1. Mixing of the Atmosphere within the EPR Design Containment in Design Basis and Severe Accident Conditions
Prof. Ali Tehrani
ONR Principal Inspector – Nuclear Safety
IAEA International Conference, Vienna - June 2017
Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants
Paper ID - 112
TRIM Ref. 2017/161531

2. Outline Multinational Design Evaluation Programme (MDEP) objectives;
Key features of the EPR design;
Background and motivation for independent confirmatory analysis;
Supporting analysis;
Design Basis Accident (DBA),
Severe Accident (SA),
Concluding remarks.

3. MDEP Initiative
Multinational Design Evaluation Programme is an OECD NEA initiative set up to enhance standardisation of safety assessment of new reactor designs by the national regulatory authorities to:
Promote understanding of participating countries’ regulatory decisions and basis for these decisions;
Enhance communication among the members and with external stakeholders;
Identify common positions among regulators reviewing new reactor designs (including EPR);
Achieve or improve harmonisation and convergence of regulations, standards, and guidance.

4. Key Features of EPR Design (1/4)
The EPR containment is a new design
Different from typical PWR containments;
Deploys a “two room” design concept;
Equipment rooms surrounding the Reactor Coolant System are isolated from the rest of the containment,
Beyond this inner region, personnel access can be provided during certain maintenance tasks with partition / separation,
In power operation, inaccessible areas within containment experience higher Temp. and radiation than accessible areas,
A number of design features promote mixing of the environment within the containment in accident conditions; referred to as the “CONVECT system”.

5. Key Features of EPR Design (2/4)
Figure courtesy of AREVA

6. Key Features of EPR Design (3/4)
The CONVECT system consists of:
Rupture foils;
Convection foils;
Mixing dampers; and
Related instrumentation and control equipment.
Rupture foils and convection foils are placed in the ceiling of SG compartments; and
Mixing dampers (8) are located in the lower part of the containment, in the In-containment Refuelling Water Storage Tank (IRWST) wall above the water level.

7. Key Features of EPR Design (4/4)
Rupturing or opening of foils and dampers is to set up circulation patterns in accessible and inaccessible areas;
Rupture foils will open at a pre-determined differential pressure across the foils,
Convection foils will open on a pressure differential and/or via melting of a fusible link,
Mixing dampers automatically open on a pressure differential, absolute containment pressure or manually from the main control room.
Increases the heat transfer surface area in DBA scenarios, and promotes mixing of hydrogen released into the containment in SA scenarios.

8. Motivation for Confirmatory Analysis
CONVECT system performance has been the subject of independent confirmatory analyses by a number of participating regulators using:
Lumped parameter codes; and
Computational Fluid Dynamics (CFD) codes.
Analyses examined the effectiveness of the CONVECT system in facilitating mixing in the containment and preventing design pressure being exceeded for DBAs and SA.
The outcome of this work has been shared amongst the participating regulators wishing to assess this feature of the EPR design, leading to improved understanding of its performance in accident conditions.
Uncertainties associated with the lumped parameter codes

9. Result of the DBA Analysis (1/2)
Double-ended guillotine break in cold leg (2A CLB)
Foils and mixing dampers opened within a few seconds of the LOCA;
Initial mixing generally good, except in pressuriser compartment and some equipment rooms;
Safety injection terminates rise in containment pressure and temperature;
Sensitivity studies showed that:
Damper failure results in elevated peak containment pressure and temperature;
Thermal stratification was observed, depending on break elevation.

10. Result of the DBA Analysis (2/2)
Main Steam Line Break (MSLB)
Foils and mixing dampers opened within a few seconds of the break.
Clear vertical stratification between dome and lower annular region.
CONVECT system produced relatively uniform pressures, but less effective in forming circulation patterns between dome / lower regions to reduce stratification.
Gas temperatures significantly higher for MSLB than LOCAs, throughout calculation duration (24hr).

11. Result of the SA Analysis (1/3)
Pressuriser (PZR) SB-LOCA
Assumes failure of emergency core cooling and containment spray, but successful partial and fast secondary cooldown and emergency feedwater operation
Generally good H2 mixing, except in spreading room
Stratification within PZR room; local PARs ineffective due to lack of air supply, but H2 ignition unlikely;
FLUENT and MELCOR exhibit similar trends, but FLUENT predicts 4-8% higher H2 at top of SG room due to stratification;
Well mixed H2 in most compartments, in agreement with EPR design supporting analysis.
Describe scenarios verbally, info in paper

12. Result of the SA Analysis (2/3)
Cold Leg SB-LOCA
Assumes failure of safety injection and containment spray, with and without delayed core re-flooding. Sensitivity studies on foil / damper failures.
Late core re-flooding results in much faster H2 release rate;
PARs are less effective at controlling H2 concentrations.
Despite good mixing in the containment, for these low frequency scenarios, there is a potential risk of flame acceleration, due to high local H2 concentrations at the top of the SG compartments and in the dome for a short period of time.
Without late core re-flooding the H2 release rate is much lower;
PARs effectively minimise risk of flame acceleration.
Describe scenarios verbally, info in paper

13. Result of the SA Analysis (3/3)
Describe scenarios verbally, info in paper
Figure courtesy of AREVA

14. Result of the Analysis
Confirmatory results provided additional confidence, since the confirmatory analyses undertaken used:
Different DBA and SA scenarios;
Different nodalisation schemes;
Different codes and methods; and
Different organisations.
Confirmatory results were broadly consistent with EPR design supporting analysis/model.
Describe scenarios verbally, info in paper

15. Conclusion (1/2) Independent confirmatory analysis has concluded:
CONVECT is effective in facilitating mixing in the containment, preventing the design pressure being exceeded in DBAs;
Stratification is possible for MSLB occurring at high elevation, the occurrence of which does not challenge the design pressure, but may lead to challenges in equipment qualification;
For SA scenarios which follow the predicted path, CONVECT enables efficient H2 mixing within the containment. Despite high local H2 concentrations, the containment integrity would not be threatened; and
The effectiveness of CONVECT has been confirmed by regulators, and their TSOs, with the analyses confirming that there is sufficient mixing within the containment to mitigate against both DBAs and SA.

16. Conclusion (2/2)
Sharing knowledge and experience between participating countries has resulted in;
Improved understanding of challenges and the outcome used as a basis for regulatory decisions;
Promoted communication among the members and with external stakeholders;
Production of a common position paper reviewing new reactor designs; and
Helped to achieve harmonisation and convergence on challenging technical and regulatory topic areas.

17. Acknowledgements
The supporting paper has been prepared and presented on behalf of the MDEP and supported by the following EPR Technical Expert Working Groups:
Accident and Transient; and
Severe Accident.
MDEP members contributing to and supporting this work include P.R. China, Finland, France, the United Kingdom and the United States.
I would like to express my gratitude to MDEP members for their help and support in preparation of this paper.

18. Questions and Discussion