Transcript Diapositive 1 - Royal Institute of Technology

EUROTRANS WP1.5 Technical Meeting
Task 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 1.5.1 Safety approach for EFIT-He – November 13-14 2007
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Task T1.5.1 Progress
 D1.20:Approach and acceptance criteria for safety
design of XT-ADS
 Issued in August 2006
 Update when design is confirmed
 D1.21:Approach 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 subcritical 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:
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
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:
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
Large load drop (e.g. during handling operations)

Rotating machinery failure
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