ru &&or@, C&ze#z, Warrington, Lams., EngZand). Most polycrystalline graphites exhibit volume shrinkage when they are irradiated at high temperatures. For temperatures up to about 1100ยฐC the rate of volume shrinkage appears to have a maximum at about 400ยฐC. For a further understanding of the mechanism of this contraction a range of graphites has been irradiated in the Belgian Materials Test Reactor BR-2 to a dose of over 3 x 102i n.cmb2, as measured by the 54Fe (n,p)54Mn reaction. The irradiation was performed at 39O"C, that is, close to the temperature where the maximum contraction rate is observed. The graphites irradiated were all manufactured from raw materials which yield well developed crystal structures at graphitization temperatures of 28OO"C, and covered a wide range of linear coefficients of thermal expansion and degrees of anisotropy. At the maximum dose, observations on highly oriented pyrolytic graphite showed that the crystals had grown 4.8% in the c-axis direction and contracted 1.8% in the a-axis direction, When the linear shrinkage of all the graphites are plotted as a function of their linear thermal expansion coefficients, they lie close to the line drawn through the two points for pyrolytic material particularly when allowance is made for increases in the linear coefficients of thermal expansion which occurred in some of the graphites during irradiation. Thus the data confirm the hypothesis that irradiation induced dimensional changes result entirely from c-axis growth and a-axis contraction of the crystals. Bulk contraction is observed in most cases at high temperatures because the crystal volume increases are small, as opposed to low temperatures where they are large; thus at high temperatures accommodation of part of the c-axis growth by oriented porosity results in the basal plane contraction being more strongly reflected in the bulk behaviour.
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The temperature dependence of the irradiation induced creep of graphite G. M. Jenkins (Central Eiectricity Generating Board, Berkeley Nuclear Labovatorits, Berkeley, Gloucester-&ire, Eng~~d). The creep rate of helical graphite springs has been measured in a core position in the "Herald" reactor. The strains are recorded continuously using a displacement transducer and the temperature is varied over a range between 100 and 300ยฐC by adjusting the gas flow. Initial results indicate a small positive temperature dependence of the steady-state creep rate as if this creep depended on the diffusion of irradiation induced defects with low activation energy (-0.15 eV). However, there is a tendency to fracture during start-up transients after a total creep strain equal to the elastic strain and so intergranular stresses may contribute, if only to the transient creep component. These and other possible mechanisms are discussed.
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The tensile fracture strain of graphite determined during neutron irradiation J. L. Jackson (Battelle Memorial Irtstitute, Richland, Washington). Graphite, when used as a moderator material, is subjected to neutron damage gradients. These damage gradients can generate internal tensile stresses which could lead to fracture. Other investigations have shown that graphite in tension in an irradiation environment will relieve some of this differential stress by undergoing "irradiation creep". However, no measurements have been reported on the fracture strain during irradiation. An investigation of this fracture strain for EGCR-type AGOT graphite has been conducted. Parallel cut samples were irradiated at 625"C_+25"C at a nominal strain rate of 0.13% per 102* n/cm'. Strains without fracture from 0.37-O-42% were obtained, compared to fracture strains of O.l-0.20/, for unirradiated material.