DEFORMATION AND MODULUS CHANGES OF NUCLEAR GRAPHITE DUE TO HYDROSTATIC PRESSURE LOADING

UoM administered thesis: Phd

  • Authors:
  • Adetokunboh Bakenne

Abstract

Graphite is used within a reactor as a moderator and a reflector material. During fast neutron irradiation, the physical properties and dimensions of nuclear graphite are changed significantly. Graphite shrinkage could lead to disengagement of individual component and loss of core geometry; differential shrinkage in the graphite component could lead to the generation of internal stresses and component failure by cracking. The latter behaviour is complicated by the irradiation induced changes in Young's modulus and strength. These dimensional and modulus change have been associated with the irradiation-induced closure of many thousands of micro-cracks associated with the graphite crystallites due to crystal dimensional change. Closure of microcracks in nuclear graphite was simulated by external pressure (hydrostatic loading, deviatory stress and dynamic loading) and not by irradiation, whilst Young's modulus was measured to check if there was any correlation between the two mechanisms. A study of the deformation behaviour of polycrystalline graphite hydrostatically loaded up to 200MPa are reported. Gilsocarbon specimens (isotropic) and Pile Grade A (PGA) specimens are (anisotropic in nature) were investigated. Strain measurements were made in the axial and circumferential directions of cylindrical samples by using strain gauges. Dynamic Young's modulus was also investigated from the propagation velocity of an ultrasonic wave. Porosity measurements are made to determine the change in the porosity before and after deformation and also their contribution towards the compression and dilatation of graphite under pressure. Graphite crystal orientation during loading was also investigated by using XRD (X-ray diffraction) pole figures. Effective medium models were also investigated to describe the effect of porosity on graphite elastic modulus.All the graphite specimens investigated exhibited non-linear pressure- volumetric strain behaviour in both direction (axial and circumferencial). In most of the experiments, the deformation was closing porosity despite new porosity being generated. Under hydrostatic loading, PGA graphite initially stiff then it became less stiff after a few percent of volume strain and then after about ~20% volumetric strain they stiffen up again, whist Gilsocarbon showed similar behaviour at lower volumetric strain (~10-13%). Gilsocarbon was stiff than PGA; this behaviour is due to the fact that Gilsocarbon has higher density and lower porosity than PGA. During unloading, a large hysteresis was formed. The stressed grains are relieved; the initial closed pores began to reopen. It is suggested that during this stage, the volume of pore re-opening superseded the volume of pores closing, the graphite sample volume almost fully recovered.In the axial compression test, PGA perpendicular to the extrusion direction (PGA-AG) was less stiff than PGA parallel to the extrusion direction (PGA-WG); in the hydrostatic compaction test, the PGA-WG sample deformed more because it had to undergo a less complicated shape change. This is because the symmetry of their anisotropy is parallel to the symmetry of the sample. The Pole figures showed an evidence of slight crystal reorientation after hydrostatic loaded up to 200MPa. The effective medium model revealed the importance of porosity interaction in graphite during loading.

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Original languageEnglish
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Award date31 Dec 2013

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