What are the power supplies for the ATWS logic circuitry?
Ch A = Y70 (120 VAC) & D11 (125 VDC)
Ch B = Y80 (120 VAC) & D21 (125 VDC)
What are the power supplies to the ARI Valves?
142A = D11 (125 VDC)
142B = D21 (125 VDC)
What are the automatic ATWS initiation setpoints?
Low-Low RPV Level of -47" for 7.3 seconds
High RPV Pressure of 1135 psig
What automatic actions occur if ATWS initiates?
11 and 12 Recirc MG set field breakers open
ARI valve for the associated channel opens
If ATWS subchannels A & C initiate, which of the following will occur?
C. Both Recirc MG set field breakers open
Each field breaker has two trip coils, one from each logic channel
Why do we want the Recirc MG set field breakers to trip open in an ATWS event?
Tripping the field breaker results in a rapid coastdown of the Recirc pumps, faster than if the drive motor breaker was tripped.
The lower flow results in more voiding which adds negative reactivity, mitigating the consequence of the event.
What is the appropriate action following an ATWS initiation where indications exist that the Recirc MG set field breakers failed to open?
Action should be taken to open the Recirc pump drive motor breaker.
Are the ARI valves:
B. Energize to open
Why does the ATWS initiation on Low-Low RPV level of -47" have a 7.3 second time delay?
If the low-low condition clears before the 7.3 second relay times out, the relay will reset to prevent an ATWS trip from affecting ECCS system performance.
Is the ATWS system logic:
B. Two out of two taken once
Sub-channels A and C for Channel A.
Sub-channels B and D for Channel B.
Describe the RPV Low-Low level ATWS inputs...
Four level transmitters which output to four analog trip units.
Describe the RPV High Pressure ATWS inputs...
Four pressure transmitters which output to four analog trip units.
How do you manually initiate the ATWS system?
Rotate arming collars for two sub-channels of one channel (A & C for A, B & D for B) clockwise until the red line is directly on top.
Then, push both sub-channel push-buttons.
Note: only one channel (A or B) has to be initiated.
If D21 power supply was out of service, and an ATWS initiated, which of the following would occur?
A. 11 and 12 Recirc MG Sets would trip. Only ARI valve 142A would open.
D21 supplies power to ARI valve 142B.
The Recirc MG Set field breakers each have TWO trip coils, one from each power supply, D11 and D21.
What is the purpose of the ATWS system?
To mitigate the consequences of a failure of the Reactor Protection System.
A scram has occured and power remains >3.5%. What is the expected system response when the OATC rotates the arming collars for A and C ATWS subchannels clockwise?
The ATWS Ch. A Pushbutton Armed annunciator will light.
Arming the ATSW pushbuttons does NOT initiate an ATWS trip.
What other signals does the ATWS circuitry provide?
Main Turbine and Reactor Feed Pump trip signals in a one-out-of-two-twice configuration
What is the plant response if the panel operator rotates the D ATWS collar clockwise, then pushes the D ATWS pushbutton?
Only the ATWS Ch. B Pushbutton annunciator will come in. No ATWS trip will occur as it needs both pushbuttons for one channel armed and pushed.
What is the expected plant response given the following pressures:
PT A = 1136 psig
PT B = 1142 psig
PT C = 1132 psig
PT D = 1133 psig
No ATWS trip will occur. With the above information, only subchannel A of Channel A and subchannel B of Channel B will have trip signals.
A two-out-of-two-taken-once logic needs both A & C or both B and D subchannels to initiate a trip.
RPV water level is -49 inches and lowering. The eight Scram Bus Lights at C-05 are lit. What action, if any, should the operator take related to ATWS?
If a scram occured, the Scram Bus Lights would be out. Since the lights are lit, indicating a Scram did not occur, it is appropriate to announce and take action to initiate an ATWS trip. (Note, this is an extreme example simply to point out what the lights mean)
What is the primary purpose of the ATWS system?
As a backup to RPS, it offers an alternate means of intiating a Scram.
Describe how the ATWS logic circuit is powered.
Each ATWS Cabinet has two independant power supplies. Each power supply converts 125 VAC to 24 VDC. One power supply receives power from D-11 (D-21) 125 VDC which is converted to 115 VAC. The other power supply receives regulated 120 VAC from Y-70 (Y-80).