Patent Number: 043022898
Section: description

DETAILED DESCRIPTION OF THE INVENTION Turning now to the drawings wherein like reference characters refer to like and corresponding parts throughout the several views, FIG. 1 illustrates a small portion of a horizontal section of a reactor core of a boiling water reactor having vertical fuel rod bundles. The reactor core section contains nine whole fuel rod bundles 10, although only one of such bundle is illustrated in the interest of clarity. The total number of fuel rod bundles in a complete cross-section amounts to several hundred. Each fuel rod bundle, for example 10a, is built up of sixty-four fuel rods 11 in the form of a square lattice. The fuel rod bundle is contained within a fuel channel 12 having a substantially square cross-section and being of a zirconium alloy material known as the "Zircaloys" or specifically "Zircaloy 4." The rods are held in their respective positions by so-called spacers 7 equally spaced between and parallel to top and bottom plates 8 and 9 of the fuel rod bundle (see FIG. 1A). Each fuel rod contains a number of pellets of uranium dioxide as fuel, stacked on top of one another and enclosed in a tube 13 of a zirconium alloy material known as the "Zircaloys", or specifically "Zircaloy 2." Spaces 14 between the fuel rods within the fuel channel are traversed in any normal manner by a coolant, as for example, light water. Gaps 15a and 15b between the fuel channels are also traversed in a normal manner by a coolant of the same type. The gaps 15b, into which control rods 16 can be inserted, are wider than the gaps 15a which contain no control rods. The reactor core cross-section also contains neutron sources 17 as well as neutron detectors 18. One or more of the fuel rods may be replaced with a non-energy producing rod, as described hereinabove. Thus, for example, a rod or rods such as 19 could be replaced with a solid or water-filled spacer holder rod 19a of "Zircaloy 2." Fuel rods 20, 21, 22 and 23 are, for example, fixed to the top and bottom plates 8 and 9 in the fuel rod bundle as shown, for example, in FIG. 1A. The spacing between the fuel rods in a bundle is primarily determined by the reactor physics demands with regard to optimum neutron economy, as well as the neutron-multiplying and thermo-hydraulic and dynamic properties of the core. In choosing the distance between the rods allowance is also made for the effect of the additional water in the gaps 15a and 15b between the fuel channels, which is of great importance for the local variations in the neutron flux. This water results in a locally increased neutron flux so that fuel rods located near the water gaps tend to be subjected to higher heat loads than other fuel rods. To equalize the power distribution within the fuel rod bundle as much as possible, fuel rods with different concentrations of fissile material, for example U 235, are used in different positions within the fuel rod bundle. FIG. 2 schematically illustrates a fuel rod bundle with the initial contents of U 235 in different fuel rods being expressed as a percentage of the initial weight of uranium in the fuel (uranium dioxide). (The percentages set forth in the following also relate to the percentage of the initial weight of uranium in the fuel.) The average fissile enrichment in the FIG. 2 example is 2.32%. Four different contents of enrichment are used when designing the fuel rod bundle, namely 1.18%, 1.85%, 2.50% and 3.07%. In the interest of clarity, the fuel rods themselves are not illustrated in FIG. 2; only their degrees of enrichment. FIG. 3 schematically illustrates the same fuel rod bundle of FIG. 2 but after three operating years. The upper numeral 24 in each square sets forth the enrichment content U 235 in percent, and the lower numeral 25 sets forth the total enrichment content of Pu 239 and Pu 241 in percent of each fuel rod in the fuel rod bundle. The plutonium has been formed under operation by capturing neutrons in U 238. The higher neutron flux previously mentioned and the consequently higher power in the rods at the water gaps 15a and 15b have resulted in the fissile material, mainly U 235, Pu 239 and Pu 241, having been consumed more rapidly here than in the central parts of the fuel rod bundle, as can be seen. With time this enhances the original enrichment distribution, and the power distribution over the fuel rod bundle is flattened, which in principle is favorable. The average content of U 235, which was initially 2.32%, is after three years of operation 0.96% and the average content of the total amount of Pu 239 (0.44%) and Pu 241 (0.07%) is 0.51%. Fission of a U 235 nucleus and a Pu nucleus provides approximately the same energy yield. The amount of fissile material in the bundle has thus been reduced to about 60% of the initial amount. The remaining fissile material is also differently distributed over the fuel rods included in the fuel rod bundles. In accordance with the technique heretofore applied, irradiated fuel rod bundles according to FIG. 3 have as a whole proceeded to storage, awaiting ultimate reprocessing for utilization of the fissile material. According to the present invention, on the other hand fuel rods in already burnt-up fuel rod bundles, for example of the type shown in FIG. 3, are used for composing reloaded fuel rod bundles. One example of such a fuel rod bundle is schematically illustrated in FIG. 4. This bundle has been composed from two fuel rod bundles which have been in operation for three years and which both have a distribution of enrichment as shown in FIG. 3. Twenty-four fuel rods, designated 31 to 54 and marked by an "X" in FIG. 5, have been removed from such a fuel rod bundle. Twenty-four encircled fuel rods, designated 61 to 84 in FIG. 6 are removed from this other bundle and are substituted for the removed rods of FIG. 5 to arrive at the new FIG. 4 bundle in an arrangement made clear from the drawing. Of course, it is possible to move fuel rods in the fuel rod bundle according to FIG. 5 after the removal of the marked fuel rods before fuel rods from the fuel rod bundle in FIG. 6 are inserted. When putting together the composed fuel rod bundle according to FIG. 4, such fuel rods in the fuel rod bundle according to FIG. 5 which have principally been replaced are located nearest to wide water gaps 15b where the enrichment of fissile material is lowest. The above-described substitution of fuel rods results in the average content of fissile material increasing from 0.96% for U 235 and 0.51% for Pu 239 and Pu 241 together in the fuel rods according to FIG. 3, to 1.26% for U 235 and to 0.53% for Pu 239 and Pu 241 together in the fuel rods according to FIG. 4. The internal power peaking factor for the composed fuel rod bundle according to FIG. 4 amounts to 1.40. Fuel rod bundles according to FIG. 4 can be used for operation for an additional year or a few years, which results in a considerable reduction of the fuel costs for the reactor. The neutron-multiplying properties of the core are greatly dependent on the volumetric relation between water and fuel. The optimum water/fuel ratio varies with fuel burn-up. Since the technical limit to the maximum extent of the burn-up is determined by the point where the contribution of the fuel to the neutron multiplication of the core becomes too small, an optimum water/fuel volume ratio is important. The volume ratio water/fuel can be increased by replacing one or more fuel rods in the central parts of the fuel rod bundle with open empty tubes to be water-filled in the reactor. This, of course, reduces the amount of fissile material, but the possibilities of utilizing the remaining material increase considerably and compensate more than enough for the material loss. FIG. 7 schematically illustrates the manner in which fuel rods in the fuel rod bundle according to FIG. 4 have been replaced with water-filled tubes, marked by empty squares 55, 56, 57 and 58, at the central portions of the fuel rod bundle. One or more of these tubes may be replaced with a rod or rods containing a burnable neutron absorber, e.g., gadolinium distributed in uranium dioxide or "Zircaloy" as a carrier material. In the application of the previously mentioned embodiment of the invention, in which at least some of the fuel rods from burnt-up fuel rod bundles, when putting together new fuel rod bundles, were turned so that those ends which had been facing downwardly in the burnt-up fuel rod bundles were facing upwardly in the new fuel rod bundle, all fuel rods which are not supporting and which are not located adjacent the water gaps 15a and 15b may, for example, be positioned in the manner described. The method offers particular advantages when applied to fuel rods at the central portions of the fuel rod bundle. The invention has been described in particular for application to a light water boiling reactor. However, it is also applicable to heavy water boiling reactors and to pressurized water reactors and to other reactors where the fuel is arranged in the form of fuel rod bundles, without departing from the scope of the invention. Obviously, many other modifications and variations of the present invention are made possible in the light of the above teachings. It is to be therefore understood that the invention may be practiced otherwise than as specifically described.