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[email protected] Technical Meeting on Phenomenology and Technologies Relevant to In-Vessel Melt Retention and Ex-Vessel Corium Cooling

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STATUS AND STRATEGIES OF EX-VESSEL CORIUM COOLING IN BELGIUM

Technical Meeting on Phenomenology and Technologies Relevant to In-Vessel Melt Retention and Ex-Vessel Corium

Cooling

17-21 October 2016, Shanghai, China

A. Malkhasyan, M. Adorni, T. Van Rompuy, D. Gryffroy (Bel V)

Technical Meeting on Phenomenology and Technologies Relevant to

In-Vessel Melt Retention and Ex-Vessel Corium Cooling

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TABLE OF CONTENTS

• Introduction

• Unit Specific EVCC strategies

• Recent Activities

• Conclusive remarks

Technical Meeting on Phenomenology and Technologies Relevant to In-Vessel Melt Retention and Ex-Vessel Corium Cooling

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OBJECTIVES

• To introduce Bel V and its activities

• To provide information about nuclear installations in Belgium

• To share information about the development of Belgian EVCC strategies

• To present the latest activities and the impact of the recent R&D


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INTRODUCTION

Bel V and Nuclear Installations in Belgium


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[email protected] Technical Meeting on Phenomenology and Technologies Relevant to In-Vessel Melt Retention and Ex-Vessel Corium Cooling

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INTRODUCTION

Bel V = Subsidiary of the FANC

(Federal Agency for Nuclear Control)

to carry out the surveillance of the Belgian nuclear installations within the frame of

the Belgian laws and regulations

In Belgium, FANC and Bel V are considered to constitute together the regulatory body

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BEL V BASIC ROLE

• Technical Support of the Federal Agency for Nuclear Control

– Nuclear Safety Assessments: Safety Evaluation Reports

– Conformity checks of new plants or modifications: issuance of licenses

– Inspections: written reports


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NUCLEAR INSTALLATIONS

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Doel and Tihange NPP’s are operated by Electrabel (ENGIE)


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Evolution of EVCC strategies in Belgium

• 1996: pilot study for melt coolability for a single unit

• 1997: report discussing reactor cavity design for all units

• End of 90s – Beginning of 2000s

– Development of severe accident guidance for all units • WOG SAMG adapted for Tihange NPP

• Custom made « BK» procedures for Doel NPP

– Summary of strategies for each unit to be used in SA guidance

• 2005: PSR – state-of-the-art of ex-vessel coolability (BE)

• 2008-2011: WENRA RLs – R&D follow-up (OECD MCCI)

• 2012-now: Belgian Stress Test

– Additional means to inject into reactor cavity

– R&D follow-up (e.g. CCI-7 & CCI-8)

9 Technical Meeting on Phenomenology and Technologies Relevant to In-Vessel Melt Retention and Ex-Vessel Corium Cooling

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UNIT SPECIFIC EVCC STRATEGIES

Evolution of the unit specific strategies 1996 - 2006


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Ex-Vessel Corium Cooling Feasibility (PWR 1000MW)

• Potential pathways to flood reactor cavity – Through drain lines between containment sump and cavity

– Through ventilation air paths

– Through access and depressurization chimney (with ejectable panels)

– Due to spray water flowing through openings into the cavity

• Spill-over of containment water into reactor cavity is not feasible (overflow at the level of primary loops) – Requires 2-7 volumes of the RWST tank (unit dependent)

– Early reactor cavity flooding through spill-over cannot be credited

• Timely flooding depends on cavity/sump relative elevation

• Steam evacuation paths are available in all the units


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Initial SAM strategy

• Main strategies proposed include: – Injection into the containment

• All relevant means - LPSI, HPSI, spray, RWST gravity drain, etc.

• Whether reactor cavity can be flooded depends on containment design and later modifications

– Injection into the RPV (after RPV failure) – as secondary strategy • Purpose: top-flooding of corium in the reactor cavity

• Additional purpose: partial IVMR (arresting melt ejection from RPV)

– Containment heat removal

• Strategies are inspired by WOG SAMG – SAG-3: Injection into the RCS

– SAG-4: Injection into containment

– SAG-8: Flood containment


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Ex-vessel Corium Cooling Strategies After Reassessment (Definitions)

• Corium Slab – Cavity is dry at the vessel failure – Corium will form a molten pool on top of the basemat – Early top flooding of the molten pool/slab

• Main cooling mechanisms - crust water ingression + melt eruption

• Particle Bed – Cavity is flooded at vessel break – Corium melt jet is broken up completely – Particle bed present in the cavity – Coolability can be ensured

• If enough water is left in the cavity and added continuously • If the residual heat is less than dry-out heat flux

• Cake structure + particle bed – Cavity is flooded at vessel break – Corium melt jet is broken up partially – Cake structure on the cavity basemat + particle bed on top – Can be coolable if both cake and particle bed are coolable


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Ex-vessel Corium Cooling Strategies After Reassessment

• 1-st strategy (early cavity flooding – before RPV break) – Preferred strategy for siliceous concrete – Reactor cavity is lower than containment floor and sump – Installation of connection between lower containment and cavity – Deep water pool can be created by gravity-driven early flooding – At RPV failure, jet break-up and corium fragmentation can be

ensured – Relatively small water volume is needed (small cavity surface) – Complete RPV flooding is achievable

• Corium melt ejection can be limited (less than 100% relocated into cavity) • Partial IVMR can be achieved (arrested melt ejection from RPV)

• Issues associated with late flooding of these units – Complete coolability of the corium slab (+ arresting MCCI) is less

probable


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Ex-vessel Corium Cooling Strategies After Reassessment

• 2-nd strategy (late cavity flooding – after RPV break) – Preferred strategy for LCS concrete – Corium slab coolability: cooling phenomena after top flooding during

CCI are more efficient (water ingression, melt eruption) – Melt spreading in dry cavity is optimized:

• Reactor cavity is isolated from the containment sump • In one unit, the reactor cavity surface is much larger

• Issues associated with early flooding of these units – Water level achievable in cavity: 10 cm – 1 m

• Not enough for corium jet break-up (no particle bed) • May impede the proper corium spread • Corium slab coolability difficult to obtain

– Even if water level can be increased more than 1m • Calculations demonstrate particle bed coolability only if melt relocation is partial • E.g. 30% of corium ejected 3h after the reactor shutdown


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SUMMARY 1996-2006


Unit Jet break-up Dry/wet cavity Principal strategy

Unit A, B Yes Cavity flooding ensured Early cavity flooding

Unit C No Cavity is always wet Late flooding is preferable

Unit D No Isolation possible Late cavity flooding

Unit E No Not isolated No clear decision on

strategy

• Ex-vessel corium cooling strategies are chosen based on the unit layout

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RECENT ACTIVITIES

WENRA + BELGIAN STRESS TEST (2006 - now)


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WENRA RL 2008

• 2008:

– PSR conclusion: Ex-vessel corium coolability remains open issue • R&D to be followed (particularly MCCI-2)

– First revision of WENRA Reference Levels published in 2008

• WENRA RL is transposed into Belgian Legislation

– Royal Decree of 30 November 2011.

– New project was lunched to ensure that Belgian plants fulfil these requirements

– RL F4.7 «Containment Degradation by Molten Fuel Shall Be Prevented or Mitigated as far as Reasonably Practicable»

– The EVCC issue has been kept in the framework of this RL


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Belgian Stress Test

• Belgian Stress Test (BEST) performed after Fukushima:

– Includes open issues of WENRA requirements

– Additional lessons learned from the accident

• Actions

– R&D follow-up • EVCC & MCCI issues (CCI-7 and CCI-8) – mainly for units with top-flooding:

confirmation that the LCS concrete is favourable for the top-flooding strategy

• Steam explosion – mainly for units with early cavity flooding (SERENA program)

– Pre-feasibility and feasibility studies for units with top-flooding • Modifications needed to guarantee a dry cavity before vessel failure

• Alternative injection means into reactor cavity after vessel failure

• These actions and SA strategies are still under discussion between regulatory body and utility


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SUMMARY – ACTUAL SITUATION


Unit Dry/wet cavity Principal strategy Concrete type

Unit A, B Cavity flooding ensured Early cavity flooding Siliceous concrete

Unit C, D, E Dry cavity ensured* Late cavity flooding LCS concrete

* Modifications are ongoing for one of the units

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CONCLUSIVE REMARKS

Status and Strategies of Ex-vessel Corium Cooling in Belgium


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Conclusions

• The Belgian EVCC strategies have evolved during last 20 years

– Continuous follow-up of R&D by utility

– Important role of international requirements and experience

• The EVCC strategies in Belgium are rather unit-specific

– Design of the containment and reactor cavity

– Concrete type

• The strategies and associated issues are still under discussion

– Several phenomenological uncertainties still exist

– Additional plant modifications are being examined


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QUESTIONS?


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