SAFETY AND LICENSING OF SPENT FUEL STORAGE AND TRANSPORT-Safety issues within spent fuel transport by AREVA

Stéphane BRUTFrançois DERLOTLaurent MILET

31 May- 4 June 2010

TN INTERNATIONAL

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TN INTERNATIONAL

Some of the main Safety issues raised during last years within French spent fuel transport:

Leaking fuel assembly transport: How to transport and demonstrate safety of package loaded with fuel assemblies with suspicion of water trapped in the cladding?

Double Barriers design for spent fuel packages: what kind of design to comply with the regulation?

Double Operations& controls associated to the water exclusion assumption: how to fulfilled requirement regarding leakage of water as a result of error?

INTRODUCTION

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What is the problem ? Cladding rupture on irradiated fuels involve:

Loss of containment with loss of internal pressure Potentially allow water to penetrate inside the claddings

Water + radiation hydrogen generation

Leaking fuel assembly transport

Accumulation of hydrogen in a sealed cavity may reached limits of inflammability non acceptable from a safety point of view

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French history with leaking fuel assembly transports December 2007: Request by French competent Authority for a systematic

measurement of hydrogen rates for packages transporting at least one non-sealed fuel rod

From this request systematic measurement of rates of hydrogen upon arrival at recycling facility (LA HAGUE) for packages transporting fuel identified as ruptured.

Measurements gave a hydrogen rate far below safety limits of inflammability (around 3%)

Beginning 2009 upon French Authority request , TN International carried out linear extrapolation of hydrogen generation to include real periods of transport + margin depending of the status of transport (national, continental transport, intercontinental)

Results according this approach: some transports that were checked as conform at the recycling site arrival would be determined as non conform regarding safety criteria

French competent authority requested a new procedure for new transport:- Measuring of hydrogen rate before transport and at least two days after closing the

cavity- Carrying out linear extrapolation taking into account real period transport and safety

margin

Leaking fuel assembly transport

New procedure extremely restrictive for transport

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TN International action plan to improve leaking fuel assembly transport

3 ways in progress Using a neutral gas to increase inflammability threshold.

- Helium atmosphere increase in the same condition the inflammability level from 3% to 5%- An application in under progress with French Competent Authority

To characterize a hydrogen generation law less penalizing as linear extrapolation.

To develop solution such as catalytic recombiner.- Such a solution experimented on a TN17T package for the wet transport of spent fuel at

TIHANGE (Belgium) showed a stabilization of hydrogen rates about 0.6%- Aim of TN INTERNATIONAL is to implement this technology on a first dry transport of spent

leaking fuel in France before long.

Leaking fuel assembly transport

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Double barriers design for packages

2005 IAEA regulation request regarding sub-criticality Sub-criticality of a package may be demonstrated assuming water

exclusion from the containment system, if and only if the design is based on a multiple high standard water barriers.

Nevertheless it is widely agreed by competent authorities that a double watertight high quality standard is enough to comply with this requirement.

Application to spent fuel cask of such a requirement Usually spent fuel casks are constituted of a thick vessel made of steel or

cast iron regarding radiation level that may involve 2 types of design: Double-lid design Double-vessel design

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Double barriers design for packages

Double-lid design

Secondary lid equipped with seal

Primary lid equipped with seal

Advantages:

- strength of the body

- high thermal conductivity of the shell

- simplification of design of drying orifices

- optimization of mass, dimensions and so cost

Drawback:

- to demonstrate the containment barrier when material subject to brittle fracture at low temperature

Thick shell in steel or iron cast

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Double barriers design for packages

Double-vessel design

Secondary vessel equipped with seal

Primary vessel equipped with seal

Advantage:

- Two complete different barriers

Drawbacks:

-Gap between the two vessel reduce the thermal performance

-If drying of the cavity is request in case of loading or unloading underwater operation, difficulties to design and perform drying orifices through the two barriers

-Risk of water trapped between the two vessels in case of loading or unloading underwater operation

-Increase of mass, size , of the package to compare a double –lid design

-More complicated to design, manufacture and use than a double-lid

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Double operations & Controls associated to the water exclusion assumption

2005 IAEA regulation request regarding sub-criticality According §677 sub-criticality shall be demonstrated assuming water flooding

within all spaces of the packaging, except if the design incorporates special features to prevent such leakage of water into or out of certain void spaces, even as a result of error.

Such features than can prevent inleakage : May be either a single barrier design licensed according 1985 IAEA regulation and

submitted to multilateral approval or a multiple barrier design licensed according 2005 IAEA regulation

Are based on design components (leaktight sealing,…) that we can call “Confinement Components" by analogy to regulatory Confinement system regarding criticality safety. And confinement operation may be defined as any operation that may result in reducing the efficiency of a confinement components as a result of error, standing for a human or a tool failure.

To prevent error resulting in leakage of water, French competent Authority require that confinement operation need to be performed twice independently (different operators and tool).

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Double operations & Controls associated to the water exclusion assumption

Application of double controls system to drying operations by AREVA

For packages loaded in pools, water is removed by draining then vacuum drying of the cavity . To prevent ice formation, vacuum pressure shall be maintained over 6mbar abs. Drying criteria is usually a pressure rise limit during a short time Control of the pressure is the key operation

Double control is then implemented by measuring continuously the pressure though a double recording measuring device Then a different operator from the one performing the test check the pressure level

and the pressure rise on the second device

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Double operations & Controls associated to the water exclusion assumption

Usually designs confinement components are Shell (s) and welded bottom

Lid (s) equipped with seals and secured by bolts

Orifice covers equipped with seals and secured by bolts

Application of double control by AREVA Draining and drying of the cavity: see previous slide

Fastening of bolts to the required torque 2 operators with a different tool for a single barrier or one different operator and tool for each barrier in case of double barriers

Check of the leak tightness of the lid and orifices 2 operators for pressure measurement for a single barrier or one different operator for each barrier in case of double barriers

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Double operations & Controls associated to the water exclusion assumption

To be noticed

That in case of sub-criticality demonstration based on a complete flooding of all of the void space of the cavity , double controls system are not required

That multiplying operations and controls has significant effect on the radiation dose of operators. Such a program should not be extended.