1. Complementarity of deterministic and probabilistic approaches
for safety assessment of French EPR design
Patricia DUPUY, Gabriel GEORGESCU
International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants
June, 6-9th Vienna

2. French context EPR FA3 French NPPs Safety case
Current fleet: 58 GEN II PWRs
GEN III EPR under construction at Flamanville
Safety case
Mainly deterministic bases, supplemented by probabilistic approach
Important place of the probabilistic approach in the safety decisions
EPR FA3: complementary use of deterministic and probabilistic approaches at each of the preoperational phases since early design
A supprimer : and one regulator
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 2

3. French context
“Technical guidelines” for plants generation III in France (endorsed by French Safety Authority in 2004)
Significant reduction of the core damage frequency (CDF) compared to current fleet, guidance value: CDF ≤ 10-5/year
Significant reduction of the radioactive releases in accidents
Severe accident situations that could lead to large early radioactive releases must be “practically eliminated”
PSA requested since the early design stage
PSA can be carried out in several steps:
Simplified assessment at the conceptual stage
More detailed studies during the engineering phases
A supprimer : and one regulator
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 3

4. PSAs carried out and reviewed in several steps
Simplified level 1 PSA (internal events)
Simplified level 1+ PSA (investigating the containment and some severe accident features)
Safety options basic design
More detailed level 1/1+ PSAs
PSA for the spent fuel pool
Studies supporting the “practical elimination”
Methodologies for level 2 and hazards PSA
Application for construction (PSAR)
Updated level 1 PSA(internal events)
Simplified PSAs for internal fire, flooding and explosion
Assessment for long LOOP/LUHS induced by extreme winds - Simplified seismic study
Semi-quantitative studies for external hazards
Level 2 PSA (internal events)
Application for operating license (SAR)
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 4

5. PSA developments: challenges and insights
Some challenges, in particular at the early design stage
Lack of design information and data
Emergency operating procedures not available,
Lack of specific reliability data
 Scope /level of details of PSAs adapted to available information
 “Iterative process” between deterministic studies and PSAs
 PSAs limitations taken into account in the decision making process
However many PSA insights for EPR design
Assessing the overall risk and the safety level compared to GEN II
Supporting the choice of design options
Adjusting the set of design extension conditions
Contributing to the « practical elimination » justifications
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 5

6. Reviews performed by IRS[N]
IRSN highly involved in the safety assessment of EPR design
For both deterministic demonstration and PSA aspects
At each of the preoperational phases
Close connection between the IRSN teams in charge of DSA and of PSA reviews  Technical exchanges, mutual benefits
As for the current fleet, IRSN developed its own EPR level 1 PSA  Valuable knowledge, sensitivity analyses
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 6

7. Main PSA insights at the design phases of EPR-FA3
Check that the risk complies with the safety probabilistic targets
PSA contributed from the early design phase to implement design improvements, in particular to add diversifications
E.g.: two additional diversified diesels generators, diversified cooling of two low pressure injection pumps, diversified cooling chain for containment heat removal system, third train on the cooling system of the spent fuel pool...
Internal hazards: results of simplified PSA globally confirmed the robustness of EPR design - PSA used to investigate particular rooms and scenarios, a few design evolutions highlighted – PSAs to be updated
External hazards: PSAs less mature to highly contribute to the decision process: efforts to develop external hazards PSAs should be strengthened
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 7

8. Design extension conditions without core melt (RRC-A)
RRC-A = Risk reduction category A
= multiple failures DEC conditions not covered by DBA
Need for particular RRC-A features to reduce the associated risk
Deterministic safety analysis
Level 1 PSA
Identification of
RRC-A features
RRC-A conditions
Safety analysis of RRC-A conditions
Methodology
To justify the appropriate design of RRC-A features
Combination of probabilistic and deterministic approaches
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 8

9. Design extension conditions without core melt (RRC-A)
Identification in PSA1 of « potential RRC-A features »
Identification of “functional sequences” (FS)
FS = group of sequences crediting a given RRC-A feature
Determination of the list of plant RRC-A features
If CDF(FS without RRC-A feature) > threshold (10-8/a.r), then RRC-A feature necessary
Identification of
RRC-A features
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 9

10. Design extension conditions without core melt (RRC-A)
Identification in PSA1 of « potential RRC-A features »
Identification of “functional sequences” (FS)
FS = group of sequences crediting a given RRC-A feature
Determination of the list of plant RRC-A features
If CDF(FS without RRC-A feature) > threshold (10-8/a.r), then RRC-A feature necessary
Identification of
RRC-A features
Within the functional sequences, selection of envelope sequences (TH/neutronic criteria)
RRC-A conditions
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 10

11. Design extension conditions without core melt (RRC-A)
Identification in PSA1 of « potential RRC-A features »
Identification of “functional sequences” (FS)
FS = group of sequences crediting a given RRC-A feature
Determination of the list of plant RRC-A features
If CDF(FS without RRC-A feature) > threshold (10-8/a.r), then RRC-A feature necessary
Within the functional sequences, selection of envelope sequences (TH/neutronic criteria)
For the envelope sequences: deterministic safety analysis and TH/neutronic calculations
Identification of
RRC-A features
RRC-A conditions
Safety analysis of RRC-A conditions
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 11

12. Design extension conditions without core melt (RRC-A)
Deterministic safety analysis
Level 1 PSA
Identification of
RRC-A features
RRC-A conditions
Safety analysis of RRC-A conditions
Insights from the
RRC-A methodology
Systematic relevant approach to identify RRC-A features necessary to cope with RRC-A conditions and to verify their appropriate design
Iterative process however necessary (preliminary PSA not enough detailed)
Evolution of the methodology defined by the designer in view of the new EPR model (same objectives but with less dependence on the level 1 PSA achievement)
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 12

13. Practical elimination
Severe accident sequences that could lead to large early radioactive releases must be “practically eliminated” (Technical Guidelines)
Situation physically impossible?
Situation extremely unlikely to arise, with a high degree of confidence?
Situation considered as “practically eliminated”
YES NO
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 13

14. Practical elimination
Situation physically impossible?
Situation extremely unlikely to arise, with a high degree of confidence?
Situation considered as “practically eliminated”
Case-by-case analysis
Deterministic and probabilistic considerations
Probabilistic approach applicable for some types of situations
PSA scope, uncertainties and limitations to be considered
No probabilistic “cut-off” value
Deterministic demonstration always requested
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 14

15. Practical elimination
Situation physically impossible?
Situation extremely unlikely to arise, with a high degree of confidence?
Situation considered as “practically eliminated”
For EPR FA3, PSA1 used for:
- Core melt accidents with containment bypass (rupture outside containment)
Fast boron dilutions
Fuel melting in the pool
Insights of PSA1 results and methods for EPR FA3:
Deep investigation of all initiating events and sequences
Identification of all the features involved in the sequences
Design and operational features improvements defined (signals, isolation devices, alarms …)
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 15

16. In some cases to point out the need for design improvements
CONCLUSION
Challenges related to the development and use of PSAs for the EPR new built especially at the early design stage  need to adapt to the available information and to be aware of PSA limitations
However, PSAs have provided useful insights for EPR FA3 at each of the preoperational phases
To confirm the design
In some cases to point out the need for design improvements
Complementarity of deterministic and probabilistic approaches for EPR-FA3 design – P. DUPUY – June 2017 16