Reduced Activation Materials

Nadine Laurence Baluc

Centre of Research in Plasma PhysicsAssociation EURATOM - Swiss Confederation

Swiss Federal Institute of Technology - Lausanne

Introduction toEffects of Radiation Damage

D-T Fusion Reaction

• The power production process which can occur at the lowest temperature and, hence, the most readily attainable fusion process on Earth is the combination of a deuterium nucleus with one of tritium.

4He, 3.5 MeV

deuteriumnucleus

tritiumnucleus

neutron14.1 MeV

On the Use of 14 MeV Neutrons

• Breeding blanket: the kinetic energy of 14 MeV neutrons is transformed into heat that is transferred to a coolant.

• The produced vapor is used to run a turbine that produces electricity.

On the Impact of 14 MeV Neutrons on Materials

• The fusion neutrons produce atomic displacement cascadesand transmutation nuclear reactions within the materials.

• The materials can become radioactive.

deuteriumnucleus

tritiumnucleus

neutron14.1 MeV

4He, 3.5 MeV

Evolution of the Microstructure

• Transmutation nuclear reactions yield the formation of impurities (e.g. H, He atoms).

• Atomic displacement cascades produce point structure defects (vacancies, interstitials).

Nuclear reactions Cascades

Production of impurities (He, H)

Production ofvacancies and

interstitials

Diffusion processes

Formation of the final microstructure

Time [s]10-610-13 - 10-810-16

Evolution of the Microstructure

• The microstructure of the irradiated material results from interactions between the various irradiation-induced defects. It can be formed of:

• Small defect clusters• Dislocation loops• Stacking fault tetrahedra• Precipitates• Voids• He bubbles

Dislocation loops

Voids

Stacking fault tetrahedra

200 nm

50 nm

10 nm

Evolution of the Properties

• Chemical composition:• Change in the chemical composition• Physical properties:• Decrease of electrical conductivity (low temperatures)• Decrease of thermal conductivity (ceramic materials)• Mechanical properties:• Hardening (H)• Loss of ductility (LD)• Loss of fracture toughness• Loss of creep strength• Dimensions:• Swelling, irradiation creep, irradiation growth• Environmental effects:• Irradiation-assisted stress corrosion cracking• Radioactivity:• Activation effects

stress

strain

H

LD

unirradiated

irradiated

Key Irradiation Parameters

• The final microstructure results from a balance between radiation damage and thermal annealing

• Key irradiation parameters:• Accumulated damage (in dpa)• Damage rate (in dpa/s)• Rate of production of impurities (e.g. He/dpa, H/dpa ratios)• Temperature

dpa = number of displacements per atom

Evolution of the Properties

• Low temperatures (e.g. < 400°C for steels):• Embrittlement effects: hardening, loss of ductility, loss of fracture

toughness, increase in DBTT (bcc materials)• Intermediate temperatures (e.g. 300-600°C for steels):• Swelling: peaks at about 450°C for RAFM steels• High temperatures (e.g. > 600°C for steels):• Enhanced precipitation effects• Enhanced creep effects

• Critical temperatures are dependent on the material and the irradiation conditions

Current Irradiation Facilities

• The existing sources of 14 MeV neutrons have a small intensity and do not allow to get important damage accumulation in a reasonable time.

• It is then necessary to simulate irradiation by 14 MeV neutrons, by using for instance fission neutrons.

• In Switzerland, we are using a mixed spectrum of high energy (590 MeV) protons and spallation neutrons (SINQ facility), which produce atomic displacement cascades and transmutation nuclear reactions, like fusion neutrons, within the irradiated material.

The Swiss Spallation Neutron Source (SINQ)

SINQ target

Irradiation Modes

• Fusion neutrons, fission neutrons, SINQ:• Strong differences in the production rate of impurities

~ 450~ 1040-50Hydrogen [appm/dpa]

~ 50~ 110-15Helium [appm/dpa]

~ 10~ 2020-30Damage rate [dpa/year]

Mixed spectrum of high energy protons

and spallation neutrons (SINQ)

Fission neutrons(BOR 60 reactor)

Fusion neutrons(3-4 GW reactor)

Defect poduction(in steels)

Materials for Fusion Reactors

Main Irradiated ComponentsBreeding blanket

Divertor

First wall

EFDA DATA

First Wall

• Plasma facing components:• First wall• Magnetic divertor

• First wall:• Forms the plasma chamber.• Serves as part and protection of the breeding blanket.

• The first wall consists of:• A structural material attached to a plasma facing (armor) material.

• Breeding Blanket:• Absorbs the 14 MeV neutrons, transforming their energy to provide

most of the reactor power output.• Shields the superconducting coils and other outer components.• Allows for neutron multiplication and breeding of Tritium to fuel the

reactor.

• A breeding blanket consists of:• A tritium breeding material (Li-containing alloy)• A neutron multiplier (Be)• A coolant (water, He and/or Pb-Li)• A structural material (to separate and contain the different materials)

Breeding Blanket

} Functional materials

• Magnetic divertor:• (a) Minimizing the impurity content of the core plasma• (b) Removing the α particle power• (c) Pumping the He ash

• The magnetic divertor consists of:A structural material (heat sink) which contains a coolant (water or He) and supports a plasma facing (armor) material.

Divertor

Structural Materials

• The qualification of structural materials is fundamental.• The thermal efficiency of a reactor is proportional to:

– The temperature of the coolant at the exit of the reactor.– The difference between the temperature of the coolant at the

exit of the reactor and the temperature of the coolant at the entrance of the reactor.

• These temperatures are mainly limited by the temperature window of use of the structural materials.

• The temperature window of use of the structural materials is mainly imposed by their mechanical properties under irradiation.

Functional Materials

• The qualification of functional materials is also very demanding:– Their mechanical properties under irradiation is not a primary

concern.– But properties, like the tritium release behavior, the thermal

conductivity or the entire structural integrity after prolonged neutron irradiation, are an important concern.

• The lack of adequate functional materials meeting very high temperature design window is an important issue for fusion power reactors.

Functional Materials

Water of heliumWater or helium and/or eutectic

Pb-Li-Coolant

-Li, eutectic Pb-Li, Li-base ceramic

material-Tritium breeding

material

-Be-Neutron multiplier material

DivertorBreeding blanketFirst wallFunction

• The choice of functional materials is very limited as it depends on the required properties.

Materials for ITER

Environmental Conditions

• Not very severe environmental conditions:• Accumulated damage at the end of life ≅ 3-5 dpa• Maximum temperature ≅ 500°C

Use of commercial materials

Candidate Materials

• Breeding blanket:• Structural material: SS 316LN• Heat sink: Cu-Cr-Zr alloy• First wall:• Be• Divertor:• Structural material: Cu-Cr-Zr alloy• Armor: W or CFC

Low activation materials are not needed

These materials have to be qualified for ITER irradiation conditions

Structural Materials for Fusion Power Reactors (DEMO and Beyond)

Key Issues for Materials Selection

• Key issues relate to physical, mechanical, chemical and neutronic properties:

• High performance• High thermal stress capacity• Good resistance to radiation damage• Compatibility with coolant• Compatibility with other materials• Long lifetime• High reliability• Adequate resources and easy fabrication• Good safety and environmental behavior

Surface Heat Capability

• The surface heat capability of a material, M, may be derived from physical properties (Young’s modulus, E, Poisson’s ratio, ν, coefficient of linear thermal expansion, α, thermal conductivity, k) and mechanical properties (ultimate tensile strength, σu):

Q.w ≅ M = σu.k (1-ν)/(α.E)

where Q is the maximum allowable heat flux and w the wall thickness.

Key Issues for Materials Selection

• Safety and environmental considerations:

• Reduced volatility, gas emission• Low specific radioactivity• Low radioactive decay heat• Small half-life radio nuclides• Controlled paths for dispersion of radioactivity• Reduced biological hazard potential• Easy waste disposal

Main Candidate Materials

• Reduced activation ferritic/martensitic (RAFM) steels• Oxide dispersion strengthened (ODS) RAFM steels• Oxide dispersion strengthened RAF steels• Vanadium-base alloys• C/C, SiC/C, SiC/SiC ceramic composites• Refractory metals and alloys (W, Cr)• Titanium-base alloys

Main candidate materials have a chemical composition that is based on low activation elements:

Fe, Cr, V, Ti, W, Si, C, (Ta)

Materials for ITER only

Examples

• RAFM steels:• F82H: 7.65 wt.% Cr, 2 wt.% W, Mn, V, Ta, Si and C below 1 wt.%

in sum total, Fe for the balance• EUROFER 97: 8.9 wt.% Cr, 1.1 wt.% W, 0.47 wt.% Mn, 0.2 wt.% V,

0.14 wt.% Ta, 0.11 wt.% C, Fe for the balance

• ODS RAFM steel:• The EUROFER 97 reinforced with 0.3 wt.% Y2O3 particles

• ODS RAF steels:• Fe-(12-14)Cr-(1-3)W-(0.1-0.5)Ti + 0.3 % Y2O3

Activation of Various Pure Metals

• Calculated radioactivity versus time after irradiation of selected elements after exposure to a typical fusion neutron spectrum (outboard first wall, 20 MWy/m2).

Activation of EUROFER 97

• EUROFER 97:• Recycling dose rate level

of 10 mSv/h is achieved after 50-100 years

• Hands-on dose rate level of 10 µSv/h is achieved after 105 years

U. Fischer and S.P. Simakov,

June 2003

26Al94Nb

53Mn

54Mn

60Co

Total

55Fe

Hands-on Limit

Recycling Limit

56Mn

Eurofer

• Assumptions:• HCLL PPCS reactor model B• Fusion power: 3300 MW • First wall made of EUROFER 97• Neutron flux: 1.53x1015 cm-2.s-1

• 5 full power year irradiation

Waste Management

• General aim: to avoid geological repository.

• Recycling of low activated and contaminated metals: current existing routes used for fission plants, provided the tritium issue is resolved (i.e., industrial detritiation processes become available).

• Recycling of highly activated and toxic materials:– Several challenges have to be overcome:

• Recycling techniques• Separation of the various parts (Be cannot be recycled in a

straightforward manner)• Availability of detritiation processes (for tritium-contaminated water,

structural and concrete materials)• Construction of recycling plants (the available capacity is too low)

– Hands-on recycling seems excluded, even after 100 years.

Out-of-Fusion Applications

Specific Environmental and Irradiation Conditions

High energy protons and neutrons(Beam window: 100 dpa/year

50 appm He/dpa, 500 appm H/dpa)Max. operating temperature: 550°C

Compatible with liquid Pb-Bi

ADS demonstrator

14 MeV neutrons(First wall: 30 dpa/year in Fe

15 appm He/dpa and 50 appm H/dpa)Max. operating temperature ≥ 650°C

Compatible with liquid Pb-17Li

Fusion reactors

fission neutrons(Core structure: 3-30 dpa/year)

Max. operating temperature: 1600°CCompatible with He

Advanced fission reactors

Materials Synergy

Reduced activation ferritic/martensitic steelsOxide dispersion strengthened steels

ADS demonstrator

Reduced activation ferritic/martensitic steelsOxide dispersion strengthened steels

Refractory metals and alloysVanadium alloys

SiC/SiC ceramic composites

Fusion reactors

Oxide dispersion strengthened steelsRefractory metals and alloys

Intermetallic alloysC/C, SiC, SiC/SiC ceramic composites

Advanced fission reactors

Potentialities of the Various Materials

Radiation hardening / fracture toughness embrittlement

(< 30 MPa.m1/2)

150 MPa creep strength (1% in 1000 hours) / chemical

compatibility

Temperature Windows of UseIdeal temperature window for the

first wall: RT-800°C or 300-1100°C

0 500 1000 1500°C

V alloys

SiC/SiC

CrTa alloys

WFirst wall

applications

Ti alloys

RAFM ODS RAFM/RAF

Candidate Materials

Water of heliumWater or helium and/or eutectic

Pb-Li-Coolant

W, W-base alloy, W-coated SiC/SiC-

W, W-base alloy, ODS RAFM or RAF

steel

Plasma facing material

ODS RAFM/RAF steels, W-base

alloy

RAFM steel, V-base alloy, SiC/SiC

RAFM steel, V-base alloy, SiC/SiCStructural material

DivertorBreeding blanketFirst wallFunction

• Various existing first wall / breeding blanket / divertor designs consider different combinations of materials.

Most Promising Material Class

• RAFM steels remain presently the most promising structural materials for plasma facing components and breeding blanket applications:

– A great technological maturity has been achieved: qualified fabrication routes, welding technology and a general industrial experience are almost available.

– Compatibility with aqueous, gaseous, and liquid metal coolants permits range of design options.

– Effects of high He/dpa ratio ?– Possible design difficulties due to ferromagnetic properties ?

Perspectives

• Temperature window of use of RAFM steels: 350-550°C.• How to manage with it ?

• Possible solutions:– Maintaining the structural materials at temperature at T ≥ 350°C:

• Coolant temperature at the entrance of the reactor: 400°C• Coolant temperature at the exit of the reactor: 550°C• A difference of 100-200°C should be sufficient to ensure an

‘adequate’ thermal efficiency of the plant.– Annealing regularly the materials to suppress radiation damage.

Critical Issues

Critical Issues

• How to account for actual irradiation conditions: fusion-relevantneutron spectrum, irradiation temperatures, accumulated doses (dpa), damage rates (dpa/s), production rates of impurities (e.g. appm He/dpa, appm H/dpa) ?

Modelling of radiation damage and radiation damage effects

Construction of the International Fusion Materials Irradiation Facility (IFMIF)

Modelling Activities

Numerical Tools

• Radiation damage is an inherently multiscale phenomenon

• Ab-initio calculations• Functional density calculations• Molecular dynamics (MD) simulations• Kinetic Monte-Carlo (kMC) simulations• 3D discrete dislocation dynamics (DDD) simulations

Evolution of radiation damage

Interatomic potentials (Fe, Fe-Cr, …)

Effects of radiation damage on the mechanical properties

Radiation Damage• Identification of small irradiation-

induced defects (type, size, number density) by combining transmission electron microscopy (TEM) observations with molecular dynamics (MD) simulations and TEM image simulations

TEM image MD simulation

TEM image simulation

50 nm

2 nm

Stacking fault tetrahedron

Radiation Damage Effects

• Molecular dynamics (MD) simulation of the interaction between an edge dislocation (left) flying at 60 m/s and a 2 nm void (centre) in Fe

• (a) dislocation approaching the void• (b) dislocation being attracted to the void• (c) just before the dislocation escapes the void

International Fusion Materials Irradiation Facility IFMIF

Main Functions

• Intense source of 14 MeV neutrons (250 mA)

• Missions:• Qualification of candidate materials up to about full lifetime of

anticipated use in a fusion DEMO reactor• Calibration and validation of data generated from fission reactors

and particle accelerators• Identify possible new phenomena which might occur due to the

high energy neutron exposure

Features

• The neutron spectrum should meet first wall neutron spectrumas near as possible

• Transmutation reaction rates ≅ 10 appm He/dpa, 40 appm H/dpa• Damage rate ≅ 20-50 dpa/year• Range of irradiation temperatures: 300-1000°C• Quasi-continuous operation• Volume of irradiation (high flux test module) = 1/2 liter

• Cost: about 1 billion $ US

Features

40-50~ 450~ 1040-50Hydrogen [appm/dpa]

10-12~ 50~ 110-15Helium [appm/dpa]

20-55~ 10~ 2020-30Damage rate [dpa/year]

IFMIF(High flux module)

Mixed spectrum of high energy protons and spallation

neutrons (SINQ)

Fission neutrons(BOR 60 reactor)

Fusion neutrons(3-4 GW reactor)

Defect poduction(in steels)

Overview

H. Matsui et al., SOFT Conference,

2004

Schematic View

D+ Accelerator

Liquid Li Target

Neutrons(~1017n/s)Li Free

Surface

EMP

D+ Beam (10MW)

Specimens

H. Matsui et al., SOFT Conference,

2004

Test Cell

Medium fluxtest modules

High fluxtest module

Low fluxirradiationtubes

Li Target

Li tank

Shield plug

H. Matsui et al., SOFT Conference,

2004

Small Specimen Test Technology

Specimen type

Present geometry

Comments

Tensile developed

Fatigue developed

Bend/Charpy DFT

Standard achieved;R&D ongoing

Creep Miniaturization needs verification

Crack growth International R&D

ongoing

Fracture toughness

International R&D

ongoing

• On the basis of miniaturized specimens, 0.5 liter (high flux test module) is sufficient to get within 15-20 yearsa representative test matrix up to about 150 dpa for a variety of materials.

1 cm

H. Matsui et al., SOFT Conference,

2004

Broader Approach

• EU-JA Bilateral Agreement to settle the issue of the ITER site:• IFMIF: EVEDA Phase.• NCT: upgrade (JA is contributing to this project also outside BA). • IFERC (International Fusion Energy Research Centre: DEMO

Design R&D Centre, Simulation/Computer Centre, Remote Experimental Centre).

460.00460.00920.00≅700MEuroTotal

172.13110.25282.38IFERC

217.5217.5435.00NCT

70.37132.25202.62IFMIF-EVEDA

JA (500M¥)

EU(500M¥)

Budget(100M¥)

Project

EFDA DATA

EVEDA Phase

• Europe: on the path to fusion power reactors the construction ofIFMIF is of the same importance as the construction of ITER.

• EVEDA Phase:• Optimized EVEDA (Engineering Validation and Engineering

Design Activities) scenario:– Considerable enlargement of the scope of Reference

EVEDA scenario in the aim to reduce the construction risks by performing beam tests with all accelerator components.

• JA could not make a commitment for the construction of IFMIF, but is interested in the optimized EVEDA scenario.

• China has expressed an interest in joining the EVEDA activities.• Other countries are welcome to join.

EFDA DATA

Time Schedule2024202220202018201620142012201020082006

Optimized Scenario

“Fast Track”

European Scenario

Reference Scenario

212019181716151413121110987654321

2024202220202018201620142012201020082006

Optimized Scenario

“Fast Track”

European Scenario

Reference Scenario

212019181716151413121110987654321

EVEDA CODA-2nd Accel.

CODA-1st Accel.

125 mA 250 mADecisions

EVEDA 1st stage Acceler.CODA-2nd Accel.

1st Accel.

125 mA 250 mADecisions

CODA-2nd Accel.125 mA 250 mADecision

ITER schedule

ITER schedule DT PlasmasHH+DD Plasmas

Construction

EVECODA-1st Accel.EVECODA-1st Accel.

EFDA DATA

Conclusion

Conclusion

• Material choices will not solve all design problems.• The design is complex by nature and it will have to be used to

overcome material limitations.• Close discussions between designers and material scientists

are strongly needed.

Materials: a key issue on the path to fusion power reactors- Plant thermal efficiency

- Public acceptance of fusion as future energy source