the use of small angle neutron scattering in the study of porosity in reactor graphites

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, UK2ISIS 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 SalfordMaterials & Physics Research Centre

Fundamentals Fundamentals of NuclearNuclear 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).

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:

I(λ,Q) = I0(λ) ΔΩ η(λ) T V dσ/dΩ(Q)

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scattering cross-section

Neutron beam hits the sample and then is scattered. The obtained diffraction pattern on a 3He detector then can be analysed.

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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

solid state

opened pore filled with

liquid

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

Contrast Matching Data and Q-dependence Density


Substance

SLD, ×10-6 Å-2

h-toluene 0,9424

d-toluene 5,654

graphite 7,5584

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

Study of pore connectivity with partial pressure variation of matching liquid


Fully saturated D -to luene

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.

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.

SANS on reactor graphites


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.

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.

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.

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).

the use of small angle neutron scattering in the study of porosity in reactor graphites

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