Transcript Diapositive 1 - Royal Institute of Technology

EUROTRANS WP1.5 Technical Meeting
Task 1.5.1 – ETD Safety approach
Safety approach for EFIT: Deliverable 1.21
Sophie EHSTER
Stockholm, May 22-23 2007
AREVA NP
Contents
 D 1.21 progress
 Main safety objectives
 Consideration of safety objectives in the design
 "Dealt with" events
 "Excluded" events
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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 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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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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Main safety objectives - 2
 Definition of the sub-criticality level: core shall remain subcritical 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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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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:
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
High reliability of function is requested: diversity of DRC system

Need for reliability study?

Risk of freezing

Emergency core unloading

Severe core damage mitigation
Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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:


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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
Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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:
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
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
Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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"Dealt with" events - 2
 "Dealt with" events:
 Risks associated with water ingress:
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
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
Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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"Dealt with" events - 3
 "Dealt with" events:
 List of limiting events:
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
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
Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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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)
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Task 1.5.1 D1.21 Safety approach for EFIT – May 22-23 2007
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