GB871068A - Moulded graphite bodies for slowing down and reflecting neutrons, and process of making same - Google Patents

Abstract

Impermeable moulded bodies, suitable for slowing down and reflecting neutrons in nuclear reactors, have a density above 1.8 g/cc., are composed of pure graphite flakes substantially all arranged parallel, and are substantially free of interstices between the flakes or have the interstices filled with polytetrafluoroethylene or graphitic material. Orientation of the graphite flakes is effected by dry rolling, magnetic fields, extrusion in admixture with a volatile liquid, e.g. alcohol or ether, in a ratio 1-1.5:1, or by trickling into a gently vibrating mould. One particular method (Fig. 3 not shown) is to extrude a broad band of graphite/liquid mixture which is folded upon itself in layers concertina-wise, and the fold edges are cut off before inserting in a mould. The oriented graphite is compressed to above 1.8 g/cc. at pressures from 300 to above 20,000 kg./sq.cm. preferably under subatmospheric pressure, with exclusion of air, while heating to 400-800 DEG C. Plates, bricks, cubes, cylinders, and tubes may be made, and various combinations of applying pressure in directions perpendicular to, parallel to, and inclined to the layer planes are described to eliminate voids by telescoping the flakes together. A rotating press die may also be used. If desired the graphite before compression may be mixed with polytetrafluoroethylene or a carbonaceous binder to ensure that interstices are filled. When a carbonaceous binder, e.g. coal tar pitch, petroleum tar, asphalt, or starch, is used it is carbonised at 1000-2000 DEG C. after compression and finally graphitised, although additional impregnations may be effected before graphitising. The graphite which may be 0.01 mm. flakes may be purified by heating to 800-1000 DEG C. in the presence of Cl2, or Na2CO3 before use, and bodies of densities 2.25 g/cc. are stated to be produced. The resulting body may, as one electrode, be passed to and fro over a carbon antipole plate, while a current of 1000-2000 amps./sq.cm. is applied, in inert gas, e.g. A, N2 or CO, at atmospheric or increased pressure to produce a very dense parallel oriented body.