Simulating Material Property Changes of Irradiated Nuclear Graphite L. Luyken, A. N. Jones, M. Schmidt, B. J. Marsden, T. J. Marrow Primarily graphite is used as a moderator Slowing down neutrons to thermal energies Is also used as a structural component. Control rod channels Coolant channels Reflector bricks Fuel Compacts

What Happens to Graphite in a Reactor Core Components Deform Material Properties Change Simple Dose Profile

Why?

Nuclear Graphite is made up of; Pechinay Bulk Structure (polarised optical image) Graphite Microstructure Filler Particles -Ordered Crystallites Binder Matrix - Disordered Crystallites Porosity - Calcination cracks - Gas evolution pores

Crystallite Structure. Crystallite Structure (Image Abbie Jones) Graphite Microstructure Mrozowski cracks Crystal

Damage Mechanism Standard ModelRuck, Tuck and Buckle Graphite Structure

Crystallite Structure (Image Abbie Jones) Damage Mechanism C a a

Initial Adsorption “unpins” layers Intercalate can then penetrate graphite planes. Intercalate concentration within microstructure dependant on partial pressure of surrounding atmosphere. Bromine can also fill dislocation ribbons and push planes further apart Widening of Dislocation Ribbons on Intercalation Simulating Irradiation Damage

Strain due to Bromination of polycrystalline graphites Strain due to Bromination of HOPG Dimensional Change by Intercalation (Presented at UNTF 2009) Single crystals experience high strain at low bromine concentration Polycrystalline graphites experience differing bulk strains depending on orientation of crystals

Tomography at the Swiss Light Source Experimental Set Up X-Ray SourceShutter Bromine RigCamera Beam Energy: 28KeV (high) Projections: 1501 (reduces noise) CCD exposure time:160ms(fast) Binning:2 x 2(reduces image resolution)

Figure 1 Strain due to bromination of Pile Grade A graphite Figure 2 Strain due to irradiation of Pile Grade A graphite

Brominating Graphite Microstructure Filler Binder

Brominating Graphite Microstructure

Binder However larger regions of the binder matrix accommodate bromine due to greater open porosity in this region Filler Bromine quickly permeates large crystalline regions Filler Nevertheless the largest deformation vectors are seen in the filler particles

Change in Young’s modulus Where E = Young’s modulus ρ = density ν = sonic velocity θ = Poisson's ratio Experimental Setup

Change in Young’s modulus TimeStrainYoung’s Modulus t=00%5.25 GPa** t= %5.29 GPa ** literature values vary from 4GPa to 7GPa

Conclusions Bromination produces bulk dimensional Initially penetrates binder phase due to large amounts of open porosity Later quickly fills filler phase Largest strains are seen in filler phase As with irradiation brominating graphite increases the young’s modulus.

Future Work Investigate crystal strains due to bromination. Applied for beam time at ILL Further develop Young’s modulus experiment to measure bulk strain insitu Use laser displacement detector

Thank you James Perrin University of Manchester David JamesUniversity of Manchester Paul TownsendUniversity of Manchester Sam MacdonaldUniversity of Manchester (Swiss Light Source) Will BodelUniversity of Manchester "This work was carried out as part of the TSEC programme KNOO and as such we are grateful to the EPSRC for funding under grant EP/C549465/1“ Questions?