ATWS TAA

Question 1

What is an ATWS

Answer 1

CFR’s - Anticipated Operational Occurrence (AOO) followed by a failure of Rx trip portion of PPS

EAL’s - Auto or manual trip fails to shutdown the reactor

Question 2

If PV does NOT analyze an ATWS in its FSAR how are we compliant with the ATWS rule?

Answer 2

• SPS

- DAFAS

Question 3

What is the worse case event for an ATWS

Answer 3

• Heat Up Event

Question 4

What is the limiting event for a decrease in Heat Removal?

Answer 4

• Loss of Condenser Vacuum

Question 5

What is the concern if RPS does NOT shut down the RX during an event

Answer 5

• RPS is credited for safety analysis

- Plant may exceed design limits

Question 6

What are “Heat Up Events”

Answer 6

• Loss of Condenser Vacuum
• Loss of MFW
• Load Rejection at power
• Inadvertent CEA Withdrawal
• Loss of normal electric power

Question 7

Why does an ATWS threaten the safety level of the plant?

Answer 7

• TOTAL reliance on operator to terminate transient
• Failure of Rx trip may result in SEVERE over pressure transient, exceeding NSSS limits causing damage
• Loss of RCS mass may uncover core
• Invalidates safety analyses

Question 8

What plant designs features minimize effects of ATWS

Answer 8

• Trip Initiation Circuitry
• Trip Relays affect BOTH UV coil and Shunt Coil
• Trip Brkr Arrangement
• SPS
• DAFAS

Question 9

How does the design of Trip Initiation Circuitry minimize the affects of an ATWS

Answer 9

• 4 independent sensing channels for each trip paprameter
• Every combo of 2 channels in matrix logic
• Deenergization of tripping relays cause RTSG relays to open brks

Question 10

How does the Trip Relays affecting BOTH UV coil and Shunt Coil minimize the affects of an ATWS

Answer 10

• UV de-energize to open
• Shunt energize to open
• redundant alternate method of protection

Question 11

How does the design of Trip Brkr Arrangement minimize the affects of an ATWS

Answer 11

• 2 parallel legs with 2 brkrs each

- Single failure will not cause a trip

Question 12

How does the design of SPS minimize the affects of an ATWS

Answer 12

• Self-supported 2/4 Hi Press signal (2409) (B/U trip)
• SPS signal sent to CEDM MG output contractors to open
• Separate and Redundant signal ensures power interrupted to CEDM’s

Question 13

How does the design of DAFAS minimize the affects of an ATWS

Answer 13

• Backup actuation of AFAS

- Will NOT actuate if either AFAS/MSIS present