BWR3 Mark I. Dr. John H. Bickel

Transcription

1 Beyond the Design Bases BWR3 Mark I Dr. John H. Bickel

2 BWR3, Mark I DBA Features: Electric di driven Mi Main Feedwater, Condensate Pumps 2 Diesels supply: 4kV power, 480V, 250VDC, 125VDC 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 Safety system designed for Small/Medium LOCA Automatic Depressurization System 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 Control Rod Drive Hydraulic Pumps (~90 200gpm) 2

3 BWR3, Mark I Beyond DBA Features: 2 trains Manual SLCS capable of injecting Sodium Pentaborate Anticipated i t dtransients Without t Scram CST can be replenished with River water or Fire Truck to prolong 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) B.5.b 5bPump (from River) 18 Hard Pipe Vent from Torus to Atmosphere (preferred) 18 Hard Pipe Vent from Drywell to Atmosphere (alternate) 18 equivalent to ~21 MWt decay heat removal (~decay heat after 1 hr) 3

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. ~110psig HardPipeVent installed to vent Torus to atmosphere Limitations: Small, compact, containment volume requires N 2 inerting for DBA LOCA Credits DBA Large LOCA containment pressure to assure ECCS pump NPSH Not designed dfor severe accidents 4

5 Mark I Containment Under Construction 5

6 Upper Reactor Building Refueling 6

7 Upper Reactor Building Operating 7

8 8 Safety/Relief Valves (S/RVs) Features: Dual Action Mechanical spring loaded overpressure safety ft function DC/N 2 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 will likely fail if Drywell Temp. > 325 F 8

9 High Pressure Coolant Injection System: Features: Can inject at full system press. (1050psia) ~1.5x10 6 lbs/hr makeup capability Designed to mitigate small, medium LOCA 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 When Torus used as water source, Temperature < 200 F 9

10 High Pressure RCIC System : Features: Can inject at full system press. (1050psia) ~2x10 5 lbs/hr makeup capability (smaller than HPCI) Designed to provide makeup after shutdown when FW unavailable 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 When Torus used as water source, Temperature < 200 F 10

11 What RCIC Turbine Driven Pump Looks Like 11

12 Station 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 hrs without charging Depletion defined as insufficient voltage to operate critical SOVs 12

13 Containment Vent for Beyond Design Bases Accidents Main Features: 1 from Suppression Pool and 1 from Drywell Redundant air operated butterfly valves (fail closed on loss of air) >56 psig rupture disk to prevent operation below design bases pressure Requires DC power, compressed air to operate 13

14 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 Steam Driven high pressure makeup sources (HPCI, RCIC) not Considered yet CRD Hydraulic makeup not considered yet AC/DC power available, compressed N 2 available Operators follow EOPs and depressurize at TAF reflooding with LPCI to achieve stable water levels Suppression Pool Cooling fails over long term Drywell Chillers, Containment Venting not considered 14

15 Loss of Feedwater with no Recovery, and Loss of HPCI, RCIC first 2 hrs. 15

16 Emergency Depressurization allows Low Pressure Coolant Injection to reflood core 16

17 At this point RPV level is restored and core cooling using injected LPCI flow fromtorus 17

18 BUT: without Torus cooling Containment Temperatures, Pressures will continually rise 18

19 Reclosure of S/RVs ~21.5hrs: causes RPV press. rise above LPCI pump pshutoff pressure 19

20 With no venting, core uncovers ~29hrs while pressure remains elevated at S/RV safety valve press. 20

21 Delayed Core Heatup starts at ~29.5hrs with Zr Water Reaction starting ~30.5hrs 21

22 Containment Failure Likely Occurs at 35 37hrs. Accompanied by H 2 burn of in reactor building 22

23 Consequences would be Large Delayed Release (~30% Core CsI) after Containment Failure 23

24 If unable to vent, how much makeup needed to keep core covered? Match boil off due to decay heat: W BOIL = Q DECAY (26hrs)/Δh Q DECAY (26hrs) = 3.88x10 7 BTU/hr Δh = h sat (1050psia) h CST (80F) ~ BTU/lbm W BOIL = (3.88x10 7 BTU/hr)/( BTU/lbm) = 34,000 lbm/hr or: = (567 lbm/min)(0.016ft 3 /lbm) = 9.07ft 3 /min Converting to gal/min, this is only: ~67.8 gpm Recall: CRD makeup flow to RPV is: gpm Running 1 electric CRD pump could easily keep the core covered for an indefinite periodof of time 24

25 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 likely Diesels fail, DC Batteries begin to discharge All AC powered equipment shuts down Feedwater and condensate pumps Drywell Coolers, room cooling LPCI, Core Spray, CRD pumps cannot be operated HPCI, RCIC potentially available provided RPV steam and DC available for starting, and until room temperatures > 150 F CST available as water source S/RVs potentially ill available to blow down if containment pressure <75 psig, and BOTH: DC power, compressed N 2 available Diesel Fire Pump (portable pumps) potentially available until out of fuel 25

26 HPCI, RCIC start on low level, RCIC throttled suction from CST, and run on local/manual l/ lwithout t DC power 26

27 RPV water level could be maintained until ~16.4 hrs when RCIC shuts down 27

28 RCIC runs until high discharge pressure trip S/RVs unavailable to depressurize and then use Fire Pump DC Power gone after ~7 8hrs, Drywell Temperatures too high ~11hrs 28

29 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 ~16.4hrs Batteries (without chargers) will all be depleted by ~7 8hrs and unavailable to open S/RVs or containment vent valves. Drywell temperatures >325 F limit for S/RV operation at ~11.25hrs Containment pressure reaches >56psig for venting containment at ~17hrs but this is 10hrs 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 In Station AC Blackout: containment venting needs to be performed earlier to preserve core cooling options. Maintaining core cooling is totally dependent d on DC power 29

30 Using these insights: Simulation studies then performed on how to successfully cope with prolonged SBO but also: Seismic SBO Seismic SBO differs from SBO: Offsite power recovery may require weeks vs. hours Possibly destroys CST (requiring i HPCI, RCIC CCsuction from Torus) This causes more rapid Torus Heatup Possibly destroys Diesel Fire Water makeup (buried piping, cast iron piping in buildings) Successful coping depends on: Gasoline powered portable DCbattery chargers (or Security Diesel ) Portable, Gasoline powered pumps Air compressor or bottled gas supply 30

31 Strategy: Break Rupture Disk below 56 psig using bottled gas cylinder and test connection 31

32 Use HPCI once, manually throttle RCIC to maintain RPV level until it stops 32

33 Vent Containment to hold pressure between psig 33

34 Initiate Manual Depressurization when Suppression Pool ~200 F (as in current EOPs) 34

35 After depressurization at ~8hrs core cooling can be provided by 100gpm portable pump 35

36 What is learned from all this: Achieving an indefinite coping time for Seismic SBO requires changing current BWR Severe Accident Management Guidelines Current guidance to use HPCI once and throttled RCIC works Alternate means will be needed for DC Battery charging ~ 5hrs 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 ascurrently in EOPs will allow use of smaller portable pumps Successfully executing this strategy requires logistics of staged equipment and supply of consumables (fuel, compressed gas) 36

37 Supplemental 37

38 How Long Can NPP Cope Without Decay Heat Removal? Coping time is: time from when heat removal is lost to onset of severe core damage (available recovery time) Coping time considers: Physical inertia built into reactor design via water to core power ratio Effects of water makeup systems Effects of support features (DC power, HVAC) which enable water makeup systems 38

39 Simplified Decay Heat Model: 39

40 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: W = = LOSS (t) Q DECAY (t) / h fg 0.15Q o t / h fg (h fg = 640 BTU/lbm.) Assume core output initially at: Q o ~6.84x10 9 BTU/hr Assume when activated: RCIC injects: W RCIC (t) ~2.0x10 5 lbm./hr. Assume initial water mass above core: M CORE ~1.2x10 5 lbm. Behavior of water inventory governed by: dm dt M CORE CORE M CORE W RCIC W LOSS t Q ot ( t ) M CORE (0) ( WRCIC ) dt h 0.15Qo (0) W RCIC t t (0.714) h 0 fg fg 40

41 After water level recovers, RCIC will stop 41

42 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 U2, U3 core output initially at: Q o ~8.13E+9 BTU/hr When RCIC stops: W RCIC (t) ~0.0 lbm./hr. Assume same initial water mass above core: M CORE ~ 1.2E+5 lbm. Behavior of water inventory governed by: dm CORE WLOSS dt M CORE M ( t ) CORE M CORE (8 hrs ) t 0.15Q ot ( h 0.15Qo (8 hrs ) ( t 8 hrs 0.714h fg 8 fg ) dt ) 42

43 Behavior of Water after RCIC stops: Slow steady boil off dictated dby decay heat curve 43

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