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University of Salford
Materials & Physics Research Centre
The use of Small Angle Neutron
Scattering in the study of porosity
in reactor graphites.
Z. Mileeva1, D.K. Ross1, D.L. Roach1, D. Wilkinson1, S.King2,
A.Jones3 and B.J.Marsden3
1Materials
& Physics Research Centre, University of Salford, UK
2ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0Q
3Nuclear Graphite Research Group, Materials Performance Centre, Pariser
Building, Room C3 The University of Manchester M13 9PL
UNTF 2011, 11-13 April, The University of Huddersfield
University of Salford
Materials & Physics Research Centre
Fundamentals of Nuclear Graphite
Graphite is a primary nuclear
component, acting as moderator
and major structural component for
90% of current UK nuclear capacity
and future international High
Temperature gas-cooled Reactors
(HTRs) capable of operating for 60–
100 years.
http://www.nuclear-graphite.org.uk/
The prediction of radiation damage in reactor grade graphites has become
a matter of considerable importance as it determines the operational
lifetime of AGR reactors. It is increasingly recognised that the standard
model for the radiation damage of reactor graphite - which was originally
established in the 1970s - variances with the situation in practice (eg. it
does not seem to predict the rate of crack development correctly).
University of Salford
Materials & Physics Research Centre
Small Angle Neutron Scattering
SANS measures the FT of the Scattering Density–Density Correlation Function.
The scattering intensity (or the number of neutrons of wavelength, λ, /unit
time/unit solid angle, scattered by a sample into a detector with wave vector
transfer, Q) is given by:
sample transmission
sample volume
detector efficiency
Solid angle element
incident flux
I(λ,Q) = I0(λ) ΔΩ η(λ) T V
dσ/dΩ(Q)
differential
scattering
cross-section
Neutron beam hits the sample and then is scattered. The obtained
diffraction pattern on a 3He detector then can be analysed.
University of Salford
Materials & Physics Research Centre
Contrast Matching
The crucial term is the differential scattering cross section which can be
written in terms of the neutron scattering length density in the carbon, δc
and the scattering density in the pore of δp:
dσ/dΩ(Q) = NpVp2 (δc - δp)2 S(Q) f2(Q) + Binc
opened pore filled
with liquid
solid state
University of Salford
Materials & Physics Research Centre
Contrast Matching
By variation the isotope content in the toluene liquid we effectively change the
scattering length density of the mixture. Thus the scattering intensity must
follow parabolic behaviour with the change of isotope content.
dσ/dΩ(Q) = NpVp2 (δc - δp)2 S(Q) f2(Q) + Binc
100% h-toluene
100% d-toluene
80% d-toluene
ˣ10-6
ˣ10-6
University of Salford
Materials & Physics Research Centre
Contrast Matching Data and Q-dependence Density
Substance
SLD, ×10-6 Å-2
h-toluene
0,9424
d-toluene
5,654
graphite
7,5584
University of Salford
Materials & Physics Research Centre
Fractal bahaviour of Activated Carbon
Basic SANS signal (empty carbon) shows two distinct fractal region:
Low Q feature - is due to the basic carbon before activation
High Q feature - is due to the activation process
University of Salford
Materials & Physics Research Centre
Study of pore connectivity with partial pressure variation of
matching liquid
Fully saturated
D-toluene
University of Salford
Materials & Physics Research Centre
SANS on reactor graphites
The scattering from unirradiated sample falls clearly into two distinct linear
regions.
The gradients are then respectively -2.86 at low Q and -3.46 at higher Q.
On the basis of the established behavior of scattering from fractal systems, we
could associate the low Q behavior with a volume fractal and the higher Q
behavior with a surface fractal.
University of Salford
Materials & Physics Research Centre
SANS on reactor graphites.
Irradiated sample data remarkably has changed to give a good linear behavior
for the full Q range measured with a gradient of -2.075, a volume fractal.
The scattering from the surface seems to have been suppressed. The obvious
conclusion is that the irradiation process has reduced the surface area.
University of Salford
Materials & Physics Research Centre
SANS on reactor graphites
University of Salford
Materials & Physics Research Centre
Measurements on Magnox graphite using the LOQ, ISIS
PGA graphite was produced by extruding needle coke and thus it is
characterized by strong preferred orientation. In order to study this, we used
two samples, one cut with the short dimension parallel to the extrusion direction
and one with the extrusion direction normal to the short dimension. Contour
plots of the resulting data are shown above.
University of Salford
Materials & Physics Research Centre
Measurements on Magnox graphite using the LOQ, ISIS
For AGR (Gilso) graphites, the SANS has a non-integer power law intensity variation
with wave vector transfer (Q) over three orders of magnitude and the non-integer index
varies with irradiation (as shown for GE1). This suggests a fractal distribution of porosity
which changes as the carbon atoms are displaced. In contrast, our recent
measurements on Magnox graphite shows that the SANS intensity peaks at 0.02Å-1,
here suggesting some periodicity in the pore distribution.
University of Salford
Materials & Physics Research Centre
Porosity in graphites. Magnox graphite.
• open pores created by decomposition gases,
• cracks between graphitic and amorphous carbon,
• internal cracks produced as the graphitic particles cool below the plastic/solid
transition, due to the anisotropic shrinkage along the c-direction (Mrozowski cracks).
A single power law exponent over a large Q range suggests that the SANS is
dominated by one porosity type. We therefore suggest that the observed SANS is due
to the Mrozowski cracks. A fractal distribution of crack widths (or inter-crack widths)
might be expected to result from the graphene sheets remaining clamped in a fractal
manner as the c-dimension shrinks.
University of Salford
Materials & Physics Research Centre
Magnox graphite. Anisotropy.
We found anisotropy in both samples. The intensity differs in the same sample
depending on which sectors were taken for an analysis: vertical or horisontal.
This is not known for other reactor type graphites and it might be the result of sample
production (eg. extrusion).
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