Diapositive 1

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EUROTRANS WP1.5 Technical MeetingTask 1.5.1
ETD Safety approach Safety approach for EFIT
Deliverable 1.21
Sophie EHSTER

• Stockholm, May 22-23 2007

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Contents

• D 1.21 progress
• Main safety objectives
• Consideration of safety objectives in the design
• "Dealt with" events
• "Excluded" events

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D 1.21 Progress

• First draft issued in February 2007
• Based on D1.20 (XT-ADS)
• w/o internal review in AREVA
• Second draft issued in April 2007
• Internal review in AREVA,
• Interactions with ANS on EFIT design
• Review from W. Mascheck
• Third draft issued mid May 2007
• Final comments are welcome by end of May
• Final issue in June 2007

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Main safety objectives - 1

• Application of defense in depth principle
  prevention and mitigation of severe core damage
  are considered
• Elimination of the technical necessity of off
  site emergency response (Generation IV objective)
• EFIT is provided with a core loaded with minor
  actinides
• Potential consequences of severe core damage
  (i.e. large degradation of the core) 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).
• Impact on prevention and mitigation of severe
  core damage

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Main safety objectives - 2

• Definition of the sub-criticality level core
  shall remain sub-critical in any event. This
  concerns severe core damage, for which there is a
  smaller margin between criticality and prompt
  criticality (except 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. This concerns also local
  melting due to the total blockage of a
  sub-assembly, for which design provisions have to
  be implemented in order to avoid the generalised
  core melting. The demonstration should benefit
  from XT-ADS operational feedback, in particular
  concerning corrosion and inspection issues,

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Main safety objectives - 3

• Severe core damage mitigation has to be
  considered. In particular, 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 (e.g.,
  in vessel phenomena as the impact of freezing
  steel on decay heat removal (paths), fuel debris
  floating/settling, decay heat source distribution
  and possible out of vessel phenomena), relevant
  risks and possible design provisions (core and
  mitigating systems),
• Regarding severe core damage situations which are
  not mitigated by severe core damage provisions
  (not possible or without a sufficient confidence
  level), they must result from a limited number of
  sequences and their exclusion justified with
  practices similar as the ones implemented for
  future nuclear plants such as EPR at least.

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Main safety objectives - 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, accelerator and lead
  coolant.

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Consideration of safety objectives in the design
- 1

• Safety functions are reviewed in the document and
  related issues are provided.
• Focus on
• Reactivity control function
• Core sub-critical in any situation (including
  core melting)
• Absorbing system during shutdown conditions
• Measurement of sub-criticality level in shutdown
  conditions
• Power control function
• High reliability of proton beam trip (2a b), in
  particular provision of sufficient grace period
  for manual trip or diverse passive means
• Adequate instrumentation (in particular, for
  local fault detection)
• Decay heat removal function
• High reliability of function is requested
  diversity of DRC system
• Need for reliability study?
• Risk of freezing
• Emergency core unloading
• Severe core damage mitigation

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Consideration of safety objectives in the design
- 2

• Confinement function
• Design strategy with regard to radiological
  releases management is not yet defined
• Normal operation
• Gas, steam, DRC secondary coolant leakage
• Severe core damage
• Hazards
• Spallation target and accelerator confinement
  system
• Core support function
• Demonstration of exclusion of large failure
• Development of methodologies considering
  corrosion and associated technical means control
  of oxide layer, detection of corrosion, leakage
• ISI with lead (opacity, temperatures, density)
• Capability of severe core damage provision if
  prevention is not sufficient

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"Dealt with" events - 1

• "Dealt with" events
• Their consequences are considered in the design
• A list of initiating faults and associated
  sequences to be studied has been determined
• Preliminary qualitative criteria on the barriers
  have been defined
• Water ingress
• Steam Generatorleakage DBC2
• Steam Generator tube rupture DBC3
• Rupture of one tube to the rupture of all Steam
  Generator tubes DBC3 except if it can be
  demonstrated that the rupture of neighbouring
  tubes can be limited (loadings assessment,
  consideration of a possible corrosion). In this
  case, the rupture of all tubes is analysed as a
  DBC4
• Steam Generator tube failure in case of thermal
  transients in the primary circuit the design
  objective is to avoid tube failure during DBC2
  and DBC3 transients

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"Dealt with" events - 2

• "Dealt with" events
• Risks associated with water ingress
• Mechanical transient due to the depressurisation
  into the reactor vessel
• Water/steam interaction with primary coolant and
  its consequences (e.g. core compaction,
  structures failure)
• Reactivity insertion (e.g., voiding effect,
  moderator effect, core compaction)
• Draining of the primary coolant outside the
  reactor vessel
• Pressurisation of the primary building
• Overcooling and subsequent freezing due to steam
  generator overflow

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"Dealt with" events - 3

• "Dealt with" events
• List of limiting events
• Leakage of main and safety vessels
• Excessive cooling leading to large lead freezing
• Large internal failure due to corrosion
  (depending on the consequences and the possible
  mitigating provisions, might be analysed as a
  severe core damage)
• Large reactor vessel failure due to corrosion
  (depending on the consequences and the possible
  mitigating provisions, might be analysed as a
  severe core damage)
• Total Instantaneous Blockage

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"Excluded" events

• "Excluded" events their consequences are not
  addressed in the design
• The exclusion has to be justified
• Physically impossible
• Practical elimination by adequate design and
  operating provision
• Preliminary list
• Large reactivity insertion due to
• Core support failure (including corrosion as
  initiator)
• Core compaction capable to approach criticality
• Voiding capable to approach criticality (e.g.,
  caused by large gas or steam ingress)
• Large fuel loading error
• Primary system damage due to large load drop
  (e.g. during handling operations)
• Large water ingress in the primary circuit (if
  consequences cannot be mitigated)
• DBC combined with complete and timely unlimited
  loss of decay heat removal function (i.e.
  Secondary Cooling System, DRC)