ECCS Flashcards
What is the ECCS Acceptance Criteria per 10CFR50.46?
- C - Maintain the core in a coolable geometry
- L - Maintain Long term decay heat removal
- O - Limit cladding oxidation to < 17% of the total cladding thickness (Zirc-hydriding concern)
- T - Maintain peak cladding temperature < 2200F (Above 2200F, the Zirc-Water reaction becomes exothermic)
- H - Limit H2 production to 1% of the total H2 that could be generated by the Zirc-Water reaction if all of the cladding reacted
CLOTH
Coolable
Long Term Cooling
Oxidation
Temperature, Max
Hydrogen generation
Discuss the Post Accident Sources of Hydrogen.
- Zirc – RCS Water reaction - Requires core uncovery and cladding temperature >2200F
- Corrosion and decomposition of metals inside Containment - decomposition of Zinc & Al due to High Temp
- Radiolysis of water - Decomposition of water due in the presence of a gamma flux -Slow but most significant source
- Hydrogen that was entrained in the RCS due to normal Hydrogen overpressure of VCT
List the SIAS actuation setpoints.
a) PZR Pressure: {1600 psia} [1736 psia]
b) CTMT Pressure: {5 psig} [3.5 psig]
Discuss the Refueling Water Tank (RWT).
{Aluminum} [Stainless steel]
* Volume: {525,000 gallons} [554,000 gallons]
* {Unit 1 Injects Sodium Hydroxide (NaOH) in outlet piping when CSAS is generated. which is corrosive to aluminum. On Unit 1 the Containment Spray Pump Recirc valves are maintained in a normally locked closed position to prevent introduction of sodium hydroxide (NaOH) into the aluminum RWT. Also, the NaOH flow path to the suction of the CS pump is isolated prior to opening the CS pump’s minimum recirculation line valves to support pump surveillance testing }
* Vented to Atmosphere
Discuss the RWT Volume Design Bases.
a) Acceptable ECCS water volume to remove decay heat, CTMT sump recirc & maintain Rx subcritical.
b) Volume to supply the HPSI, LPSI, & CS pumps in injection mode for 20 minutes, {305,600} [330,000] After ~ 20 minutes, RAS actuates and 1 HPSI on sump can remove Decay Heat
c) The RWT is sized to contain sufficient water to fill the refueling water canal, transfer tube, and the refueling cavity to 24 feet greater than Rx vessel flange
d) Since the refueling water tank is not provided with missile shielding, the safety injection tanks have been credited as a backup water source for RCS makeup during safe shutdown.
Discuss RWT Level Transmitters.
- 4 Transmitters
- Alarms & Actuations:
a) Tank Overflow @ ~ 38’ 3” (overflow to a local catch basin and eventually enter plant storm drains
b) Hi-Hi level alarm @ 37’ 9”: - {Closes V3459 – RWT return header isolation valve. Normally locked closed anyway}
- [Closes LCV-07-12- RWT return header isolation valve. Normally closed]
- Interlocks prevent overfilling the RWT during Rx Cavity pump down using the by LPSI pump
c) Hi level alarm @ 37’ 6” (~ 6” from spillover piping)
d) Lo level alarm @ 33’
e) 2 out of 4 detectors {≤ 4 feet} [6 feet] - Shuts RWT outlet isolation valves (MV-07-1A & B); and
- Opens CTMT Sump suction valves (MV-07-2A & B)
Discuss RWT Tech Spec Requirements.
RWT boron concentration
≥ 1900 ppm and ≤ 2200 ppm
RWT borated water temperature
≥ 55°F and ≤ 100°F
Water volume is
≥ 477,360 gallons
Discuss the HPSI Pumps.
- The HPSI pumps function to inject borated water into the reactor coolant system if a break occurs in the reactor coolant pressure boundary
- Used to fill the SITs when RCS pressure is > 1500 psia and plant is not on solid plant pressure control
- Powered from 4160V A3/B3
- Auto Start on a SIAS signal
- Sequence on 6 second load block following LOOP
- Design Flow 345 gpm (includes recirc flow)
- Recirc Lines – combined with LPSI pumps (~ 30 gpm) (Discussed below)
- Run Out Flow ~ 650 gpm {640} [685] gpm
- Shutoff Head 1250 psig
- {The CTMT Spray Pumps can be used to supply additional NPSH to the HPSI Pumps from just downstream of SDC HXs via motor operated valves V3662 & V 3663, ensuring NPSH post RAS}
- Seal water is self-supplied from the pump discharge
Discuss HPSI Pump Design Basis.
- For small break LOCA’s: With a higher RCS pressure, the HPSI pumps will ensure that the injected flow is sufficient to meet the ECCS acceptance criteria. Since sump recirc is unlikely due to the break size, the HPSI pumps continue injecting into the RCS to provide makeup for spillage out the break while a normal cooldown is implemented.
- For a Large break LOCA: The HPSI pumps are sized such that one pump will deliver saturated water at a flow rate sufficient to maintain the core flooded, matching decay heat boil-off at the time the safety injection system switches to the recirculation mode not less than 20 minutes after the LOCA.
- For Recirc Phase: The HPSI pumps are also used during recirculation to maintain water level over the core following the LOCA. For long term core cooling, the HPSI pumps are manually realigned for simultaneous hot and cold leg injection. This ensures flushing and ultimate subcooling of the core coolant independent of break location.
Discuss HPSI Minimum Flow Recirc Lines.
Unit 1:
* Each ECCS pump recirc line connects to a shared LPSI & HPSI pump min flow RWT return line.
* Recirc flow returns through two in-series MOV’s (MV-3659 & 3660 - ‘A’ Safeguards Room)
* Valves are normally open with power disabled via control room key switches (shown below)
* Procedurally, power is restored to the valves via the key switch prior to RAS
* Once power is restored, the valves auto close signal on an RAS signal.
* The purpose of the closure is to prevent the highly radioactive water most like present during a LOCA from recircing back to the RWT, creating a rad concern and a potential unmonitored release path
* Because of the common line / valve arrangement, single failure concerns are present. “OFF/ON” key switch (56 & 57) operated power lockout switches are located on RTGB 106. They are used to remove power to the valves making the valves passive components such that a single active failure can NOT disable both trains and prevent premature isolation
Unit 2:
* Two recirc lines exist with two isolation valves on each line. One MOV & One SOV. No single failure concern exists]
* A train (V-3659 & V-3495)
* B Train (V3660 & V-3496)
* Automatically close on a RAS signal. Same closing basis as Unit 1
* The SOV’s (V-3495 & 3496) fail closed on a loss of power
* The SOV’s can be operated from RTGB 206 or locally using Close / Locked Open key switches
Discuss HPSI Injection Header / Throttle Valves (8) (7’s - A Train; 6’s - B Train).
- 8 valves total. 4 valves on each train. Supplies all 4 Cold legs
- A Hdr – {Aux Hdr} [A Hdr], A Hdr relief at 2485 psig and 132 gpm capacity for alternate CHG thru A Hdr
- B Hdr – {HP Hdr} [B Hdr], B Hdr relief at 1750 [1585] psig at 5 gpm capacity
- Normally Closed - Open on SIAS
- Controlled from RTGB switch (CLOSE / AUTO / OPEN) or by Local PBs
- MOV’s can be throttled closed to control HPSI flowrate, (once throttling criteria is met)
- Powered from associated MCC A5/B5 & A6/B6
- On a LOOP following a SIAS, if the valves were throttled, they will fully re-open once power is restored
Discuss HPSI Throttling Criteria.
- 1 SG available for heat removal with NR Level being maintained or restored to between 60-70%
- RCS subcooling greater than or equal to 20F subcooling
- PZR Level > 30% and not lowering
- RVLMS Sensors 4-8 are covered OR no abnormal differences (> 20F) between Th and Rep CET
Discuss Hot Leg Injection.
- Used to flush boron precipitation from the top of the core and prevent flow blockages
- Large LOCA in Cold Leg could result in bulk boiling in core)
- Start 4-6 hours following LOCA if not on SDC
- Time Basis:
a) 4 Hrs – Sufficient minimum time for decay heat removal to try and prevent the hot leg injection from flashing to steam which could steam void the core
b) 6 Hrs – Maximum time limit ensures hot leg injection is effective. Start the hot leg injection before the flow blockage is too severe and complete blockage is present. - {Unit 1 was not originally designed for hot leg injection. Three potential flowpaths were later created to fufill this function. The 3 methods will be discussed in detail later}
- [Unit 2 was designed with dedicated Hot and Cold leg injection lines. System design provides simultaneous flow]
- On either Unit, simultaneous hot and cold leg injection is initiated 4-6 hours after a LOCA as long as any ONE of the following conditions exist:
a) RCS subcooling is less than minimum subcooling based on REP CET temperature.
b) PZR level is less than 30%.
c) RVLMS indicates hot leg nozzles are not full; sensors 4 through 8 are NOT covered, or the difference between THOT and REP CET temperature is greater than 20ºF
Discuss where Hot Leg injection flow is coming from.
Unit 1:
* Primary : A or B LPSI Pump via warmup line to opposite train Th suction line
Otherwise flow would short cycle back to the running LPSI pump suction
HPSI pumps provide cold leg injection
* 1st Alternate: 1A HPSI Pump via the Charging System through both the regenerative heat exchanger (RHX) and the RHX bypass line to the pressurizer auxiliary spray line.
1B HPSI pump provides cold leg injection
* 2nd Alternate: CTMT Spray Pump: provides hot leg injection via A or B hot leg supply headers
HPSI pumps provide cold leg injection
Unit 2:
* [Unit 2 was designed with dedicated Hot and Cold leg injection lines. System design provides simultaneous flow]
* A or B HPSI pump provided simultaneous Hot leg and Cold Leg injection from HPSI pumps via a split 50/50, at least 220 gpm to each (orifices balance flow)
* The other HPSI pump provides cold leg injection only
Discuss LPSI Pumps.
- Primary function is to injecting large quantities of borated water into the RCS during an emergency involving a large pipe rupture. Sufficient flow is delivered under these conditions to satisfy ECCS acceptance criteria
- The second function of the LPSI pumps is to provide shutdown cooling flow through the reactor core and shutdown cooling heat exchangers
- Powered from 4160V A3/B3
- Sequence on 3 second load block following LOOP. Will restart only if previously running from a SIAS
- Design Flow: {3000 gpm} [3100 gpm] (includes recirc flow)
- Run out Flow: {4500 gpm} [4600 gpm]
- Recirc flow:{40 gpm} [100 gpm]
- Shutoff Head: 185 psig
- Auto start on SIAS
- Auto stops on RAS - Designed to prevent long term dead head operation of the LPSI pumps during small break LOCA’s
- {Seal coolers on Unit 1 LPSI pumps supplied from CCW}
