Nuclear Graphite: The End of the Beginning?

A.J. Wickham, Nuclear Technology Consultancy;

Visiting Professor, The University of Manchester

and

G.B. Neighbour, Oxford Brookes University

December 2011

December 2011

Structural Material

December 2011

A UK AGR in the

course of construction:

Graphite blocks

~1 metre high form the

structure which

supports the fuel (with

which graphite sleeves

are also used in this

case)

AGR Fuel Elements with Graphite Sleeves

December 2011

A reactor is a VERY large machine

University of Chicago Squash Court “CP-1” December 2nd 1942

December 2011

The Beginning - CP1

Example Fission Process

December 2011

Moderation of Nuclear Fission Reaction

December 2011

Moderators slow down

neutrons so that they can

interact with further

fissionable atoms

Low atomic number

elements are good

moderators: hydrogen (or

deuterium) frequently used,

but graphite was seen to

offer some particular

advantages

Numerous sources of design information are available...

December 2011

Designs:

•Air Cooled

•Water Cooled

•CO2 Cooled

•Helium Cooled

•RBMK mixes graphite

and tubes with boiling

water

Basic ‘Magnox’ Technology

December 2011

AGRs have a more

complex gas-flow system

around the core, but the

basic principal is the same

Advantages and Disadvantages of Graphite Moderators

Large passive thermal resistance (high heat capacity) gives long period of ‘grace’

to deal with any malfunction such as air ingress to a hot reactor;

AGRs tolerant to significant degradation of the structure;

Can function without enrichment of fuel (e.g. Magnox)

On-load re-fuelling is possible;

BUT...

Classical designs are very large;

Expensive to construct;

Does not utilise the nuclear fuel as efficiently as (for example) water reactors and

fast breeders;

Quite low efficiencies compared with other reactor types (economic issue)

December 2011

Graphite-moderated reactors world-wide (major plant only) UK: 2 small experimental (‘40s); 2 plutonium producers (air cooled - 50s); 24 Magnox

commercial reactors, 1 prototype AGR; 14 AGRs: currently operating 3 Magnox and 14

AGRs (all CO2 cooled)

USA: Early Chicago Piles; 9 Plutonium producers at Hanford, WA, also X-10 in Tennessee –

all closed

France: 3 Plutonium producers and 5 UNGG (rather similar to UK Magnox) – all closed

Italy: 1 UK-designed Magnox reactor (closed)

Japan: 1 UK-designed Magnox reactor (closed); 1 experimental HTR (in operation)

Germany: 2 experimental ‘pebble-bed’ HTR designs (AVR and THTR)

Former USSR: ~6 plutonium producers and 27 RBMKs or forerunner designs thereof

North Korea: 1 plutonium producer: miniature version of UK Calder Hall design

China: a number of essentially defunct plutonium producers; 1 experimental HTR (in

operation); commercial modular HTRs under construction

South Africa designed PBMR but government removed funding December 2011

The Learning Curve: Windscale Pile 1, 1957

December 2011

Windscale after the fire

December 2011

Points to note regarding Windscale:

Original cause of the incident was lack of understanding of ‘Wigner energy’ in

which displaced carbon atoms are in higher energy positions than in the normal

hexagonal lattice: this phenomenon is important at low irradiation temperature,

when potential rate of release of energy per unit temperature rise can exceed the

specific heat capacity of the graphite: deliberate anneals ‘went wrong’;

Reducing the air flow allowed temperatures to rise even further whilst increasing

the air flow ‘fanned the flames’; we no longer use such low temperatures, and we

no longer use air as coolant;

Fire originated in failed fuel and isotope cartridges;

Graphite did not burn (you need a temperature in excess of 3300C to achieve

that): the ‘blue flames’ were carbon monoxide burning from the thermally-induced

oxidation reaction (in a limited supply of air)

2C + O2 = 2 CO

December 2011

Looking closer at Nuclear Graphite

December 2011

Example

Pile Grade A (PGA)

Magnox Reactors

Anisotropic

Needle Coke

IM1-24

Advanced Gas-cooled Reactors

Near isotropic

Gilsocarbon filler particles

500μm 800μm

Filler Particle (parallel to extrusion)

Polygranular graphites differs from single crystal graphite in that it has:

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.

Historically, nuclear graphite (density r0)

has been viewed as an assembly of single

graphite crystals (density rc) with

interconnected porosity.

~0.5 mm

AGR Moderator Graphite

IM1-24

(Density = 1.8 g/cc,

Flex. Strength = 34 MPa)

The Graphite Moderator

Moderation of neutrons (<2MeV)

Structural Component (CO2 cooled)

Safety Case (functionality)

Structural integrity

Support unhindered movement of the Insertion 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)

Nuclear Graphite is also Porous...

December 2011

...this allows radiation-

induced oxidation to occur

throughout the structure

in air or CO2 coolants

Irradiation Damage in Graphite

(from Fast Neutrons)

December 2011

a

c

a

‘Classical’ forms of

irradiation damage

• 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

Polygranular Graphite: A Classical View

Mrozowski Crack Observations

AIMPRO Modelling reveals new stable structures

December 2011

... and examination of old micrographs offers

evidence of bridges and ‘ruck and tuck’...

...graphite irradiation behaviour

Is VERY complicated...

Examples of Irradiation-Induced Property Changes

December 2011

Irradiation induced changes in Gilsocarbon at 550oC (EDT)

Dose (EDND) n/cm2 x10

20

0 20 40 60 80 100 120 140 160 180 200

a K

-1 x1

0-6

1

2

3

4

5

E/Eo

-1

0

1

2

3

4

V/

V

-8

-6

-4

-2

0

Ko/K

-1

0

1

2

3

4

5

6

7

8

Dose vs CTE

Dose vs E/Eo-1

Dose vs DV/V

Dose vs Ko/K-1

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.

Differential Stressing

Compressive

Tensile

KeywayPotential

crack

END OF

LIFE

Compressive

Tensile

Keyway

START OF

LIFE

LIFE

Before turn-around, bore shrinks faster than periphery

After turn-around, periphery shrinks faster than the bore

Neutron Flux, Temperature and Radiolytic Oxidation Gradients

Differential Property Changes

Generation of Internal Stresses

Enhance understanding of the ‘feature’ strength, notch sensitivity and fracture initiation.

Thermal transients

Ultimately you want …

Fracture model to predict which bricks will fail under reactor

conditions (radiolytic oxidation and neutron irradiation).

Better demonstrate the continued functionality of the reactor core.

Ensure safe operation as well as life extension.

But…!

Nuclear Graphite

Highly heterogeneous microstructure (at microstructural level).

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

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)

Complications in Understanding Performance

Crack Paths & Surfaces

Polygranular graphite is a quasi-brittle materials just like others!

Examples

Poly-granular graphite

Bone

Concrete (& Rock)

& Others

(Ouagne, 2001)

Quasi-brittle materials exhibit size and scale effects

Mic

ros

tru

ctu

re S

imu

lati

on

-Co

mp

uta

tio

nal M

od

el

G

F

B

S

C

P

G: Gilsocarbon particle, F: fragmented Gilsocarbon particle, P: high concentration of porosity, B: Binder phase, C: calcination cracks, S: filler particle boundary region,

G

F

B

S C

P

‘Probabilistic’ Microstructure IM1-24 Micrograph

Ra = 90.9mm

White Light Interferometery

Ra = 65.2mm

Width = 1.5 cm

Microstructure

Simulation-

Computational

Model

Modelling Degradation

Actual

size

10 cm2

30%

40%

Crack Propagation Through FEA model

Complex, bifurcation, multiple locations, process zone, etc.

Tensile Load

Key Lessons

Graphite is a life limiting issues

Design flaws (sharp corners) / consistent reactor design!

Material selection is extremely important in terms of understanding

performance through life (tailor).

Attention early on for structure-property relationships and multi-scale

principles as well as the inter-relationship of properties.

Need to understand mechanistically the initiation and propagation of

fracture any industrial application of polygranular graphite, especially

NNP, to predict performance and that needs coupling to well

understood plant operating rules.

The Future Reactor Designs?

December 2011

And some lessons!!!

Chernobyl Unit 4 (RBMK)

A combination of poor design (mixed moderators) and lack of

any safety culture...

December 2011

The Chernobyl

Catastrophe: a

prototype

for nuclear terrorism

?

December 2011

December 2011

“My house: forgive me, and farewell”

Prospects for the future of graphite reactors: HTR designs

Offer high temperature outlet gas and therefore high-grade process heat which can be used

(for example) to produce hydrogen from water for motor fuel using the Bunsen reaction;

Higher efficiency of energy production anyway, using ceramic fuel particles, so overall

utilisation efficiency is much higher;

Possibility to use gas turbine rather than steam boilers;

Inherently safe systems from reactor-physics (accident) point of view;

Two basic design options:

prismatic graphite blocks with embedded fuel tubes containing particles

‘pebble’ fuel: 60mm diameter ‘graphite’ balls also containing embedded fuel particles

Countries currently interested: China, Japan, Korea, USA, Russia, (South Africa), other

developing economies in SE Asia: other countries involved in the technology include The

Netherlands, Germany (unofficially, they developed the pebble-bed concept), even UK

design companies such as AMEC

December 2011

Japanese HTTR Concept

December 2011

Chinese Prototype Pebble Bed under Construction (HTR-10)

December 2011

Reactor

Core Barrel

Conditioning

System Maintenance Isolation/Shutdown Valve

Generator

Power

Turbine

Recuperator

High Pressure

Compressor

Low Pressure

Compressor

Gearbox

Inter-Cooler

Core Conditioning

System

Pre-Cooler

PBMR Concept: Pebble-Bed Modular Reactor

Harvesting the Knowledge on Nuclear Graphite

December 2011

IAEA project pages

Hosted MS SharePoint

Scalable solution

International Portal

Modules: Member states

Wiki

Reports

IAEA file servers

-Existing DB

-INGSM

-Integrated training

www.iaea.org/NuclearPower/Graphite/

Levels of access:

- General public

-Project members

Wiki

Introductory

Simple to navigate & edit

No software required

High availability

Levels of access

-Project members

Detailed Reports

-Current technical

reports

-Historical scanned

reports

Navigation

-Context searching

-Tagging

-Related documents

The (Radioactive) Waste Issue

~250,000 tonnes of irradiated graphite world-wide to be disposed of;

Most is destined (as “Intermediate Level Waste”) for deep repositories (which do not exist);

Tendency to leave it in reactor pressure vessels for short half-life radioactivity to decay;

Alternative plans are under investigation:

CARBOWASTE (EU Initiative)

IAEA Collaborative Research Programme on ‘Treatment Options’

Independent research initiatives (e.g. Universities of Idaho and Manchester)

Possibilities include:

Recycling within the nuclear industry;

Extraction of useful isotopes

Separation of troublesome isotopes, so ‘downgrading’ the residual waste category

December 2011

HSE and EA Generic Safety Assessment and thus most likely:

UK-EPR (European pressurised water reactor) by Areva and EDF

Westinghouse AP-1000 (AP=Advanced Passive)

Completed July 2011

Opportunities for New

Construction:

•Evolutionary Light Water Reactors,

e.g. ABWR

•Generation III+ (designs that can be

built this decade), e.g. AP-1000,

ESBWR, PBMR

•Generation IV (Advanced with

integrated nuclear energy systems

with goals: sustainability, safety and

reliability economics)

•Fusion

How ‘Green’ is this?

December 2011

State of Play

“At present nuclear power (which is carbon-free at point of generation)

provides some 15% of the world’s electricity, around 15% of the EU’s

electricity, and 20% of Britain’s. This figure represents more than 80% of all

UK’s present low-carbon-power.”

NIA Nuclear Link 25 pg 1 Sept 2009

Hazardous Materials IV

How do we dispose of hazardous wastes?

Land, Sea or Air

Have to ensure no leaching, migration,

etc. (double liners, collection systems,

monitoring facilities)

“Concentrate and contain” or “dilute and

disperse”

Approaches to hazardous waste - hierarchy

of priorities.

Eliminate Waste (Prevent)

Reduce Generation

Recovery / Recycle

(inc. energy recovery)

Treatment

Disposal

Reuse

The Waste Hierarchy

Waste Hierarchy

Sellafield Adjusted Waste Management Hierarchy. From Nuclear Future, 4, [6], pg 334

Sustainability

Managing to be environmentally responsive

Given that our planet is not a sustainable system by itself, a sustainable product is a

product, which will give as little impact on the environment as possible during its lifecycle.

Sustainability is (i) doing the right thing and (ii) doing the thing right!.

In order to be sustainable, accepted actions should be 'environmentally sustainable,

economically viable, technologically feasible, socially desirable/tolerable, legally

permissible, administratively achievable and politically expedient. In other words, “Use

and development that meets today’s needs without preventing those needs from being

met by future generations”. Sustainability encompasses the Triple Bottom Line of

economic, environmental and social responsibility. In other words it involves human,

financial and natural capital.

Datschefski’s “the total beauty of sustainable products”. He found that 99% of all

environmental innovations use one or more of these five principles: cyclic; solar; safe;

efficient; social.

Hitting the Wall

Non-sustainable

activity Sustainable activity Restoration

Population and Demand (increase)

Capacity and quality of: (i) water, oceans and fisheries; (ii) arable land;

(iii) climatic stability; (iv) capacity of environment to absorb waste;

(v) forest cover; and (vi) food (decrease).

Think about the Triple Bottom Line

Brundtland Report: “Humanity has the ability to make development sustainable – to

ensure that it meets the needs of the present without compromising the ability of future

generations to meet their own needs”.

December 2011