Verfahren zur Bildung eines Agglomerates aus umhüllten Kernbrennstoffpartikeln

1,265,974. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 4 Dec., 1969 [18 Dec., 1968], No. 60122/68. Addition to 1,195,206. Heading G6C. An agglomerate of nuclear fuel particles, each particle of which bears a fission product retentive coating, is formed by applying an overcoating of a powder composed of between 40 and 60 wt. per cent synthetic resin powder and between 60 and 40 wt. per cent graphite powder, vibrocompacting the overcoated particles into a mould cavity, and maintaining the vibration whilst zone heating the particles by causing a thermal front to pass through the cavity. The mould cavity may be formed by a blind hole in an elongate graphite member which constitutes a container for the agglomerate after moulding. In an example, the fuel had a kernel composed of UO 2 /C-10 in which the uranium contained 39 wt. per cent U-235, the entire uranium content of the coated particles being 21À1 wt. per cent. Fuel kernels of this composition were coated with several layers of pyrolytic carbon and silicon carbide in known manner. The mould cavity formed in a graphite member was cylindrical with spaced inner and outer cylindrical walls. A sleeve of polyethylene film was secured around the inner cylindrical wall and the overcoated particles were vibration filled into the annular cavity. An annular electric heating element was placed around the graphite member and moved upwardly throughout the length of the member which at the same time was vibrated. Loss of volume which occurred during the settling of the fuel particles was made good. Resin/ graphite bonds were formed between the compacted fuel particles which were agglomerated into a coherent mass, and the mass was heated to 800‹ C. in argon to carbonize the resin which had been cured during the zone heating process. The agglomerate was then heated in situ in the graphite at 1800‹ C. in vacuo. The presence of the polyethylene film, which carbonizes during heat treatment, allows the annular agglomerate to shrink radially without cracking longitudinally.