JP2005265696A - Fuel assemblies for boiling water reactors - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel assembly for boiling water type atomic reactors, in which the void coefficient becomes a proper value and the adjustment of reactivity can be realized in a proper flammable poison amount in a reactor core having high burnup and continuously operating for a long term. <P>SOLUTION: A fuel assembly for boiling water type atomic reactors, comprising a plurality of fuel bars 101 aligned in a square grid form, and a channel box 104 which encloses the outer periphery of these fuel bars as a whole, wherein the fuel bars 101 includes a plurality of first fuel bars (groups 1 to 4) to which no flammable poison is added, at least one second fuel bar (G) to which gadolinium alone is added as a flammable poison, and at least one third fuel bar (Er) to which a rare earth oxide alone, other than gadolinium, is added as a flammable poison. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a boiling water reactor fuel assembly, and more particularly to a boiling water reactor fuel assembly that includes a fuel rod to which a flammable poison is added and is suitable for high burnup and long-term cycle operation.

  In a boiling water reactor, a flammable poison is generally added to a part of a fuel rod and used as a fuel rod containing a flammable poison. In most cases, gadolinium oxide (gadolinia) is used as a flammable poison. Among rare earth oxides other than gadolinium, nuclides with large cross-sectional areas such as erbium, samarium, europium, and dysprosium are also considered to be used in some nuclear reactors because of their properties as flammable poisons ( Patent Literatures 1 to 3).

  Here, using FIG. 3 to FIG. 6, characteristics of flammable poisons of gadolinium and rare earth oxides other than gadolinium (oxides of erbium, samarium, and europium), flammable poisons in fuel assemblies for boiling water reactors How to use the fuel rods will be described.

  Gadolinium has the largest absorption cross-section in light water reactors. For example, in gadolinium 157 (Gd-157), the cross-sectional area of the heat group is 250,000 burns, which is the largest among all isotopes. It has characteristics as a flammable poison. FIG. 3 shows the energy dependence of the Gd-157 cross section. As indicated by the one-dot chain line (CAPTURE) in the figure, the heat absorption cross section is very large on the thermal energy side, and if many peaks with sharp resonance absorption are excluded, the absorption cross section decreases as the neutron energy increases. To go.

  Next, FIG. 4 shows the energy dependence of the cross-sectional area of erbium 167 (Er-167). As indicated by the alternate long and short dash line (CAPTURE) in the figure, the absorption cross-section of the heat group of Er-167 is about 600 barn and smaller than Gd-157, but has an absorption cross-section that can be used as a flammable poison. Unlike Gd-157, the energy dependence has a large absorption peak in the resonance region, and the value of the resonance integral is about 4 times as large as 3000 burns compared to 760 burns of Gd-157.

  Next, FIG. 5 and FIG. 6 show the energy dependence of the cross-sectional areas of samarium 149 (Sm-149) and europium 151 (Eu-151), respectively. These isotopes, like erbium 167, have a large absorption peak in the resonance region unlike Gd-157, and the value of the resonance integral is larger than that of Gd-157. Although dysprosium 162 does not show a cross-sectional area diagram, it shows energy changes in cross-sectional areas similar to those of samarium 149 (Sm-149) and europium 151 (Eu-151).

  3 to 6 are JENDL- posted on the website of the Nuclear Data Center of the Japan Atomic Energy Research Institute Energy System Research Department (http://wwwndc.tokai.jaeri.go.jp/jendl/j33/J33_J.html). Quoted from the cross-sectional area diagram of 3.3.

A fuel rod containing a flammable poison in a boiling water reactor fuel assembly is used as a so-called solid body obtained by mixing gadolinia with a base material such as uranium oxide and sintering it. The concentration of gadolinia to be added is determined in consideration of the operation period of the reactor, the burnup degree, the enrichment of uranium, the position in the assembly, and the like, and is usually used in the concentration range of 2 to 10 wt%. Various arrangements of fuel rods containing flammable poisons are used in the assembly. There are cases where fuel rods containing flammable poisons are arranged so as not to be adjacent to each other, some are adjacent in an L shape, some are adjacent to a water rod, some are adjacent to a channel box, etc. .
Japanese Patent Laid-Open No. 4-212093 JP-A-8-201555 JP 2000-298187 A

  In the fuel assemblies using only the above-mentioned conventional gadolinium or only rare earth oxides other than gadolinium as a flammable poison, the following points have been problems.

  (1) When gadolinium is used alone, it has a large cross-sectional area only in the thermal energy region, so the void reactivity coefficient may be on the positive side in the case of a fuel assembly with particularly high flammable poison reactivity. . For example, in a core with a take-off burnup of about 80 GWd / t or more, or in a core with a continuous operation period of 5 years or more, the number of fuel rods containing combustible poisons increases, and the void coefficient is negative and small or positive at the beginning of combustion. It may become. In addition, when gadolinium is used for a long continuous operation of 10 years or more, the absorption cross-sectional area is large, so it is necessary to quickly deplete the concentration, and in the case of natural gadolinium, it is several tens wt%. Concentration is required. For this reason, there is a drawback that the amount of fissile material such as uranium is reduced.

  (2) When erbium is used alone, the absorption cross section in the thermal energy region is not sufficiently large, so the amount of flammable poison added is significantly increased compared to gadolinium to suppress the initial reactivity. There is a need. Further, when used alone, only the absorption in the resonance region is large, so that there is a problem that the void coefficient becomes too large on the negative side.

  The present invention has been made to solve the above-mentioned problems of the prior art, and in particular, in a high burnup core and a long-running continuous core, the void coefficient becomes an appropriate value and the reactivity is adjusted appropriately. The objective is to obtain a boiling water reactor fuel assembly that can be realized with a toxic amount.

  The present invention achieves the above object, and the invention according to claim 1 is a boiling device having a plurality of fuel rods arranged in a square lattice and a channel box surrounding the outer periphery of the fuel rods as a whole. In the fuel assembly for a water reactor, the plurality of fuel rods include a plurality of first fuel rods to which no combustible poison is added and at least one second fuel to which only gadolinium is added as a combustible poison. It includes a rod and at least one third fuel rod to which only a rare earth oxide other than gadolinium is added as a combustible poison.

  The invention according to claim 6 is a boiling water nuclear reactor fuel assembly comprising a plurality of fuel rods arranged in a square lattice and a channel box surrounding the outer periphery of the entire fuel rods. The plurality of fuel rods includes a plurality of first fuel rods to which no flammable poison is added, at least one second fuel rod to which both gadolinium and a rare earth oxide other than gadolinium are added as a flammable poison, It is characterized by including.

  According to the present invention, the ability to absorb flammable poisons can be greatly increased, so that a fuel assembly with a reduced number of fuel rods containing flammable poisons can be provided. Further, it is possible to provide a fuel assembly that can maintain the toxic reactivity for a long period of time as compared with the case of Gadolinia alone.

  A first embodiment of a boiling water nuclear reactor fuel assembly according to the present invention will be described with reference to FIG. FIG. 1 is a plan view schematically showing the arrangement of fuel rods 101 and water rods 102 in the fuel assembly 100. The fuel rods 101 are arranged in 12 rows and 12 columns in a square lattice shape, and the whole is surrounded by a channel box 104 having a rectangular tube shape. As the water rod 102, one having a circular cross section is arranged at the center of the fuel rod array. FIG. 1 is a view for illustrating the arrangement of the fuel rods 101 and the like, and it is preferable that the actual cross section of each fuel rod 101 is circular.

  The fuel rod 101 includes a fuel rod filled with concentrated uranium pellets to which no combustible poison is added, and a fuel rod containing a combustible poison. Fuel rods to which no flammable poison is added are classified into four types of groups 1 to 4 according to uranium enrichment. In FIG. 1, symbols 1 to 4 indicate positions corresponding to the groups 1 to 4. The enrichment of the fuel rod of group 1 is the highest, and the enrichment is lower in the order of groups 2, 3 and 4.

  There are two types of fuel rods containing flammable poisons, and one fuel rod G is obtained by adding only gadolinia to the base material of enriched uranium, and is represented by the symbol G in the figure. In the other fuel rod Er, only erbium oxide (erbia) is added to the base material of the enriched uranium, and is represented by the symbol Er in the figure.

  In the example of FIG. 1, group 1 is 58, group 2 is 24, group 3 is 8, group 4 is 4, fuel rod G is 12, fuel rod Er is 22.

  In the enriched uranium fuel rod groups 1 to 4 having no flammable poison, the fuel rod groups 3 and 4 having low enrichment are arranged at the outer peripheral position. The group 2 fuel rods are arranged at the outer peripheral position and at a position adjacent to the water rod 102. The fuel rods G are arranged in the second layer from the outer periphery of the fuel rod arrangement, and the fuel rods G are adjacent to each other. The fuel rods Er are arranged at positions within the second layer so as not to be adjacent to each other. However, here, “adjacent” means adjacent in the vertical or horizontal direction, and does not include those facing diagonally. The number of fuel rods G and the number of fuel rods Er are smaller in the former.

  Light water of the coolant containing steam flows between the fuel rods 101 in the channel box 104 from the lower part to the upper part. Inside the water rod 102, a coolant containing almost no vapor flows. Further, outside the channel box 104 of the fuel assembly 100, cooling water not containing steam flows. Further, since the fuel assemblies 100 are regularly arranged in the nuclear reactor, similar fuel assemblies are lined up at a predetermined interval between adjacent assemblies.

  In the fuel assembly having the above configuration, the fuel rod Er has a neutron absorption cross section in a large resonance region, and therefore absorbs neutrons of resonance energy. In general, the toxic reactivity cannot be sufficiently secured only by absorption in the resonance region, but the fuel rod G has a very large absorption cross section of thermal neutrons and has a large toxic effect with thermal energy, and a great toxic effect can be obtained in both cases. For example, compared to the fuel rod G alone or the fuel rod Er alone, the poisonous reactivity is exerted in both heat and resonance energy even if the total amount is the same. Great effect.

  The fuel rod G and the fuel rod Er are converted into nuclides having a large absorption cross section by neutron absorption. Since the cross section of the fuel rod G is very large, the conversion speed is fast. In comparison, the fuel rod Er is converted at a relatively slow speed. In this embodiment, since the fuel rods G are adjacent to each other, the neutrons are locally recessed as compared with the case where they are not adjacent. Therefore, the gadolinia conversion speed is converted at a slower speed than when the fuel rods G are not adjacent to each other. On the other hand, since the fuel rods Er are not adjacent to each other, the conversion speed does not decrease as in the case where the fuel rods G are adjacent.

  As described above, according to the present embodiment, the toxic effect can be enhanced even with the same number of fuel rods G and fuel rods Er as in the case of single use of fuel rods Er, so that the number of flammable poison-containing fuel rods can be reduced. In general, there is an example of limiting the uranium enrichment of fuel rods containing flammable poisons to an average level because the addition of flammable poisons deteriorates the heat transfer of the fuel rods, but if the fuel rods containing flammable poisons can be reduced, the enrichment level This makes it possible to realize a fuel assembly with improved Therefore, a fuel assembly with a higher burnup can be obtained, and as a result, fuel cost can be reduced.

  Further, since the fuel rods G are adjacent to each other, the gadolinia conversion speed in the fuel rod G is reduced, and the fuel rod G can be used for a longer period of time. On the other hand, the conversion rate of the elvia in the fuel rod Er is slow because the neutron cross section is smaller than the gadolinia. Therefore, it is possible to approach the conversion speed of the fuel rod G by not adjoining the fuel rods Er. By doing so, it is possible to avoid that only elvia or only gadolinia is converted first and one of them remains.

  Furthermore, since the neutron absorption effect of the fuel rod G is much greater than that of the fuel rod Er, the absorption capacity per fuel rod G is greater than that of the fuel rod Er. Therefore, thermal neutrons and resonance neutrons can be absorbed in a balanced manner by making the number of fuel rods G smaller than that of the fuel rod Er. However, in order to suppress the initial reactivity, the number of fuel rods G can be set to be approximately the same as the number of fuel rods Er depending on the design.

  The gadolinia weight ratio of the fuel rod G is generally used in the range of 2 to 10 wt%, but if the weight ratio of the elvia in the fuel rod Er is larger than the weight ratio of gadolinia, one of them does not remain significantly, The so-called residual reactivity penalty can be minimized. Further, when the elvia is mainly used for long-term operation, it is conceivable to set the gadolinia concentration to be smaller than the elvia concentration. For example, the case where it is used for a core for continuous operation for 10 years corresponds.

  Furthermore, the fuel rod Er can be used by adding not only Elvia alone but also gadolinia. In this case, the total amount of combustible poisons can be increased without increasing the number of fuel rods Er. Of course, it is possible to add erbia to the fuel rod G, and in this case, the total amount of the flammable poison can be further increased. Thus, when both are added and used simultaneously, what is called a residual reactivity penalty can be minimized by making the weight ratio of gadolinia smaller than elvia. However, when used for long-term operation, it may be possible to design the elvia concentration to be larger than the gadolinia concentration.

  In the above description, the arrangement of 12 rows and 12 columns has been described as an example, but the present invention can be applied to a fuel assembly for boiling water reactors in general. Further, the fuel rods containing the flammable poisons can be disposed at the outermost peripheral position of the assembly. For example, either the fuel rod G or the fuel rod Er can be disposed at the outermost peripheral position as necessary. is there.

  In this embodiment, the water rod has a single circular cross section, but any cross section shape of the water rod can be applied. For example, a square cross section or a plurality of circular cross sections may be used. . Furthermore, the present invention can also be applied to a fuel assembly in which a flow path for non-boiling water that substantially serves as a water rod is formed in a channel box, instead of a circular or square cross section like a water rod. As such an example, an internal cross gap type fuel assembly is known.

  In the above description, erbia is used as the flammable poison as the rare earth oxide, but the same effect can be obtained by using an oxide of samarium, europium, or dysprosium instead of erbia.

  Next, a second embodiment of the boiling water nuclear reactor fuel assembly according to the present invention will be described with reference to FIG. However, parts common to the first embodiment are denoted by common reference numerals, and redundant description is omitted.

  FIG. 2 schematically shows the arrangement of the fuel rods 101 and the water rods 202 in a plan view of the boiling water nuclear reactor fuel assembly 200 according to the second embodiment. In the second embodiment, the fuel rods 101 are arranged in a 10 × 10 square lattice. In addition, two water rods 202 having a circular cross section are arranged at substantially the center of the fuel rod arrangement.

  The configurations of the fuel rod groups 1 to 4 to which no flammable poison is added and the fuel rod Er to which only Elvia is added to the base material of the enriched uranium are the same as those in the first embodiment.

  In the second embodiment, fuel rods in which only gadolinia is added to the base material of enriched uranium are displayed in three types, fuel rods G1, G2, and G3, depending on their positions. The fuel rod G1 is in the second row from the outer periphery, and is adjacent to only the fuel rod to which no flammable poison is added. The fuel rod G2 is in the third row from the outer periphery and is adjacent to only the fuel rod Er. The fuel rod G3 is adjacent to the water rod 202 and the fuel rod to which no flammable poison is added. Regarding the fuel rods G1, G2, and G3, the gadolinia concentrations are higher in the order of G1, G2, and G3, and the concentrations of G2 and G3 may be equal. The concentration of uranium is set such that G1 and G2 are equal and G3 is the lowest.

  44 in group 1, 8 in group 2, 8 in group 3, 4 in group 4, 4 fuel rods G1, 4 fuel rods G2, 2 fuel rods G3, and fuel rod Er 22.

  According to the above configuration, flammable poison-containing fuel rods G1, G2, and G3 containing only gadolinia and flammable poison-containing fuel rods Er containing only elbia are not adjacent to each other, so that neutrons in the same energy region are locally Absorbs. Further, since the fuel rod G3 is adjacent to the water rod 202 having a large amount of neutrons in the thermal energy region, the neutron absorption amount of the fuel rod G3 becomes larger than that in the case where the fuel rod G3 is not adjacent to the water rod 202.

  According to this second embodiment, by adjoining fuel rods containing different types of combustible poisons, the toxic effect per number can be enhanced rather than adjoining fuel rods containing the same kind of combustible poisons, The number of fuel rods containing flammable poisons can be reduced. In particular, fuels for long-term operation cycles that require a significant reduction in reactivity require the placement of a large number of fuel rods containing flammable poisons.By adopting this method, the number of fuel rods containing flammable poisons can be increased. Restrictions are relaxed. Further, since the fuel rod Er has a large absorption cross-sectional area of resonance neutrons, even if it is adjacent to the water rod 202, the absorption effect is rather small. Therefore, the fuel rod G3 including gadolinia is preferentially adjacent to the water rod 202. Thereby, the absorption effect of the whole fuel rod containing a flammable poison is enhanced.

  In this embodiment, the 10-row 10-column arrangement has been described as an example, but it goes without saying that the present invention is applicable to general boiling water nuclear fuel assemblies. Furthermore, it is clear that the effect can be enhanced by arranging the fuel rods containing gadolinia at the outermost peripheral position of the assembly. For example, at the outermost peripheral position, any one of the fuel rod G1, the fuel rod G2, and the fuel rod G3 can be arranged as necessary. In this embodiment, two water rods having a circular cross section are used, but any cross section shape of the water rod can be applied. For example, a square cross section or a single circular cross section can be used. good.

  Even in this embodiment, erbia is used as the flammable poison as the rare earth oxide, but the same effect can be obtained by using samarium, europium, or dysprosium oxide instead of erbia.

1 is a schematic horizontal sectional view of a first embodiment of a boiling water nuclear reactor fuel assembly according to the present invention. FIG. The typical horizontal sectional view of 2nd Embodiment of the fuel assembly for boiling water reactors which concerns on this invention. It is sectional drawing of gadolinium 157, Comprising: A vertical axis | shaft represents a cross-sectional area (burn), and the horizontal axis represents neutron energy (eV). It is sectional drawing of erbium 167, Comprising: The vertical axis | shaft represents a cross-sectional area (burn), and the horizontal axis represents neutron energy (eV). It is sectional drawing of samarium 149, Comprising: A vertical axis | shaft represents a cross-sectional area (burn), and the horizontal axis represents neutron energy (eV). It is sectional drawing of Europium 151, Comprising: A vertical axis | shaft represents a cross-sectional area (burn), and the horizontal axis represents neutron energy (eV).

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100,200 ... Fuel assembly, 101 ... Fuel rod, 102 ... Water rod, 104 ... Channel box, 202 ... Water rod

Claims (7)

In a boiling water nuclear reactor fuel assembly having a plurality of fuel rods arranged in a square lattice shape and a channel box surrounding the outer periphery of these fuel rods,
The plurality of fuel rods include a plurality of first fuel rods to which no flammable poison is added, at least one second fuel rod to which only gadolinium is added as a flammable poison, and other than gadolinium as a flammable poison. Including at least one third fuel rod to which only rare earth oxide is added,
A fuel assembly for boiling water reactors characterized by   2. The boiling water reactor fuel assembly according to claim 1, wherein the number of the second fuel rods is smaller than the number of the third fuel rods. 3. Reactor fuel assembly.   The boiling water nuclear reactor fuel assembly according to claim 1 or 2, wherein at least a part of the second fuel rod and at least a part of the third fuel rod are adjacent to each other. A fuel assembly for boiling water reactors.   The boiling water nuclear reactor fuel assembly according to any one of claims 1 to 3, wherein at least one water rod is disposed near a center, and at least a part of the second fuel rod is the water fuel. A fuel assembly for a boiling water reactor, wherein the fuel assembly is disposed adjacent to a rod, and the third fuel rod is disposed not adjacent to the water rod.   5. The fuel assembly for a boiling water reactor according to claim 1, wherein there are a plurality of the second fuel rods, and at least some of the second fuel rods are adjacent to each other. A fuel assembly for a boiling water reactor characterized by the above. In a boiling water nuclear reactor fuel assembly having a plurality of fuel rods arranged in a square lattice shape and a channel box surrounding the outer periphery of these fuel rods,
The plurality of fuel rods includes a plurality of first fuel rods to which no flammable poison is added, and at least one second fuel rod to which both gadolinium and a rare earth oxide other than gadolinium are added as a flammable poison. Including,
A fuel assembly for boiling water reactors characterized by   7. The fuel assembly for a boiling water reactor according to claim 6, wherein the second fuel rod contains gadolinium oxide and erbium oxide, and a weight ratio of gadolinium oxide in the second fuel rod is an erbium oxide weight. A fuel assembly for a boiling water reactor characterized by being smaller than a ratio.

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