233.40-2

Reactor, Boiler, & Auxiliaries - Course 233

CONTAINMENT SYSTEM

I. INTRODUCTION TO CONTAINMENT

The containment system is provided specifically tocater for a rupture of the HT system high pressureboundary, and is designed to be able to contain essentiallyall energy (in the form of overpressure and heat) andradioactivity released during the maximum credible pipingrupture of a loss of coolant accident (LOCA).

The system is basically an envelope which containswithin it the high pressure piping components of the HT andHT auxiliary systems. It is rupture of these componentswhich could lead to a large release of HT D20 and activity.

Tritium activity will of course be released insidecontainment on a LOCA with some fission product activityfrom the soluble fission products (mainly iodine) alwayspresent in small concentrations in the HT 020. Largeamounts of fission product activity will not be released ona LOCA unless the ECCS system fails to provide adequatecooling to match the heat production from decay heat, inwhich case, fuel failure is likely. If fuel failuresoccur, then substantial quantities of fission products(mainly gases and vapours) will be released insidecontainment~

The criteria for determining the containment envelopeeffectiveness is the time integrated leak rate during andafter a large loss of coolant accident. This simply meansthere are limits set on the leak rate out of containmentduring an accident situation. The containment is the lastphysical boundary available to prevent the escape ofact i vi ty to the environment if the other barr iers havefailed (fuel pellet, fuel sheath, heat transport piping).

The basic principles associated with the containmentsystem, which is applied by the operator at all times totry to ensure that leak rate limits are met, is to minimizethe leak rate out of containment that may be occurring fromany possible source of leaks.

Depending upon the station the containment system mayactually form an envelope around more components than justthe high pressure HT piping. Older stations in particulartend to have larger containments than newer stations.

July 1981 - 1 -

PICKUING

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Figure 1· Relative Sizes of PNGS/BNGS Containment

Figure 1 shows the smaller size of the Bruce NGS-Acontainment relative to pickering NGS-A containment toillustrate this. The reason for smaller containmentenvelopes is that newer stations have more equipmentlocated outside containment. In particular at Bruceprimary heat transport pumps and steam generators arelocated mainly outside of, but penetrating, containment.Figure 6 shows an example of a Bruce NGS-B steam generatorcontainment penetration seal. In addition, at Bruce, thereare no areas normally accessible on power within contain­ment. l\t Pickering, however there are various shieldedareas within containment which can be entered duringreactor operation. This feature is one of the reasonsPickering has a larger containment volume than Br~ce.

II. TYPES OF CONTAINMENT SYSTEMS

Two major types of containment are used on large CANDUreactOrs:

(a) Pressure Suppression System

(b) Vacuum Containment System

The first type is used on large single-unit plants,eg, Douglas Point and the Candu 600 MW(e). The seco'ld typeis used on the multi-unit plants, eg, Pickering N:;S, BruceNGS, and Darlington NGS.

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(a) Pressure Suppression System

Pressure Suppression containment is illustratedfor the Candu 600 MW(e) system in Figure 2. Thereactor building, have thick (1-2 m) concrete wallsand a thick steel dome, is designed to withstand aninternal pressure of 124 kPa(g) and to leak not morethan 0.5% of total volume/day. The Douglas Pointspecifications are less severe due to the smallerreactor size). The walls and dome in this system arethe containment 'envelope' around the HT systems.

To suppress the pressure surge following a LOCA,light water spray dousing inside containment is usedfrom a large water storage facility built into thereactor building as shown.

Dousing provides a number of functions during aLOCA:

(1 ) It absorbs heat energythereby suppressing thecontainment.

from thepressure

lostrise

coolantinside

(2) It also reduces the time period of theoverpressure excursion within containmentcompared to the time for overpressure if nodousing were used.

(3) The dousing water will entrain insoluble fissionproducts released into the vacuum building, andhelp to prevent contamination spread by retainingthese fission products on the vacuum buildingfloor. Soluble fission products such as 1-131will be dissolved to a certain extent in thedousing water, but noble gases such as Kryptonand Xenon will be unaffected by dousing, and veryprone to atmospheric release in the event of acontainment leak.

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233.40-2= • __ •

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1 MAIN STEAM SUPPL V PIPING2 BOILERS'J MAIN PRIMARY SYSTEM PUMPS.I CAlANDRIA ~SSEM8LY

5 FEEDERS6 FUEl CH"NNEL A5SEM6L)7 DOUSI~~G N." rEP SU!'P,','8 CRANE R"!l~

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Figure 2: Reactor aui Id ing Cu taway (Pressu rcSuppression Containment System)

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The capital cost of containment at the presenttime, favours independent containment for up to twounit stations. Three or more units are requiredbefore the multi-unit "vacuum" containment (below) canbe justified economically. All the presentlycommitted plants using 600 MW(e) units employ only oneof two such units with independent pressuresuppression containment.

(b) Vacuum Containment System

This system a unique Canadian design, sometimescalled a negative pressure containment system, is afeature of our multi-unit Candu Stations. Theprinciple involved in negative pressure containment isthat within minutes after any accident resulting inoverpressure in a reactor building, the pressurewithin the containment boundary would be below that ofthe surrounding atmosphere. Hence, after the initialoverpressure transient, no outward leakage would takeplace. The complete system consists of the reactorbuildings, the vacuum building, the pressure reliefsystem, the vacuum system and the dousing system.(See Figure 3 and 4.)

PICKERING GS A PICKERING GS B

Relief Louvres

Vacuum Ducts

Pressure Relief Duct

Vacuum Building

Figure 3: Major Components of Vacuum ContainmentSystem at Pickering NGS

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The reactor build ings at Pickering and Bruce use con­ventionally reinforced containment structures, cylindricalat Pickering NGS and rectangular at Bruce NGS (to scale inFigure 1). The PNGS reactor buildings are connected via apressure relief duct to the large cylindrical vacuum build­ing, see Figure 3. The Bruce NGS buildings are joined bymeans of an underground fuelling machine duct, which itselfis joined by an underground duct to the vacuum building.In both stations, the vacuum building is isolated from thereactor building and relief duct by pressure relief valves.

The containment volume at Bruce is smaller than atpickering, as mentioned earlier, because the stearn gener­ators and some other equipment eg, primary pump motors arelocated outside containment at Bruce unlike pickering.This means that less air is carried with any escaping steaminto the vacuum building, so that the vacuum building canalso be smaller at Bruce than at pickering despite the 50%higher reactor power of the Bruce units.

An advantage of the vacuum system over the pressuresuppression system is the reduced requirement on leaktightness, because of the shorter time for which the over­pressure exists after a LOCA.

Operational Features

Detailed operation of vacuum containment will not bediscussed in this courSe but some of the basic features arediscussed below.

During normal operation, the vacuum building is main­tained at 10 kPa(a) pressure, and the reactor building andpressure relief duct up to the pressure relief valves iskept subatmospheric (-0.3 kPa(g) PNGS, -2.5 kPa(g) BNGS) tominimize outleakage. The D20 vapour recovery system main­tains this subatmospheric pressure.

Containment system operation, ie, the opening of thepressure relief valves and subsequent onset of dousing, istriggered by high pressure in the boiler room/reactor vaultarea. The pressure rise itself will open the pressurerelief valves (typically at 4 kPa(g)) in the pressurerelief duct. No pressure sensing instrumentation thereforehas to be relied upon for the opening of the pressurerelief valves under a loss of coolant accident condition.Typical pressure transients in the vacuum building and inthe ~eactor building are shown in Figure 5.

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140Reactor Building

120

Instrumented PRV's.,-......_- ModulatingPRY's

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50 Vacuum Building------__Dousing

Starts

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40 60 80 100Time from LOCA -Is)

Figure 5: Typical Pressure Transients in Reactocand Vacuum Buildings Following a LOCA,(50% HT D20 Loss).

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As the pressure rises in the vacuum build ing due tothe entrance of steam and air, dousing begins by syphoningaction when the vacuum build ing pressure is large enough('\..40 kPa(a)). Pressures in both the vacuum building andreactor bUilding peak "'v 20 to 30 seconds after the eventbegins (depending on the size of the LOCA) then decrease.Within a few minutes the pressures should stabilize in thereactor building and vacuum building. When the reactorbuilding pressure falls back to 'V 10 kPa(g) most of thepressure relief valves close. The remaining pressurerelief valves (designated as instrumented PRV 1 s) can beopened and closed by remote manual cont rol to enable thepressure in the reactor building to be maintained slightlybelow atmospheric pressure. This can be done using theremaining vacuum in the vacuum building which should bebelow atmospheric pressure.

The final subatmospheric pressure of the vacuum build­ing will depend on the amount of D20 steam released in theLOCA and the amount of condensation achieved by vacuumbuilding dousing. The more steam released, the higher thefinal vacuum building pressure will be.

III. IMPORTANT CONTAINMENT CONSIDERATIONS

Some important general features of containment are nowdiscussed:

(a) Types of Containment Penetrations

(i) Permanently Installed Equipment penetration

The concrete and steel containment structurehas to be penetrated by a number of items. Theseinclude:

(a) various permanently installedsupplies such as service air,air, electrical power and control

equipmentinstrumentcables.

( b) piping connections and processinstrument lines (Bruce) forsystems located partially insidement.

parameternumerouscontain-

(c) system components such as primary pumps andboilers, eg, Bruce NGS-A.

All these penetrations have specially de­signed seals to maintain containment integrity,ie, prevent leakage. For example the large pene­tration at Bruce for the steam generators, shownin Figure 6, is located around the tube sheetarea of the boiler by a specially designed seal.

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Note that the boiler tubes containing HT 020 arephysically outside containment, and that thetubes themselves are the containment envelope atthat location. This is considered acceptable asa large LOCA due to catastrophic rupture of largenumbers of boiler tubes simultaneously isextremely unlikely.

At Pickering, the main steam lines andboiler feedwater inlet lines penetrate contain­ment. Even though boiler tubes here are physic­ally still inside containment, large scale boilertube rupture would, as at Bruce, effectivelyviolate the containment principle, by allowing HT020 and possibly fission product activity to leakinto the steam-feedwater loop outside contain­ment. The only way to eliminate such a paththrough containment would be to place the wholesteam-feedwater system wi thin containment.Obviously this is not practical nor, fortunately,necessary.

In the case of hot high pressure heat trans­port system process parameter instrument lines atBruce, their penetration of containment alsoappears to violate the containment principle.However these lines are considered to produceonly minor leaks in the event of a rupture. Inorder therefore to have accessibility to themwithout having to enter containment and also tominimize the probability of contamination spreadoutside containment they are located in instru­ment rooms with closed doors, outside contain­ment.

(ii) Equipment and Personnel Airlocks

The purpose of these is to provide for move­ment of personnel and equipment into and out ofcontainment.

The airlocks from part of the containmentenvelope, and consist of a pair of doors withinflatable seals, separated by a passageway.Electric and/or pneumatic interlocks allow onedoor only to be opened at one time, the otherremaining closed to maintain containmentintegri ty.

The closed or open status of airlock doorsis monitored in the control room. A break ofcontainment via an airlock door fault would beindirectly indicated by a decrease of reactorbuilding 6P with respect to atmospheric pressure.

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(iii) Fuel Bundle TransferMachine and 'FuellingOut of Reactor Vault

Into and Out of...--fue1l..-i!!3­Machine Transfer Into and

New fuel bundles are transferred into thefuelling machine from the new fuel'bundle loadingmechanism, and irradiated fuel bundles are trans­ferred out of the fuelling machine into theirradiated fu-el transfer mechanism. At each ofthese mechanisms, provision is made by means ofseals and interlocks so that containment is notviolated during fuel transfer operations.

The fuelling machine service room is part ofcontainment while the fuelling machine is intransit from the service room to the vault forrefuelling. While the F/M is in the servicearea, the fuelling machine service room mayormay not still be part of the containment system,depending upon whether there is a containmentseal provided for this room.

(iv) Penetration of Containment by the 020 Vapou~

Recovery System

The purposes of the vapour recovery systemare:

to recover D20 leakage from reactor areaatmosphere, and hence to reduce D20 losses andtritium contamination in the D20 areas.

to maintain 020 areas at a pressure less thannon-020 areas by exhausting dried air to thecon tarnina ted exha ust stack.

There may be several recovery systems serv­ing various D20 areas, depending on station lay­out, eg, at Bruce there are reactor vault,fuelling duct, and central fuelling area vapourrecovery systems. Each system consists of a loopof ducting from the containment area being dried,to the vapour recovery driers, a-nd back to thedried areas (Figures 7 an-d 8). Under normaloperation, a continual exhaust to atmosphere, viathe contaminated exhaust stack, using a purge orexhaust drier keeps pressure within the driedareas slightly less than atmospheric pr:essure.This minimizes outward leakage of activity. Thevapour recovery system penetrates containment andbecause of this the system isolation points(dampers/valves) automa tica 11y close in order toisolate containment from the environmpnt I on

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Vault Cooling System

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condi tions which could releaseenvironment. This feature,containment "button Upll or "boxin more detail ~n (b) below.

activity to thereferred to asup II is discussed

(v) Penetration of Containment by the Reactor Build­ing Ventilation System (Pickering)

Under normal operating condi tions, thereactor building ventilation system, by means ofexhaust fans, circulates air through the ooo-D20areas on a once-through basis, and exhausts it upthe stack. These 000-D20 areas are normallyaccessible during operation. Exhaust fans, seeFigure 8, maintain the noo-D20 areas at slightlyless than atmospheric pressure to minimize out­leakage, although the pressure will still begreater than that in the 020 areas. This systemthen is open to the environment during normaloperation, see Figure 8.

Under conditions which could release activ­ity to the environment, the ventilation systemwill be isolated from the environment by theclosing of inlet and outlet isolation dampers inorder to box up containment. Details of this arediscussed in (b) below.

(b) Containment Box UpBruce NGS (Figure 7)

At Bruce NGS, because the vapour recovery sys ternpenetrates containment, automatic closing of therecovery system isolation points is necessary when .qnyof the following occurs:

- high containment activity- high containment pressure- loss of stack activity monitoring.

The isola tion points are dampers (i) at the exhaus t toatmosphere on the purge drier outlet and valves (i) onthe drier condensate line, leading to the condensatecollection tank. Note that the driers, condensatesystem, and system piping up to the isolating pointsare an integral part of containment. On containmentbox up, the above dampers and valves automaticallyclose.

(i) Usually two in series are used for reliabili ty.

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Pickering NGS (Figure 8)

At Pickering NGS the principle of box up is thesame as at Bruce NGS but design differences result indifferent isolation points. This is because, unlikeBruce, the Pickering ventilation system, which servesthe non-D40 areas within containment, shares itsexhaust wlth the vapour recovery system exhaustdriers. Also the main driers are physically insidethe reactor building containment structure. However 1

the containment envelope extends to the isolationdampers in the inlet and outlet lines connected to theexhaust driers, located physically outside the reactorbuilding. In addition, isolation for the condensatesystem is provided by a U loop water seal, which isalways boxed up in the sense that the loop seal isalways intact. This seal is designed to withstandpressure surges associated with LOCAls.

On containment box up, the following isolationdevices close:

inlet and outlet dampers in the exhaust drierloop external to the reactor building.

outlet isolation damperson the combined vapourexhaust.

leading to the filterrecovery/ventilation

inlet isolation dampers of the ventilationsystem.

(c) Reactor Building Cooling System (Vault Cooling System)

This system is designed to cool and dehumidifythe reactOl:" building atmosphere, particularly in thereac tor va ul t. Typica I max imum tempera ture and dew­point limits are 'V 40°C and 'V-IaoC respectively.Atmospheric heating is via heat losses (sometimescalled ambient losses) from hot equipment such asboilers, pumps, motors and pipes.

The system, located inside containment, consistsof air cooling units cooled by service water andsupplied with air by fans. During normal opera.tion,no condensation is expected on the coolers, althoughhigh D20 and H20 leakage rates will result in somecondensa tion.

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233.40-2

During a LOCA, however, this cooling system has acontainment function, namely to provide a heat sink inthe reactor building to condense steam. It can removetypically up to 1% reactor thermal power. This isadequate to match decay heat a few hours after shut­down. This heat sink would be particularly importantin the absence of any other heat sink, eg, the ECCSheat exchanger, even though the heat transfer from thefuel to the vault atmosphere, and hence to thecoolers, is not very effie ient. Therefore anyreduction in the vault cooling capacity is consideredto contribute to containment unavailability, seebelow.

(d) Containment Unavailability

Safety systems, of which containment is anexample, are required to meet certain unavailabilityta rgets. For example, a t Bruce the target is 10-3 .The units of this number ari years per year. There­fore this means containment should not fail (beunavailable) for longer than 10- 3 years (~8 hours) inone year.

A containment failure (sometimes called a loss orbreach of containment) is an event which would impairthe operation of containment during a LOCA. Contain­ment failure during a LOCA might result in the releaseof activity to the environment or to the stationaccessible areas.

Highly reliable containmentimportant. Examples of containmentbelow. Notice that not all thesecally involve leakage as such.

is thus veryfailure are givenfailures specifi-

Examples of containment failures are:

- failures of automatic containment isolation mechan­isms to button up (note this may be due to failureof activity monitors and containment pressureswitches, as well as to damper or valve failure).

- failure of airlock integrity allowing air passagethrough the airlock.

- failure of more than a specified number of pressurerelief valves to operate.

- loss of dousing capability.

- leak paths in containment.

- loss of reactor vault cooling capability.

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Containment components and subsystems are testedperiodically to demonstrate that the containmentreliability target 'is met~ Examples of such tests arethe airlock pressure test, damper leak test and pres­sure relief valve tests. Any actual leaks not detect­ed by such routine tests may be detected by a rise incontainment pressure, which is monitored in the con­trol room.

ASSIGNMENT

1. State the importance of the following systems withregard to containment:

- reactor building D20 vapour recovery system- reactor ventilation system- reactor building cooling system.

2. For your own plant, Iaca te on a f lowsheet the conta in­ment isolation points for the systems of question 1.

3. StatewhicL

one specific examplecould lead to a hazard

of containment failure

(a) to the environment, and(b) to plant equipment

if a LOCA occurred.

4. State what the containment reliability target is foryour own plant. From your station's last 4 quarterlyreports, find what the sources of containment unavail­ability were, and what the actual unavailability was.

5. List three conditions which initiate contain~ent but­ton up.

D.J. Winfield

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