Microstructural characterisation of Irradiated
Nuclear Graphite using FE Analysis
A. T. Bakenne, A. N. Jones, B. J. Marsden and G. N. Hall
Nuclear Graphite Research Group, The University of Manchester
UNTF 2010 University of Salford 14th - 16th April 2010
Background
• Graphite is an allotrope of carbon.
• Nuclear graphite is used within a reactor as a
moderator and reflector material.
• NBG-10 and PCEA - new nuclear graphites, candidates
for Gen-IV technology reactor components.
• During operation graphite will experience significant property changes
such as dimensional change and Young’s modulus.
• Property changes occur as a function of fast neutron irradiation and
temperature can lead to component distortion and high internal stresses.
UNTF 2010 University of Salford 14th - 16th April 2010
Project aim
• To characterise the structure of two grades of graphite; NBG-10 (SGLcarbon group) and PCEA (GrafTech). Using high resolution X-ray
tomography to provide an input for FE modelling. Changes in material
properties before and after irradiation will then be validated using DIC
technique
UNTF 2010 University of Salford 14th - 16th April 2010
Manufacturing process
UNTF 2010 University of Salford 14th - 16th April 2010
Porosity in Virgin NBG10 and PCEA
Polarised light micrograph
SEM of virgin nuclear graphite: A) NBG-10 and B) PCEA
UNTF 2010 University of Salford 14th - 16th April 2010
Bulk physical properties of NBG-10 and PCEA
Forming
process
Grain or cell size
Density (g/cm3)
Pitch coke graphite Near
isotropic
Extrusion
1.6 mm (Max.)
1.8
Petroleum Coke graphite
Near Isotropic
Extrusion
0.8 mm (Max.)
1.79
Graphite
Manufacturer
Coke type
NBG-10
SGL Carbon
PCEA
GrafTech
NBG-10
•
PCEA
Irradiated in materials test reactor (MTR) programmes (Raphael project)
courtesy of NRG Petten. PCEA and NBG-10 samples have been irradiated
to 8.66 and 9.16dpa respectively
UNTF 2010 University of Salford 14th - 16th April 2010
Methodology
UNTF 2010 University of Salford 14th - 16th April 2010
Finite Element Modelling (Ongoing work)
UNTF 2010 University of Salford 14th - 16th April 2010
Effect of porosity shape and size on Young’s
modulus using simple model
• 2D meshes will be created, each mesh with different porosity shape (circle, oblate spheroid)
and size (20- 40% porosity).
• The mesh will be given isotropic properties (i.e HOPG properties), each mesh will be
loaded, the behaviour will be checked and validated. The same procedure will be repeated
for 3D mesh
UNTF 2010 University of Salford 14th - 16th April 2010
Finite element predictions
• A linear displacement is expected in the Finite element analysis due to the
elastic property input.
• The Young’s modulus results obtained from small models size will be more
scattered (variation) than those obtained with larger models.
• The effective Young’s modulus calculation in different areas of a virgin
specimen should highlight some differences across the specimen, since
pore volume fractions can differ slightly between the 8mm3 models.
• The Young’s modulus of irradiated graphite will be higher than virgin
graphite due to the closure of porosity during irradiation and pinning.
UNTF 2010 University of Salford 14th - 16th April 2010
Microstructural characterisation of Virgin
PCEA and NBG10
Optical microscopy images
NBG-10 (A and C)
UNTF 2010 University of Salford 14th - 16th April 2010
PCEA (B and D)
PCEA
4th baked
8.66DPA
Virgin
UNTF 2010 University of Salford 14th - 16th April 2010
NBG-10
Virgin
9.16 DPA
UNTF 2010 University of Salford 14th - 16th April 2010
SEM at low resolution
NBG-10 (A and C)
UNTF 2010 University of Salford 14th - 16th April 2010
PCEA (B and D)
SEM at high resolution
NBG-10 (A and C)
UNTF 2010 University of Salford 14th - 16th April 2010
PCEA (B and D)
X- ray diffractometer
Crystallite dimensions
Xrd Analysis
UNTF 2010 University of Salford 14th - 16th April 2010
Sample
a spacing
(Å)
c spacing
(Å)
Crystallite size
(110 peak)
La (Å)
Crystallite size
(002 peak)
Lc (Å)
Virgin NBG-10
2.46
6.72
242
177
PCEA Baked
2.46
6.70
120
133
Virgin PCEA
2.46
6.72
242
233
Future work
•
Young’s modulus of sub-models at different areas in both graphites will be
calculated and compared with each other. The numeric Young’s modulus
will be compared with experimental Young’s modulus (12.8GPa and
11GPa for NBG-10 and PCEA respectively). Effect of mesh density, model
length and X- ray Tomography resolution on Young’s modulus will be
examined.
•
Eventually in this project, the plan is to validate the Abaqus modelling by
using DaVis codes which can be also be used to make the strain analysis
and failure prediction of nuclear graphite components in HTR/VHTR.
•
More secondary validation characterisation techniques for PCEA and
NBG10 will be carried out using scanning electron microscopy (SEM),
optical microscopy and X-ray diffraction (XRD).
UNTF 2010 University of Salford 14th - 16th April 2010
Acknowledgement
•
NRG @ Petten for completing the irradiation of this graphite within the
Raphael Program and making these samples available to Manchester
•
SGL and GrafTech for supplying the graphite and helpful discussions
•
The author will also like to acknowledge KNOO for their support
UNTF 2010 University of Salford 14th - 16th April 2010