Patent Number: 054328292
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly and a reactor core, and more particularly to a fuel assembly for use in a boiling water reactor and a core of such a reactor. A conventional fuel assembly loaded in a boiling water reactor comprises a channel box in the form of a rectangular tube and a fuel bundle housed in the channel box. The fuel bundle comprises upper and lower tie plates respectively fitted to upper and lower portions of the channel box, a plurality of spacers installed in the channel box with intervals therebetween in the axial direction, a number of fuel rods penetrating through the spacers and arrayed in a square lattice pattern with their opposite ends fixed to the tie plates, and at least one water rod. Recently, raising a degree of burn-up of a fuel assembly has been attempted from the standpoints of prolonging the operating time, effectively utilizing uranium resource, and reducing the amount of spent fuel generated. For achieving a higher degree of burn-up, it is required to increase enrichment of a fuel assembly. With enrichment increasing, however, larger mean energy of neutrons has raised the problem that reactivity change due to void variations may increase, or effective utilization of fissionable material (fuel economy) may be impeded. The increased reactivity change due to void variations not only enlarges an absolute value of the void coefficient and lowers core stability, but also reduces a shutdown margin because of an increase in the hot-cold swing. Such a tendency is dealt with by increasing a moderator proportion (i.e., a ratio of moderator to fuel) in the fuel assembly and reducing mean energy of neutrons (i.e., making the neutron spectrum softer). In a boiling water reactor, control rods and neutron detecting counters are disposed outside the channel box. Therefore, a gap is defined between fuel assemblies for allowing those units to be inserted therein. Since the gap is filled with saturation water, those fuel rods which are positioned in a peripheral portion of the fuel assembly (i.e., in a region nearer to the gap) and those fuel rods which are positioned in a central portion of the fuel assembly are affected by the saturation water in the gap in different ways. Specifically, in the peripheral portion of the fuel assembly nearer to the gap, the ratio of moderator to fuel is so large as to increase a moderating effect, thus making nuclear fissions in the fuel rods at such a position more active. On the contrary, the fuel rods positioned in the central portion of the fuel assembly are less affected by a moderating effect due to the saturation water in the gap. Thus, the ratio of moderator to fuel, as a factor of determining nuclear characteristics of a fuel assembly, is different depending on the position of the fuel assembly. There are two methods of raising the ratio of moderator to fuel; i.e., a method of reducing a fuel inventory and a method of increasing a moderator region or moderator density. Practically, these methods are carried out by (1) increasing a boiling water region (e.g., diminishing the number of fuel rods or thinning the diameter of fuel rods), and (2) increasing a non-boiling water region (water rod region or gap water region). In the fuel assembly prepared by adopting one of the above methods, however, the fuel inventory is reduced in any case, meaning that fuel economy is improved from the aspect of enlarging the ratio of moderator to fuel, but fuel economy is impeded in terms of the fuel inventory. Eventually, an improvement in fuel economy is not achieved. Furthermore, the above-mentioned methods give rise to new problems. With the method (1), the reduced total length of fuel rods increases a linear heat generation rate and decreases a thermal margin. With the method (2), the reduced area of flow paths for a coolant makes a pressure drop larger. In a conventional fuel assembly, fuel rods are arrayed into a lattice pattern of 8 rows and 8 columns (i.e., 8.times.8). If the number of unit lattices in the fuel rod array is increased to 9.times.9 or 10.times.10, it would be possible to reduce a linear heat generation rate, enlarge a heat conducting area, and increase a thermal margin. Also, as illustrated in FIGS. 3 and 4 of JP, A, 52-50498, it is known to construct a fuel assembly by using partial length fuel rods which have a shorter fuel effective length. With this type fuel assembly, since the flow path area of a two-phase flow (in an upper portion of the core) having a large friction loss is enlarged, the pressure drop can be suppressed without reducing the fuel inventory. Consequently, by adopting the aforesaid methods (1) and (2) in addition to those two approaches, a fuel assembly can be obtained which is suitable for raising a degree of burn-up. In view of the above, there has been proposed a fuel assembly that fuel rods are arrayed in a lattice pattern of 9.times.9 or 10.times.10 with each fuel rod having a larger outer diameter but the number of fuel rods being increased, the cross-sectional area of a water rod is made larger than that of a unit lattice, and further a plurality of partial length fuel rods are arranged, as disclosed in JP, A, 62-276493, JP, A, 64-31089 and U.S. Pat. No. 5,068,082, for instance. More specifically, JP, A, 62-276493 discloses a fuel assembly having the increased number of unit lattices in the fuel rod array, in which a number of water rods or large-diameter water rods are arranged, and a plurality of partial length fuel rods are arranged in a row along a diagonal including corners of the lattice array of fuel rods. The partial length fuel rods are denoted by 14 FIGS. 1 and 5. In FIGS. 1, 7 and 8, etc. of JP, A, 64-31089, there is disclosed a fuel assembly having the increased number of unit lattices in the fuel rod array, in which large-diameter water rods are arranged and one or a plurality of partial length fuel rods P are arranged at one or more corners of the lattice array of fuel rods. In FIGS. 41 to 56 of U.S. Pat. No. 5,068,082, there are disclosed a fuel assembly having the increased number of unit lattices in the fuel rod array, in which a plurality of partial length fuel rods P are arranged together adjacently to large-diameter water rods. U.S. Pat. No. 5,068,082 also describes a fuel assembly having the increased number of unit lattices in the fuel rod array, in which large-diameter water rods are arranged, and a plurality of partial length fuel rods P are arranged in a row along a line bisecting each side of the lattice array of fuel rods at its outermost layer (e.g., FIGS. 2B, 6 and 10, etc.) or along a diagonal including corners of the lattice array of fuel rods (FIG. 5 and 12, etc.). Furthermore, U.S. Pat. No. 5,068,082 discloses another layout example of the partial length fuel rods P in which the partial length fuel rods P are arranged at each corner and the middle of each side of the lattice array of fuel rods in its outermost layer. SUMMARY OF THE INVENTION A first object of the present invention is to provide a fuel assembly which enables a higher degree of burn-up and reduces a void coefficient without lowering reactivity, and a reactor core loading such a fuel assembly therein. A second object of the present invention is to provide a fuel assembly which enables a higher degree of burn-up, reduces a void coefficient without lowering reactivity, and further makes a local power peaking flat. A third object of the present invention is to provide a fuel assembly which enables a higher degree of burn-up, reduces a void coefficient without lowering reactivity, and further makes a moderator's cross-sectional area of a neutron moderating rod optimum. A feature of the present invention to achieve a fuel assembly meeting the above first object resides in that the fuel rods include a plurality of first fuel rods and one or more second fuel rods having a shorter fuel effective length than said first fuel rods; (b) said second fuel rods are arranged in an outermost layer of a fuel rod array in the square lattice pattern at positions other than corners of the outermost layer; and (c) among the fuel rods inside said outermost layer of said fuel rod array in the square lattice pattern and arranged in a layer adjacent to said outermost layer, those fuel rods adjacent to said second fuel rods in said outermost layer are said first fuel rods. A feature of the present invention to achieve a reactor core meeting the above first object resides in that (a) said core includes a plurality of first fuel assemblies and a plurality of second fuel assemblies; (b) said first fuel assemblies each comprise a number of fuel rods arrayed in a square lattice pattern and at least one neutron moderating rod having a cross-sectional area of a moderator larger than a cross-sectional area of a unit lattice of the fuel rod array, said fuel rods including a plurality of first fuel rods and one or more second fuel rods having a shorter fuel effective length than said first fuel rods, said second fuel rods being arranged in an outermost layer of said fuel rod array in the square lattice pattern at positions other than corners of the outermost layer, among the fuel rods inside said outermost layer of said fuel rod array in the square lattice pattern and arranged in a layer adjacent to said outermost layer, those fuel rods adjacent to said second fuel rods in said outermost layer being said first fuel rods; and (c) said first fuel assemblies and said second fuel assemblies are loaded in a core central portion and a core circumferential portion, said first fuel assemblies having a smaller loading ratio in the core central portion than in the core circumferential portion. The second object of the present invention is achieved by that when said neutron moderating rod is projected in two directions orthogonal to each other in said fuel rod array in a square lattice pattern, said second fuel rods arranged in said outermost layer are lacated inside a projected range of said neutron moderating rod including the outermost opposite regions of the projected range. The third object of the present invention is achieved by setting the cross-sectional area of the moderator in said neutron moderating rods to 7-14 cm.sup.2. The present invention has been made based on the following studies. A description will now be given of results of the studies. Taking into account application to existing cores, it is required in development of fuel assemblies having a higher degree of burn-up that a pressure drop, a thermal margin (linear heat generation rate, critical power) and other parameters remain the same as those in existing fuel assemblies. As stated before, thinning the outer diameter of fuel rods and increasing the number of lattice cells in the fuel rod array are advantageous in achieving a fuel assembly with a higher degree of burn-up. However, such approaches raise new problems. If the number of lattice cells in the fuel rod array is simply enlarged, the degree of freedom in fuel array becomes larger, but the length of the peripheral edges is increased and so is a pressure drop. Also, the reduced outer diameter of fuel rods increases a time constant of fuel rods and hence makes stability (channel stability, core stability) more marginal. In order to solve those problems, an absolute value of the void coefficient is required to be smaller than that of the existing fuel assembly. Stated otherwise, while reducing an absolute value of the void coefficient has been discussed so far from the standpoint of increasing an enrichment of fuel, an absolute value of the void coefficient must be made still smaller in the case of enlarging the number of unit lattices in the fuel rod array. As stated in connection with the above methods (1) and (2), a reduction in the reactivity coefficient such as the void coefficient requires increasing the ratio of moderator to fuel, i.e., increasing the water rod region and reducing the fuel inventory. However, reducing the fuel inventory impedes fuel economy and hence should be avoided. Accordingly, it is important in development of fuel assemblies having a higher degree of burn-up to realize a new reactivity control method (for reducing an absolute value of the void coefficient and a hot-cold swing) with no need of reducing the fuel inventory. One reactivity control method without reducing the fuel inventory is to select positions of partial length fuel rods, as disclosed in the above-cited JP, A, 62-276493, JP, A, 64-31089 and U.S. Pat. No. 5,068,082. In one part of U.S. Pat. No. 5,068,082 and JP, A, 64-31089, a plurality of partial length fuel rods are arranged together at positions adjacent to large-diameter water rods or at corners of the lattice array of fuel rods. In another part of U.S. Pat. No. 5,068,082 and JP, A, 62-276493, a plurality of partial length fuel rods are arranged in a row along a diagonal including corners of the lattice array of fuel rods or along a line bisecting each side of the lattice array of fuel rods at its outermost layer. Any of these prior arts intends to promote a neutron moderating effect and reduce both the void coefficient and the hot-cold swing, by making the non-boiling water region (water rod region or gap water region) and the partial length fuel rods adjacent to each other. With those prior art schemes, a reactivity control capability is improved with a reduction in the reactivity coefficient such as the void coefficient, but sufficient cares have not been paid to change in reactivity itself and a local power peaking depending on positions where partial length fuel rods are set. More specifically, with the prior arts, singe a plurality of partial length fuel rods are arranged together in such a manner that at least a part of the partial length fuel rods is adjacent to the water rod or the gap water region at the corner, there accompanies a problem that resonance neutrons are absorbed more and the reactivity loss is so increased as to impede fuel economy. Also, in a cross-section above the partial length fuel rods, there arises a problem that the local power peaking of the fuel rods adjacent to the partial length fuel rods is increased and hence the thermal margin is reduced. Further, the above-cited prior arts have paid no considerations on how to make, in fuel assemblies aiming a higher degree of burn-up, the cross-sectional area of the water rods optimum in relation to the arrangement of the partial length fuel rods. Meanwhile, the following has been found from studies conducted by the inventors of this application. In the case of restrictively arranging or localizing the non-boiling water region, as a moderator, in a fuel assembly that the number of unit lattices in the fuel rod array is increased to 9.times.9 or more, localizing the moderator in the outer region of the fuel assembly facing the channel box is more effective (to provide higher sensitivity) than localizing it in the inner region of the fuel assembly facing the water rods for the purpose of improving the reactivity control capability (i.e., making smaller reactivity change due to variations in the void coefficient and hot-to-cold transition, and reducing the void coefficient in its absolute value) (see FIG. 1). Also, in the case of arranging those fuel rods which have a shorter fuel effective length than ordinary fuel rods, namely, partial length fuel rods, this is effective in a cross-section above the partial length fuel rods for improving the reactivity control capability (i.e., reducing the void coefficient) similarly to the above case of localizing the non-boiling water region. The sensitivity, which represents a rate of reduction in the void coefficient depending on positions where the partial length fuel rods are set, changes in the following order from a higher to lower level (see FIG. 2): (1) Fuel at corners of an outermost layer of the fuel assembly facing the channel box, PA1 (2) Fuel in the outermost layer of the fuel assembly facing the channel box other than (1), PA1 (3) Fuel in the inner region of the fuel assembly adjacent to the water rods, and PA1 (4) Fuel adjacent to neither the channel box nor the water rods. Moreover, in the case of arranging the partial length fuel rods in the outermost layer of the fuel rod array, a control rod worth is affected depending on their set positions such that the larger control rod worth is obtained by arranging the partial length fuel rods in the outermost layer at any positions facing the channel box rather than arranging them adjacently to the water rods (see FIG. 3). Accordingly, by arranging one or more second fuel rods in the form of partial length fuel rods in the outermost layer of the fuel rod array in the square lattice array, an effect of reducing the void coefficient can be obtained to improve the reactivity control capability. An effect of enhancing the control rod worth can also be expected, which contributes to an improvement in safety. Furthermore, for the case of arranging partial length fuel rods in the outermost layer of the fuel rod array in a fuel assembly that the number of unit lattices in the fuel rod array is increased to 9.times.9 or more, the following has been found about an influence of set positions of the partial length fuel rods upon reactivity and a local power peaking from studies conducted by the inventors of this application (see FIG. 4). When a partial length fuel rod is arranged at a corner position of the outermost layer, the reactivity loss and the local power peaking of that fuel rod which is adjacent to the partial length fuel rod are both large. When a partial length fuel rod is arranged at a lattice position adjacent to the corner position of the outermost layer, the reactivity loss is remarkably improved, but the local power peaking of both the fuel rod adjacent to the partial length fuel rod and the corner fuel rod (i.e., the fuel rod positioned at the corner) remains substantially large. When a partial length fuel rod is arranged at a third lattice position counting from the corner position of the outermost layer, the reactivity loss is further improved, but the local power peaking of both the fuel rod adjacent to the partial length fuel rod and the corner fuel rod is still large. When a partial length fuel rod is arranged at a fourth lattice position counting from the corner position of the outermost layer, i.e., when a partial length fuel rod is arranged inside a projected range of the water rods including both lattice positions at the outermost opposite regions of the projected range, the reactivity loss is almost zero and the local power peaking of both the fuel rod adjacent to the partial length fuel rod and the corner fuel rod is reduced to a large extent. Consequently, the reactivity loss is reduced by arranging the second fuel rods in the form of partial length fuel rods in the outermost layer of the fuel rod array in a square lattice array other than its corner positions. Also, the reactivity loss is reduced by arranging the second fuel rods in the outermost layer of the fuel rod array in a square lattice array other than its corner positions and those positions adjacent to the corner positions. More preferably, when the water rods or neutron moderating rods are projected in two directions orthogonal to each other in the fuel rod array in the square lattice pattern, by arranging the second fuel rods in the outermost layer of the fuel rod array in a lattice pattern inside the projected range including the outermost opposite regions of the projected range, it is possible to reduce both the reactivity loss and the local power peaking, thereby improving fuel economy and a thermal margin. In addition, the following has been found from studies conducted by the inventors of this application. Specifically, to maximally utilize an effect resulted from localizing the moderator region, ordinary fuel rods are required to be localized. The localization of the ordinary fuel rods reduces probability that resonance neutrons are absorbed, and hence contributes to a further improvement in fuel economy. If, among the fuel rods inside the outermost layer of the fuel rod array in the square lattice pattern and arranged in a layer adjacent to the outermost layer, those fuel rods adjacent to the second fuel rods in the outermost layer are the first fuel rods, the region of the first fuel rods is surrounded by the moderator region and, as a result, thermal neutrons efficiently decelerated through the moderator region are caused to flow into the region of the first fuel rods with higher efficiency. Therefore, resonance absorption is reduced to improve not only the reactivity control capability but also fuel economy. This effect is further enhanced by making the region, where the first fuel rods are arranged, spread over one entire layer adjacent to the outermost layer, and is still further enhanced by making that region spread over two layers adjacent to the outermost layer. Meanwhile, preferably, by arranging, inside the outermost layer of the fuel rod array in the lattice pattern, one or more third fuel rods in the form of partial length fuel rods in a layer adjacent to the outermost layer at its corners, there is obtained an effect of rendering distribution of a coolant flow rate and distribution of a vapor volume rate more uniform within the channel box. In the region facing the channel box, particularly, in the region near its corners, friction resistance is generally so large that the coolant flow rate tends to decrease. This tendency can be overcome by arranging the partial length fuel rods at respective corners of the layer adjacent to the outermost layer. Incidentally, the expression "A is adjacent to B" used here in connection with the arrangement of fuel rods implies all such conditions that A is adjacent to B not only in the row and column directions, but also in oblique directions. Further, based on studies conducted by the inventors of this application, the following has been found about how fuel economy is affected by localizing the moderator region (i.e., the non-boiling water region), and how the cross-sectional area and shape of water rods are made optimum. For a fuel assembly that the number of unit lattices in the fuel rod array is increased to 9.times.9 or more, comparing the case of enlarging the water rod region inside the fuel rod assembly and the case of enlarging the gap water region outside the fuel rod assembly on a condition that the fuel inventory is kept constant, the water rod region is more effective (to provide higher sensitivity) than the gap water region in a point of increasing a neutron infinite multiplication factor (i.e., improving fuel economy). Accordingly, the cross-sectional area of the water rods requires to be enlarged for an improvement in fuel economy (see FIG. 5). In this case, an optimum range of the cross-sectional area of the water rods is from 7 to 14 cm.sup.2. Making the cross-sectional area of the water rods optimum is also related to stability. Stability is evaluated in terms of two modes; i.e., channel stability and core stability. An increase in both the uranium inventory and the cross-sectional area of the water rods reduces a margin of the channel stability, while an increase in the uranium inventory and a decrease in the cross-sectional area of the water rods degrades the core stability. Therefore, the allowable zone from the viewpoint of stability ranges from 9 to 11 cm.sup.2 in terms of the cross-sectional area of the water rods, the range being defined by a limit line of the channel stability and a limit line of the core stability (see FIG. 6). To enlarge the cross-sectional area of the water rods, adopting a large-size water rod is advantageous in reducing the number of fuel rods which must be sacrificed, and reducing the coolant flow passage area which is less effective to cool fuel rods (i.e., increasing a critical power). Assuming that the spacings between the water rods and the fuel rods adjacent to the water rods are constant, it is most preferable in the case of circular water rods to use the unit lattices of 2.times.2 as a water rod for effective utilization of the space. Therefore, preferably, by setting the moderator's cross-sectional area of one or more neutron moderating rods to the range of 7 to 14 cm.sup.2 the reactivity is enhanced and fuel economy is further improved. Also, by setting the moderator's cross-sectional area of the neutron moderating rods to the range of 9 to 11 cm.sup.2 the channel stability, the core stability, as well as fuel economy are improved. Additionally, the improved stability renders equipment installed for higher stability unnecessary. Moreover, by arranging the neutron moderating rods in such a region as able to accommodate 7 to 12 fuel rods, and locating the water rod region such that two or more of four lattice positions adjacent to each of the fuel lattice positions in the water rod region are those positions where the water rod region adjoins, a large-size circular water rod with the size corresponding to 2.times.2 cells can be arranged three or four in a fuel assembly having the fuel rod array of 10.times.10, and two in a fuel assembly having the fuel rod array of 9.times.9. Therefore, the coolant flow passage area which is less effective to cool fuel rods is diminished and the critical power is increased. In addition, the following has been found by considering the above results of the studies together. Arranging the partial length fuel rods adjacently to the water rods is substantially equivalent to enlarging the water rod region at the center of the fuel assembly and, therefore, has an effect of increasing a neutron infinite multiplication factor of the fuel assembly. Also, by making the number of the partial length fuel rods adjacent to the channel box larger than the number of partial length fuel rods adjacent to the water rods, the number of partial length fuel rods required in terms of the reactivity control capability can be cut down, which provides an effect of increasing the control rod worth. Therefore, preferably, by arranging one or more third fuel rods in the form of partial length fuel rods adjacently to the neutron moderating rods, the neutron infinite multiplication factor is further increased and both the reactivity and fuel economy are improved. Also, preferably, by making the number of second fuel rods arranged in the outer layer of the fuel rod array in a square lattice array larger than the number of third fuel rods given by the partial length fuel rods arranged adjacently to the neutron moderating rods, the number of partial length fuel rods used can be cut down while ensuring a necessary level of the reactivity and increasing the control rod worth. In addition, it has been found from studies conducted by the inventors of this application that, by arranging the partial length fuel rods adjacently to each other, there can be obtained a greater effect of improving both the reactivity control capability and the control rod worth than resulted from summing an effect obtainable with one partial length fuel rod alone. Therefore, preferably, by arranging two second fuel rods in the form of partial length fuel rods in at least one side of the outermost layer of the fuel rod array in a square lattice pattern, the effect of enhancing both the reactivity control capability and the control rod worth is doubled. Also, by arranging those two second fuel rods adjacently to each other, there can be obtained an effect twice or more as much as that in the case of arranging two second fuel rods not adjacently to each other, in point of enhancing both the reactivity control capability and the control rod worth. Additionally, it has been found from studies conducted by the inventors of this application that, to improve the channel stability and the core stability, upper ends of the partial length fuel rods are advantageously positioned at a level from 4th-stage spacer to 6th-stage spacer. This spacer level corresponds to 1/2-3/4 in terms of a ratio of the fuel effective length to the full fuel rod length. Therefore, preferably, by setting the second fuel rods in the form of partial length fuel rods to have a fuel effective length in a range of 1/2 to 3/4 of that of the ordinary fuel rods, there can be obtained an effect of improving the channel stability and core stability. Reactor cores are primarily grouped into C lattice cores of the type that the gap water region on the side, through which a crucial control rod is inserted, has the same gap width as that of the gap water region on the opposite side, and D lattice cores of the type that the gap water region on the side, through which a cross-shaped control rod is inserted, has a larger gap width than that of the gap water region on the opposite side. Preferably, by arranging second fuel rods at least one for each of two adjacent sides of the outermost layer of the fuel rod array in a square lattice pattern, there can obtained a fuel assembly suitable for being loaded into D lattice cores. A spectral shift rod can adjust a neutron moderating effect with a water level therein changed depending on the core flow rate and, as a result, it can be utilized to control reactivity or power. Meanwhile, in a BWR fuel assembly, burn-up reactivity is generally controlled by gadolinia. To effectively perform reactivity control or power control with a water level in the spectral shift rod, the amount of gadolinia requires to be reduced. Since the reactivity control effect is enhanced and the shutdown margin is improved by arranging the partial length fuel rods, the amount of gadolinia can be reduced. As a result, it is possible to improve fuel economy and achieve the best use of an effect of the spectral shift rod. If the number of unit lattices in the fuel rod array is increased from 8.times.8 to 9.times.9 or more, the number of layers of fuel rods constituting a fuel assembly is enlarged and hence time degree of freedom in layout for distributing the fuel rods in the fuel assembly is increased. Accordingly, fuel or moderators can be localized in the fuel assembly, meaning that the above-explained arrangement can easily be realized. Incidentally, the term "localization (localized or localizing" used herein implies that, in the fuel or moderator region surrounded by boundary lines between fuel and moderators, the length of the boundary lines per unit volume is shortened. Finally, a method of loading fuel assemblies according to the present invention will now be described. The fuel assembly (first fuel assembly) of the present invention has a feature that, since many partial length fuel rods are used, the fuel inventory largely varies in the axial direction. Accordingly, supposing a retrofitted core based on an existing core in which conventional fuel assemblies (second fuel assemblies) are loaded, an effect of axial neutron flux distribution due to an axial difference in fuel inventory must be taken into consideration. If the first fuel assembly is loaded among the second fuel assemblies having no partial length fuel rods or the smaller number of partial length fuel rods than the first fuel assembly, there is found a tendency for the second fuel assemblies to increase the power in a core lower portion and, on the contrary, for the first fuel assembly to increase the power in a core upper portion. With the method of loading fuel assemblies according to the present invention, the first fuel assemblies and the second fuel assemblies are loaded in the core central portion and the core circumferential portion such that the first fuel assemblies have a smaller loading ratio in the core central portion than in the core circumferential portion, whereby the linear power generation rate can be held not larger than a set value.