Patent Number: 043022898
Section: summary

BACKGROUND OF THE INVENTION A nuclear reactor core normally contains several hundred fuel rod bundles. Each fuel rod bundle consists of a plurality of fuel rods. Boiling water reactors normally use fuel rod bundles containing 8".times.8" fuel rods; sometimes 6".times.6", 7".times.7" or 9".times.9" fuel rods are used. Pressurized water reactors use fuel rod bundles containing 15".times.15", 16".times.16" or 17".times.17" fuel rods. One or more of these fuel rods may be substituted by inert rods or tubes having a different function than producing energy. Each fuel rod contains a large number of fuel pellets stacked on top of one another in a cladding tube, which is normally of a zirconium alloy known as "Zircaloy." In each fuel rod bundle the fuel rods are arranged between a bottom and a top plate, to which some fuel rods, so-called tie rods, are secured. In a boiling water reactor the fuel rod bundle is surrounded by a fuel channel, which is normally of "Zircaloy." Inside the channel the fuel rods are kept at the desired distance from each other in the lateral direction by spacers located at suitable distances in the vertical direction. When the burn-up in a reactor has progressed so far that the smallest acceptable core reactivity margin has been reached, a partial recharge of fuel is carried out. By balancing, in a suitable manner, the fuel quantity to be replaced as well as the fissile enrichment of the replacement fuel, an excess reactivity is realized that permits a certain energy output until the next refuelling occasion. During the partial recharge of a boiling water reactor, it is possible to exchange, for example, one-fifth of the fuel each operating year (or any other suitable operating period), as normally from the end of the second year of operation. This means that the fuel for example remains in the core for 5 years during equilibrium conditions, but that the part of the fuel that is exchanged during the initial state is used for a shorter period of 3 to 4 years. So far, refuelling has always been performed in such a manner that irradiated fuel rod bundles have been removed from the core and that new fuel rod bundles with unirradiated fuel have been inserted into the empty spaces formed, usually after a suitable relocation of the remaining fuel rod bundles within the core. This relocation of fuel rod bundles is done in order that the reactor may have an optimum power distribution within the core and may have optimum reactivity. The irradiated fuel rod bundles which have been removed from the reactor core then proceed to storage, awaiting ultimate reprocessing for utilizing the remaining fissile material. SUMMARY OF THE INVENTION The present invention is based on the realization that it is possible to make considerable savings of fuel costs by composing new fuel rod bundles out of fuel rods from already burnt-up fuel rod bundles and using the fuel rod bundles thus composed for an additional or a few additional operating periods in the reactor. The fuel cost savings per year which can be thus accomplished are considerable for each reactor. The present invention relates more particularly to a method of refuelling in a nuclear reactor having a core containing a plurality of fuel rod bundles which are built up from a plurality of fuel rods, the method being characterized in that at least one burnt-up fuel rod bundle is replaced by a fuel rod bundle which is at least partially composed of fuel rods from fuel rod bundles already burnt-up in a reactor, the mean content of fissile material in the fuel rod bundle thus composed being higher than the mean content of fissile material in the fuel rod bundle which is replaced by such composed fuel rod bundle in the reactor. For light-water boiling reactors using uranium dioxide and possibly plutonium dioxide as fuel, the burnt-up rod fuel bundles which are used when putting together the new fuel rod bundle have preferably a maximum content of fissile material in the form of U 235, Pu 239 and Pu 241 of 1.75% of the initial total weight of uranium and possibly plutonium in the fuel. The content of the fissile material, however, should not be below 1.20% of the initial weight. To utilize the invention in full, during refuelling in this way, at least some twenty or so burnt-up fuel rod bundles in the reactor should be replaced with fuel rod bundles composed in the manner described. For optimum utilization of the fuel in the new composite fuel rod bundle, the different fuel rods are located in the new composite fuel rod bundle so that the resulting internal power peaking factor, i.e., the ratio of the maximum local value of the rod power and its mean value in a horizontal section through the fuel rod bundle, is at least 1.20 and preferably 1.30 to 1.50. To put together a composed fuel rod bundle from already burnt-up fuel rod bundles according to the invention, preferably some fuel rods are removed from one fuel rod bundle already burnt-up in a reactor and fuel rods from one or more of other already burnt-up fuel rod bundles are inserted into empty locations in the first fuel rod bundle, the last-mentioned fuel rods having a higher mean content of fissile material than those removed from the first-mentioned fuel rod bundle. At the same time a rearrangement can be made of the fuel rods which have been kept in the fuel rod bundle from which some fuel rods were first taken out, so as to achieve an optimum power distribution in this fuel rod bundle. When putting together the composed fuel rod bundle it is suitable to retain such fuel rods in the first fuel rod bundle that constitute supporting elements in this fuel rod bundle. Also spacers, spacer holder rods, and top and bottom plates are preferably retained in the fuel rod bundle. According to one embodiment of the invention, when putting together a new fuel rod bundle, open tubes--to be water-filled inside the reactor--are used instead of fuel rods in some positions for fuel rods in the fuel rod bundle, or empty spaces are left in these positions, to be occupied by water in the reactor. Such a measure, as will be explained in more detail in the following, may influence the burn-up of the fuel in an advantageous manner. In connection with putting together a composed fuel rod bundle one or more rods or tubes containing burnable neutron absorber material such as gadolinium, boron or samarium distributed in a carrier material such as uranium dioxide, zircaloy or steel may be placed in some positions for fuel rods in the fuel rod bundle instead of fuel rods. In this way it is possible to achieve an improved reactivity control during the earlier part of the operating period and to simultaneously obtain at the end of the operating period a favorable influence similar to that obtained with a water-filled tube. According to another embodiment of the invention relating to reactors having vertical fuel rods, when putting together a new fuel rod bundle at least some of the fuel rods, preferably those which are located centrally, are arranged with those ends facing upwardly that previously in a reactor were facing downwardly. In a boiling water reactor the high content of steam bubbles at the upper part of the core results in the neutron flux being somewhat depressed there so that the fissile material is consumed more slowly than in parts located at a lower core level. It is therefore possible, when putting together composed fuel rod bundles, to increase the energy output from a boiling water reactor by turning some of the fuel rods from burnt-up fuel rod bundles upside down when reinserting them into composed fuel rod bundles. This embodiment entails advantages also for pressurized water reactors, among other things, because of the fact that the coolant shows a density gradient in the longitudinal direction of the fuel rods.