Slide 2

Graphite core components and the graphite core assembly

of a High Temperature Gas-cooled Reactor (HTGR)

Mike Davies*

Member of the Subgroup on Graphite Core Components

(SG-GCC)

Presented at SG-MF&E 15 May 2013, Miami, USA

*Specialist Engineering Design,

AMEC Nuclear, UK

Slide 3

Contents

• Graphite manufacture, material properties, ASTM specifications

• Graphite core components – machining, examination, and

testing

• Defects in graphite and what is allowable

• Graphite core component installation (to form a core assembly)

• Graphite core assembly inspection

• Cleanliness requirements

Slide 4

Graphite manufacture (1)

Impregnated

carbon block

Basic carbon

block

Finished

graphite block

Green block

Graphitisation (2800oC)

Impregnation with pitch

Baking (1000oC)

Mixing

Grinding

Calcining

CokePitch

Extrusion/Moulding

Slide 5

Graphite manufacture (2)

Slide 6

Extrusion of cylindrical block

Slide 7

Extrusion of rectangular block

Slide 8

Typical baking furnace

Slide 9

Graphitizing furnace - loading

Slide 10

Graphitizing furnace - operating

Slide 11

Typical microstructure of a medium grain

extruded graphite

Slide 12

Material Specifications

ASTM Standard Material Specifications - Graphite:

○ D 7219-08 Standard Specification for Isotropic and Near-isotropic Nuclear Graphites:

- Suitable for reactor core application where neutron irradiation induced dimensional changes are a significant design consideration.

○ D 7301-08 Standard specification for Nuclear Graphite Suitable for Components Subjected to Low Neutron Irradiation Dose

- Suitable for reflector/shielding blocks and core support structures

• Incorporated as Mandatory Appendix 1.

Slide 13

Examples of nuclear grade graphite

Manuf. Grade Coke Forming Method

GrafTech PCEA Petroleum Extrusion

PPEA Pitch Extrusion

PCIB Petroleum Iso-moulding

SGL NBG-18 Pitch Vibro-molding

NBG-17 Pitch Vibro-molding

NBG-25 Petroleum Iso-molding

Toyo Tanso IG-110 Petroleum Iso-molding

IG-430 Pitch Iso-molding

Slide 14

Typical properties

• Density: 1.75 to 1.85 g/cm3

• Tensile strength: 20 to 30 MPa

• Bend strength: 30 to 45 MPa

• Compressive strength: 70 to 90 MPa

• Young’s modulus: 9 to 12 GPa

• CTE: 3.5 to 4.5 (10-6/K)

• Thermal conductivity: 120 to 150 W/m/K

Slide 15

Machining

Slide 16

Examination (dimensional checks)

Slide 17

Examination for defects

• Graphite defects

– Cracks, inclusions, high porosity, blow holes

• Machining/handling defects

– Chips

• Visual examination can only pick up surface defects, and these are compared with allowables

• Specification/sketches produced for allowables

• Ultrasonic/eddy current techniques can be used for detecting internal defects, but a bit limited, so not being implemented yet

Slide 18

Allowable defects (1)

Slide 19

Allowable defects (2)

Qualifications of examination personnel

• HHA-5000 states that:

– All personnel performing nondestructive examination operations under

this Article shall be competent and knowledgeable of the applicable

examination requirements and procedures.

– All examinations required by this Article shall be performed and the

results evaluated by qualified examination personnel.

– All nondestructive examination personnel shall be qualified in

accordance with the requirements of Section V, Article 1, T-120(e).

Slide 20

Slide 21

Testing

• Testing may be a specified requirement after machining to

ensure a Graphite Core Component is acceptable, i.e. there are

no significant internal defects or low strength

• Internal pressure testing – used for the cylindrical fuel sleeves

in the UKs AGRs – essential for preventing defective sleeves

entering service

• Deadweight testing – particularly important for Graphite Core

Components with a support function

Slide 22

Flow diagram for machining

Start with manufactured

graphite block

(labelled with unique

number for traceability)

Before machining, check

details

Machine feature(s)

Inspect for defects and

check dimensions

OK?QuarantineFinish

machined?

No

No

Yes

Yes

Carry out any testing on part,

e.g. deadweight or pressure

OK? Reject

Yes

No

Package (complete with

appropriate paperwork)

Store/transport to site

Reject

Repair

Slide 23

Graphite core design – AGR (UK)

Slide 24

Graphite core assembly – AGR (UK)

Slide 25

Graphite core components forming the

assembly

Graphite Core Assembly Construction

Tolerances

Slide 26

Slide 27

Layer/assembly checks

Slide 28

Flow diagram for installation

Transport package from site

store to unpackaging area

Unpackage block and check

features and any defects or

damage against accompanying

paperwork

Load into reactor vessel for

installation

OK ? Quarantine

Assemble part in correct

location ( as per the

construction drawing ) and

record unique number

Graphite layer complete

Install side restraints and

adjust core boundary to

plan position

Undertake complete layer

check

All layers assembled ?

Undertake complete check

of channels etc

No

Yes

Yes

No

Yes

No

Accept

Reject

Mandatory Appendix - Cleanliness

requirements (1)

• This Appendix addresses the contamination control

requirements for graphite core components and assemblies.

• The requirements apply from billet manufacture to packaging

and storage at the material manufacturer; from storage through

to machining, examination, testing and packaging at the

component manufacturer; and from storage through to

unpackaging, examination and installation of components to

form the graphite core assembly at the construction site.

Slide 29

Slide 30

Mandatory Appendix - Cleanliness

requirements (2)

• Two standards shall be used in the design process for new

facilities, or for the assessment of the suitability of existing

facilities, and the development of operation procedures.

• These standards identify documentation that should be

maintained for verification of contamination control in the

design of the facility and its operations.

• The two standards are:

(a) ASTM E 2217, “Standard Practice for Design and Construction of

Aerospace Cleanrooms and Contamination Controlled Areas, and

(b) ISO 14644-4, “Cleanrooms and associated controlled environments.”