DEVELOPMENT OF A GRAPHITE (HX30) WITH IMPROVED GAS DIFFUSIONAL CHARACTERISTICS.

The specification of a graphite suitable for DRAGON fuel tubes which operate on the ‘back-sweep’ purge principle includes the two requirements: (i) the viscous permeability constant B0 < 3 × 10−11 cm2, and (ii) the fission-gas transmission coefficient D' < 1 × 10−6 cm2/sec at 20 atm., 7500, and 2 psi pressure drop. Calculations, based on the results of a study of the counter-diffusion of gases in graphite, suggested that, providing the first criterion was satisfied, the second requirement would be met by a material whose pores were on the average at least 5·5 μm in diameter. A series of experimental graphites based on grists of scrap nuclear-grade graphite was made with the object of producing artefacts whose pore sizes were in the region of 10 μm, without exceeding the specified limit on permeability. After several adjustments to the size distribution of the grist, one was obtained which yielded an artefact which, after subsequent pitch impregnation, satisfied the reactor specification. The family of graphites based on this particular grist is known as HX30. A second series of experiments demonstrated that this material could be made in full-sized rods 8 ft long by 3·25 in. dia. Any of four baking routes was possible, depending on whether the primary bake and the re-bake after impregnation were carried out under atmospheric pressure or at 100 psig. An appropriate number (1 to 3) of impregnations yielded an acceptable graphite. The superior properties of HX30 are considered to be due chiefly to the exclusion of fines (below 300 BSS mesh) from the grist but the nature of the grist (graphite and not coke) and the use of re-extrusion are probably also significant. The original theoretical approach appeared to be largely vindicated by the pore-size distribution of the successful product.