Shutdown Cooling Flashcards
Discuss Shutdown Cooling Design Basis.
Safety Related
* Removes heat (from injection flow boil-off) from CNMT atmosphere when HPSI and CS systems are aligned for long term recirc cooling following a large break LOCA
* [Remove heat from RCS during small break LOCA (break flow not sufficient for DHR)]
* [Sufficient overpressure protection at low temps, prevent Rx Vessel NDT limit violation – 350 psig relief’s]
* During shutdown conditions, the normal heat sink (Steam Generators) becomes unavailable below approximately 270oF.
Discuss SDC Entry Criteria.
- Pressure {< 267 psia} [< 275 psia]
- Temperature (Th) < 325F Both Loops
Discuss SDC Heat exchangers.
CCW outlet AOV’s open on SIAS.
Outlet AOV fully opens, but total flow is limited by a normally throttled downstream manual valve.
Discuss Shutdown Cooling Hot Leg Suction Valves.
Suction Path
Unit 1
From both Hot Legs via two MOV’s
No cross tie capability
Unit 2
From both Hot Legs via three MOV’s
Has cross-tie capability
Power Supply
Unit 1
One valve in each line powered from opposite train
Valve closest to the RCS hot leg is powered from B train power & the inner from A train power
A Hot Leg - V3480 (B) - V3481 (A) - A LPSI
B Hot Leg - V3652 (B) - V3651 (A) - B LPSI
MCC A5 / B5 / B6
Unit 2
One valve in each line powered from opposite train
Valve closest to the RCS hot leg is powered from the opposite train
A Hot Leg - V3480 (B) - V3481 (A) – V3664 (A) – A LPSI
B Hot Leg - V3652 (A) - V3651 (B) - V 3665 (B) -B LPSI
MCC A5 / B5 / B6
Cross-tie (V3545) is powered by the “2AB” power supply.
Downstream of the closest loop isolation valves 3480 & 3652
a) This enables SDC to be operable in the event of a loss of one train’s AC power source and the operable train has the capability of using the inoperable train’s suction flowpath via the crosstie.
Effects of opposite train power?
Unit 1
Single failure isolation criteria: Requires two valves in series with opposite train power so that isolation is possible.
Loss of power to either train results in a loss of availability of both SDC trains if not already in service
Unit 2
Single failure isolation criteria: Requires two valves in series with opposite train power so that isolation is possible.
Loss of power to either train results in a loss of availability of one train only
Inputs
Unit 1
One pressure transmitter controls two valves (One on each train)
* PT-1103 supplies V3481 & V3651
* PT-1104 supplies V3480 & V3652
Fail AS-IS on loss of power
If PT fails high while closed, valves won’t open
If PT fails high while open, valves stay open
Unit 2
Each pressure transmitter controls only valve
* PT-1103 (V3652),
* PT-1104 (V3481)
* PT-1105 (V3480),
* PT-1106 (V3651)
Fail AS-IS on loss of power;
If PT fails high while closed, valves won’t open
If PT fails high while open, valves go closed
Open Permissive
Unit 1
Requires RCS pressure ≤ 267 psia to open
Unit 2
Requires RCS pressure ≤ 276 psia to open
By procedure, not opened until ≤ 275 psia
Closing
Unit 1
No auto close signal = Once open they will not close on pressure
Unit 2
Auto close signal: If RCS pressure rises to ≥ 500 psia on associated transmitter.
Discuss Unit 1 LPSI Pump Discharge Flowpath.
Arrangement
Individual pump discharge lines combine into a common header through an air operated flow control valve (FCV-3306) and a motor operated bypass valve (MV-03-2)
Function
Passes flow during injection mode from LPSI pumps
Regulates the amount of primary coolant that bypasses the SDC heat exchangers and maintains a constant design flow rate through the core during plant cooldowns
{FCV – 3306}
Common discharge flow control valve
AOV – Fails open on loss of air or power
{MV-03-2}
It is the motor operated bypass valve around FCV-3306.
Powered from MCC-1AB
MV-03-2 is normally open to pass a sufficient amount of flow to meet the hot and cold leg injection requirements of the LPSI system in the event that a failure of valve FCV-3306 isolates the main flow path.
MV-03-2 is CLOSED when on SDC.
LPSI header isolation valves control flow to each cold leg
FCV-3657
Used in conjunction with FCV-3306 to control SDC temperature (Basically both valves are throttled to control how much SDC flow goes through the SDC HX.
AOV - fails closed on loss of air or power
Valves are pinned in position once final desired RCS temperature is reached and stabilized. Therefore a loss of air or power would not have a detrimental effect on SDC temperature. (This is addressed in SDC 1-NOP-03.05)
Operation
Flow control valves balanced to control temperature
LPSI header isolation valves control SDC flow to each cold leg
When FCV-3306 is in Auto, the valve will throttle to maintain flow through FT-3306 at the dialed in setting.
Auto mode is not currently used.
Why Not?
Valve seat wear concerns on FCV-3306 and HCV-3657
By procedure, only one valve can be throttled at a time. Therefore the system is operated in Manual
Discuss Unit 2 LPSI Pump Discharge Flowpath.
Arrangement
Individual pump discharge lines each containing a motor operated flow control valve (‘A’: FCV-3306) (‘B’: FCV-3301)
Function
Passes flow during injection mode from LPSI pumps
Regulates the amount of primary coolant that bypasses the SDC heat exchangers and maintains a constant design flow rate through the core during plant cooldowns
FCV- 3306 (A)
FCV- 3301 (B)
LPSI discharge flow control valves
MOV’s – Fails as is on loss of power
FCV-3657 (A)
FCV-3512 (B)
Used in conjunction with the train specific FCV-3306 (A) or FCV-3301 (B) to control SDC temperature
Basically both valves are throttled to control how much SDC flow goes through the SDC HX.
MOV - fails as-is on loss of power
Operation
Flow control valves balanced to control temperature
LPSI header isolation valves control SDC flow to each cold leg
Discuss SDC Purification.
Unit 1
* SDC line enters the CVCS upstream of the Letdown HX, enabling this flow to pass by TE-2224,
* Allows automatic ion exchangers protection in the event of high temperatures.
* Flow is controlled from the Control Room by adjusting PCV-2201P and PCV-2201Q.
* Only lined up to one SDC Train at a time
* High temperature will bypass the IX’s (procedure limit is still 140 degrees)
* On both Units, SDC purification flow is indicated on FT-2202, letdown flow indicator, located on RTGB 105 [205]
Unit 2
* SDC purification line enters the CVCS downstream of both the Letdown HX and TE-2224.
* No automatic protection of purification ion exchangers in the event of the introduction of high temperature SDC flow.
* SDC purification cannot be placed in service on Unit 2 until RCS temperature is below 140oF.
* Flow is controlled locally by having the SNPO throttle V3712 or V03012 (depending on the train)
* Only lined up to one SDC Train at a time
* High temperature will NOT bypass the IX’s (procedure limit is 140 degrees)
* On both Units, SDC purification flow is indicated on FT-2202, letdown flow indicator, located on RTGB 105 [205]
Discuss Pressurizing SDC System.
- Used to try and minimize the pressure surge on the system when it is unisolated which could lift the {350 psig} & [365 pisg] SDC relief valves
- If SDC is isolated from the RCS and RCS pressure is greater than 200 psia;
AND
The differential pressure between RCS and SDC is greater than 100 psi,
Then PRESSURIZE SDC using one of the following three methods to achieve the desired P:
a) Preferred: Cntmt Spray
b) First alternate {Letdown} [Jogging open Hotleg Suction]
c) Second alternate: Stop all RCPs , depressurize the RCS and open the hotleg suction valves
Discuss Unit 2 SDC Full Flow Relief Valve.
- Each SDC suction line contains a full flow shutdown cooling relief valve (SDCRV) with a lift setpoint of 350 psia and 2300 gpm capacity each
- Full Flow Relief’s in CNMT between 2nd and 3rd isolations (350 psia / 2300 gpm)
- Designed to protect the RCS against overpressurization during full safety injection flow following a spurious SIAS while on SDC.
- SDCRVs (V3667 and V3666) are part of the Tech Spec RCS overpressurization system (T/S 3.4.12) and discharge to the containment sump.
a) Required to be aligned for LTOP (w/Th isolations open) when Tc < 132F
Discuss SDC Cooldown Operations.
Start aligning SDC when <1500 psia & <500°F
What is the purpose of the SDC Standby Lineup?
To allow quick start of the standby SDC train when the running train is lost.
Which train is preferred to be in Standby during reduced inventory (< 33’) and mid-loop (< 31’ 3”) Ops? Why?
Train B is preferred to be in Standby Quick Start (during reduced inventory or mid-loop ops).
SDC “A” suct. high point 11 ft > its HL suction
SDC “B” suct. high point 3 ft > its HL suction
SDC “A” loses prime easier so prefer keeping “B” in standby when < 33 ft more likely to be recoverable
Discuss Reduced Inventory/Mid Loop.
- A Reduced Inventory Condition exists when fuel is in the Reactor and the reactor vessel water level is between 3 feet below the top of the reactor vessel flange and equal to the top of the flow area of the RCS hot leg nozzles at the reactor vessel
- A Mid-Loop Condition exists when fuel is in the Reactor and the reactor vessel water level is lower than the top of the flow area of the RCS hot legs nozzles at the reactor vessel. RCS level is reduced to allow installation of S/G nozzle dams.
- If during mid-loop or reduced inventory, RCS loss due to boil-off is not made up, RCS inventory and level may drop and uncover the fuel. Fuel temperatures would rise and the clad may fail, causing a breach of the clad barrier (and if RCS is vented, that barrier is already lost).
- Reduced Inventory = 33’ = 3’ below vessel flange (36’)
- Mid Loop Operation = 31’ 3” (part of loops filled definition) (SG nozzle dam installation). Defined as lower than the top of Hot Leg
- Lowering level reduces NPSH and can cause vortexing or air induction to LPSI / SDC suction
- If SDC is lost especially at low RCS levels the RCS will heat up. If operating at a mid loop or reduced inventory S/Gs may be unavailable as a heat removal mechanism. During a sustained loss of SDC the RCS temperature will rise until boiling occurs (at 212ºF if RCS vented, saturation temperature for current RCS pressure if not).
- LPSI suction will be lost if level drops below {29’ 9.5”} [29’ 8.2”].
- If SDC lost at Mid Loop – raise level by:
a) CHG pumps
b) HPSI pumps
c) LPSI pumps
d) Gravity fill from RWT (0” = 20’), (9’ 6” = mid loop) - Flow Rates When Reducing RCS Level With SDC in Service:
a) LPSI mini-flow line to RWT (~125 gpm max)
b) SDC Purification to the HUT’s (<145 gpm)
c) LPSI through 6” line to RWT (>125 gpm) - Used when draining the refueling cavity
Discuss Reduced Inventory/Mid Loop Level Indication.
- Tygon Tube (from bottom of B1 intermediate Leg – connected to top of PZR)
- RVLI and Tygon must agree within 3”
- LI 1117/1117-1 (0-48”), (0-400”) – ERDADS display/ {CRAC} [PACB-2] A hot leg to Pzr
Discuss 1[2]-AOP-03.02.
Used when SDC is lost due to:
a) Loss of SDC flow
b) Increasing SDC temperature
c) Fluctuating LPSI pump amps
d) [Closure of hot leg suction valves (High RCS pressure)]
Upon entry into the AOP, the procedure will address time to boil, RCS level, containment closure, and restoring SDC
Subsequent actions will direct the completion of the safety function status check per the Low Mode ONP and calculate a time to boil
Low Mode Off-Normal Procedure for the current plant condition should be implemented if conditions continue to degrade or the SDC procedure is not stabilizing plant conditions.
