Patent GB-1185816-A (GB)
Title: Nuclear Fuel Element
Inventor: LEROUGE GEORGES
IPC: G21C3/18
Filed: 1969-03-10
Abstract: 1,185,816. Fuel elements. COMMISSARIAT A L'ENERGIE ATOMIQUE. 10 March, 1969 [22 March, 1968], No. 12408/69. Heading G6C. A fuel element comprises a tube of fuel material, a metallic can in intimate contact with the external wall of the tube and closed by metallic end caps, and a graphite rod which takes up substantially the entire internal cavity of the tube and is in intimate contact with the internal wall of the tube, the end portions of the graphite rod being applied against the end caps either directly or through end pallets which are secured to the end faces of the rod. Such a fuel element 2<SP>1</SP> comprises a tube 4<SP>1</SP> of metallic fuel (e.g. low-alloy U) clad with an outer can 6<SP>1</SP> of Mg alloy containing a small proportion of Zr. Grooves 10<SP>1</SP> for anchoring the can are formed in the outer surface of the tube 4<SP>1</SP> in order to prevent a ratchetting effect. The can 6<SP>1</SP> is sealed off by end-caps 12<SP>1</SP>, 13 of substantial thickness and formed of Mg alloy containing a small proportion of Mn. The entire central cavity 16<SP>1</SP> of the tube 4<SP>1</SP> is occupied by a rod 14<SP>1</SP> of graphite which terminates at the same level as the tube 4<SP>1</SP>. The rod 14<SP>1</SP> is preferably coated with a thin film of inert refractory material, e.g. ZrO 2  or Al 2 O 3 , prior to casting the uranium around it so as to prevent carburization of the uranium to form a bond which would cause difficulty in carrying out decanning operations. Circular projections 18 in the end portions of the rod 14<SP>1</SP> fit in recessed portions of the internal face of the tube 4<SP>1</SP> to form a tight bond between the rod 14<SP>1</SP> and the tube 4<SP>1</SP>. In consequence, the bottom end-cap 12<SP>1</SP> which is attached to the can in such a position as to be in contact with both the rod 14<SP>1</SP> and the tube 4<SP>1</SP> is not liable to be in contact with only one of these two components after a certain number of thermal cycles nor to be subjected to localized punching forces. In order to further reduce the risk of punching of end-caps during canning, or irradiation, endcaps of sintered Mg having superior mechanical strength may be employed instead of machined Mg-alloy end-caps. The upper end-cap 13 has a central orifice in which a plug 20 is inserted after the end-caps 12<SP>1</SP>, 13 have been welded to the can, the fuel element being placed in a vacuum chamber prior to welding of the plug 20 to remove any gases which may have been occluded or trapped within the can. Graphite having an open-pore volume of the order of 5% is preferably used for the rod 14<SP>1</SP>, being formed by machine-forging a standardgrade graphite, followed by impregnation with a furfuryl binder or densification by means of a pyrocarbon deposit. Alternatively, a conventional graphite may be used by projecting an A1 coating on the tube 4<SP>1</SP> by the spray-gun process prior to canning. By this means, the consequences of fissures in the can or end-caps are minimized inasmuch as the volume of gas liable to be trapped within the pore structure of the graphite is much reduced. Refractory end-pellets may be interposed between the graphite rod and the end-caps.