Severe Core Damage Progression within a CANDU 6 Calandria Vessel

1
Severe Core Damage Progression within a CANDU 6
Calandria Vessel

• P.Mani Mathew,T. Nitheanandan and S. Bushby
• Head, Severe Accidents Section
• Reactor Safety Division
• AECL Chalk River, Canada
• ERMSAR 2008
• 23-25 September 2008
• Nesseber, Bulgaria

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Introduction

• Severe Core Damage Accident
• Accident in which substantial damage is done to
  the reactor core structure whether or not there
  are serious off-site consequences
• Reactor Cooling System and Moderator back-up heat
  sinks are unavailable in a CANDU 6.
• This presentation on phenomenology/progression

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CANDU-6 Reactor Core
4
CANDU Fuel Channel
Severe Core Damage when Reactor Cooling System
and Moderator heat sink are lost in current CANDUs
5
Severe Core Damage Progression

• Slow progression of Severe Core Damage in CANDU-6
• Significant quantity of water surrounds the core
• Moderator Plays an Important role as a Heat Sink
  in LOCA/LOECC (Design Basis)

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LOCA/Loss of ECC but Moderator Heat Sink Available

• Primary system depressurizes, cooling to fuel
  reduced
• Fuel heats up, deforms and transfers heat to
  pressure tubes
• Pressure tubes heat up and sag into contact with
  calandria tubes
• Heat from fuel is removed by moderator
  circulation system
• Core geometry is maintained, but Fuel can be
  severely damaged
• Moderator Plays an Important role as a Heat Sink

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In-Vessel Core Damage

• Loss of Coolant Events with ECC and Loss of
  Moderator Heat Sink
• Fuel Channels Heat Up
• Moderator Boils Off
• Core Disassembly Occurs
• Debris relocate to water-cooled Calandria Vessel
  Bottom
• Reactor Vault Cooling and Make-up Water systems
  Play an Important role as a Heat Sink

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LOCA-LOECC, loss of Moderator heat sink

• Typical sequence of events
• Primary system depressurizes, cooling to fuel
  reduced
• Fuel heats up, deforms and transfers heat to the
  pressure tube
• Pressure tubes heat up and sag into contact with
  calandria tubes
• Heat load from fuel channels slowly boils off the
  moderator
• Uncovered fuel channels gradually collapse, break
  up and are quenched in remaining moderator
• After all moderator is expelled, debris bed heats
  up
• Reactor vault water inventory keeps calandria
  vessel intact
• RCS inherently depressurized before Core
  Disassembly

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A schematic showing the uncovery of top fuel
channels following moderator expulsion
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IN VESSEL SCD ACCIDENTSCHANNEL DISASSEMBLY

• CHANNELS BREAK UP BY SAGGING
• Analyses Small Scale Tests

submerged channels support uncovered channels
MODERATORLEVEL TRANSIENTGOVERNS RATE
OFDISASSEMBLY
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IN VESSEL SCD ACCIDENTSSUSPENDED DEBRIS
steam can accesshot, unoxidized Zrsurfaces in
annulus
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IN VESSEL SCD ACCIDENTSSUSPENDED DEBRIS

• suspended debris mass builds up with time
• steam access into debris interior more difficult
  with time
• debris weight supported by first submerged row of
  calandria tubes
• load-bearing capacity of CT is not unlimited

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A schematic showing the various phenomena inside
the calandria vessel during the transient
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A schematic showing the collapse of the core into
the residual water below
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Consolidated terminal debris bed, beginnings of
molten corium formation near the top surface and
the evolution of natural circulation in the
reactor vault water
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Formation of solid crust surrounding molten
corium on the cooler surfaces of the calandria
vessel
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Core Disassembly Test Facility Test Chamber
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Schematic of Channel Layout

• Small scale tests underway, 1/5 scale
• 12 heaters to simulate fuel bundles of a CANDU 6
  channel

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Three Channel Test
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Post-test Close-up View of Channel Break-up
(CD-10)
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CANDU-6 Reactor Core
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MAAP4-CANDU Code

• MAAP4 CANDU has models of horizontal CANDU-type
  fuel channels and CANDU-specific systems such as
  Calandria Vessel, Calandria Vault, Reactor
  Cooling System (Primary Heat Transport System),
  Containment Systems such as Dousing, LACS, etc.
• Can assess influence of Severe Accident
  Management strategies to mitigate and recover
  from an accident state
• Sequences, resulting in severe core damage, that
  can be simulated by MAAP4-CANDU
• Station Blackout sequence
• Large LOCA
• Small LOCA
• Steam Generator Tube Rupture
• Feeder Stagnation Break
• Main Steam Line Break

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SCD ACCIDENTSMAAP4-CANDU CORE
COMPLEX NODALIZATION FOR CORE DISASSEMBLY

• Channels heat up break up at different rates
• Intact channels debris coexist
• Same CV waterlevel in all axial nodes
• Suspended debris mass differs inaxial nodes

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Generic CANDU 6 SBO Analysis Assumptions

• AC power and all onsite standby/emergency power
  unavailable
• Reactor shutdown after accident initiation
• Moderator-, Shield-, Shutdown cooling unavailable
• Main and Auxiliary Feed water unavailable
• ECCS (high, medium and low pressure) unavailable
• Dousing and Crash cool-down not credited
• LACS not available
• No Operator Interventions are credited
• Failure criteria used to fail certain
  components/systems
• No make-up to the Reactor Vault

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UO2 Mass/Loop (Generic CANDU 6 SBO)
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CANDU-6 Reactor Core
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IN VESSEL SCD ACCIDENTSDEBRIS COOLABILITY

• cylinder geometry well suited for external
  cooling flooding
• large surface-to-volume ratio
• surrounded by water jacket
• Make-up to reactor Vault for in-vessel retention

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Summary

• The CANDU core damage progression is slow and
  predictable candling-type of behavior is not
  expected
• MAAP4 CANDU is a useful tool to calculate severe
  accident progression in a CANDU 6 plant
• Corium can be contained in the calandria vessel
  by in-vessel retention