC’s?

Answer 2

CPCs shift to and from a calculated value of ASI (Rule of 51/45)
Uses a fixed value or calculated ASI value based on NI power. (Fixed value at low powers)
↑ power: ASI swaps to calculated value when sum of the three excore detectors (one safety channel) is > 51% (~17% Reactor Power).
o Uses Pseudo Hot Pin ASI at low power. Validate ASI within limits prior to swapping to calculated ASI.
↓ power: ASI swaps to fixed value when sum of the three excore NIs < 45% (~15% RTP).

Question 3

How is Azimuthal Shape Index (ASI) calculated?

Answer 3

LULU

Question 4

List the Core Protection Calculators (CPC) Auxiliary Trips:

Answer 4

Trips with no pre-trips

Question 5

How is Local Power Density (LPD) calculated in Core Protection Calculators (CPC)?

Answer 5

C.R.A.P.

CEAC Penalty Factor
Radial Peaking Factor
Axial Peaking Factor
Power

Question 6

How is Departure from Nucleate Boiling Ratio (DNBR) calculated in Core Protection Calculators (CPC)?

Answer 6

C.R.A.P. Plus 3 (water parameters)

CEAC Penalty Factor
Radial Peaking Factor
Axial Peaking Factor
Power

Plus: RCS Mass Flow, Tcold (hottest), and Pzr Pressure

Question 7

Does CPC generate a CWP for Hi Pzr Pressure Pre-trip?

Answer 7

No

This CWP is generated by PPS not CPC.

Question 8

Does CPC use Incores or Excores?

Answer 8

Excores

COLSS uses Incores

Question 9

What are the Safety Limits?

Answer 9

In MODES 1 & 2: DNBR shall be maintained at ≥ 1.34. (Protects the cladding)

The peak fuel centerline temperature shall be maintained < 5080°F (decreasing by 58°F per 10,000 MWD/MTU) (4901°F (decreasing by 13.7°F per 10,000
MWD/MTU). Note that this SL is assured as long as Peak Linear Heat Rate (LPD) does not exceed 21.0
kW/ft. (LPD protects fuel centerline SAFDL)

In MODES 1, 2, 3, 4, and 5, the RCS pressure shall be maintained at ≤ 2750 psia.*

Question 10

How is the Safety Limit for Peak fuel centerline satisfied?

Answer 10

Fuel Centerline Melt: peak linear heat rate (LPD) in the limiting fuel pin maintained less than or equal to that value which precludes fuel centerline melting. < 5080°F (drops by 58°F every 10,000 MWD/MTU).
* Safety limit met if LPD ≤ 21 kW/ft

Question 11

What is the purpose of the Core Protection Calculators?

Answer 11

CPC/CEAC Purpose
* Provide reactor trips to prevent exceeding SAFDLs (prevents violating safety limits during AOOs / normal Ops).

Question 12

Does CPC use the hottest or coldest Tcold when calculating Coolant Temperature Shadowing Factor?

Answer 12

o (Lowest Tc) Coolant Temperature Shadowing Factor. Used for Calibrated Neutron Flux Power.

Question 13

How much can a Reactor Coolant Pump (RCP) slow down before the Reactor trips?

Answer 13

The Coolant Mass Flow Program (FLOW) (CPC)
The program will reduce the calculated DNBR to less than 1.34 whenever fewer than four reactor coolant pumps are operating at greater than or equal to 95% of rated speed.

o Causes Low DNBR trip
o Ensures no fuel damage from four pump loss of flow event (AOO)

Question 14

What parameters does the CPC Auxiliary trip “Operating Space” evaluate? What are the ranges?

Answer 14

Outside Analyzed Range (Operating Space Auxiliary Trip)
* Tcold (≤ 505°F or ≥ 590°F)
* PZR Pressure (≤ 1860 psia or ≥ 2388 psia)
* ASI (> ± 0.5)

Question 15

How would CPC detect an Asymmetric Steam Generator Transient? How would CPC respond?

Answer 15

CPC Aux Trip
Asymmetric Steam Generator Transient Trip (ASGT)
* Based on difference in Tcolds (loop 1 vs loop 2).
* Protects against inadvertent closure of both MSIVs on a SG.
* If one MSIV closes, setpoint will NOT be reached.

Question 16

Do CEAC’s see all CEA’s or just their quadrant?

Answer 16

• Each CEAC receives RSPT inputs for all CEAs.

Question 17

How many CEAC’s does a CPC channel need?

Answer 17

• 2 CEACs required per CPC since a single RSPT is used as input for 1 CEAC in all 4 channels, so no single sensor failure will prevent a required trip from occurring.

Question 18

Does a CPC see all CEA’s or just their quadrant?

Answer 18

• Each CPC receives CEA position (RSPT) for the 22 target rods associated with specific CPC (core quadrant).

Question 19

What do the CEA Position Processors (CPP) do?

Answer 19

CEA Reed Switch Position Transmitter (RSPT) outputs are sent to CEA Position Processors (CPPs). The CPPs convert the RSPT analog voltage input to a digital CEA position output. The CPP digital position output is sent through high speed fiber optic lines to CEA Calculators CEACs.

Question 20

What is CEAC Penalty Factor used for?

Answer 20

DNBR and LPD

The penalty factor becomes a multiplier in the DNBR and LPD calculations.

Question 21

Will the CPC detect a sheared shaft on an RCP?

Answer 21

No
CPC’s use RCP speed sensor.
* Would NOT detect a sheared shaft (RPS low RCS flow trip would detect it)

Question 22

What do the DNBR/LPD margin meters display on B05?

Answer 22

DNBR Margin (Analog)
* DNBR Calculation compared to 1.34 setpoint and difference is sent to the DNBR Margin meter on B05.
* 1.34 + Margin Meter Reading = Actual DNBR

LPD Margin (Analog)
* LPD Calculation compared to 21.KW/ft setpoint and difference is sent to LPD Margin meter on B05.

• Margin meters indicate ZERO when trip setpoint is reached.

Question 23

Is the CPC VOPT (Variable Over Power Trip) more or less conservative than the RPS (Reactor Protection System) VOPT?

Answer 23

More conservative

Variable Overpower Trip (VOPT)
* Trips on ± rate of change, step change or reaching max value (110%)
o 1 %/min ↑
o 16.67 %/min ↓
o 8% step
* Protects the core during rapid positive reactivity excursions (ejected CEA is the limiting fault)
* NI input to CPC VOPT is compensated for temp/CEA shadowing, RPS trip is not.
* CPC VOPT should actuate prior to RPS VOPT.

Question 24

What causes a CWP in CPC’s?

Answer 24

CEA Withdrawal Prohibit (CWP) signal
* Disables outward CEA motion (can be manually bypassed on CEDMCS)
* 2 of 4 CPC logic. Outputs to PPS, then to CEDMCS. (any 2, can be different parameters, etc. C Channel Low DNBR and B Channel High LPD will generate a CWP)
* CWP generated by:
o Low DNBR pre-trip (2 of 4 logic)
o High LPD pre-trip (2 of 4 logic)
o Hi PZR pressure pre-trip (2 of 4 logic, generated in PPS)
o RPCB (flag from selected CEAs that drop / bit set ~ 25 seconds)
o CPC Pre-penalty (CEA deviation)

Question 25

How do CPC’s determine Azimuthal Tilt?

Answer 25

COLSS
The CPCs do not communicate directly with COLSS and COLSS does not communicate
directly with the CPCs. However, COLSS does calculate a value for Azimuthal Tilt (Tq) which
is required for CPC operation.

Azimuthal Power Tilt (Tq)
* Power asymmetry between azimuthally symmetric fuel assemblies (similar to QPTR)
* Azimuthal tilt is an addressable constant manually inserted in the CPCs (typically 1.02 or 2% tilt).
* Tq is calculated by COLSS
* Alarm (Plant Monitor Computer) if calculated value exceeds value in CPCs
* COLSS calculated value is manually entered into CPCs as addressable constant
* CPC cannot measure tilt (only sees one quadrant). Must be calculated by COLSS or manually using NIs.

Question 26

What modifies the “static penalty factor” and how does that change with time?

Answer 26

**Xenon ** Redistribution Penalty Factor -
Subgroup Deviation (9”)
o Subgroups within a group not within band (not applicable to Gr. 5, only one subgroup)
o Penalty Factor worsens over time (Xenon Redistribution penalty factor)
 Ramps up for 6 hours then stays at higher value
o Dropped 12 finger: trips that CPC channel DNBR/LPD immediately, CEACs sends penalty factor to the other CPCs.
o No penalty for dropping a 4 finger CEA
o Higher penalty for a larger deviation (+9” will cause a trip)*

Question 27

Does a Reactor Trip cause a CPC Trip?

Answer 27

Yes

• Reactor trip: CEA do not all insert at the same time/rate/sequence. DNBR/LPD trip will be generated from penalty factors.

Question 28

How does CPC’s calculate a Radial Peaking Factor?

Answer 28

Radial Power Profile (RPF) Calculation
* Core divided into **20 axial slices or “nodes” **(top to bottom)
* Uses CEA target positions (RSPTs) to determine RPF of each node.
* RPF is the ratio of peak to average power in a node.
* Value determined from a RPF table (not actually calculated, would take too long)

Question 29

Does the CPC Temperature Shadowing Correction use the hottest or coldest Tcold?

Answer 29

Temperature Shadowing Correction
* NI signal sensitive to Tcold changes (density affects neutron leakage).
* Automatically compensated for by comparing Min Tcold to temperature used during NI calibration.
o Lowest Tcold used because it would block more neutrons (more conservative)
* Alarm for high deviation between CPC calculated and excore power.

Question 30

During a Reactor downpower, with no rod motion, how does ASI move?

Answer 30

Down power (no rod motion) causes ASI to go more negative due to Thot ↓. The change in Thot density has more of an affect than the change in Tcold. (think of it like a boration, power is moving to the top)

Question 31

To calculate ASI, COLSS uses __________, and CPC uses _____________.

A. Incores, Incores
B. Incores, Excores
C. Excores, Incores
D. Excores, Excores

Answer 31

B. Incores, Excores

Question 32

What parameters are used in CPC “Mass Flow” calculation?

Answer 32

Thot, Tcold, RCP Speed, and Pzr Pressure

RCS Flow is calculated using reactor coolant pump (RCP) speed sensors as well as hot and cold
leg temperatures and pressurize pressure.

Question 33

Where is the compensated neutron flux
power (output from CPC) displayed?

Answer 33

Calibrated Neutron Flux Power (Analog)
* Sent to a dual pen recorder on** B05.**
* Red pen: CPC calibrated linear power
* Green pen: NI uncorrected power to VOPT
* May differ slightly because of the shadowing correction applied to CPC NI power.

Question 34

What is CEA Shadowing Correction in CPC’s?

Answer 34

CEA Shadowing Correction
CEA position (From RSPT) affects flux to the NIs. Rod shadow factors checked during start-up.
CPC corrects the excore signal based on CEA insertion.*

Question 35

What determines the magnitude of a CPC penalty factor?

Answer 35

The magnitude of the penalty factors generated is dependent upon several factors:
* Magnitude of the CEA deviation in a subgroup
* The direction of the CEA deviation
* CEA configuration

Question 36

If the operator sets the INOP code for CEAC 1 has an (addressable constant), how does the CPC determine a CEAC penalty factor?

Answer 36

CPC will use the last valid value from failed CEAC 1 and then auctioneer them against good CEAC 2 value

Question 37

How does lowering pressurizer pressure affect the DNBR and LPD POL’s?

Answer 37

DNBR goes down (gets worse). LPD is unaffected by pressurizer pressure changes.

Question 38

Describe the Operational Bypass for CPC trips:

Answer 38

Trips are automatically placed in service when power rises to 1E-4%.

Trip may be bypassed when THERMAL POWER is < 1E-4% RTP. Bypass shall be automatically removed when THERMAL POWER is > 1E-4% RTP.