24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 1

CANDU SafetyCANDU Safety#6 #6 -- Heat RemovalHeat Removal

Dr. V.G. SnellDr. V.G. SnellDirectorDirector

Safety & LicensingSafety & Licensing

24/05/01 CANDU Safety - #6 - Heat Removal.ppt Rev. 0 vgs 2

OverviewOverviewλλ the steam andthe steam and feedwaterfeedwater system is similar in most respects to system is similar in most respects to

that of other reactor typesthat of other reactor typesλλ 4 steam generators4 steam generators

–– 2 in each heat transport system loop, in series2 in each heat transport system loop, in seriesλλ 3 main and 1 auxiliary3 main and 1 auxiliary feedwaterfeedwater pumpspumpsλλ overpressure protection provided by 16 Main Steam Safety overpressure protection provided by 16 Main Steam Safety

Valves (Valves (MSSVsMSSVs), 4 on each steam line), 4 on each steam lineλλ Atmospheric Steam Dump Valves (Atmospheric Steam Dump Valves (ASDVsASDVs) can dump 10% ) can dump 10%

steam to atmosphere (if condenser is unavailable)steam to atmosphere (if condenser is unavailable)λλ Condenser Steam Discharge Valves can dump 100% transient Condenser Steam Discharge Valves can dump 100% transient

and 60% steady steam to the condenserand 60% steady steam to the condenser

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To AuxiliarySystem

-0-

Bypass to ToCondenser Turbine

-0- -0-

FromL ...: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

ND • Normally OpenNC· Normally Closed

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

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e Valve Normally Open

~Valve Normally Closed

apumpSteamReheater

".--.... SteamSeparator ~-/-

To Feedwater System

FromSteam 1Generators +

I ---.. -IS~ Low Pressure Turbines~ 1. L - r---.::....-.., ,..J

+ rri-.l..rl..... Generator Main PowerCondenser l;:;;: High,Steam (@) Pressur~ I ..... Output

Discharg e I Turbine _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

Condensate

Tank

High Pressure Auxiliary Low PressureHeaters Pump Deaerator Heaters

".---..., ".---..., ".---...,,,.---..., Condensate

c:To=St=ea:;;Em=G=en=er=ato=rs===*_f-I/\.r ........~-v---"1~~c::::.Q -A..r~ ~~.....,Ex~a~~n--l=:::::;[E;;;;;;;;;;;:::-:JFeed

Pumps 131

Feedwater Heating System

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Steam RemovalSteam Removalλλ Main Steam Safety Valves sized to remove 115% of full steam Main Steam Safety Valves sized to remove 115% of full steam

flow (12 / 16 valves)flow (12 / 16 valves)λλ dual actuation:dual actuation:

–– spring loaded for overpressure protectionspring loaded for overpressure protection–– pneumatic actuators for controlled openingpneumatic actuators for controlled opening

λλ used for forced rapid depressurization of steam generators used for forced rapid depressurization of steam generators (“crash(“crash cooldowncooldown”) for:”) for:–– LOCALOCA–– loss of heat sinkloss of heat sink

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Small LOCASmall LOCA

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Forced DepressurizationForced Depressurizationλλ prevents heat transport system pressure in a small LOCA from prevents heat transport system pressure in a small LOCA from

“hanging up” at the boiler pressure and blocking ECC“hanging up” at the boiler pressure and blocking ECCλλ allows rapid depressurization of the heat transport system allows rapid depressurization of the heat transport system

following a loss of heat sink, so lowfollowing a loss of heat sink, so low--pressure heat removal pressure heat removal systems can be usedsystems can be used

λλ does not add reactivity (does not add reactivity (cooldowncooldown reduces reactivity)reduces reactivity)λλ does not require opening valves on the primary side does not require opening valves on the primary side

(converting a small LOCA or a loss of heat sink into a larger (converting a small LOCA or a loss of heat sink into a larger LOCA)LOCA)

λλ signals are fully duplicated & diverse (two subsystems) for signals are fully duplicated & diverse (two subsystems) for increased reliabilityincreased reliability

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Steam Main IsolationSteam Main Isolationλλ tradeofftradeoff::

–– provide reliable heat removal pathway, orprovide reliable heat removal pathway, or–– stop escape of radioactivity through leaking boiler tubestop escape of radioactivity through leaking boiler tube

λλ CANDU has a low radioactive inventory in heat transport CANDU has a low radioactive inventory in heat transport system and a good record on boiler tube integrity, so prefer system and a good record on boiler tube integrity, so prefer assured heat removalassured heat removal

λλ in long term, if there is a leaking boiler tube, can manually in long term, if there is a leaking boiler tube, can manually isolate the affected boiler through Main Steam Isolation Valve isolate the affected boiler through Main Steam Isolation Valve (MSIV)(MSIV)

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High Pressure Heat SinksHigh Pressure Heat Sinksλλ 3 3 ×××××××× 50% main50% main feedwaterfeedwater pumps, Class IV powerpumps, Class IV powerλλ 1 1 ×××××××× 4% auxiliary4% auxiliary feedwaterfeedwater pump, Class III power (diesels)pump, Class III power (diesels)λλ shutdown cooling system:shutdown cooling system:

–– can remove decay heat andcan remove decay heat and cooldowncooldown the heat transport the heat transport system without requiring depressurizationsystem without requiring depressurization

–– normal operation:normal operation:λλ cooldowncooldown via steam generators to 177via steam generators to 177°°°°°°°°CCλλ cooldowncooldown via shutdown cooling system from 177via shutdown cooling system from 177°°°°°°°°C to C to

5454°°°°°°°°C and hold it thereC and hold it there–– maintenance: remove decay heat when heat transport maintenance: remove decay heat when heat transport

system is open for maintenancesystem is open for maintenance

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Shutdown Cooling System Shutdown Cooling System -- EmergencyEmergencyλλ can cool down heat transport system from 260can cool down heat transport system from 260°°°°°°°°CC

–– no depressurization requiredno depressurization required

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Low Pressure Heat SinksLow Pressure Heat SinksEmergency Water SystemEmergency Water Systemλλ seismically qualifiedseismically qualifiedλλ supplies water to:supplies water to:

–– steam generatorssteam generators–– heat transport systemheat transport system

λλ powered by separate seismicallypowered by separate seismically--qualified diesel generatorsqualified diesel generatorsλλ draws from dousing tank or external ponddraws from dousing tank or external pond

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EWSReservoir

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Low Pressure Heat Sinks Low Pressure Heat Sinks -- ModeratorModeratorλλ moderator normally removes 5% thermal powermoderator normally removes 5% thermal powerλλ separate pumps and heat exchangersseparate pumps and heat exchangersλλ short pathway from fuel to moderator liquidshort pathway from fuel to moderator liquidλλ can remove decay heat from fuel without melting the UOcan remove decay heat from fuel without melting the UO2 2

following an accident which leaves NO water in the channel following an accident which leaves NO water in the channel (LOCA + LOECC)(LOCA + LOECC)

λλ can remove heat from fuel without failing fuel sheaths after a can remove heat from fuel without failing fuel sheaths after a month or so, with NO water in the channelmonth or so, with NO water in the channel

λλ can be used after a very long shutdowncan be used after a very long shutdown

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ModeratorModerator

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Feeder PipesFeeder Pipesλλ 2 long feeder pipes connected to 2 long feeder pipes connected to

each channel give a huge surface each channel give a huge surface areaarea

λλ with water in the channels, decay with water in the channels, decay heat can be rejected to air in the heat can be rejected to air in the reactor vault after a long shutdown reactor vault after a long shutdown (without primary side heat removal (without primary side heat removal or circulation)or circulation)

λλ used at Pointused at Point LepreauLepreau during the during the long shutdown after wood was left long shutdown after wood was left in the boilerin the boiler

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SummarySummaryλλ conventional secondary side heat removal (main and auxiliaryconventional secondary side heat removal (main and auxiliary

feedwaterfeedwater pumps)pumps)λλ atmospheric discharge of steam for LOCA or loss of heat sink atmospheric discharge of steam for LOCA or loss of heat sink

to depressurize steam generatorsto depressurize steam generatorsλλ shutdown cooling system which can be used at high pressureshutdown cooling system which can be used at high pressureλλ low pressure emergency water system to remove heat from low pressure emergency water system to remove heat from

steam generators in earthquakes and other accidentssteam generators in earthquakes and other accidentsλλ moderator as backup to ECC for severe accidentsmoderator as backup to ECC for severe accidentsλλ very longvery long--term heat removal to moderator and/or feeder pipesterm heat removal to moderator and/or feeder pipes