| 摘要 |
1,118,332. Fuel elements. UNITED STATES ATOMIC ENERGY COMMISSION. 1 Dec., 1965 [7 June, 1965], No. 50896/65. Heading G6C. A fuel element comprises an elongated tubular cladding having upper and lower closed ends defining a fuel zone in which from two to five fuel segments are serially arranged, each segment having one end fixed relative to the cladding and the other end free to slide within a void space containing a spring. The segments are thus free to thermally expand and contract in unison in the same direction. In the case of the two-segment fuel element, the void at the top of the lower segment is above the transverse mid-plane of the element and the segments are of unequal length, whereas in the case of the three-, four-, and five-segment fuel elements, the segments are of equal length. The thermal expansion coefficient of reactivity approaches zero with an increase in the number of fuel segments, reaching zero when there are four segments. An increase beyond five segments results in the coefficient remaining near zero or going slightly positive with the disadvantage that an excessive amount of neutron-absorbing material is introduced into the fuel element. The use of such fuel elements in a reactor greatly simplifies the control of reactivity excursions by means of Doppler broadening. Each fuel segment may comprise a plurality of serially arranged subsegments, each subsegment being an individually clad compact containing fissile fuel, e.g. UO 2 , PuO 2 or a mixture of these. In a particular sodiumcooled fast reactor, the fuel comprises U-238 (1598 kg.), Pu-239 (296 kg.) and Pu-240 (26 kg.) in UO 2 -PuO 2 form clad in stainless steel. The elongated tubular cladding contains upper and lower end reflectors, e.g. Ni or C, and insulator pellets, e.g. Al 2 O 3 , MgO or sapphire, at each end of the fuel segments. The fuel segments are arranged in hexagonal arrays around central moderator rods of BeO. An extension rod is coupled to the upper end of each element to facilitate its insertion into and removal from the reactor core. |
