Digital Feedwater Control (DFWCS)

Question 1

When will the DFWCS Master Controller enter and exit tracking on a Reactor Trip?

Answer 1

Exits at >51.9%NR SGWL
Enters at < 26%NR SGWL

Question 2

If the 1E button on the DFWCS Screen is green and power is raised > 14% what will happen?

Answer 2

DFWCS will NOT automatically swap to 3E. One of the prerequisites to swap is that the 3E button is green (selected).

Question 3

When does DFWCS swap from 3E to 1E?

Answer 3

13.5% and decreasing

Question 4

What causes an “ATUN Lockout”?

Answer 4

8% deviation between Reactor Power instruments or

8% deviation between Feedwater Temperature Typically a transmitter failure will be the cause of this.

Question 5

Describe the response of the DFWCS on a shutdown.

Answer 5

100%: DCCVs passing 10% flow, ECCVs maintaining SGWL
50% - Downcomer Control Valves close
15% - Swapover (ECCVs immediately close. DCCVs immediately open to 20% then ramp to 80%)
13.5% - DFWCS shifts from 3E to 1E
13% - indicated feed flow changes from the Total Feed Flow transmitters to the Downcomer.

Question 6

What is High Level Override in the DFWCS?

Answer 6

When SG is too high (>88%NR), the DFWCS will close the Downcomer and Economizer Control Valves in an attempt to lower SGWL.

• ≥ 88% NR
• Closes feedwater control valves on affected SG (Downcomer and Economizer)
• Sends a “zero” signal to MFP controller on the affected SG FWCS.

Question 7

What is the power supply to the DFWCS?

Answer 7

**Normal power is NNN-D11 **and NNN-D12 is the alternate power. Power is normal-seeking.

(Loss of NNN-D11 will cause the following:
o Loss of one Reactor Power input (Excore Control Channel #1) Low TAVE will cause DFWCS to NOT feed in RTO
* Loss of NNN-D12 will cause the following:
o Loss of one Reactor Power input (Excore Control Channel #2) Low TAVE will cause DFWCS to NOT feed in RTO)

Question 8

Describe the response of the DFWCS on a startup.

Answer 8

< 13%: ECCVs closed, DCCVs maintaining SGWL, MFPs at minimum speed, 1E Control
13% - indicated feed flow changes from the Downcomer to the Total Feed Flow transmitters
14% - DFWCS shifts from 1E to 3E
15%-16.5% - Swapover
50% - Downcomer Control Valves open to ~65%

Question 9

When will the Master Controller go to “tracking” mode (i.e. display “tracking” on the screen)?

Answer 9

• Anytime the Economizer and Downcomer Control Valves are placed in Manual
• When a Reactor Trip occurs the Master Controller will track the output of the RTO Controller

Question 10

If Total Feed Flow reaches ______% deviation, DFWCS will swap to ___ Control and generate a _________ __________.

Answer 10

8%
1E
3E Lockout

Question 11

When will DFWCS cycle in RTO after a Reactor Trip?

Answer 11

It will **exit when SGWL > 51.9% NR **and will re-enter RTO when SGWL < 26% NR. This will cycle until the under voltage relays on the CEDMCS (UV 1&2 OR 3&4) bus are reset.

Question 12

At what Tave will all feed flow be stopped to the SGs after a Reactor Trip?

Answer 12

Less than 564F

Question 13

If Feedwater Temperature reaches ___% deviation, DFWCS will generate a _______ Lockout and ___________________.

Answer 13

8% (40F)
ATUN Lockout
insert a canned value of 425F

Question 14

If Steam Flow reaches ____% deviation, DFWCS will swap to ___ Control and generate a _________ __________.

Answer 14

50%
1E
3E Lockout

Question 15

MFP Arrow control - How big are the increments for a single arrow push vs. a double arrow push?

Answer 15

• Manual Control
  o Single arrow - 1% change (10 RPM), Double arrow - 5% change (50 RPM)

Question 16

If Reactor Power reaches _____% deviation, DFWCS will generate a ______ Lockout and _________ ________________.

Answer 16

• 8%
• ATUN
• insert a canned value.
  
  • 5% if below 16.5%
  • 60% if above 16.5%

Question 17

What are the “Elements” in 1E vs 3E control in Digital Feedwater Control System?

Answer 17

1E = Steam Generator Level

3E = Steam Generator Level + Steam Flow + Feedwater Flow

Question 18

How does the Steam Bypass Control System support the Digital Feedwater Control System?

Answer 18

Steam Bypass Control System
* SBCS Demand (output) and TLI provide anticipatory transient control. (Reliable indication of steam demand.)
* 1E controller only.

Question 19

When RTO (Reactor Trip Override) logic clears, what is the status of the Digital Feedwater Control System (DFWCS) master controller (auto or manual)?

Answer 19

• When RTO logic clears, at ≥ 51.9% (NR), then the master controller will shift to 1E control (in “auto”).
• Even if the master controller was in “manual” at the time of the trip, it will be in “auto” when the RTO logic clears.

Question 20

What happens to the master controller on a reactor trip?

Answer 20

• Master controller shifts to “local” and “tracking” on a trip.
• Will track, and pass, the signal from the RTO controller. Controller will be in “tracking” as long as the RTO logic is present.

Question 21

How many Steam Generator Water Level transmitters does DFWCS use? How does it pick a transmitter to use?

Answer 21

• Control function in 1E and 3E (4 total transmitters, two for each generator)
  * Auto selects the highest reading Level Transmitter on a SG
• If one is OOS, uses the remaining LT
• 15% deviation between SGs:
  o Selects the lowest LT for control on the SG that has the higher level

Question 22

What event does RTO prevent?

Answer 22

When the reactor trips, SG levels normally will lower rapidly due to the shrink caused by an increase in steam header pressure. DFWCS (in RTO) will feed
the SGs much more slowly than the large level deviation from setpoint would call for. RTO
prevents an overcooling transient.

During natural circulation (loss of RCP’s), RTO will overfeed.

Question 23

How does DFWCS compensate for a steam flow transmitter being placed in maintenance?

Answer 23

If a steam flow transmitter is removed from service by placing it in “maintenance” on the FWCS 1
Redundant Transmitter Logic screen, the remaining transmitter’s value will be doubled in order to approximate total steam flow and that value will be used by 3 element control.

Question 24

Is Control Channel adaptive tuning used in 1E control in DFWCS?

Answer 24

No

Uses Average of the inputs
o Can select either channel or average on DFWCS (only FWCS 1)
* Tunes the responsiveness by adjusting the proportional band and integration rate based on the reactor power level.
o More responsive at higher power levels. (3E only)

Question 25

Explain swapover logic

Answer 25

Swapover: the shifting of feedwater flow from through the Downcomer control valves to
through the Economizer control valves (during a increase in reactor power) or from through
the Economizer control valves to through the Downcomer control valves (during a decrease
in reactor power) is known as swapover.

Swapover during an increase in reactor power will
occur if reactor power is above 16.5% OR
if reactor power is between 15 and 16.5% and
**either **Downcomer control valve reaches 80% open. Swapover occurs on BOTH Steam Generators.

Swapover during a decrease in reactor
power will occur if reactor power is below 15%.

Question 26

Explain the downcomer and economizer control valve positions as reactor power increases

Answer 26

DCCV’s open as power rises to 13%
Swapover occurs (15-16.5%), DCCV’s full close over 15 minutes. Economizer opens to supply feedwater
At 50% steam flow, DCCV opens to pass 10% steam flow (50% open valve position).

Question 27

What parameters does Adaptive Tuning look at and affect?

Answer 27

• Uses Feedwater Temperature (1E and 3E) and reactor power (3E only).
• Adjusts the 1E and 3E controller responsiveness (varies program gains and time constants).
• More responsive at higher powers than at low powers.

Question 28

How does the Digital Feedwater Control System (DFWCS) respond to both the economizer and downcomer controllers being placed in Manual?

Answer 28

• If both ECCVs and DCCVs are in manual, the master controller will be in “local” and “tracking”.
• Master controller tracks the valve controller with the highest output signal.
• **If either DCCV or ECCV controller is returned to “auto”, **the master controller will stop tracking and the master controller output will be the last tracking signal received from the controlling valve when either of the valves was returned to “auto”.

Question 29

What are the limitations on the output of the master controller for the Digital Feedwater Control System?

Answer 29

• Controller output limits:
• <** 15% power (NI): 25% max** output
• RTO: 9% max output (due to tracking RTO controller)

Question 30

Do Main Feed Pump controllers receive inputs from both DFWCS’ controllers while in RTO?

Answer 30

• RTO: MFPs only receive input from their associated DFWCS.

Question 31

If DFWCS substiutes a canned value for a Control Channel(at 100% power), will Steam Generator water levels change?

Answer 31

No, SG levels will be stable