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MAAP4 0 6 Simulation of Beyond DBAMAAP4.0.6 Simulation of Beyond DBA BWR3 Mark I
Dr. John H. Bickel
Monticello Nuclear Generating Plant, Monticello, Minnesota 1775 MWt (currently) → 2004 MWt (EPU) Xcel Energy
Original License: 9 1970 Re Licensed to operate until 2030Original License: 9‐1970 Re‐Licensed to operate until 2030
2
BWR3, Mark I – DBA Features:• Electric driven Main Feedwater, Condensate Pumps• 2 Diesels supply: 4kV power, 480V, 250VDC, 125VDC• 2 Trains Electric Driven LPCI with 2 RHR Heat Exchangers• 2 Trains Electric Driven LPCI with 2 RHR Heat Exchangers
– Safety system designed for Large Design Bases LOCA
• LPCI also has Drywell and Suppression Pool Spray Function• 2 Trains Electric Driven Core Spray Pumps
– Safety system designed for Large Design Bases LOCA
• 1 Train HPCI with suction from CST1 Train HPCI with suction from CST– Safety system designed for Small/Medium LOCA
• Automatic Depressurization SystemS f d i d f S ll/M di LOCA i h HPCI f il– Safety system designed for Small/Medium LOCA with HPCI failure
• 1 Train RCIC with suction from CST– Non‐Safety system designed for heat removal when main condenser
unavailable
• 2 Trains HP Control Rod Drive Hydraulic Pumps (~90‐200gpm)3
BWR3, Mark I – Beyond DBA Features:BWR3, Mark I Beyond DBA Features:• 2 trains Manual SLCS capable of injecting Sodium Pentaborate
– Anticipated Transients Without Scram
• CST can be replenished with River water or Fire Truck to prolong HPCI, RCIC, CRD injectionHPCI, RCIC, CRD injection
• “Security Diesel” 480V power can be connected to Battery Chargers (pre‐staged cable spools with quick‐connect plugs)
• Alternate Makeup to Reactor via existing piping connections:– RHR Service Water (from River)
– Diesel Driven Fire Pump (from River)Diesel Driven Fire Pump (from River)
– “B.5.b Pump” (from River)
• 18” Hard Pipe Vent from Torus to Atmosphere (preferred) – 18” equivalent to ~21 MWt decay heat removal (~decay heat after 1 hr)
4
BWR‐3 with Mark I Containment(Courtesy of General Electric)
Features:‐Engineered in 1960’s to address Large DBA LOCA‐ Design Press: 56psig‐ Ultimate Press. ~110psigHard Pipe Vent installed to‐ Hard Pipe Vent installed to
vent Torus to atmosphere
Limitations:‐Small, compact, containment volume requires N2 inerting for DBA LOCA‐ Credits DBA Large LOCACredits DBA Large LOCA containment pressure to assure ECCS pump NPSH‐ Not designed for severe
id
5
accidents
Mark I Containment Under Construction
6Courtesy TVA Browns Ferry NPP
What Reactor Building Looks Like in Refueling withDrywell Head Removed
7Courtesy TVA Browns Ferry NPP
What Reactor Building Looks Like During Operation
8Courtesy TVA Browns Ferry NPP
8 Safety/Relief Valves (S/RVs)Features:‐ Dual Action‐Mechanical spring‐loaded
f t f tioverpressure safety function‐DC/N2 operated relief valve function (for automatic depressurization)‐3/8 S/RVs used to rapidly depressurize RPV to allow core flooding.
Limitations:‐Physically located in Drywell‐ On loss of DC, or Press>75psig: relief mode is inoperable‐Electro‐pneumatic controller can failcan fail if Drywell Temp. > 335˚F(current Qualification Temp )(current Qualification Temp.)
9Monticello NGP
S/RV Controls for Emergency Manual DepressurizationEmergency Manual Depressurization
10Monticello NGP
High Pressure Coolant Injection System:Features:‐ Can inject at full system press. (1050psia)‐ ~1 5x106 lbs/hr makeup1.5x10 lbs/hr makeup capability‐CST is water source, Torus is alternate water source‐Designed to mitigate small, medium LOCA‐ Can be started locally, run without DC powerwithout DC power
Limitations:‐Needs RPV supply press. > 50 i50psig‐Needs exhaust (Torus) press. <75psig‐DC power needed to remote pstart‐When Torus used as water source, Temperature < 200˚F11/29/2011 11
HPCI Indications on Control Board
11/29/2011 12
High Pressure RCIC System :Features:Can inject at full system‐ Can inject at full system
press. (1050psia)‐ ~2x105 lbs/hr makeup capability (smaller than HPCI)‐ Designed to provide makeup after shutdown when FW unavailable‐CST is water source Torus is‐CST is water source, Torus is alternate water source‐ Can be started locally, run without DC power
Limitations:‐Needs RPV supply press. > 50psig50psig‐Needs exhaust (Torus) press. <75psig‐DC power needed to remote t t
13
start‐When Torus used as water source, Temperature < 200˚F
RCIC Indications on Control Board
14Monticello NGP
What RCIC Turbine Driven Pump Looks Like
15Courtesy TVA Browns Ferry NPP
Example of Pre‐Staged Equipment for Beyond Design Bases AccidentsBeyond Design Bases Accidents
Monticello NGP 16
Station DC BatteriesFeatures:‐2x 250V Batteries‐2x 125V Batteries‐Typical Lead Acid storage cells
Limitations:‐250V Batteries deplete in 7‐8 hrs without charging‐125V Batteries deplete in 5.5‐5.75 hrs without chargingcharging‐ Depletion defined as insufficient voltage to operate critical SOVs
C t TVA B F NPP
17
Courtesy TVA Browns Ferry NPP
Containment Vent for d dBeyond Design Bases Accidents
Main Features:
• Suppression Pool Vent
• Redundant air operated butterfly valves (failbutterfly valves (fail closed on loss of air)
• >56 psig rupture disk to prevent operation below normal design bases pressures
• Requires DC power, compressed air to operatep
18
Control Room Layout
19Monticello NGP
Loss of Feedwater with no Heat Removal• Initially more severe than Loss of Offsite Power
• Reactor trip, MSIV closure is Delayed
• Rate of initial RPV Level Drop is Faster• Rate of initial RPV Level Drop is Faster
• Steam Driven high pressure makeup sources (HPCI, RCIC) not Considered ‐ yetRCIC) not Considered yet
• CRD Hydraulic makeup not considered ‐ yet
• AC/DC power available compressed N availableAC/DC power available, compressed N2 available
• Operators follow EOPs and depressurize at TAF reflooding with LPCI to achieve stable water levelsreflooding with LPCI to achieve stable water levels
• Suppression Pool Cooling fails over long term
• Drywell Chillers Containment Venting not consideredDrywell Chillers, Containment Venting not considered
•• This is a dominant PRA accident sequenceThis is a dominant PRA accident sequence20
Loss of Feedwater with no Recovery, and Loss of HPCI, RCIC – first 2 hrs.,
21
Executing Emergency Depressurization with LPCI or Core Spray Reflood at ‐126” is well practiced procedureSpray Reflood at ‐126 is well practiced procedure
22
RPV Level Indicator on Control Board
23Monticello NGP
Emergency Depressurization allows L P C l I j i fl dLow Pressure Coolant Injection to reflood core
24
At this point RPV level is restored and corecooling using injected LPCI flow from Toruscooling using injected LPCI flow from Torus
25
Without Any Suppression Pool Cooling: Containment Temperatures Pressures continually riseContainment Temperatures, Pressures continually rise
26
Reclosure of S/RVs ~21.5hrs: causes RPV press. rise above LPCI pump shutoff pressurep p p
27
If Torus cooling fails, EOPs currently direct venting when pressure >56psig:venting when pressure >56psig:
28
If venting fails: Core Uncovery ~29hrs while pressure remains elevated at S/RV safety valve pressremains elevated at S/RV safety valve press.
29
Delayed Core Heatup starts at ~29.5hrs withZ W R i i 30 5hZr‐Water Reaction starting ~30.5hrs
30
Containment Failure Likely Occurs at 35‐37hrs. Accompanied by H2 burn of in reactor buildingp y 2 g
31
Consequences would be Large Delayed Release(~30% Core CsI) after Containment Failure( )
32Note: Degree of retention of fission products within reactor building not modeled.
Mitigation Strategy: Mitigation Strategy: If unable to vent, how much water makeup needed to cool core?water makeup needed to cool core?
• Match boil off due to decay heat:
• WBOIL = QDECAY(26hrs)/Δh
• QDECAY(26hrs) = 3.88x107 BTU/hr
• Δh = hsat(1050psia) – hCST(80F) ~1142.3 BTU/lbm
• WBOIL = (3.88x107 BTU/hr)/(1142.3 BTU/lbm) = 34,000 lbm/hr‐‐or: = (567 lbm/min)(0.016ft3/lbm) = 9.07ft3/min
• Converting to gal/min, this is only: ~67.8 67.8 gpmgpm
• Recall: CRD makeup flow to RPV is: 90 90 –– 200 200 gpmgpm
• 1 electric CRD pump could easily keep the core covered for an i d fi it i d f ti if t i t f il dif t i t f il dindefinite period of time if containment overpressure failure does if containment overpressure failure does not impact CRD pump operating environment.not impact CRD pump operating environment.
33
Evaluation of Station AC Blackout
• Loss of offsite power causes: Loss of Feedwater. Loss of Recirculation Flow, MSIV closure, Reactor Trip, Recirc. Pump Seal Leakage (37 – 165gpm) likelyg ( gp ) y
• Diesels fail, DC Batteries begin to discharge• All AC powered equipment shuts down
F d t d d t• Feedwater and condensate pumps• Drywell Coolers, room cooling• LPCI, Core Spray, CRD pumps cannot be operated
• HPCI RCIC potentially available provided RPV steam and DCHPCI, RCIC potentially available provided RPV steam and DC available for starting, and until room temperatures > 150˚F
• CST available as water sourceS/ V i ll il bl bl d if i• S/RVs potentially available to blow down if containment pressure <75 psig, and ‐ BOTH: DC power, compressed N2 available
• Diesel Fire Pump (portable pumps) potentially available until out of fuel
34
HPCI/RCIC start on low level. RCIC throttled Suction from CST, and run on local/manual without DC power, / p
Notes: (1) Running HPCI
l idlone cycle rapidly refills RPV causing HPCI, RCIC auto shutdown.
(2) Running with HPCI in “pull to lock” and RCIClock” and RCIC throttled minimizes on/off cycles – and thus conserves DC batteries.
35
RCIC will shutdown when Suppression Pool Pressure exceeds 64 7psia at ~ 13 9 hrsPressure exceeds 64.7psia at 13.9 hrs.
36
Core Uncovery: ~15.5 hrs, Core Melt: ~17 hrs
1.1 103
1.2 103
RPV Pressure (psia) vs. Level (ft)
800
900
1 10340
600
700 30
ress
ure
(psi
a)
PV L
evel
(ft)
PPS
XWCORE
XWSH
TAF
BAF
300
400
500
10
20Pr RPBAF
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 360
100
200
0
10
37
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
Time in Hours
time
Containment Overpressure Failure: ~20.4 hrs
Issues: (1) EOP C.5‐1200 calls for ventingcalls for venting containment at 56 psig (70.7 psia)At ~15hrs.(2) Without charging, last battery is depleteddepleted~ 8hrs 8hrs.
38
SRVs Possibly Could not be operated >8hrsdue to 335˚F equipment qualification uncertaintiesdue to 335 F equipment qualification uncertainties
1 103
Drywell, Suppression Pool Temp (F)
800
900
600
700
(psi
a) PoolTemp
DWTemp
300
400
500
Pres
sure
SRVLimTemp
ContFailTemp
100
200
300
39
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 360
Time in Hours
TIME
EOP C.5‐1200 Directs Operators to maintain Torus Temperature within Heat Capacity Temperature LimitTemperature within Heat Capacity Temperature Limit
by Controlled RPV Depressurization
11/29/2011 jhbickel@esrt‐llc.com 40
What is learned from all this:• If there is no containment heat removal working, high pressure steam
driven pumps will eventually shutdown on backpressure at ~13.9hrs
B tt i ( ith t h ) ill ll b d l t d b ~7 8h d• Batteries (without chargers) will all be depleted by ~7‐8hrs and unavailable to open S/RVs or containment vent valves.
• Drywell temperatures >335˚F challenge EQ limit for S/RV operation at ~8 hrs
• Containment pressure reaches >56psig for procedural containment venting at ~15hrs ………………but this is 7hrs7hrs afterafter battery depletionbattery depletionventing at 15hrs ………………but this is 7hrs 7hrs afterafter battery depletionbattery depletion
• Venting at this time could allow local manual restarting of RCIC pump in time to recover water level but there is no DC powerbut there is no DC power
M i i i li i h d d DC• Maintaining core cooling is thus dependent on DC power
In Station AC Blackout: containment venting needs to be considered earlier In Station AC Blackout: containment venting needs to be considered earlier to preserve core cooling optionsto preserve core cooling optionsto preserve core cooling options.to preserve core cooling options.
41
Using these insights:
• Simulation studies then performed on how to successfully cope with prolonged SBO but also: Seismic SBOp g
• In both cases: no possibility for Torus Coolingno possibility for Torus Cooling
• Seismic SBO differs from SBO: – Offsite power recovery may require weeks vs. hours
– Possibly destroys CST (requiring HPCI, RCIC suction from Torus)
– This causes more rapid Torus Heatupp p
– Possibly destroys Diesel Fire Water makeup (buried piping, cast iron piping in buildings)
• Successful coping depends on:• Successful coping depends on:– Gasoline powered portable DC battery chargers (or “Security Diesel”)
– Portable, Gasoline powered pumps
– Air compressor or bottled gas supply
42
Strategy: Break Rupture Disk below 56 psigusing bottled gas cylinder and test connectionusing bottled gas cylinder and test connection
Availability of compressed air compressed air and DCDC becomes essential tomaintain Containment Vent
43
Path open for heat removal.
Vent Containment, but keep pressure: 15 – 25 psig(If any hydrogen were present in containment it would be inerted via excess Nitrogen and lacking ( y y g p g gsufficient Oxygen to ignite. Thus: we don’t want outside atmospheric air entering containment – so keep pressurized)
44
Initiate Manual Depressurization when Suppression Pool heats up (as in current EOPs)Suppression Pool heats up (as in current EOPs)
45
After depressurization at ~8hrs core cooling can be provided by low press 100gpm portable pumpprovided by low press. 100gpm portable pump
46
What is learned from all this:• Current guidance to use HPCI once and throttled RCIC: worksworks
• Alternate means will be needed for DC Battery charging ~ 5hrs• Alternate means will be needed for DC Battery charging 5hrs
•• Venting Containment is needed earlier (~ 8hrs) Venting Containment is needed earlier (~ 8hrs) to maintain lower Drywell Temperatures to operate S/RVs allowing emergency RPV depressurization
• Earlier venting requires defeating 56 psig rupture disk
d i i l i ill ll• Emergency RPV depressurization as currently in EOPs will allow use of smaller portable pumps
• Successfully executing this long term coping strategy requiresSuccessfully executing this long term coping strategy requires logistics of staged equipmentstaged equipment and supply of “consumables” (fuelfuel, compressed gascompressed gas)
47
“Supplemental”Supplemental
48
How Long Can NPP Cope Without Decay Heat Removal?Without Decay Heat Removal?
• Coping time is: time from when heat removal is lost toCoping time is: time from when heat removal is lost to onset of severe core damage (available recovery time)
Coping time considersCoping time considers:
• Physical inertia built into reactor design via water to water to core power ratiocore power ratio
• Effects of water makeup systemsEffects of water makeup systems
• Effects of support features (DC power, HVAC) which enable water makeup systemsenable water makeup systems
49
Simplified Decay Heat Model:
50
Behavior of RPV Level with RCIC:• Assume pressure ~1050 psia , constant due to relief valve operation
• At constant pressure water mass loss is related to decay heat, thus: WLOSS(t) = QDECAY(t) / hfg = 0.15Qot‐0.286 / hfg (hfg = 640 BTU/lbm.)WLOSS(t) QDECAY(t) / hfg 0.15Qot / hfg (hfg 640 BTU/lbm.)
• Assume core output initially at: Qo~6.84x109 BTU/hr
• Assume when activated: RCIC injects: WRCIC(t) ~2.0x105 lbm./hr.
• Assume initial water mass above core: MCORE~1.2x105 lbm.
• Behavior of water inventory governed by:
dM
286.0
)15.0()0()( dttQWMtM
WWdt
dM
to
LOSSRCICCORE
714.0
0
15.0)0(
)()0()(
tQtWM
dth
WMtM
oRCICCORE
fg
oRCICCORECORE
)714.0()0( t
htWM
fgRCICCORE
51
After water level recovers, RCIC will stop
52
Behavior of Water after RCIC stops:• Assume RCIC can not be operated after 8hrs• Pressure ~1050 psia and constant due to relief valve operation• At constant pressure water mass loss still related to decay heatAt constant pressure water mass loss still related to decay heat• If core output initially at: Qo~8.13E+9 BTU/hr• When RCIC stops: WRCIC(t) ~0.0 lbm./hr.• Assume same initial water mass above core: MCORE~ 1.2E+5 lbm.• Behavior of water inventory governed by:
dM
)15.0()8()(286.0
dttQhrsMtM
Wdt
dM
to
LOSSCORE
)8(15.0)8(
)()8()(
714.0714.0
8
hrstQhrsM
dth
hrsMtM
oCORE
fg
oCORECORE
)8(714.0
)8( hrsth
hrsMfg
CORE
53
Behavior of Water after RCIC stops:l d b l ff d d b d hSlow steady boil‐off dictated by decay heat curve
54
55
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