Diapositive 1

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EUROTRANS WP1.5 Technical MeetingTask 1.5.1
Safety approach
Sophie EHSTER

• Madrid, November 13-14 2007

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Contents

• Task T1.5.1 progress
• Main features of the gas-cooled EFIT safety
  approach
• "Dealt with" events
• "Excluded" events

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Task T1.5.1 Progress

• D1.20Approach and acceptance criteria for
  safety design of XT-ADS
• Issued in August 2006
• Update when design is confirmed
• D1.21Approach and acceptance criteria for safety
  design of EFIT
• Issued in August 2007
• Addresses generic ETD (LBE cooled)
• T1.5.1 covers also gas-cooled EFIT back up option
• Task to be performed in 2008
• AREVA contribution to be included in gas-cooled
  EFIT specific report
• Main features of the gas-cooled EFIT safety
  approach are provided hereafter

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Main features of gas-cooled EFIT safety
approach-1

• General safety objectives and principles are the
  same as the ones defined for lead-cooled EFIT, in
  particular
• Application of defense in depth principle
  prevention and mitigation of severe core damage
  (i.e. large degradation of the core) are
  considered,
• Severe core damage is considered since it reveals
  specific risks of the technology which have to be
  dealt with.
• EFIT is provided with a core loaded with minor
  actinides
• Potential consequences of severe core damage are
  expected larger as the amount of minor actinides
  in the core increases (e.g., lower fraction of
  delayed neutrons, lower Doppler effect, lower
  critical mass).

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Main features of gas-cooled EFIT safety approach-2

• Definition of the sub-criticality level core
  shall remain sub-critical in any event.
  Concerning severe core damage, criticality could
  occur if consequences are demonstrated
  acceptable.
• Prevention of severe core damage sequences
  leading to severe core damage shall be extremely
  rare. The confidence in the prevention provisions
  must be very high.
• Issues/gas-cooled design
• Limited operational feedback available (HTR
  technology),
• High reliability requested for beam trip system
  and DHR system (mainly active means),
• Very short grace periods.
• Advantages/gas cooled design
• No significant void effect,
• ISI facilitated by He environment.

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Main features of gas-cooled EFIT safety approach-3

• Severe core damage mitigation has to be
  considered. In particular, prompt criticality has
  to be excluded. This could lead to the limitation
  of content of minor actinides and/or to a
  lowering of the sub-criticality level. At the
  pre-conceptual design phase of EFIT, studies
  associated with severe core damage should focus
  on the determination of the main phenomena,
  relevant risks and possible mitigating provisions
  (core design and dedicated systems).
• Regarding severe situations which cannot
  mitigated by plant design (not technically
  possible a reasonable cost or with an
  insufficient confidence level), a specific
  demonstration showing that the associated risk is
  acceptable has to be provided
• Practices similar as the ones implemented for
  future nuclear plants such as EPR at least can be
  considered,
• If the situations are not physically impossible,
  their occurrence has to be made sufficiently rare
  in order to not consider the consequences of the
  situation in the design. This will mainly rely on
  the implementation of a sufficient number of
  diverse practical prevention provisions. The
  adequacy of provisions can be justified by
  probabilistic insights.

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Main features of gas-cooled EFIT safety approach-4

• The demonstration that the objectives related to
  severe core damage prevention are met can be
  performed by means of Probabilistic Risk
  Assessment. The cumulative severe core damage
  frequency should be lower than 10-6 per reactor
  year.
• At the early stages of EFIT, the Line Of Defense
  (LOD) method can be used to provide adequate
  prevention of severe core damage at least two
  "strong" lines of defense plus one "medium" LOD
  are requested for each sequence.
• Unique EFIT safety issues have to be considered
  such as the potential radiological impact on the
  public due to minor actinides and spallation
  products and such as the protection of workers
  with respect to target and accelerator.

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"Dealt with" events

• "Dealt with" events
• Their consequences are considered in the design
• A list of initiating faults and associated
  sequences to be studied has been determined in
  PDS-XADS contract for a similar design (except
  for Power Conversion System water-steam,
  super-critical CO2 cycles). This list has to be
  updated.
• Preliminary list of limiting events
• Bounding reactivity insertion core compaction,
  large water ingress
• Total Instantaneous Blockage

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"Excluded" events

• "Excluded" events their consequences are not
  mitigated by design provisions
• A list of severe situations beyond those
  considered in the design is established and the
  associated consequences are assessed. If the
  consequences cannot be made reasonably and
  confidently mitigated, the situation is
  excluded when it is
• Physically impossible
• Practically eliminated by adequate design and
  operating prevention provisions
• Preliminary list
• Large reactivity insertion due to
• Core compaction capable to approach criticality
• Moderator effect capable to approach criticality
  (e.g., caused by steam ingress)
• Large fuel loading error
• Core support failure due to challenges on
  internals, primary circuit and primary circuit
  support, in particular
• Large load drop (e.g. during handling operations)
  
• Rotating machinery failure