| 摘要 |
778,881. Nuclear reactors. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. April 27, 1956 [April 30, 1955], No. 12591/55. Class 39(4). A fuel element for a nuclear reactor comprises a fuel content of tubular form contained between inner and outer tubular sheaths, the inner sheath being formed of a different material from that of the outer sheath so as to collapse at a lower temperature than the outer sheath to discharge under gravity at least some of the fuel content along the inside of the outer sheath when predetermined temperature conditions are exceeded in the element. The outer sheath 12 may be of niobium and the inner sheath 11 of vanadium. Enriched uranium is contained between the sheaths and two molybdenum washers (not shown); above and below these washers are breeder tubes 14 of natural uranium. Fig. IC shows the lower end of a fuel element. The bottom of tube 14 rests on a natural uranium packing tube 16 supported on a niobium end plug 17 threaded into a stainless end steel fitting 18. An annulus 20, duct 21 and pluggable hole 22 are provided for filling the element with liquid sodium to improve heat transfer from the fuel to sheaths 11, 12. At the top end of the fuel element, Fig. 1A, there is a niobium end plug 27 screw fitted in a stainless steel end fitting 29 which is supported by a flanged end resting on a top plate 35. Liquid sodium enters through the mouth 40 and divides at channels 39, part flowing inside and part flowing outside the fuel element. At between 1000 and 1100‹C. there is a rapid breakdown of the vanadium inner sheath which forms an alloy with the uranium, and the fuel is then discharged by gravity so that there is no danger of a super-critical condition arising. Tungsten, zirconium, molybdenum, titanium or tantalum may be used for the outer sheath in placed of niobium, and chromium or nickel for the inner sheath. As an alternative the inner sheath may be of stainless steel, an 80-20 nickel chromium alloy, or a molybdenum /nickel twoply with the molybdenum next to the uranium. In an alternative method the inner sheath is of a material which melts at a suitable temperature or of a two-ply material which forms a binary entectic at a suitable temperature ; examples of this are a titanium /nickel or zirconium /nickel two ply material with the titanium or zirconium adjacent the uranium. In a further alternative the inner sheath is made of a material which alloys with the fuel content at one temperature but is prevented from so doing until a higher temperature is reached by a barrier layer which inhibits solid diffusion, for example a nickel tube coated with tantalum. |
