Something about nuclear graphite

Tongxiang LIANG

Nuclear Graphite Research Group

Institute of Nuclear and New Energy Technology

Tsinghua University

txliang@tsinghua.edu.cn

北京 陶瓷

OVERVIEW

• History

• Requirements

• Manufacturing

• Microstructure

• Irradiation performance

• Disposal of spent graphite

History of nuclear graphite

• In 1919 Rutherford discovered the proton, and put out the idea that there could be a particle with mass but no charge. He called it a neutron.

• James Chadwick discovered the neutron in 1932, received the Nobel Prize for his discovery in 1935.

Neutron is the golden key to open the gate

of nuclear energy

• Physicists soon found that the neutron is an ideal "bullet" for

bombarding other nuclei. In 1934, Enrico Fermi found that

slow (thermal) neutrons striking the target will be more likely

to collide with silver atoms; the increased collisions result in

higher radioactivity.

• 1938, Otto Hahn, Lise Meitner and Fritz Strassmann

discovered that uranium nuclei split when bombarded with

neutrons.

• The nuclear cross section of U235 for slow neutrons is about 1000 barns, while for fast neutrons it is in the order of 1 barn.

Therefore thermal neutrons are more likely to cause U235 to fission than to be captured by U238.

• How to obtain slow neutron? use a neutron moderator such as graphite, Be, water, to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutron.

• In 1942 the Manhattan Engineer Project was set up in

the United. Scientists recruited to produce an atom bomb

included Robert Oppenheimer (USA), David Bohm (USA), Leo

Szilard (Hungary), Eugene Wigner (Hungary), Rudolf Peierls

(Germany), Otto Frisch (Germany), Niels Bohr (Denmark), Felix

Bloch (Switzerland), James Franck (Germany), James Chadwick

(Britain), Emilio Segre (Italy), Enrico Fermi (Italy), Klaus Fuchs

(Germany) and Edward Teller (Hungary).

• In order to produce Pu, Chicago Pile 1 (CP-1) reactor was built.

Fuel of CP1 CP-1 reactor was a pile of uranium and graphite blocks, graphite is neutron moderator

Graphite from CP 1

Then, graphite has been widely used as a

moderator, reflector and fuel matrix in various

types of nuclear reactors, such as gas cooled

reactor (e.g. AGR, MAGNOX), Russian RBMK

reactors, high temperature gas cooled reactor

(Dragon, Peach Bottom, AVR, THTR-300, Fort

St. Vain, HTTR, HTR-10 ) and so on.

Magnox AVR

THTR 300

Dragon reactor

HTR-10

M.S.T. Price, Nucl Eng and Design, 251 (2012) 60-68

Requirements

Nuclear designer requires:

six high and four low •High purity (neutronic and waste point of view, Boron

content)

•High density (The greater the density, the greater its

moderation of neutron flux)

•High irradiation stability (The irradiation behavior is

strongly influenced by the source of the pitch, the coke

and the manufacturing process)

•High thermal conductivity

•High strength

•High oxidation resistivity

• low anisotropy, less than 1.1, defined by

coefficient of thermal expansion (CTE) in

orthogonal directions

• low CTE, ~4 x 10-6 K-1 (20 - 120℃)

• low elastic modulus

• low cost

The graphite crystal is anisotropic; i.e. its properties are different in perpendicular and parallel directions relative to the principal alignment of the basic planes

Nuclear graphite IG11 NBG18

Filler petroleum coke Pitch coke

Mean particle size ( µm) 20 300

process isostatic pressing vibration molding

Density (g/cm3) 1.78 1.85

Flexural strength (N/mm2) 37 30

Tensile strength (N/mm2) 25 24

Compressive strength (N/mm2) 77 77

Thermal conductivity (W/mK) 78

CTE (10-6/℃) 4.1

Anisotropy 1.05 1.03

Basic properties of nuclear graphite

Philippe Beghein et al, Nuclear Eng and Design, 251 (2012) 146-149

Philippe Beghein et al, Nuclear Eng and Design, 251 (2012) 146-149

Nuclear Graphite Manufacturing

• cold isostatic pressing or vibration molding is used for shaping in order to get low anisotropy.

• After baking (carbonization), the artifact is typically impregnated with a petroleum pitch and re-baked to get densification. Impregnation and re-bake may perform several times to attain the required density.

• Graphitization typically occurs at temperature >2,500°C.

• Additional halogen purification may be required.

W Winders, et al, Project 23747, Graphite Technology Development Plan, PLN-2497. Idaho Falls:

Idaho National Lab.2007

Raw materials: 1) Calcined coke-----petroleum or pitch coke (size and distribution) 2) Binder----coal tar pitch (soft point and carbon yield, thermoplastic) 3) impregnants---coal tar pitch or polymer 4) additives

mixing

How to get isotropic graphite?-----coke/manufacture

process

•A needle coke known as AGOT graphite was used in the Hanford

piles and achieved some undesirable results. Needle coke is

anisotropic, and when irradiation it develops large internal stresses

in one direction, which results in cracking and a decreased

irradiation lifetime.

Magnox, windscale, WAGR

piles used needle coke and

extrusion processed graphite

as moderator

• Gilsonite coke from natural mine in Utah USA,

particles have “onion skin” structure, molding

process gives isotropic property.

Needle coke

Isotropic cokes are favored for nuclear graphite

AGR, THTR 300 used, no longer exist

• Germany developed a “second coke” method:

coke + binder----blended----baked----milled,

mix with binder----blended----baked---

impregnated----graphitized

• In order to get isotropic behavior, Isostatic

pressing or Vibration molding is used

Isostatically presser

oil

Vibration molding machine

Baking or carbonization 1000~1250℃, Binder to carbon

A N Jones and B J Marsden, CARBOWASTE Abbeye St Jacut 25th -28th Oct 2010

Autoclave Impregnation By pitch to improve density

Using low soft point, high carbon yield pitch

A N Jones and B J Marsden, CARBOWASTE Abbeye St Jacut 25th -28th Oct 2010

Acheson and lengthwise graphitization

furnace

Acheson lengthwise

Acheson and LWG

semi-products themselves act as resistor

Philippe Beghein et al, Nuclear Eng and Design, 251 (2012) 146-149

machining

The final product

• There is a big shrink rate difference between pitch

and coke particles during the baking period, as a

result internal stress will be produced that may lead to

the cracking of graphite products.

• The green coke’s or mesocarbon microbead (MCMB)

surface is chemically active, which makes it easy to

combine with binder. Furthermore, coke and binder

have a same shrink rate during the heat treatment

process. Graphol series graphite designed by Oak

Ridge National Laboratory belong to green coke

graphite, the damage strain of graphol graphite is

larger than that of conventional graphite.

Coke size: •Particle size, pore or crack size and distribution have great effect on the performance of graphite. Isostatic pressing graphite (e.g. IG110) used fine-particle-coke, about 20 m, vibration forming graphite (e.g. NBG18) has a large particle size, 300 m, and they have different fracture behavior.

•For coarse-particle graphite, it did not break immediately when the crack grow to the critical value. But the fine-particle graphite damaged quickly as the crack began to grow.

• The fracture toughness is increased due to energy dissipating mechanisms such as crack deflection, crack bridging and pull out.

•Coarse-particle graphite exhibits low sensitivity on the crack than the fine-particle graphite. Upon repeatedly loading, coarse graphite fracture toughness increased but fine-particle graphite decreased.

Microstructure

J. Kane, et al, Boise State University

Very complex structure

C Karthik, et al, Boise State University

G: Giloscarbon filler particle

B: binder phase

C: calcination cracks

E: gas entrapment pores

needle coke filler particle

Optical micrograph of two graphite

Paul J Hacker, J. Phys. D: Appl. Phys. 33 (2000) 991–998

Mrozowski cracks

Mrozowski cracks, with lengths from tens of nanometres to more than 10 µm and width from several nanometres to about 200 nm.

TEM of Virgin Graphite

(a) Graphite lattice; (b) turbostratic graphite

Upon heating, graphite increases

its order of crystallinity

Mrozowski cracks were formed due to the thermal expansion coefficient difference along and normal to the grain of graphite during slow cooling process of graphitisation.

Irradiation performance

• Under irradiation, graphite undergo changes in its thermo-

mechanical properties, especially via swelling and irradiation-

induced creep, which affects the graphite’s in-service life time.

• understanding of these life-limiting phenomena is very

important for the development of new nuclear graphite. The

current understanding is that the ballistic displacement of carbon

atoms caused by irradiation results in the accumulation of

interstitial atoms between the basal planes. These interstitial

clusters eventually rearrange to form new basal planes resulting in

the expansion along the c-axis; however, this explanation has been

disputed by several researchers.

• The historic nuclear graphite no longer exist; must

characterize the microstructures of new nuclear graphite

and demonstrate that they exhibit acceptable properties in

both the non-irradiated and irradiated state.

Upon neutron irradiation a neutron will knock carbon

atoms from the basal plane and cause the formation of a

vacancy

New plane forms, c-axis expansion

Irradiation : the <a>-axis shrinkage and <c>-axis growth

46

B.T. Kelly et al, IAEA-TECDOC-1154, 2000

The in-plane c-c bond is very strong, the interplanar bonding is weak

Atom is difficult insert into the basal plane

C. Karthik, et al, Boise State University

the cracks tend to close upon heating

closure of the cracks by electron irradiation

Keyun Wen, et al, Journal of Nuclear Materials 381 (2008) 199–203

In-situ e-Beam Irradiation Effects of NBG 18 graphite Crack closing by the swelling of c direction

J. Kane, et al, Boise State University

(1) (2)

(3)

In-situ HRTEM of NBG-18

graphite under e-Beam

irradiation

Mrozowksi cracks

J. Kane, et al, Boise State University

At the beginning of irradiation, a-axis shrinking and c-axis swelling, but the swelling is taken up by the accommodation cracks.

The dimensional change is shrink

After the cracks closed, the c-axis accommodation has been filled, turnaround

swell

Ne

utro

n d

ose

How to determine the lifetime of reflector graphite?

Shrink—swell, at the point of “0” changes

53

dim

ensi

on

al v

aria

tio

ns

Neutron dose

Lifetime

low anisotropy

has a longer

lifetime

Turnaround. After turnaround pore generation and expansion will continue until internal stresses are large enough to propagate cracks

Dimensional instability under irradiation is one of the main problems for nuclear graphite

For the moderator and reflector of HTGR, the

lifetime of graphite is about 40 years, it must

have a higher irradiation stability.

The irradiation behavior is strongly influenced

by its crystallographic structure, i.e., by the

source of the pitch, the coke and the

manufacturing process.

Which kind of raw materials and process is

benefit for irradiation? Unknown! need further

research.

has strong bonds in the basal plane, a-axis; weak bonds in “c” direction

Modulus is greater in “a” direction

The coefficient of thermal expansion is much greater in “c” direction

Heat is transferred by lattice vibration (phonon) along the basal plane, thermal conductivity is greater in “a” direction

Other behavior Graphite crystal Polycrystalline graphite

Always have some degree of preferred orientation,---with grain (WG) or against grain (AG)

Higher modulus in WG direction

Lower CTE in WG direction

Higher thermal conductivity in WG direction

Irradiation induce defects, phonon-phonon scattering, thermal conductivity decrease

Cracks closure lead to the CTE rise

Irradiation defects pinning dislocation, and crack closure, so modulus increase; high dose, large pores and crack, modulus decrease

Irradiation

（200）peak as the irradiation dose (at 30 ℃), irradiation reduce the graphitization degree

56

Irr. dose

Change in Young’s modulus

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200

CTE

α/α

0

Dose n/cm2 EDND x 102

430oC

600oC

940oC

1240oC

Just like Young’s modulus, Initial increase of CTE attributed to closure of Mrozowski cracks. Subsequent decrease attributed to opening of new porosity.

Thermal conductivity decreases as the neutron dose increasing

59

Disposal of spent graphite

After decommissioning, a HTGR with 200MW will

produce about 600 tons radioactive nuclear graphite,

and during the annual operation, the discharged spent

fuel contains about 60 tons graphite.

Up till now about 250,000 tons irradiated graphite

has to be disposed as radioactive waste, though with

low radioactivity. How to dispose the large volume of

contaminated graphite reasonably needs to be addressed

if we want to develop HTGR reactor sustainably.

Low Level Waste (LLW).

• Most radioisotopes can be removed by high

temperature annealing.

• 14C has a half-life of 5730 years, and is a

biologically hazardous substance because of its

readily assimilation by the human body, how

to dispose 14C becomes the key issue for

management of nuclear graphite during

decommissioning of graphite moderated

reactors.

J. Fachinger et al，Nuclear Engineering and Design 238 (2008) 3086

Experimentally determined 14C specific activity

was 130±20 kBq/g for the RBMK-1500 reactor

14C activity measured by LSC as a function

of fast neutron fluence. (D. Lexa, J of

Nuclear Materials 2006,348,122)--the

ASTRA research reactor

There are several principal solutions possible:

•Packing and disposal of graphite in appropriate repositories

•Incineration of graphite with the exhaust of CO2 gas

•Recycling.

Every management route has its strong and weak aspects.

Disposal option is simple and cheap, but this requires building specialized repositories. Disposal is not very attractive from the point of view of nuclear energy sustainability.

Incineration means that all 14C would be emitted to the atmosphere. Incineration of one RBMK-1500 reactor graphite (1700 tons of graphite, ∼7×1014 Bq of 14C)would increase the amount of 14C in the atmosphere by 0.6%.

is the most challenging option

• Recycling of nuclear graphite is of great practical value. However, it is very difficult to separate 14C from the graphite matrix.

• Steam pyrolysis is an effective technology for the remove of 14C

• The mechanism of steam pyrolysis for removing 14C is based on the fact: most of 14C is located on the graphite surface, the inner surface of pores or grain boundary, then it becomes 14CO2 and 14C O when oxidized by water or oxygen.

14 N + n ----14C

Schematic drawing of steam pyrolysis

Summary • HTGR graphite’s life is about 40 years, if prolong to 60 years, much

more research works should be done on graphite.

• Research should focus on the following materials: Graphite using

green coke as raw material has a high fracture strain, vibration

molding of coarse particle graphite shows special fracture

toughness.

• In recent decades, materials science has made great progress, but

nuclear graphite research came to a standstill since the 1980s.

Improvement on performances of nuclear graphite may be inspired

by introducing new ideas, including nano-carbon materials, anti-

oxidation coating, mesophase carbon, and so on.

• Steam pyrolysis method is the effective technology for removing

14C from spent graphite. Recycling of waste graphite for nuclear

graphite fabrication is of great practical value due to sustainability.

Nuclear graphite research institutions

• The University of Manchester

• University of Bath

• Oak Ridge National Laboratory

• University of Oxford

• SGL

• INET---will publish some papers in three years

INET’s plan

Research program in my group:

•Manufacture and properties of nuclear graphite

•Irradiation of graphite, including modeling and simulation by the first principle, Molecular Dynamics, MC…..

•Disposal of nuclear graphite waste