Professor Gareth B. NeighbourCEng CPhys MInstP MIMMM FHEA PGCHE

Head of Department

Department of Mechanical Engineering and Mathematical Sciences

Editor: CARBON (Elsevier), Impact Factor 6.196

UTC Swindon Governor / Director

Chair: UK Graphite Core Committee

Nuclear Graphite:

Past and Future Perspectives

My Perspective!!!

A View of the Carbon World

Tiny!

A Not So Short History

Notable Carbon Dates (1)

• 1564 – The pencil!

• 1789 – Lavoisier coins the word ‘carbone’ based on the latin carbo meaning

‘charcoal’

• 1879, The name Graphite proposed by Werner and Harsten in the same year

from the greek graphien meaning ‘to write’

• 1899 - Acheson Graphite Company

• 1924 – Bernal characterised hexagonal crystal structure of graphite

• 1942 – First self-sustaining reaction (CP1)

• 1943 – Lipson and stokes rhombohedral lattice (in natural graphite)

accounting for extra lines in XRD.

• Post – 1945 Coking evolved from the Dubb’s cracker driven by the switch from

steam to diesel locomotives and the demand for heavy fuel oil sharply

declined. More than twice the rate of increase in crude distillation capacity.

Between 1950 and 1970 coking capacity increase five fold. Coking

increases distillate production & minimizes heavy fuel oil.

Acheson

The Tubes Alloys &

Manhattan Projects and

then CP1

Notable Carbon Dates (2)

• 1949-57: Rosalind Franklin’s work on graphitizing and non-graphitizing carbons

• 1956: Calder Hall Nuclear Power Station

• 1956: Mrozowski cracks (fine shrinkage cracks that connected to the

globular porosity)

• 1957: Windscale Fire

• 1965: Taylor publishes the mesophase observations in coal

• 1985: C60 (mass spectroscopy)

• 1986: Chernobyl

• 1990: C60 (isolated crystals in soot)

• Post 1990: Fullerenes and nanotubes

• 1996: Nobel Prize in Chemistry to Professors Curl, Kroto and Smalley for their

discovery of fullerenes

• 2004: Paper published in Science on ‘isolating graphene’

• 2010: Nobel Prize to Professors Geim and Novoselov "for groundbreaking

experiments regarding the two-dimensional material graphene"

Calder Hall: 27 Aug 1956

(Magnox / PIPPA Design)

Mrozowski

Crack

Observations

1956

1957 Windscale Piles

The First Nuclear Disaster

(Uncontrolled Release of Wigner Energy)

• 1947 – Build

began

• 1950 - critical

Windscale (1957)

My old office

Chernobyl, 1986… Sometimes

Things Go Very Wrong!

Reactors Commissioned Decommissioned

Magnox Berkeley 2 1962 1989 Bradwell 2 1962 2002 Calder Hall 4 1956 2003 Chapelcross 4 1959 2004 Dungeness ‘A’ 2 1965 2006 Hinkley Point ‘A’ 2 1965 2000 Hunterston ‘A’ 2 1964 1990 Oldbury 2 1967 Sizewell ‘A’ 2 1966 2006 Trawsfynydd 2 1965 1993 Wylfa 2 1971 AGR Dungeness ‘B’ 2 1984 Hartlepool 2 1984 Heysham I 2 1984 Heysham II 2 1988 Hinkley Point ‘B’ 2 1976 Hunterston ‘B’ 2 1976 Torness 2 1988

2015

2012

UK - 7 AGRs + 1 PWR

Advanced Gas-cooled

Reactor (AGR)

Table 1.1. British AGR Graphites

Station Manufacturer Grade

Dungeness B AGL IM1-24

Hinkley Point B AGL IM1-24

Hunterston B AGL IM1-24

Hartlepool BAEL GCMB

Heysham I BAEL GCMB

Heysham II UCAR GCMB

Torness UCAR GCMB

AGL - Anglo Great Lakes Corporation Limited

BAEL - British Acheson Electrodes Limited

UCAR - Union Carbide (successor company to BAEL)

Each had the same specification!

Core Construction

There are issues with the QA and

acceptance of bricks perhaps and the

inducing damage during the

construction as well as variability on the

‘heat’ / batch properties.

Development of Nuclear

Graphite Grades

(adapted from Ball, 2008)

Year Country Reactor Type Coolant Graphite grade

1942-1950s USA (USSR) CP1, X10, W Air, H2O AGOT, TSX

1950-1960s UK (France) Magnox C02 PGA

1960s-1980s UK AGR C02 GCMB/IM1-24

1960 - UK AGR Sleeves C02 NNEA, Nittetsu

P2, ATR-2E,

PPEA, NBG-10

1980 - UK AGR Sleeves C02

1964 - 1976 UK HTR-Dragon Helium Test-bed

1966 - 1988 Germany Pebble Bed Helium GCMB

1967 - 1989 USA HTR Prismatic Helium H451

1991 Japan HTR Prismatic Helium IG110, PGX

1990’s South Africa PBMR Helium NBG-17

2000’s China PBMR Helium IG110

SEM Images of Machined Surface SEM Images of Fracture Surface

RMBK (Grade GR280)

Large Structures…

AGR Moderator Graphite

IM1-24

(Density = 1.8 g/cc,

Flex. Strength = 34 MPa)

So What are the Qualities

of Nuclear Graphite?

• Density >1.7g/cc

• High degree of graphitization (ind. by thermal conduct. >100 W/m/K @RT)

• Low neutron absorption cross-section of <4mbarns (also feeds into decom

costs) – controlled by purification strategies & minimising Boron & ash content.

• High irradiation dimensional stability indicated by CTE bet. 4 and 5.5 x 10-6 K-1

• Long irradiation time, indicated by critical fluence, at operating temperature

• Optimised strength (> 20MPa to perform structural duties) and elastic modulus.

• Low air (moisture) reactivity levels (LOCA)

CALCINATION AT ~ 1300-1400oC

COKE ‘FLOUR’

(2-300m)

COKE PARTICLES

(0.4-0.8mm)

MILLED

AND SIZED COAL TAR

PITCH BINDER

MIXED AT ~ 165-170oC

EXTRUDED, PRESSED AND MOULDED

RAW PETROLEUM COKE

CALCINED COKE

NUCLEAR GRAPHITES

~57%w/w~19%w/w

~24%w/w

BAKED TO ~ 800-1000OC

IMPREGNATED WITH ‘LIGHT FRACTION’ COAL TAR

PITCH

GREEN ARTICLE

BAKED ARTICLE

GRAPHITIZED AT ~ 2400-3000OC

INDUSTRIAL GRAPHITES

RE-BAKE AND RE-

IMPREGNATE TO REQUIRED

DENSITY

PURIFIED

Gilsonite & Gilsocarbon

Coke

Businessmen go down with their businesses because they like the old way so well they cannot bring themselves to change. …

Henry Ford, My Life and Times, 1922

Being disruptive…

By the way, he used lots of Gilsonite!

Gen IV Reactors & Beyond

Opportunities…

Chinese Pebble Bed HTR-10

VNEC Graphite

IG110 Graphite

source: David Ball - Presentation INGSM - 16

NBG-18 Graphite

source: David Ball - Presentation INGSM - 16

SGL Carbon - R4340

(Density = 1.72 g/cc, Flex. Strength = 40 MPa)

The Current Problem in the UK

The Graphite Moderator

Moderation of neutrons (<2MeV)

Structural Component (CO2 cooled)

Safety Case (functionality)

Structural integrity

Support unhindered movement of theInsertion of control rods / fuel stringers

Ensure adequate heat removal from the fuel(coolant flow)

Key Properties [f(T,g,r)]

Dimensional Change

Strength

Elastic Modulus

Coefficient of Thermal Expansion

Thermal Conductivity

Irradiation Creep

Kelly, B. T. (1985).

Prog. Nuc. Energy,

16, [1], 73-96.

(Neutron irradiation and radiolytic oxidation)

Fission Chain Reaction

n• Irradiation

– c-axis expands– a-axis shrinks

ac- 27.00*10-6 K-1 (STP)

aa- -1.5 * 10-6 K-1 (STP)

C

A

Spiro-interstitial (Heggie, Sussex)

Differential Stressing

Components subject to extreme conditions.

Internal stresses

Potential cracking

In essence, a critical stress criterion is

currently used to predict failure in an AGR

moderator brick.

And for Nuclear Cores its

Complex!!!

An ‘Unified’ Model

Thermal Transients

Future?

Multi-Scale Fracture Modelling

(Issues of Scale)

In Simple Terms…

Log sn

Log D

Strength

criteria

WeibullLEFM

transitional

Scaling Factor = lm = (D/D0)m

Gilsocarbon Graphite

Consistent view of what we see?

Two or more carbonaceous species

originating as filler, binder or

impregnant;

A wide range of crystallite perfection

and crystallite sizes in different parts

of the microstructure, dependent upon

raw materials and manufacturing

processes;

Complex networks of pores of

different types that originate at

different stages in the manufacture of

the graphite.~0.5 mm

But Nuclear Graphite is …

• Highly heterogeneous microstructure (at

microstructural level) and deviates from classical

behaviour (non-ideality value of 0.65)

• Complex networks of pores of different types that

originate at different stages in the manufacture of

the graphite.

Open Pores

Closed Pores

Transport Pores

Blind Pores

• Shape and size of filler / porosity dictate mechanical

performance thus difficulty in predicting mechanical

performance.

Especially in extreme environments

r

r

Complications in

Understanding Performance

• Complexity of Microstructure

• Crack/Fracture Surface Morphology &

Machined Surfaces

• Fracture Process Zone/Sub Critical

Processes & Non-Ideal Behaviour

• Scale

• Degradation Mechanisms (e.g.

Radiolytic Oxidation & Fluence)

Microstructure Simulation-

Computational Model

G

F

B

S

C

P

G: Gilsocarbon particle, F: fragmented Gilsocarbon particle, P: highconcentration of porosity, B: Binder phase, C: calcination cracks, S: fillerparticle boundary region,

G

F

B

SC

P

‘Probabilistic’ Microstructure IM1-24 Micrograph

Modelling Degradation and Scale

Actual size

10 cm2

30%

40%

Microstructure Simulation-Computational

Model

Ra =

90.9m

White Light

Interferometery

Ra = 65.2m

Width = 1.5 cm

Microstructure Simulation-

ANSYS Model Principal

Stress Distributions

200x200 Unit

Model

Applied Load / Stress

via displacement

Large areas stress freeTension: High stress regions are

often found along discontinuities

between various constituents –

“stiffness attracts load”.

Porosity is not always the

dominant factor, i.e. initiation

sites for fracture.

Transmission of forces are highly

non-uniform / complex.

3D Model

CT scans input into AMIRA to generate 3D microstructure model

Crack Propagation

Through FEA model

Tensile Load

0

200

400

600

800

1000

1200

1400

1600

0 0.05 0.1 0.15 0.2 0.25 0.3

Load

(N

)

Load-Point Displacement (mm)

Predicted Load-Deflection

Curve

(Ouagne, 2001)

Use elemental data to determine bulk

mechanical properties such as total strain

energy and so loads can be calculated and

plotted against displacement (input)

Key Lessons

Nuclear graphites still presents a challenging

task… and I haven’t even talked about disposal!

Hindsight:

• Suitable cokes with a stable future (structure, size and purity).

• Coke with low impurities lessening the need for purification.

• Optimization of resultant properties (IAEA Database) … material selection is

extremely important in terms of understanding performance through life.

Graphite is a life limiting issues

• Design flaws (sharp corners) / consistent reactor design!

• Understand complex (micro-) structure-property relationships & multi-scale

principles as well as the inter-relationship of properties with irradiation

performance (plant operating rules) with a view to predict performance of large

scale components.

• Coupled investigations which link models, theory and experiment ultimately will

pay dividends… for the next generation of graphite materials… need

collaboration of graphite producer, reactor designer, irradiation agency &

others.

Disruptive Technology

Further Information!

For further information:

http://mems.brookes.ac.uk ;

http://www.journals.elsevier.com/carbon/

www.facebook.com/brookes.mems

@OBU_MEMS, @GNEIGHBOUR

Email – gneighbour@brookes.ac.uk

www