Download - Lyon, October 10-11 2006
AREVA NP
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)
Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20065 5AREVA NP
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
Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20066 6AREVA NP
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
Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20068 8AREVA NP
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
Task 1.5.1 D1.21 Safety approach for EFIT – October 10-11 20069 9AREVA NP
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