Design Features of ACR in Severe Accident Mitigation

H. Shapiro, V.S. Krishnan, P. Santamaura, B. Lekakh and C.BlahnikACR Process, Safety and Risk Assessment

Presented to NEA/IRSN Workshop on Evaluation of Uncertainties in Relation to Severe Accidents and Level 2 Probabilistic Safety Analysis

Aix-En-Provence, France, 7-9 November 2005

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Presentation Outline

• Severe Accident Management (SAM)• ACR Design• ACR SAM-related design features• Conclusions

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Severe Accident Management (SAM)

• Actions to:− Prevent core damage, terminate progress of core damage and

retain core within vessel− Maintain containment integrity− Minimize offsite releases

• ACR core damage characteristics different from pressure vessel reactors

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ACR ACR Design Overview• Horizontal fuel channels

surrounded by heavy water moderator

• Slightly enriched uranium fuel and light water reactor coolant

• Two fast acting, fully capable, diverse and separate shutdown systems physically and functionally independent

• Emergency Core Cooling consists of emergency coolant injection (ECI) system and long term cooling system (LTC)

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ACR ACR Design Overview • Reserve Water System

− emergency source of water by gravity to the steam generators, moderator system, shield cooling and heat transport system

• Secondary control area − monitoring and control capability to

shutdown reactor and maintain the plant in a safe shutdown state following events that render the main control room unavailable

• Electrical power systems supply− safety-related portions are seismically

qualified and consist of redundant divisions of standby generators, batteries, and distribution to the safety-related loads.

SG

RESERVE WATER TANK

SPRAYHEADERS

SUMP SUMP

LTC GRADELEVEL TANK

LTC GRADELEVEL TANK

TO LTC TO LTC

REACTOR

SG

HTS M

AK

E-UP

HTS M

AK

E-UP

MO

DER

ATO

R M

AK

E-UP

CA

LAN

DR

IA VA

ULT M

AK

E-UP

EMER

GEN

CY

FEEDW

ATER

EMER

GEN

CY

FEEDW

ATER

RIH RIHROH ROH

ONLY ONE LOOP OF HTS IS SHOWN

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ACR Design Overview • Containment system: strong

structure, (steel lined and low leakage) with isolation, hydrogen control and heat removal

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ACR Core Damage States

• Limited Core Damage (CANDU-specific)

• In-Calandria Core Damage (analogous to In-Vessel Core Damage in ALWRs)

• Ex-Calandria Core Damage (generic to all designs)

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Prevention of Core Damage

• Conventional AM provisions− Keep inside of pressure tubes flooded with liquid water to

prevent fuel and pressure deformations• HTS depressurization (Engineered and Inherent

features)− Two independent, reliable, engineered systems to

depressurize HTS to below ECIS injection pressure using MSSVs

− Inherent “thermal fuse” (pressure tube failure) allows flooding of fuel by passive water supplies from ECIS or RWS

• SAMDA − Engineered emergency cross-connections of safety grade

systems

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Core Retention Design Provisions

• 2 vessels to retain hot core− Fuel channels− Calandria

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Retention in Fuel Channels

• Fuel channels depressurized before fuel heats up

• Heat rejection paths established by fuel and pressure tube deformations while the fuel is still solid

• Channels submerged in liquid water ensures retention of hot, solid fuel debris within the fuel channels.

NOMINAL

SAGGED with SLUMPED FUEL BALLOONED with SLUMPED FUEL

calandria tube

pressure tube

fuel element

annulus gap

approximately to scale

H2O coolant

D2O moderator

SAGGED with DEFORMED FUEL BALLOONED with DEFORMED FUEL

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SAM Design Provisions

• 2 steam relief paths• Water make-up by gravity from RWS• Level measurement in calandria and

RWS tank• Means to control calandria water

make-up• Cross sections for alternate services• SAMDA – calandria vessel design

optimization to reject heat to shield water

SUSPENDEDDEBRIS

properties& mobility

DEBRIS-WATERINTERACTIONS

GAS FLOWPATTERNS &HYDROGEN

TERMINAL DEBRISproperties & coolability

CHANNELFAILURE

BROKENCHANNEL

dissassembly

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Retention in Calandria Vessel

• Strong calandria vessel• Outer walls in contact with

water• Passive heat sink provided

by RWS

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Core Damage Termination

• Termination by flooding core materials with water and keeping them flooded thereafter.

• Design provisions for termination in:− Fuel channels (LTC from sumps; RWS water by gravity)− Calandria vessel (RWS water by gravity)− Calandria vault (ACR layout meets EPRI core debris

spreading criterion)• SAMDA

− Recirculate sump water through RWS tank

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Containment Integrity Maintenance

Challenges:• Pressurization• Flammable gas control• Core concrete interaction

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Prevention of Containment Pressurization

• Design features:− Strong containment− Containment sprays− Local Air Coolers to condense steam (SG vault, RB dome,

moderator room and other locations)− Post-accident heat sinks for HTS, calandria vessel, and shield

water to stop steaming into containment• SAMDA

− Containment venting

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Prevention of Containment Pressurization Sprays

RWT (2500 m3 Total)

1000 m3

REACTOR

SPRAYHEADERS

SPRAYHEADERS

SUMP SUMP

LTC SYSTEM

LTC GRADELEVEL TANK

LTC GRADELEVEL TANK

FROM ROH(SDC)

FROM ROH(SDC)TO RIH

(SDC)

TO RIH(ECC)

LTC SYSTEM

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Prevention of Containment PressurizationLocal Air Coolers

SG VAULTENCLOSURES

SG VAULTLACS

FANS

COOLINGCOILS

RWTDOMELACS RWT

REACTOR

SG VAULTLACS

FANS

COOLINGCOILS

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Offsite Release Minimization

• Isolated ACR containment is leak tight (less than 0.2% per day at design pressure)

• Containment structure has a large margin (more than twice the design pressure)

• Fail safe design• SAMDA

− Containment venting

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Flammable Gas Control

• Post-accident hydrogen control strategy− Remove hydrogen from containment atmosphere− Prevent hydrogen detonation

• Design provision− Passive autocatalytic recombiners and igniters− Containment structure designed to provide forced and natural

circulation mixing

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Conclusions

• Severe accident management design provisions ongoing from early stages of ACR design

• Active heat sinks for process vessels capable of operating under severe accident conditions

• Passive backup heat sinks will provide SAM more than 1 day to diagnose accident and establish ultimate heat sink

• Robust containment design

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