Patent Number: 053373375
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below, referring to embodiments. In FIG. 1, a fuel assembly according to a first embodiment of the present invention, when applied to a boiling water-type nuclear reactor, is shown. A fuel assembly 10 comprises an assembly of fuel rods 1 to 3 each comprising a zircaloy cladding 20 and a fuel material of enriched uranium oxide filled therein, and a zircalory channel box 21 of square cross-section which encases the fuel rods. Fuel rods 1 and 2 have a uranium enrichment of 4.9 wt .%, and fuel rods 3 have a uranium enrichment of 4.0 wt. %. Fuel rods 2 and 3 contain about 0.04 wt. % of natural boron, and are provided at the corners in the outermost peripheral region of the fuel assembly. Twelve fuel rods 11 containing gadolinia have a uranium enrichment of 4.4 wt. % and a gadolinia concentration of about 4.5 wt %. According to the first embodiment, fuel rods 2 and 3 containing burnable poison elements having a relatively small neutron absorption cross-section such as boron, etc. are provided in the region having a soft neutron energy spectrum and a large neutron flux such as the outermost peripheral region of the fuel assembly, and fuel rods 11 containing burnable poison elements having a relatively large neutron absorption cross-section such as gadolium, etc. and fuel rods 1 containing no such burnable poisons are provided in other regions having an average neutron energy spectrum. Numeral 8 indicates a water rod. The effect of the first embodiment will be given below in contrast to a conventional fuel assembly shown in FIG. 4 on the basis of the enrichment distribution of the conventional fuel assembly (which will be hereinafter referred to as prior art example). In the prior art example, fuel rods 1 have an uranium enrichment of 4.9 wt. %, fuel rods 12 have a uranium enrichment of 4.4 wt. % and fuel rods 13 have a uranium enrichment of 3.4 wt. %. Eighteen fuel rods 22 containing gadolinia have a uranium concentration of 4.4 wt. % and a gadolinia concentration of about 4.5 wt. %. In FIG. 5 changes in the local power peaking with burnup of the first embodiment is shown in contrast to the fuel assembly of the prior art example, where the power distribution is flattened by making the uranium enrichment distribution lower toward the peripheral side of the fuel assembly. In the fuel assembly of the first embodiment, burning rate is made gentler by adding boron to the fuel rods in the outermost peripheral region of the fuel assembly, and the power can be reduced throughout the fuel lifetime. Boron has a neutron absorption ratio smaller by about 1/100 than that of gadolinium, and thus the boron burning rate is gentler. That is, even if boron is burnt out at the final stage of operation cycle, the power of fuel rods is not considerably increased in contrast to gadolinium. Particularly by providing fuel rods containing boron in the outermost peripheral region of a fuel assembly having a large neutron flux, no unburnt boron remains even if the enrichment of fuel rods at the corners as in the first embodiment is made higher than that of fuel rods of the prior art example (FIG. 4), and the neutron flux in the outermost peripheral region can be suppressed. Even if the boron is burnt out, the increasing degree of the neutron flux in the outermost peripheral region is small. Thus, in the first embodiment, the power can be flattened as in the case of providing a uranium enrichment distribution as in the prior art example. When boron is added to the fuel rods provided in the center region in the horizontal cross-section of a fuel assembly to control the excess reactivity, boron remains unburned at the final stage of operation cycle due to the small neutron flux in the center region. It is preferable to add gadolinium having a higher burning rate to the fuel rods provided in the center region in the horizontal cross-section of a fuel assembly. When a case of adding boron to the fuel rods provided in the outermost peripheral region excluding the corners is compared with a case of adding boron to the fuel rods provided in the outermost peripheral region including the corners, the enrichment of the latter fuel rods can be made higher than that of the former, because the fuel rods provided at the corners have the lowest enrichment. The latter (the case of adding boron to the fuel rods provided in the outmost peripheral region including corners) can have a higher average enrichment of a fuel assembly than the foyer (the case of adding boron to the fuel rods provided in the outermost peripheral region excluding corners), if the maximum enrichment of the latter fuel assembly is equal to that of the former. In the case of the latter, a difference between the maximum enrichment and the minimum one in a fuel assembly can be made much smaller. Even if there is an upper limit to the applicable enrichment (for example, even if the upper limit of the enrichment is 5.0 wt. %), the smaller difference leads to a higher burnup within the limit. In FIG. 6, changes of neutron infinite multiplication factor with burnup in the fuel assembly according to the first embodiment is shown in contrast to a conventional fuel assembly using only gadolinia as a burnable poison. In the fuel assembly of the first embodiment, burning reactivity characteristics of almost the same level as that of the conventional fuel assembly can be obtained in the initial period of fuel lifetime, and such neutron absorbing material as .sup.156 Gd and .sup.158 Gd can be reduced at the final stage of operation cycle. Furthermore, the average enrichment can be increased by about 0.1 wt. %, and thus the neutron infinite multiplication factor can be increased by about 1.5% .DELTA.k. In other words, in the first embodiment, the neutron infinite multification factor can be increased while keeping the maximum uranium enrichment below the upper limit. In the boiling water-type nuclear reactor, a control rod having a cross-type cross-section is inserted in a gap region formed among the fuel assemblies. Boron is added to the fuel rods provided at the positions facing the gap regions, into which the control rod is not to be inserted, in the outermost peripheral regions of fuel assemblies and no boron is added or boron may be added at a lower concentration, to the fuel rods at the positions facing the gap regions, into which the control rod is to be inserted. In this embodiment, the neutron flux in the regions, into which the control rod is to be inserted, is larger, as compared with the first embodiment, and thus the control rod reactivity can be increased. In FIG. 7, the horizontal cross-section of a fuel assembly according to another embodiment of the present invention, when applied to a boiling water-type nuclear reactor, is shown. In this embodiment boron is added to fuel rods 2 around water rods 8 (for example, fuel rods adjacent to the water rods) in addition to the fuel rods at the corners in the outermost peripheral regions of a fuel assembly 14. Similar effect is that of the first embodiment can be obtained in the structure of this embodiment. In the embodiment shown in FIG. 7, solid moderator rods of zirconium hydride, etc. having an equivalent neutron moderating effect to that of water can be provided in place of the water rods 8 and boron-containing fuel rods 2 may be provided around the solid moderator rods. In FIG. 8, a further embodiment of the present invention where the present invention is applied to a fuel rod 31 is shown. Boron is added to pellets in the lower level section 6 of a fuel rod 20 having a low void coefficient and a large thermal neutron flux, whereas gadolinia is added to pellets in the upper level section 7 of the fuel rod 20. The burnup at which the burnable poison is burnt out can be retarded, as compared with a fuel rod where gadolinia is uniformly added in the axial direction, and it is easier to control the excessive reactivity in case of obtaining higher burnup. According to other embodiment of the present invention a reactor core charged with the present fuel assemblies can reduce the amount of gadolinia to be added and consequently can reduce the neutron absorption by the remaining gadolinia and correspondingly reduce the amount of fissile material, e.g. .sup.235 U, as compared with a reactor core charged with the conventional fuel assemblies. In the foregoing embodiments, gadolinium and boron are used as a combination of a burnable absorber having a relatively large thermal neutron absorption cross-section and a burnable absorber having a relatively small thermal neutron cross-section. Other combinations of gadolinium and erbium, gadolinium and dysprosium, gadolinium and a transuranium element such as neptium, americium, etc. can be used in the present invention. In the foregoing embodiments, enriched uranium oxides are used as fuel, zircaloy is used as a structural material for the reactor core, and water is used as a moderator and a coolant. Other fuel, structural material, moderator and coolant can be used in the present invention. According to the present fuel assembly, local power peak can be decreased throughout the operation cycle and the reactivity loss by strong neutron absorber can be reduced while keeping the enrichment of fissile material of the fuel uniform.