Patent Number: 050680820
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a fuel assembly for a nuclear reactor and more particularly to a fuel assembly for a boiling water reactor capable of ensuring a long operation cycle and maintaining a high shut-down margin. A fuel assembly for a boiling water reactor (BWR) is constructed by a square channel box in which a number of fuel rods each comprising a metallic clad in which nuclear fuel material is packed are regularly arranged. The reactor core of the BWR includes a plurality of cells each comprising a cruciform control blade and four fuel assemblies surrounding the control blade and these cells are arranged in a regulated manner in the core. Namely, each fuel assembly and control blade have axes perpendicular and parallel to each other, and a coolant operated as a moderator flows from the lower portion towards the upper portion of the reactor core. Concerning the BWR, steam void is not formed in a portion near the lower end of the core effective portion, i.e., the lower end of a heat generating portion at which an exothermic reaction is performed, but a lot of voids is generated at the area above the central portion of the reactor core, and the generated voids move up towards the upper portion of the reactor core. Accordingly, the void fraction in the BWR becomes high towards the upper portion of the reactor core, and as a result, the moderation characteristics to neutrons are lowered and hence the output power is also lowered. In order to obviate these defective matters, in a conventional technique, it has been performed to increase the enrichment of the fissile nuclide to be contained in the fuel at a portion of high void fraction or it has been performed to mix a burnable poison with the fuel element to suppress the increasing of the power output at a portion of low void fraction. For the reasons described above, in the BWR, the burn-up at the upper portion of the core is liable to be delayed, and hence the concentration of U-235 becomes relatively higher than that of the other portion of the core. In addition, since a fissile nuclide such as Pu-239 is generated by the voids, it is difficult to maintain the shut-down margin of the reactor core at the upper portion thereof. Moreover, recently, many efforts have been made for elongating the reactor operation cycle of the reactor and improving the degree of burn-up of the fuel in order to satisfy the economical requirement. In these cases, however, the enrichment of the fuel is necessarily increased, so that the maintenance of the shut-down margin of the reactor is made further difficult. The fuel assembly which has been conventionally used and a fuel assembly which is expected to be used in the near future for the boiling water reactor (BWR) will be described hereunder by way of typical examples with reference to the drawings. FIG. 73A is a perspective view of a fuel assembly of conventional type and FIG. 73B is a schematic vertical sectional view of a fuel rod consisting of the fuel assembly. Referring to FIG. 73A, the fuel assembly comprises water rods, not shown, and fuel rods 2 secured by an upper tie plate 4, a spacer 5 and a lower tie plate 6, and a channel box 1 surrounding the outer periphery of the thus secured water rods and fuel rods 2. Each of the fuel rods 2, as shown in FIG. 73B, comprises a clad or sheath 7, a plurality of fuel pellets 8 arranged in the clad 7, a spring 9 located in a gas plenum disposed above the pellets 8 in the clad 7, an upper plug 10 for closing the upper opening of the clad 7, and a lower plug 11 for closing the lower opening of the clad 7. FIG. 74 is a cross sectional view of the conventional fuel assembly shown in FIG. 73A, in which sixty-two fuel rods 2 and two water rods 3 are arranged in the channel box 1 to constitute the fuel assembly. The water rods 3 serve to suppress the shortage of the water acting as the moderator in the interior of the fuel assembly, but the water rods 3 are axially uniformly arranged, so that there may arise such problems as excessive water condition at the lower portion of the reactor core or water shortage condition at the upper portion thereof. FIG. 75 also shows a cross sectional view of a fuel assembly which has been developed for improving the characteristics of the fuel assembly shown in FIG. 74, and the fuel assembly shown in FIG. 75 includes one water rod 12 having a diameter larger than that of the water rod 3 to pass non-boiling water therethrough. However, even in this example, there arises a problem of the excessive water condition at the lower portion of the reactor core and the water shortage condition at the upper portion thereof as described with respect to the former example shown in FIG. 74. FIG. 76 shows a cross sectional view of a further example of a conventional fuel assembly developed for improving the fuel assembly of the type shown in FIG. 74, and the fuel assembly of FIG. 76 comprises four square channel boxes 13 each containing sixteen fuel rods 2 which are arranged to constitute a water area of a boiling moderator material and a cross-shaped space 14 defined by the respective channel boxes 13 constitutes a water area of a non-boiling moderator material to thereby aim the uniform distribution of the output power in the horizontal direction. With the fuel assembly of this character, however, there also arise problems of the excessive water condition at the lower portion of the reactor core and the water shortage condition at the upper portion thereof. FIG. 77 shows a cross section of a still further example of the conventional fuel assembly of the type improving that shown in FIG. 75. The fuel assembly of FIG. 77 is constructed by nine sub-bundles 15 each comprising nine fuel rods 2, and relatively wide gaps 16 are defined between the respective sub-bundles 15. With the fuel assembly of this example, the problems of the excessive water condition and the water shortage condition at the lower and upper portions of the reactor core have not been solved. As described hereinabove, concerning the BWR, steam voids are formed in the area of the location of the fuel assemblies except the lowest portions thereof and the voids move up towards the upper portion of the reactor core, and accordingly, the void fraction in the BWR becomes high towards the upper portion of the reactor core. As a result, the moderation characteristics to neutrons are lowered and hence the fission rate is also lowered. In other words, the burning progresses at the lower portion of the reactor core and the burning is delayed at the upper portion thereof. In order to obviate this phenomenon; that is, in order to suppress the lowering of the output power at the upper portion of the reactor core, it has been performed to increase the enrichment of the fissile nuclide to be contained in the fuel disposed at the upper portion of the reactor core. However, the increasing in the void fraction at the upper portion of the reactor core and the increasing in the enrichment of the fissile nuclide of the upper portion of the reactor core will result in the difficulty for maintaining the shut-down margin at the upper portion of the reactor core in the shut-down period of the BWR. On the other hand, in order to elongate the reactor operation cycle to meet the economical requirement, it will be desired to further increase the enrichment of the fuel. However, these facts result in the further reduction of the subcriticality at the upper portion of the reactor core, and finally, there may arise a case where the reactor is not shut-down. Because of this problem, in the conventional technique, it is considerably difficult to elongate the operation cycle of the reactor. SUMMARY OF THE INVENTION An object of this invention is to substantially eliminate the drawbacks and defects encountered in the conventional technique described above and to provide an improved fuel assembly particularly constituting a reactor core of a water boiling reactor (BWR) capable of ensuring the maintenance of reactor shut-down margin even in the increasing in the enrichment of a fuel, and improving an axial output power distribution. This and other objects can be achieved in one aspect according to this invention by providing a fuel assembly of the type in which a number of fuel rods each constructed by filling a fuel material in a clad are arranged, the fuel assembly comprising at least one first fuel rod having a partial effective fuel area filled with a fuel material and having a portion in which enrichment of a fissile nuclide is significantly reduced in a clad of the fuel rod or the fissile nuclide does not exist at all at an axial level including a portion (called shutdown zone) at which subcriticality is made small at a period in which maintenance of reactor shut-down margin is made difficult during a reactor operation period, and a second fuel rod having a total effective fuel area filled with a fuel material throughout an entire axial length of the clad of the fuel rod. According to the preferred embodiment of this invention, the first fuel rod having a partially effective fuel area may be constructed as a fuel rod provided with a partially disposed interposed zone in which the enrichment of the fissile nuclide is significantly reduced in the clad or the fissile nuclide does not exist at all. The first fuel rod may be further constructed according to the preferred embodiment of this invention a short fuel rod having an axial length shorter than that of the second fuel rod having a total effective fuel area. Further according to the preferred embodiment of this invention, tube means through which a moderator passes are arranged symmetrically with respect to at least one diagonal line, in cross section, of the fuel assembly and the first fuel rod having the partial effective fuel area is disposed in an area defined between these tube means. According to this invention, there is also provided, in another aspect, a fuel assembly of the type in which a number of fuel rods each constructed by filling a fuel material in a clad are arranged, the fuel rod comprising at least one first fuel rod having a partial effective fuel area filled with a fuel material and having a portion in which enrichment of a fissile nuclide is significantly reduced in a clad of the fuel rod or the fissile nuclide does not exist at all at an axial level including a first portion at which subcriticality is made small at a period in which maintenance of reactor shutdown margin is made difficult during a reactor operation period and a second portion located between the first portion and a lower end of the effective fuel area, and a second fuel rod having a partial interposed zone in which enrichment of a fissile nuclide is significantly reduced in a clad of the fuel rod or the fissile nuclide does not exist at all at the second portion mentioned hereinabove. According to the fuel assembly of the characters described above, in regions or zones axially located adjacent to an interposed zone of a fuel rod in which the enrichment of the fissile nuclide is significantly reduced or the fissile nuclide does not exist at all, the neutron interaction (binding effect) is weakened at a reactor cold period and is increased in a reactor high temperature operation period, particularly, during the occurrence of voids. This phenomenon will be explained with reference to the action of the thermal neutrons each having a short diffusion length. Namely, since the density of water is large (i.e., about 1.0 g/cm.sup.3) in the reactor cold period, the diffusion length of the thermal neutron becomes small and the interaction of the thermal neutrons in the zones adjacent to the interposed zone of the fuel rod is reduced, and as a result, the neutron multiplication characteristics are lowered. With the boiling water reactor, in the reactor high temperature operation period, the temperature of the water is about 286.degree. C. (reference value) and the density thereof is about 0.74 g/cm.sup.3 even when no void is generated, and the migration length of the neutron in the water is increased to about 1/0.74 (i.e. 1.35) times to that in the reactor cold period. Moreover, the density of the steam-water mixture in the occurrence of the voids is lowered to an extent of about 0.3 g/cm.sup.3, and as a result, the thermal neutron diffusion length in the gas-water mixture is increased to 1/0.3 (.apprxeq.3) times. Consequently, the neutron mutual interaction in the fuel areas adjacent to the interposed zone is increased and hence the neutron multiplication characteristics are also increased. According to the functions described above, by introducing the interposed zone into the fuel rod, the effective multiplication factor K.sub.eff is lowered in the reactor cold period, that is, the reactor shut-down margin (subcriticality) is made large, and on the other hand, in the reactor high temperature operation period, the effective multiplication factor K.sub.eff can be prevented from becoming lower even if the fuel amount is reduced by the introduction of the interposed zone, and in a certain case, it may even become possible to increase the effective multiplication factor by suitably designing the fuel rod inclusive of the interposed zone in comparison with that in the case of the fuel rod with no interposed zone. In addition, during the operation period of the BWR, since the void ratio is high at the upper portion of the reactor core, the moderator is insufficient, but according to this invention, the amount of the fuel at the upper portion of the reactor core is reduced, so that the water-to-fuel volume ratio is increased, thus resolving the insufficiency of the moderator. The output power is therefore increased and the power axial distribution can be improved. Moreover, since the water much exists in the upper portion of the reactor core, the void factor (i.e. large load) can be alleviated. The coolant flows upwardly from the lower portion of the reactor core, and the voids are not generated in the lower portion thereof but are generated in the other portion, and particularly, in the upper portion of the reactor core, the void factor is increased. This fact means that the flow rate of the coolant in steam-water mixture state increases largely. Since the pressure drop is usually in proportion to about the square of the coolant flow rate, the pressure loss in the upper portion of the fuel assembly is made large. The pressure loss is varied in accordance with the wetted areas of the fuel rods or channel box and the structure or numbers of the spacers. In this connection, according to this invention, the fuel rod is partially removed at an area upper than a portion having a length of two-third (2/3) to five-sixth (5/6) of the effective fuel length measured from the lower end of the effective portion of the fuel to thereby locate the vanishing zone, so that the number of the fuel rods in the upper portion of the fuel assembly where the pressure loss is made large can be reduced. Accordingly, the wetted area is reduced, thus effectively reducing the pressure loss, and as a result of this, the reduction of the driving power of a recirculation pump is made possible. The stability of the channel box is improved and the flow of the coolant in the channel is also made stable, which results in the reduced variation of the void factor and the improvement of the reactor core stability. Since the thermal neutron flux is increased around the tube through which the moderator passes, the thermal neutron flux increasing region can be widened by arranging a plurality of such tubes in the fuel assembly. The reactivity of the fuel is also increased in the thermal neutron flux increasing region, thus increasing the effective multiplication factor K.sub.eff. The tubes may be preferably arranged symmetrically with respect to the diagonal line of the fuel assembly in cross section to make easy the design thereof. Preferred embodiments of this invention will be further described hereunder in detail with reference to the accompanying drawings.