| 摘要 |
1396767 Fuel elements GENERAL ELECTRICCO 15 Sept 1972 [30 Sept 1971] 42844/72 Heading G6C A nuclear fuel assembly has a central cluster of its fuel rods fuelled with initial Pu and is provided with a water tube extending through the cluster to increase the neutron moderation in the region of the Pu fuel and also to provide axial support for fuel rod spacers. The invention is described in connection with a boiling water reactor, the core of which comprises an array of spaced fuel assemblies submersed in lightwater moderator-coolant, each fuel assembly comprising a plurality of fuel rods 21 supported between upper and lower skeletonized tie plates 22, 23. Lateral support is provided by spacers 24. Each fuel rod 21 has end plugs 26, 27 formed with extensions supported in cavities 29, 31 in the tie plates 22, 23. Certain of the peripheral fuel rods are secured to the lower tie plate 23 by screw threaded end plugs 27<SP>1</SP> and to the upper tie plate 22 by retaining nuts 32. In this manner the upper and lower tie plates and the fuel rods are formed into a unitary structure. A thin-walled flow channel 33 is sized to form a sliding fit over the upper and lower tie plates. An inner cluster of fuel rods 21(1) contains initial Pu and is surrounded by fuel rods 21(2) containing initial enriched U fuel free of significant initial Pu. A fuel rod in a central position within the inner cluster is replaced by a water tube 41 similar to a fuel rod but containing no fuel and provided with apertures in its end plug extensions to permit a throughflow of water. The increased neutron moderation provided by the water results in a cooler neutron spectrum for the Pu-fuelled rods 21(1) to take advantage of the reduced capture-to-fission ratio in Pu-239 and Pu-241 and the reduced neutron capture in the fertile Pu-240. Advantageously, the Pu in rods 21(1) is mixed with natural or depleted U. Burnable poison such as Gd or Sm may be mixed with the Pu-fuel in rods 21(4) to control excess reactivity. Alternatively, the poison may be included in a plurality of the U-fuelled rods. In addition to the above-mentioned apertures, the tube 41 may be provided with coolant inlet holes 52 and outlet holes 53. The tube 41 is formed with radially extending, axially spaced fins or lugs (Figs. 11 and 12, not shown), by means of which the spacers 24 are retained in fixed axial position. |
