CANDU Safety #6 - Heat Removal Dr. V.G. Snell Director Safety & Licensing

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1 CANDU Safety #6 - Heat Removal Dr. V.G. Snell Director Safety & Licensing 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 1

2 Overview the steam and feedwater system is similar in most respects to that of other reactor types 4 steam generators 2 in each heat transport system loop, in series 3 main and 1 auxiliary feedwater pumps overpressure protection provided by 16 Main Steam Safety Valves (MSSVs( MSSVs), 4 on each steam line Atmospheric Steam Dump Valves (ASDVs( ASDVs) ) can dump 10% steam to atmosphere (if condenser is unavailable) Condenser Steam Discharge Valves can dump 100% transient and 60% steady steam to the condenser 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 2

Q! Check Valve INO) :IJ Steam Safety Relief Valve INC) :0l Main Steam Isolation Valves INO) 8 Atmospheric Steam Discharge Valves INC) Condenser Steam Discharge Valves INC) ~ Turbine Stop Valves (NO)

3 Q! Check Valve INO) :IJ Steam Safety Relief Valve INC) :0l Main Steam Isolation Valves INO) 8 Atmospheric Steam Discharge Valves INC) Condenser Steam Discharge Valves INC) ~ Turbine Stop Valves (NO) o Isolation Valves (NO) -.,- ~ Flow Measurement Bypass to To Condenser Turbine To Auxiliary System -0- ND Normally Open NC Normally Closed From L...:F:..:.r:::om::.:..:..F:::ee::.:d::w:,:a:,::te::.:r..::C::o::,:nt:::ro::.:I..:V,::a:.:lv.::,e.:::S,::ta:,::ti:::on:.::s:.....::::::==========::;: }-, Feedwater - \-l Heating System 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 3

To Feedwater System e Valve Normally Open ~Valve Normally Closed From Steam 1 /- ".--... Steam Separator ~ - Steam Reheater Generators + I ---.. -IS~ Low Pressure Turbines ~ 1. L - r---.::...-..,,.

4 To Feedwater System e Valve Normally Open ~Valve Normally Closed From Steam 1 /- " Steam Separator ~ - Steam Reheater Generators + I IS~ Low Pressure Turbines ~ 1. L - r---.::...-..,,..j apump + rri-.l..rl... Generator Main Power Condenser l;:;;: High, Steam (@) Pressur~ I... Output Disc harg e I Turbin e _1-..., Il.WJJJu..w.J.J,l.l<llllIo.wJ..WlIlfllllLU..loOUJ.J.J,lJ.<lllJ.l-l.l.WllJ-I JllJ..U..U.l-lJ.JJu,IJ<.l.llJ..l.'-J..UJ.l.llIl ~ Valves t Ir~:-::::t:;-:::t::-~::~;;::~~~~ CSDV's) \... _ I I /I \. Cooling Bypass Steam "t/~~:gll'~~~~~~~~~i~~~~r.;nr~n,",rl~"""~,n",,"~",,,-,r~~~~~~~~~~~~~~~~='~~~====:::::j.water Condenser Hot Well c:to=st=ea:;;em=g=en=er=ato=rs===*_f-i/\.r... High Pressure Auxiliary Low Pressure Heaters Pump Deaerator Heaters " , " , " ,,, , Condensate ~-v---"1~~c::::.q Feed Pumps 131 Tank Feedwater Heating System Condensate -A..r ~ ~~...,Ex~a~~n--l=:::::;[E;;;;;;;;;;;:::-:J 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 4

5 Steam Removal Main Steam Safety Valves sized to remove 115% of full steam flow (12 / 16 valves) dual actuation: spring loaded for overpressure protection pneumatic actuators for controlled opening used for forced rapid depressurization of steam generators ( crash cooldown ) for: LOCA loss of heat sink 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 5

6 Small LOCA 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 6

7 Forced Depressurization prevents heat transport system pressure in a small LOCA from hanging up at the boiler pressure and blocking ECC allows rapid depressurization of the heat transport system following a loss of heat sink, so low-pressure heat removal systems can be used does not add reactivity (cooldown( reduces reactivity) does not require opening valves on the primary side (converting a small LOCA or a loss of heat sink into a larger LOCA) signals are fully duplicated & diverse (two subsystems) for increased reliability 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 7

8 Steam Main Isolation tradeoff: provide reliable heat removal pathway, or stop escape of radioactivity through leaking boiler tube CANDU has a low radioactive inventory in heat transport system and a good record on boiler tube integrity, so prefer assured heat removal in long term, if there is a leaking boiler tube, can manually isolate the affected boiler through Main Steam Isolation Valve (MSIV) 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 8

9 High Pressure Heat Sinks 3 50% main feedwater pumps, Class IV power 1 4% auxiliary feedwater pump, Class III power (diesels) shutdown cooling system: can remove decay heat and cooldown the heat transport system without requiring depressurization normal operation: cooldown via steam generators to 177 C cooldown via shutdown cooling system from 177 C C to 54 C C and hold it there maintenance: remove decay heat when heat transport system is open for maintenance 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 9

10 Shutdown Cooling System - Emergency can cool down heat transport system from 260 C no depressurization required 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 10

11 Low Pressure Heat Sinks Emergency Water System seismically qualified supplies water to: steam generators heat transport system powered by separate seismically-qualified diesel generators draws from dousing tank or external pond 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 11

12 EWS Reservoir 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 12

13 Low Pressure Heat Sinks - Moderator moderator normally removes 5% thermal power separate pumps and heat exchangers short pathway from fuel to moderator liquid can remove decay heat from fuel without melting the UO 2 following an accident which leaves NO water in the channel (LOCA + LOECC) can remove heat from fuel without failing fuel sheaths after a month or so, with NO water in the channel can be used after a very long shutdown 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 13

14 Moderator 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 14

Feeder Pipes 2 long feeder pipes connected to each channel give a huge surface area with water in the channels, decay heat can be rejected to air in the reactor vault after a long shutdown

15 Feeder Pipes 2 long feeder pipes connected to each channel give a huge surface area with water in the channels, decay heat can be rejected to air in the reactor vault after a long shutdown (without primary side heat removal or circulation) used at Point Lepreau during the long shutdown after wood was left in the boiler 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 15

16 Summary conventional secondary side heat removal (main and auxiliary feedwater pumps) atmospheric discharge of steam for LOCA or loss of heat sink to depressurize steam generators shutdown cooling system which can be used at high pressure low pressure emergency water system to remove heat from steam generators in earthquakes and other accidents moderator as backup to ECC for severe accidents very long-term heat removal to moderator and/or feeder pipes 24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 16

Lecture 7 Heat Removal &

Lecture 7 Heat Removal & Containment Dr. V.G. Snell Nuclear Reactor Safety Course McMaster University Containment R4 vgs 1 Where We Are (still) Deterministic Requirements Experience Chapter 3 Chapter 1 Safety Goals Chapter 6 Probabilistic