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EUROTRANS WP1.5 Technical MeetingTask 1.5.1 – ETD Safety approach

Safety approach for EFIT: Deliverable 1.21

Lyon, October 10-11 2006

Sophie EHSTER

Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20063 3AREVA NP

Contents

Main safety objectives

Safety functions

"Dealt with" events

"Excluded" events

Conclusions

Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20064 4AREVA NP

Main safety objectives

Application of defense in depth principle: prevention and mitigation of severe core damage are considered

Elimination of the necessity of off site emergency response (Generation IV objective)

Probabilistic design targets:

Higher level of prevention than XT-ADS is aimed at since the core is loaded with a high content of minor actinides (low fraction of delayed neutons, low Doppler effect). Cumulative severe core damage frequency:

10-6 per reactor year

If LOD approach is used: 2a + b per sequence

At the pre-conceptual design phase (EUROTRANS), severe core damage consequences are assessed in order to determine the main phomena, associated risks and possible design provisions (core and mitigating systems)

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Safety functions

Reactivity control function:

Definition of sub-criticality level (dealt with by WP1.2, checked further by WP1.5):

Consideration of most defavorable core configuration (possible adaptation)

Consideration of reactivity insertion: Keff to be justified through reactivity insertion studies

Consideration of hot to cold state transient

Consideration of uncertainties

Consideration of experimental devices

Use of aborber rods (design in WP1.2): during shutdown conditions to be moved preferentially by

dedicated mechanisms

(in case of critical core configuration)

Measurement of sub-criticality level To be performed before start-up with accelerator, target and

absorbers inserted

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Safety functions

Power control function:

Power control by the accelerator

Proton beam must be shut down in case of abnormal variation of core parameters, in particular in case of failure of heat removal means

High reliable proton beam trip is requested:

at least 2a+b LOD are requested: b must be diversified (passive devices (target coupling) and operator action (large grace time needed))

Implementation of core instrumentation:

Neutron flux

Temperature at core outlet (each fuel assembly if efficient for flow blockage)

DND (very efficient in the detection of local accidents for SFR)

Flowrate

Implementation of target instrumentation

Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20067 7AREVA NP

Safety functions

Decay heat removal function:

Performed by Forced convection: 4x (1primary pump + 2 Steam Generators)

provided for power conditions. Use to reach "cold" shutdown state?

Natural convection: 3 + 1 safety trains (redundancy) cooled by two-phase oil system

Reactor Cavity Cooling System would not be capable to remove decay heat at short term

A high reliability of the function is requested e.g. number of systems, redundancy, diversity, duty of the

cavity walls cooling system

Consideration of common modes (e.g. freezing, corrosion, oil induced damage) to be prevented by design

Definition of safe shutdown state/mission duration

EFR background: 3 trains 100% or 6 trains 50% and diversification

Need for a reliability study?

Emergency core unloading

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Safety functions

Confinement function:

Performed by three barriers

Fuel cladding

Reactor vessel and reactor roof

Reactor building

Design must accommodate

The radiological releases

The pressure if any (cooling system lekage)

Specific issues:

Coupling of the reactor, spallation target and the accelerator needs to be assessed

No generation of polonium 210

Control of radiological releases to the atmosphere has to be performed

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Safety functions

Core support function:

Performed by

The reactor internals

The reactor vessel and its supports

Exclusion of large failure?

Is the demonstration credible?

Checking of the capability of severe core damage mitigation provisions on this scenario

Specific issues:

ISIR of in-vessel structures under a metal coolant (e.g. core support inspection inside or outside the reactor vessel?)

Consideration of oxide formation (design, monitoring, mitigation provisions)

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"Dealt with" events

"Dealt with" events: their consequences are considered in the design

Determination of the "dealt with" initiating faults list and associated sequences:

assessment of XT-ADS list and consideration of EFITdesign features

ANSALDO task: to confirm the list of initiating faults

sequences (success/failure of mitigating means) will be determined in accordance with the main safety objectives

Same practical analysis rules as XT-ADS ones

Consideration of EFIT specific features: increase of the core power density, consideration of core loaded with a high content of minor actinides, risk of water/steam ingress (Steam Generator), much higher risk of freezing (327°C)

Radiological consequences: use of method?

Determination of barriers (e.g. fuel, cladding, structures) criteria: to be preliminary defined and confirmed by R&D about the knowledge of material behaviour for higher temperatures

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"Dealt with" events/ Consequences of implementation of a steam cycle

Additional initiators (in accordance with the European background) :

Steam Generator leakage: DBC2

Steam Generator Tube Rupture: DBC3

Several SGTR has to be considered at least as a limiting event (assessment of the phenomenology e.g. combination of corrosion and loading due to DBC)

DHR HX leak (two phase oil): DBC2 (1 tube) or DBC3 (multiple tube rupture)

Feedwater system malfunction: DBC2

Secondary steam system malfunction: DBC2

DHR cooling system malfunction: DBC2

Feedwater leakage/line break: DBC3 or DBC4 depending on the size of the leak

Secondary steam leakage: DBC3 or DBC4 depending on the size of the leak

DHR cooling system leakage: DBC2 or DBC3 depending on the size of the leak

Combination of SGTR and steam line break has to be considered as a limiting event (DEC)

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"Dealt with" events/ Consequences of implementation of a steam cycle

Associated risks:

Reactivity insertion: moderator effect, void effect, core compaction

Mechanical transient due to the depressurisation into the reactor vessel

Steam explosion

Draining of the primary coolant outside the reactor vessel

Pressurisation of the reactor buiding

Overcooling and subsequent freezing (SG overflow)

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"Excluded" events

"Excluded" events: their consequences are not considered in the design

Their non consideration had to be justified

Preliminary list:

Large reactivity insertions

Core support failure

Complete loss of proton beam trip function

Complete loss of decay heat removal function

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Conclusions

D1.21:

First draft to be issued at the end of October 2006 (FANP)

To be reviewed by ANSALDO (design) and partners involved in the safety analyses