Patent GB-1482439-A (GB)
Title: Pressed spherical fuel elements of graphite for high-temperature nuclear reactors
IPC: G21C3/28
Filed: 1974-12-13
Abstract: 1482439 Fuel elements; processing spent nuclear fuel HOBEG HOCHTEMPERATURREAKTOR - BRENNELMENT GmbH 13 Dec 1974 [14 Dec 1973] 53957/74 Headings G6C and G6R A spherical fuel element for use in a gas-cooled high temperature reactor comprises a graphite core 1 containing only fertile particles 4 and surrounded both by a graphite zone 2 containing only fissile particles 5 and by an outer shell 3 of pure graphite. In one example, spherical cores of natural VO 2  210 Ám in diameter were used as the fissile particles and coated with two layers of pyrolytic carbon with an overall thickness of 175 Ám. Optionally, an intermediate layer of SiC was provided to increase fission product retention. The particles of fertile material (ThO 2 ) with a core diameter of 617 Ám were also coated with two layers of pyrolytic carbon with an overall thickness of 145 Ám. A mixture of 64% by weight of natural graphite, 16% of graphited petroleum coke and 20% of a novolak binder resin was used as the graphite pressing powder. In separate operations, the fissile and fertile particles were coated with the graphite pressing powder, an enveloping layer 160 Ám being formed around the fertile particles and one 240 Ám thick around the fissile particles. To produce the spherical core, 48 g. of the fertile particles thus coated and enveloped were introduced into a rubber mould with 30 g. of graphite pressing powder, mixed and formed into a spherical core under a pressure of 50 kg./cm.<SP>2</SP>. This core was then introduced into a second rubber mould and held centrally by three spacers, the rest of the mould being filled with a mixture of 41 g. of enveloped, coated fissile particles and 20 g. of graphite pressing powder. The core and mixture were then consolidated under a pressure of 80 kg/cm.<SP>2</SP>. This prepressed sphere was then provided with a shell of the same graphite pressing powder and finally pressed under high pressure (3 t./cm.<SP>2</SP>). In order to coke the binder resin, the spheres were heated to 800‹ C. for 18 hours and, after cooling, were heat-treated at a temperature of up to 1800‹ C. After the heat treatment, the spheres were machined to the prescribed diameter of 6 cm. In the first stage in the reprocessing of the spent fuel elements, the elements are subjected to combustion in oxygen at 1000‹ to 1200‹ C. to burn up the graphitic structural material and so expose the fertile and fissile particles, advantageously in a rotary tubular furnace of the kind shown in Fig. 2. In one embodiment, in a first tubular rotary furnace 11, the two outer graphite layers of the fuel elements are burnt off with atmospheric oxygen enabling fissile particles 13 to drop through a perforated (40 mm. diameter perforations) furnace wall 14 and a sloping grid 15 arranged beneath it with holes 5 mm. in diameter. They are collected in a container 16. The remaining spherical fuel element cores have a diameter of 40 mm. and drop through the wall 14 on to the grid 15 and roll down it into a second tubular rotary furnace 17 where the rest of the graphite matrix is burnt off. Fertile particles 18 containing U-233 then drop into a container 19. In a modification, the zone 2 of the spherical fuel element is made weaker than the shell 3 and the core 1, as for example by using no pressing powder other than that which envelops the coated fissile particles. Such weakened fuel elements, after they have been used in the reactor, are crushed and the fragments which break off, coming from the shell and the fissile zone, are subjected to combustion separately from the cores containing the fertile material.