Elbaz - IRSN Approach to Safety of SNF storage pools after Fukushima

IRSN approach of the safety of the

spent nuclear fuel storage pools after

Fukushima accident

Virginie ELBAZ

Institute of Radioprotection and

[N]uclear Safety

Fontenay aux Roses - France

International Experts’ Meeting on Reactor and Spent Fuel Safety in the Light

of the Accident at the Fukushima Daiichi Nuclear Power Plant

Vienna March 19 – 22, 2012

IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –

IAEA Meeting - Vienna 21st March 2012 – Virginie ELBAZ 2 2

Contents

1. Introduction

2. French approach to the stress test

3. Spent fuel storage pools

4. Safety assessment on spent fuel storage pools

5. IRSN assessment approach and outcomes

6. R&D program on Spent Fuel Pool accidents at IRSN

7. Conclusion


IAEA Meeting - Vienna 21st March 2012 – Virginie ELBAZ 3

▌The complementary safety assessment CSA called stress test consists in reassessing the safety margins of the nuclear installations for extreme natural events (earthquake, flood) and total loss of the safety system (loss of power supply, loss of cooling)

After the Fukushima accident in March 11th in 2011, a review of the

safety of the facilities has been undertaken at the European level

based on European specification produced by WENRA and approved

by ENSREG

1. Introduction



Background : Periodic safety review of a facility

Process defined

by the

French Law

PSR

≈Every 10 years

Safety reassessment Ageing and conformity examination

Avoid discrepancy between the facility’s

state and new safety approaches,

practices and regulations,

Assess the robustness in the actual safety

requirement

Facilitate the on-going improvement of

the facilities' safety and their operations




▌Updating of safety guide (definition of earthquake, seismic design…)

▌Taking into account the future operation context: lifetime

activity evolution

equipment characteristic evolution (ageing)

▌Operating feedback - events related to similar facilities: criticality accident of Tokaï-Mura in 1999

flooding at Le Blayais NPP in 1999

Periodic Safety Review




Stress Test

Specific

examination of

the conformity with

the design

requirements

Extreme natural events

taken into account

in the safety

requirement

Robustness

beyond the safety

design requirement

(severe accident)


Identification of a list of Systems Structures and Components

(SSC) need to be qualified for hazard levels higher than those

considered in the existing safety framework

For extreme situations, this « Hardened Safety Core » allows to

bring back the plants in a safe state

In addition

to the PSR



Loss of Power sources

Flood

Loss of all the sources of cooling

Earthquake

Deterministic approach

Loss of supply even back-up,

and ultimate back-up

IRSN assessment available on the website: www.irsn.fr

Beyond the design basis

usually taken

in French assessment

+

Combination of scenarios


Assessment of succession of events even in situation of

degraded site and isolated from outside

Potential induced events (fire, explosion, industrial site around…)

http://www.irsn.fr/



Nuclear Fuel cycle

Pool

Pool


▌ In France, NPP fuels are stored in pools located in the nuclear power plants of EDF and before treatment in pools of the reprocessing plants of AREVA La Hague



▌EDF PWR reactors pools

▌AREVA La Hague pools

▌EDF Superphenix pool

Spent fuel storage facilities in France

Under water storage

Dry storage ▌CASCAD, CEA (French Alternative

Energies and Atomic Energy) Commission) Cadarache




EDF PWR REACTOR STORAGE POOLS

▌ Fresh nuclear fuel used in PWR is manufactured with uranium, slightly enriched (4.5 %) with fissile 235U isotope and MOX

▌ Locations between 380 and 630 of fuel elements according to PWR plants

▌Time of decay before going to AREVA La

Hague ≈ 2-3 years for UOX fuel

▌ Stainless steel liner with reinforced concrete walls

▌The pool is situated in a specific building (BK), near the reactor building




▌The maximum thermal power is between 8 and 14 MW

▌Water capacity between 1150 and 1900 m3

▌The pool cooling system is provided by an external system to the pool (PTR) and earthquake proof

▌The cooling system has 2 redundant loops (each one consists of pump and heat exchanger)

▌The water is drawn from the pipe located about 4 m below the surface of the pool

Cooling

system

EDF PWR REACTOR STORAGE POOLS




▌Pools built between 1976 and 1988

▌500 and 1000 PWR baskets per pool

C. Cieutat – Copyright AREVA

Transport

BK La

Hague





fuel cladding

basket

water

▌Volume of water is between 10 000 and 15 000 m3 (10 times more than NPP pools)

▌Height of the water: 9 m (twice the height of a basket)

▌Maximal Thermal Power authorized is between 8 to 16 MW per pool (similarly to NPP pools)


▌PWR design basket with initial 235U fuel enrichment of 4,5% and MOX

▌Main assemblies come from EDF power plants

AREVA LA HAGUE SPENT FUEL POOLS




▌The reinforced concrete basin rests on a slab, independent of any adjacent structure, on neoprene bearings pads, and permits free thermal expansion.

▌Pool water is permanently cooled and purified with cooling exchanger C. Cieutat – Copyright AREVA

AREVA LA HAGUE SPENT FUEL POOLS




Diagram of cooling water system of storage spent fuel pools

UNDER WATER STORAGE – AREVA LA HAGUE SPENT FUEL POOLS

POOL

Outdoor installation

Air cooling tower

Indoor installation


Water temperature 35°C

Fuel Assembly

Thermal exchanger

slab



Damage the fuel cladding integrity

Safety of

spent fuel pools Pool water must be continuously

cooled to remove the heat

produced by spent fuel

assemblies

ACCIDENT

Keeping water level

Release of radioactive materials

to environment

Leak in the pool

Loss of the water

evaporation

draining

4. Safety assessment on spent fuel storage pools

SAFETY REQUIREMENT

•Robustness of the civil engineering and of the cooling

system

•Monitoring system

•Redundancy of equipment

•Backup generators/supply



Monitor the situations of pools (water level, temperature of water) in case of a severe accident (degraded site)

Lots of questions involved the spent fuels storage pools during Fukushima accident


Water Supply taking into account the increase of the radiation

Prevent the loss of the water level



Consequences

of initiating

events

Total loss of

cooling

Total loss of

power supplies

Dewatering of pool

- Leakage

- Drainage through a pipe

Failure of the

instrumentation

and control system




Parameter of the

spent fuel storage

Initiating

events

Identification

of scenario of accident

- Robustness

- Conformity with

design requirement

Water level

Temperature of Water

Redundancy of equipment

Crisis

organization

Defence in depth

D

E

S

I

G

N

H

A

R

D

C

O

R

E

Current water supply - make-up

means

- Fire network

- Demineralized water

- Safety tanks

Instrumentation/measure Prevention Emergency plan

TO BE DEFINED

IRSN approach

- Strengthten make-up water (pre-assembly pipe) in

any condition (earthquake, high level of radiation…)

-Study more accident scenarios

-Identification of operational and accessible devices

set up in any conditions even for extreme situations

5. IRSN assessment approach and outcoming





Definition of the “HARDENED SAFETY CORE”

5. IRSN assessment approach and outcomes for NPP pools

Means of crisis to be investigated

- Accessibility to actions means in case of a high level of radioactivity due to sky

effect

-Hydrogen production ?

Means of actions based on hardware and operations

-Isolating or minimizing of leaks or breaks consequences (anti-siphon devices,

new isolating devices)

-Water supplies (strengthen the design of water supplies in any condition)

-Instrumentation (monitor the water level on the full range of the pool height)

-Possibility of restarting a cooling train

Prevention

-Robustness of the design (transfer tube)

-In-service inspections

-Compliance to the current requirements

-Important For Safety (IFS) components and systems availability

Sources: I. MIRAMON and L. GILLOTEAU IRSN



R&D program on Spent Fuel Pool

accidents at IRSN

IRSN undertook R&D actions since several years concerning the issue of spent

fuel pool accidents, especially concerning the behaviour of clad material in air

Some studies and exploratory calculations have been done with the

ICARE/CATHARE code (ASTEC Module), for very specific air undercooling

scenarios, mainly focused on the thermo-mechanical behaviour of fuel

assemblies.

The MOZART experimental program was launched (2005-2009) to address the

phenomenology of zircaloy nitriding and oxidation in air.

Development of calculation code




Devoted to the study of ignition conditions and thermal

runaway propagation in Air on BWR and PWR fuel assembly

mock-ups in pool storage conditions

IRSN is fully involved in the OCDE Spent Fuel Pool program (US-NRC) at Sandia

National Laboratory (2009-2013)

Validation of ASTEC code for PWR geometry

Rupture of cladding

New ARAMIS R&D programme on spent fuel pool accident

(2012-2016)




Recommendations leaded by IRSN

Complementary attempt must be provided by the operators in

order to ensure the spent fuel pools (“hard core”) in a safe state

for scenarios beyond the safety requirements

Capacity to restore water in any conditions (i.e degraded

situation, after an earthquake) in order to increase the

robustness of the facility

Conclusion

7. Conclusion


IAEA Meeting - Vienna 21st March 2012 – Virginie ELBAZ 24 24

Thank you for your attention…





Safety investigations dealing with postulated initiators (seism, flood) :

Overheating of the storage pool of spent fuel until boiling

Decay of water inventory of the pool by steaming (or due to a leak or a

piping break in case of an accidental draining)

Risk of degradation of the radiological conditions (1 meter above a fuel

assembly is necessary to guaranty good radiological conditions)

Dewatering of fuel assembly being handled leading to a racing of

zirconium oxidation reaction

Boiling of the stored fuel assemblies leading to :

• a risk of restarting the chain reaction (criticality), and then

• a risk of important radiolysis of water leading to a hydrogen

accumulation

Dewatering of the stored fuel assembly in the bottom of the pool

leading to a severe accident





Definition of safety criteria : in order to avoid worst events

Safety assessment performed during PSRs

No dewatering of fuel assembly, even partially (in order to prevent

from damaging the fuel cladding which causes a severe accident) :

Assemblies in stored position

an assembly being handled

Safety assessment performed during CSAs (to be investigated)

No localized boiling in the storage area of spent fuel assemblies (in

order to prevent criticality risk leading to a risk of radiolysis of water

and then, a risk of hydrogen accumulation)





Definition of functional criteria in order to obtain safety assessments

outcomes leading to the implementation of hardware modifications

fundamental design principles

A leak or break in any system connected to the pools should not cause direct dewatering of stored spent fuel assemblies, even if no isolating action is launched.

If a drainage occurs via a piping connected to the pools, it must be possible either to isolate the drainage process before direct dewatering of an assembly being handled or to put the spent fuel assembly in safe position before its dewatering.

When drainage causes loss of pool cooling, an emergency water supply should prevent stored fuel assemblies from being dewatered later

Elbaz - IRSN Approach to Safety of SNF storage pools after Fukushima

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