Patent Number: 054229227
Section: summary

BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to boiling water reactors (BWR), particularly, to preferable fuel assemblies and reactor cores for labor-saving fuel shuffling operation by increasing size of the fuel assembly and reducing number of the fuel assemblies with ensuring thermal margin and reactor shut down margin. (2) Description of the Prior Art A fuel assembly for BWR is, in general, composed of a bundle of fuel rods forming a square lattice, each of the fuel rods is manufactured by inserting a plurality of fuel pellets containing fissile material into a cladding tube and sealing, and a channel box having a hollow square cross section, an outer side of which is about 14 cm, which covers the above fuel bundle. A reactor core is formed in a cylindrical shape by further bundling of the above fuel assemblies. As for fuel, enriched uranium or/and plutonium-enriched uranium is used in a chemical form of oxide. As reactivity of reactor core decreases with burning of fuel, the fuel is loaded into the reactor core more than the critical amount at beginning of the reactor operation cycle so that the reactor maintains criticality. Excess reactivity yielded by loading of the excess fuel is controlled by adjusting neutron absorption in the reactor core with mixing burnable poison such as gadolinia etc. into the fuel, and inserting control rods having cruciform cross section, which comprise boron carbide or hafnium, among a plurality of adjacent fuel assemblies. In order to allow inserting the cruciform control rod into the reactor core, water gap regions being filled with non-boiling water, of which gap size is almost twice of a blade thickness of the control rod, are provided around the channel box of the fuel assemblies. Moreover, water rods filled with non-boiling water are provided at center of the fuel assembly in view of neutron flux flattening. Atomic numbers ratio of hydrogen to uranium in the reactor core average (optionally it is called H/U ratio hereinafter) which depends on a size of the above non-boiling water region and the amount of fissile material is adjusted in a range of 4-5 in order to make necessary enrichment of the fuel lowest mainly in view of uranium resource saving. On the other hand, the excess reactivity increases at shut down of the reactor because of increase in water by phase change of steam to water. Accordingly, it becomes important to ensure reactor shut down margin. Regarding to methods for increasing reactor shut down margin of the fuel assembly and the reactor core, the following two methods are well known as prior art; (1) JP-A-63-231293 (1988) In accordance with this prior art, neutron average energy in the reactor core is reduced, a difference in neutron moderating effect between upper portion and lower portion of the reactor core is reduced, and consequently, the reactor shut down margin is increased, by making a ratio of transverse cross section area of the water gap region which is a saturated water region outside the channel box to transverse cross section area of pellets in all fuel rods in the channel box at least one. (2) JP-A-2-12088 (1990) In accordance with this prior art, an excess reactivity of the reactor core is reduced and, consequently, the reactor shut down margin is increased, by composing the fuel assembly so as to have a non-boiling water region of which area is at least 9.1% of the transverse cross section area of the channel box. Hitherto, increase of output power has been achieved in general by increasing in the number of fuel assemblies. However, the increase in the number of fuel assemblies in the reactor core causes increase in the numbers of fuel assemblies to be shuffled and to be transferred in periodical inspection of the reactor core, and consequently, necessary period and man-hour for fuel exchange operations increase and an utilization factor for the plant can be lowered. Therefore, there is a problem that a scale merit which is expected by the increase of the output power is not necessarily obtained. Accordingly, in view of labor saving for fuel exchange operation, it is effective to increase size of a fuel assembly for reducing total number of fuel assemblies in the reactor core. On the other hand, the increase in size of a fuel assembly causes increment of local power peaking factor in a diametral direction because of increase in heterogeneity of the reactor core. Further, the number of the fuel assemblies in the reactor core decreases by increasing size of the fuel assembly under a condition that the reactor core has a constant size. Accordingly, the number of control rods being inserted among the fuel assemblies also decreases. It means relative decrease in total length of the control rod blade, reducing in control rods worth, and decrease in the reactor shut down margin. Therefore, it is necessary to have means for preventing above described problems when increasing size of the fuel assembly. When the above described prior art are applied for increasing size of the fuel assembly, the following defects exist; In accordance with the prior art, JP-A-63-231293 (1988), the reactor shut down margin can be increased by reducing the neutron moderating effect, but thermal margin is decreased by increase in the ratio of the transverse cross section area of the water gap region to the transverse cross section area of the total fuel pellets. In accordance with the prior art, JP-A-2-12088 (1990), the transverse cross section of the non-boiling water region is defined by taking the internal transverse cross section of the channel box as a base. Therefore, there are some cases in which effective increment of the reactor shut down margin can not be achieved depending on a loading condition of the fuel rods in the fuel assembly. As for the thermal margin, the situation is the same. Further, the above defined value for the transverse cross section of the non-boiling water region varies depending on the kind of the fuel material such as uranium-plutonium mixed oxides, or enriched uranium. SUMMARY OF THE INVENTION (1) Objects of the Invention The first object of the present invention is to provide a fuel assembly and a reactor core therefor which are capable of increasing size of the fuel assembly. The second object of the present invention is to provide a fuel assembly and a reactor core therefor which are capable of increasing size of the fuel assembly with ensuring thermal margin. The third object of the present invention is to provide a fuel assembly and a reactor core therefor which are capable of increasing size of the fuel assembly with ensuring reactor shut down margin. In the present invention, increasing size of the fuel assembly is aimed at about 1.5 times of the conventional fuel assembly in consideration of reducing the number of the fuel assemblies about a half of the conventional one. (2) Methods for solving the Problems In order to achieve the above first and the second objectives of the present invention, a fuel assembly having a plurality of fuel rods which are composed by inserting a plurality of fuel pellets containing fissile material into cladding tubes and sealing the cladding tubes, and at least a moderating rod filled with a moderator for moderating neutrons which are generated by nuclear fissions of the fissile material, characterized in having an average ratio at least 0.4 in the axial direction of a sum of transverse cross section area of the portion filled with the moderator of the moderating rods to a sum of transverse cross section area of the fuel pellets is provided. In order to achieve the above first and the third objectives of the present invention, the above fuel assembly preferably having the transverse cross section area for the portion filled with the moderator of 14-50 cm.sup.2 per moderating rod is provided. Further, preferably, the above fuel assembly characterized in having a value in a range of 2.7-3.4 for a ratio of a sum of transverse cross section area of the moderator at a horizontal cross section surrounded by hypothetical planes which are imaginarily formed by extending outer hem of upper tie plate, which bundles upper ends of a plurality of the above fuel rods, downward vertically to the horizontal cross section to a sum of transverse cross section area of the fuel pellets is provided. Further preferably, in order to achieve the first objective of the present invention, the above fuel assembly characterized in having at least a double wall tube in which water level goes up and down depending on flow rate of the moderator as for one of themoderating rods is provided. Further, in order to achieve the above first to third objectives of the present invention, a reactor core having the above fuel assembly according to the present invention is provided. Further preferably, in order to achieve the first and the second objectives of the present invention, the above reactor core characterized in having a ratio utmost 0.7 of a sum of transverse cross section area for the moderator being filled in the water gap region around the fuel assembly to a sum of transverse cross section area for the above fuel pellets is provided. In order to achieve the first and the third objectives of the present invention, the above reactor core preferably having a control rod which is composed of a plurality of absorbing rods containing neutron absorber bundled in a shape having a cruciform cross section, and being inserted into the water gap region around the fuel assemblies, and a ratio at least 0.20 for a sum of surface area of the above absorbing rods to a sum of surface area of the above fuel rods is provided. Further preferably, the above reactor core having a control rod which is composed of a plurality of absorbing rods containing neutron absorber bundled in a shape having a cruciform cross section, and being inserted into the water gap region around the fuel assemblies, and a ratio at least 0.4 for a sum of transverse cross section area of the above absorbing rods to a sum of transverse cross section area of the above water gap region is provided. Further preferably, the above reactor core having a value between 3.0-3.5 for a ratio of a sum of transverse cross section area of the above moderator to a sum of transverse cross section area of the above fuel pellets is provided. In accordance with the present invention which is composed as above described, a transverse cross section area of the moderator in the moderating rod is increased, a transverse cross section area of the water gap region is decreased, values of local power peaking factor is decreased, and thermal margin is ensured, by making a ratio of transverse cross section area of the portion filled with moderator in the moderating rod to transverse cross section area of the fuel pellets averaged in the axial direction at least 0.4. Moreover, the excess reactivity is reduced and the reactor shut down margin is ensured, by making the transverse cross section area of the moderator per moderating rod 14-50 cm.sup.2. Further, the excess reactivity is reduced and the reactor shut down margin is ensured, by making a ratio of a sum of transverse cross section area of the moderator at a horizontal cross section surrounded by hypothetical planes which are imaginarily formed by extending outer hem of upper tie plate downward vertically to the horizontal cross section to a sum of transverse cross section area of the fuel pellets a value in a range of 2.7-3.4. By using a double wall tube water rod as for at least one of the moderating rods, effects of spectrum shift are multiplied to reduce necessary uranium enrichment, and an operation without inserting control rods can be performed. The local power peaking factor is reduced and the thermal margin can be ensured by making a ratio of a sum of transverse cross section area of the moderator being filled in the water gap region around the fuel assemblies to a sum of transverse cross section area of the fuel pellets utmost 0.7. The control rod worth is increased and the reactor shut down margin is ensured by making a ratio of a sum of surface area of the absorbing rods to a sum of surface area of the fuel rods at least 0.20. Further, the control rod worth is increased and the reactor shut down margin is ensured by making a ratio of a sum of transverse cross section area of the absorbing rods to a sum of transverse cross section area of the water gap region at least 0.4. The excess reactivity is decreased and the reactor shut down margin is ensured by making a ratio of a sum of transverse cross section area of the moderator to a sum of transverse cross section area of the fuel pellets a value in a range 3.0-3.5.