MAAP4.0.6 Simulation of Beyond DBA BWR3 Mark I rev03

1. MAAP4.0.6 Simulation of Beyond DBA MAAP4 0 6 Simulation of Beyond DBA BWR3 Mark I  Dr. John H. Bickel 2.…
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  • 1. MAAP4.0.6 Simulation of Beyond DBA MAAP4 0 6 Simulation of Beyond DBA BWR3 Mark I  Dr. John H. Bickel
  • 2. Monticello Nuclear Generating Plant, Monticello, Minnesota    1775 MWt (currently) → 2004 MWt (EPU)                 Xcel Energy Original License: 9‐1970                                                   Re‐Licensed to operate until 2030 Original License: 9 1970 Re Licensed to operate until 2030 2
  • 3. 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 CST 1 Train HPCI with suction from CST – Safety system designed for Small/Medium LOCA • Automatic Depressurization System – S f Safety system designed for Small/Medium LOCA with HPCI failure d i d f S ll/M di LOCA i h HPCI f il• 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
  • 4. 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 injection HPCI, 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
  • 5. 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. ~110psig ‐ Hard 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 LOCA Credits DBA Large LOCA  containment pressure to assure  ECCS pump NPSH ‐ Not designed for severe  accidents id 5
  • 6. Mark I Containment Under Construction Courtesy  TVA Browns Ferry NPP 6
  • 7. What Reactor Building Looks Like in Refueling with Drywell Head Removed Courtesy  TVA Browns Ferry NPP 7
  • 8. What Reactor Building Looks Like During Operation  Courtesy  TVA Browns Ferry NPP 8
  • 9. 8 Safety/Relief Valves (S/RVs) Features: ‐ Dual Action ‐Mechanical spring‐loaded  overpressure safety function f t f ti ‐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 fail if Drywell Temp. > 335˚F (current  Qualification Temp.) (current Qualification Temp ) Monticello NGP 9
  • 10. S/RV Controls for Emergency Manual DepressurizationEmergency Manual Depressurization Monticello NGP 10
  • 11. High Pressure Coolant Injection System: Features: ‐ Can inject at full system  press. (1050psia) ‐ ~1 5x106 lbs/hr makeup 1.5x10 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 power without DC power Limitations: ‐Needs RPV supply press. >  50psig 50 i ‐Needs exhaust  (Torus)   press. <75psig p ‐DC power needed to remote  start ‐When Torus used as water 11/29/2011 11 source, Temperature <  200˚F
  • 12. HPCI Indications on Control Board11/29/2011 12
  • 13. 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. >  50psig ‐Needs exhaust  (Torus)   press. <75psig ‐DC power needed to remote  start t t ‐When Torus used as water  source, Temperature <  200˚F 13
  • 14. RCIC Indications on Control Board Monticello NGP 14
  • 15. What RCIC Turbine Driven Pump Looks Like Courtesy  TVA Browns Ferry NPP 15
  • 16. Example of Pre‐Staged Equipment for  Beyond Design Bases Accidents  Beyond Design Bases Accidents Monticello NGP 16
  • 17. Station DC Batteries Features: ‐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  charging ‐ Depletion defined as  insufficient voltage to  operate critical SOVs Courtesy  TVA Browns Ferry NPP C t TVA B F NPP 17
  • 18. Containment Vent for Beyond Design Bases Accidents d d Main Features: • Suppression Pool Vent • Redundant air operated  butterfly valves (fail  butterfly 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  operate p 18
  • 19. Control Room Layout Monticello NGP 19
  • 20. 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  RCIC) not Considered ‐ yet• CRD Hydraulic makeup not considered ‐ yet• AC/DC power available compressed N2 available AC/DC power available, compressed N available • Operators follow EOPs and depressurize at TAF  reflooding with LPCI to achieve stable water levels with LPCI to achieve stable water levels• Suppression Pool Cooling fails over long term• Drywell Chillers Containment Venting not considered Drywell Chillers, Containment Venting not considered• This is a dominant PRA accident sequence 20
  • 21. Loss of Feedwater with no Recovery,  and Loss of HPCI, RCIC – first 2 hrs. , 21
  • 22. Executing Emergency Depressurization with LPCI or Core  Spray Reflood at ‐126 is well practiced procedure Spray Reflood at ‐126” is well practiced procedure 22
  • 23. RPV Level Indicator on Control Board Monticello NGP 23
  • 24. Emergency Depressurization allows Low Pressure Coolant Injection to reflood coreL P C l I j i fl d 24
  • 25. At this point RPV level is restored and corecooling using injected LPCI flow from Toruscooling using injected LPCI flow from Torus 25
  • 26. Without Any Suppression Pool Cooling: Containment Temperatures, Pressures continually riseContainment Temperatures Pressures continually rise 26
  • 27. Reclosure of S/RVs ~21.5hrs: causes RPV press. rise  p p p above LPCI pump shutoff pressure 27
  • 28. If Torus cooling fails, EOPs currently direct  venting when pressure >56psig: venting when pressure >56psig: 28
  • 29. If venting fails: Core Uncovery ~29hrs while pressure  remains elevated at S/RV safety valve press. remains elevated at S/RV safety valve press 29
  • 30. Delayed Core Heatup starts at ~29.5hrs with Zr‐Water Reaction starting ~30.5hrs Z W R i i 30 5h 30
  • 31. Containment Failure Likely Occurs at 35‐37hrs. Accompanied by H2 burn of in reactor building p y g 31
  • 32. Consequences would be Large Delayed Release ( (~30% Core CsI) after Containment Failure )Note: Degree of retention of fission products within reactor building  not modeled. 32
  • 33. 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 gpm 67.8 gpm• Recall: CRD makeup flow to RPV is: 90 – 200 gpm 90 – 200 gpm• 1 electric CRD pump could easily keep the core covered for an  indefinite period of time if containment overpressure failure does  i d fi it i d f ti if containment overpressure failure does  if t i t f il d not impact CRD pump operating environment. 33
  • 34. 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) likely g ( gp ) y• Diesels fail, DC Batteries begin to discharge• All AC powered equipment shuts down • F d t Feedwater and condensate pumps d d t • Drywell Coolers, room cooling • LPCI, Core Spray, CRD pumps cannot be operated• HPCI RCIC potentially available provided RPV steam and DC HPCI, RCIC potentially available provided RPV steam and DC  available for starting, and until room temperatures > 150˚F • CST available as water source• S/ V S/RVs potentially available to blow down if containment pressure  i ll il bl bl d if i <75 psig, and ‐ BOTH: DC power, compressed N2 available• Diesel Fire Pump (portable pumps) potentially available until out  of fuel 34
  • 35. HPCI/RCIC start on low level. RCIC throttled Suction from CST, and run on local/manual without DC power , / p Notes:  (1) Running HPCI  one cycle rapidly  l idl refills RPV  causing HPCI,  RCIC auto  shutdown.  (2)  Running with  HPCI in “pull to  lock” and RCIC  lock” and RCIC throttled  minimizes on/off  cycles – and thus  conserves DC  batteries. 35
  • 36. RCIC will shutdown when Suppression Pool  Pressure exceeds 64.7psia at  13 9 hrs Pressure exceeds 64 7psia at ~ 13.9 hrs. 36
  • 37. Core Uncovery: ~15.5 hrs, Core Melt: ~17 hrs RPV Pressure (psia) vs. Level (ft) 3 1.210 3 1.110 3 110 40 900 800 ressure (psia) 30 XWCORE RP Level (ft) 700 XWSH PPS 600 TAF PV BAF Pr 500 20 400 300 10 200 100 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 time Time in Hours 37
  • 38. Containment Overpressure Failure: ~20.4 hrs Issues:  (1) EOP  C.5‐1200   calls for venting  calls for venting containment at  56 psig (70.7 psia) At ~15hrs. (2) Without  charging, last  battery is depleted depleted ~ 8hrs. 8hrs 38
  • 39. SRVs Possibly Could not be operated >8hrsdue to 335 F equipment qualification uncertaintiesdue to 335˚F equipment qualification uncertainties Drywell, Suppression Pool Temp (F) 3 110 900 800 700 Pressure (psia) PoolTemp 600 DWTemp SRVLimTemp 500 ContFailTemp 400 300 200 100 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 TIME 39 Time in Hours
  • 40. EOP C.5‐1200 Directs Operators to maintain Torus Temperature within Heat Capacity Temperature Limit Temperature within Heat Capacity Temperature Limit by Controlled RPV Depressurization 11/29/2011 jhbickel@esrt‐ 40
  • 41. 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 Batteries (without chargers) will all be depleted by ~7‐8hrs and  ) ill ll b d l t d b ~7 8h d 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 7hrs  venting at ~15hrs ………………but this is 7hrs after battery depletion 7hrs after battery depletion• Venting at this time could allow local manual restarting of RCIC pump in  time to recover water level but there is no DC power but there is no DC power• M i i i Maintaining core cooling is thus dependent on DC power li i h d d DCIn Station AC Blackout: containment venting needs to be considered earlier  to preserve core cooling options. to preserve core cooling options 41
  • 42. Using these insights:• Simulation studies then performed on how to successfully cope  with prolonged SBO but also: Seismic SBO p g• In both cases: no possibility for Torus Cooling no 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 Heatup p 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
  • 43. 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 DC becomes essential to DC maintain  Containment  Vent  Path open for heat removal. 43
  • 44. 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 g sufficient Oxygen to ignite. Thus: we don’t want outside atmospheric air entering containment – so keep  pressurized) 44
  • 45. Initiate Manual Depressurization when Suppression Pool heats up (as in current EOPs)Suppression Pool heats up (as in current EOPs) 45
  • 46. After depressurization at ~8hrs core cooling can be  provided by low press. 100gpm portable pump provided by low press 100gpm portable pump 46
  • 47. What is learned from all this:• Current guidance to use HPCI once and throttled RCIC:  works works• 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• Emergency RPV depressurization as currently in EOPs will allow  d i i l i ill ll use of smaller portable pumps• Successfully executing this long term coping strategy requires Successfully executing this long term coping strategy requires  logistics of staged equipment and supply of “consumables” (fuel staged equipment fuel,  compressed gas) compressed gas 47
  • 48. “Supplemental” Supplemental 48
  • 49. How Long Can NPP Cope  Without Decay Heat Removal? Without Decay Heat Removal?• Coping time is: time from when heat removal is lost to Coping time is: time from when heat removal is lost to  onset of severe core damage (available recovery time)Coping time considers:Coping time considers• Physical inertia built into reactor design via water to  water to  core power ratio• Effects of water makeup systems Effects of water makeup systems• Effects of support features (DC power, HVAC) which  enable water makeup systems enable water makeup systems 49
  • 50. Simplified Decay Heat Model: 50
  • 51. 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 (t)  Q (t) / h 0.15Q / hfg                 (hfg = 640 BTU/lbm.) 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 CORE  WRCIC  WLOSS dt 0.15Qot 0.286 t M CORE (t )  M CORE (0)   (WRCIC  )dt 0 h fg 0.15Qo 0.714  M CORE (0)  WRCIC t  t (0.714)h fg 51
  • 52. After water level recovers, RCIC will stop 52
  • 53. 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 heat At 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 CORE  WLOSS dt 0.15Qot 0.286 t M CORE (t )  M CORE (8hrs )   ( )dt 8 h fg 0.15Qo 0.714  M CORE (8hrs )  (t  8hrs 0.714 ) 0.714h fg 53
  • 54. Behavior of Water after RCIC stops: Slow steady boil‐off dictated by decay heat curve l d b l ff d db d h 54
  • 55. 55
  • 56. 56
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