JP2013217678A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly capable of improving core thermal limits by flattening an axial direction output distribution throughout an operation cycle period.SOLUTION: Four out of 74 fuel rods provided in a fuel assembly 1 are short size Gd fuel rods 14, 15, whereas 14 fuel rods are long size Gd fuel rods 12, 13. A second long size fuel rod 12 of the long size Gd fuel rods 12, 13 has separate concentration distributions in two areas in a lower region "C" and gadolinia concentration is higher the lower in the axial direction. The second short size fuel rod 15 is filled on its upper part side of a fuel effective length with uranium fuel having as base material natural uranium without gadolinia added, and filled on its lower part side with uranium fuel having low gadolinia concentration. The concentration of gadolinia added to the short size Gd fuel rods 14, 15 is suppressed to be lower than minimal values of the long size Gd fuel rods 12, 13.

Description

  The present invention relates to a fuel assembly loaded in a boiling water reactor (BWR), and more particularly, to a fuel assembly including a MOX fuel that is a mixed oxide of uranium and plutonium.

  In the MOX fuel assembly, the axial power distribution at the beginning of combustion is flattened to improve the thermal characteristics of the core, while reducing the control rod value and preventing the reactor shutdown margin from decreasing. In order to provide a fuel assembly capable of simplifying the distribution and reducing the number of types of MOX fuel rods, a long fuel rod having a relatively long active fuel length and a fuel active length longer than that of the long fuel rod In a fuel assembly in which short short fuel rods are arranged in a square grid of 9 rows and 9 columns, the long fuel rods are first long fuels filled with MOX fuel, which is a mixed oxide of uranium and plutonium. And a second long fuel rod filled with uranium fuel to which no flammable absorbent is added, and a third long fuel rod filled with uranium fuel to which a flammable absorbent is added, the short fuel Back with flammable absorbent added to the rod There is filled with fuel (see Patent Document 1).

JP 2003-194978 A

  By the way, in the fuel assembly, recently, in order to improve the economic efficiency of nuclear power generation, the degree of burnup of fuel has been increased. In order to improve the economic efficiency of the fuel loaded in the nuclear reactor, it is necessary to increase the burnup by extending the period of loading in the nuclear reactor as much as possible. Also, the more nuclear fuel material contained in the fuel assembly, the more the burnup of the fuel assembly will increase. Therefore, increasing the fissile material contained in the fuel assembly, that is, increasing the average concentration of the fuel material. An increase has been made.

  As the degree of burnup increases, the reactivity of the fuel increases, so that the number of fuel rods and the amount of the flammable absorbent mixed with the flammable absorbent in order to suppress the increase in reactivity tend to increase. In addition, as the average enrichment of the fissile material in the fuel assembly increases, the reactivity at the initial stage of combustion increases, so that a fuel containing a combustible absorbent is ensured in order to ensure a furnace shutdown margin and a thermal margin. The loading ratio of the rods also tends to increase.

  Here, in the case of MOX fuel assemblies, considering that flammable absorbents such as gadolinia are mixed only in enriched uranium fuel rods, the increase in the number of fuel rods mixed with flammable absorbents is the amount of uranium used. Will increase. For this reason, from the viewpoint of reducing the amount of uranium used per fuel assembly, it is desirable to reduce the uranium enrichment of the gadolinia fuel rods as much as possible. In addition, when the enrichment distribution and the gadolinia concentration distribution are distributed in the axial direction, the molding of the fuel becomes complicated and the manufacturing cost increases. For this reason, it is desirable that the uranium enrichment and gadolinia concentration of the uranium fuel rod containing gadolinia be as small as possible in the axial direction.

  In view of such needs specific to MOX fuel, it is necessary to optimize the arrangement of uranium fuel rods to which a combustible absorbent is added in the MOX fuel assembly.

Here, the average fuel removal degree E depends on the number of batches N, the operation period T, and the fuel loading amount M, and there is the following relationship among them.
E∝N * T / M
Therefore, in order to improve the fuel economy, it is better that the batch number N is large (the number of new fuel bodies is small), but if the operation period T is lengthened, the batch number N is small (the number of new fuel bodies is large). In the direction. Further, when the batch number N is small, the variation in the burnup of fuel in the core is reduced, and the number of fuel bodies that perform combustion changes in the same tendency is increased. For this reason, the power distribution in the axial direction becomes severe, and the core power distribution tends to be distorted downward from the initial cycle to the middle cycle.

  In the case of MOX fuel, in order to improve the fuel economy, the uranium enrichment of gadolinia-added uranium fuel rods is reduced or natural uranium with a low uranium concentration is used. To increase the utilization rate of Pu. For this reason, since the output of the gadolinia-added uranium fuel rod is reduced and the output of the MOX fuel rod is increased, the local output peaking of the fuel assembly is increased.

  For this reason, in the gadolinia-added uranium fuel rod, using natural uranium and extending the operation cycle period increases axial power peaking and local power peaking, which decreases the thermal margin of the core. There was a problem.

  In Patent Document 1, although the axial power distribution at the beginning of the cycle is flattened by adding a combustible absorbent such as gadolinia to the short fuel rod, flattening of the power distribution over the operation cycle period cannot be achieved. .

  An object of the present invention is to provide a fuel assembly capable of flattening an axial power distribution throughout an operation cycle period and improving a thermal margin of a core.

  In order to achieve the above object, the first invention provides first and second long fuel rods having a long effective fuel length, and first and second short fuels having a shorter effective fuel length than these long fuel rods. A fuel assembly for a boiling water reactor in which rods are arranged in a square grid of 9 rows and 9 columns, and is a first long fuel rod filled with MOX fuel, which is a mixed oxide of uranium and plutonium And a uranium fuel that is divided into at least two or more in a region having the same length as the short fuel rod, and in which a combustible absorbent having a higher concentration of combustible absorbent than that in the upper region is added to the lower region. A filled second long fuel rod, a first short fuel rod filled with uranium fuel with a combustible absorbent added to the entire effective fuel length, and a combustible absorbent in the lower region of the effective fuel longitudinal axis. Fill with the added uranium fuel and add a flammable absorbent in the upper region A second short fuel rod filled with uranium fuel that has not been added, and the concentration of the combustible absorbent added to the first short fuel rod and the second short fuel rod is the second long fuel rod. It is below the minimum value of the concentration of the combustible absorbent added to the above.

  Further, the second invention is the flammability of the first invention, which is divided into at least two in a region longer than the short fuel rod, and the concentration of the combustible absorbent is lower in the upper region than in the lower region. A third elongate fuel rod filled with uranium fuel to which an absorbent material was added was further provided.

  According to a third invention, in the first or second invention, the uranium fuel in the long fuel rods and the short fuel rods has a base material of natural uranium, deteriorated uranium, or recovered uranium.

  According to the fuel assembly of the present invention, the axial power distribution can be flattened throughout the operation cycle period, and the thermal margin of the core can be improved. Therefore, the MOX fuel assembly can be operated for a long time and the Pu load can be increased. Can do.

1 is an elevational view showing a partial cross section of a first embodiment of a MOX fuel assembly of the present invention. It is sectional drawing which shows the AA arrow cross section of 1st Embodiment of the MOX fuel assembly of this invention shown in FIG. It is sectional drawing which shows the BB arrow cross section of 1st Embodiment of the MOX fuel assembly of this invention shown in FIG. It is a figure which shows arrangement | positioning in the cross-sectional direction of the fuel rod in the BB arrow cross section of 1st Embodiment of the MOX fuel assembly of this invention shown in FIG. It is a figure which shows the structure of the axial direction of the fuel rod of 1st Embodiment of the MOX fuel assembly of this invention shown in FIG. It is a figure which shows the relationship between the gadolinia density | concentration of a gadolinia addition uranium fuel rod, the number of gadolinia fuel rods, and the reactivity. It is a figure which shows the mode of transition in the cycle of a core average axial direction power distribution when the gadolinia density | concentration of a 2nd elongate fuel rod is made into 1 area | region in an axial direction. It is a figure which shows the mode of the transition in the cycle of a core average axial direction power distribution when the gadolinia density | concentration of the 2nd elongate fuel rod in the lower area | region "C" of a fuel assembly is made into 2 area | regions in an axial direction. Changes in the maximum linear power density during the cycle in the fuel assembly in which the gadolinia concentration of the second long fuel rod in the lower region “C” of the fuel assembly is two regions in the axial direction and in the fuel assembly in one region. FIG. When the gadolinia concentration of the second short fuel rod in the lower region “C” of the fuel assembly is set to two regions in the axial direction and the core average axial power distribution at the initial stage of the cycle in the fuel assembly in the case of one region It is a figure which shows a mode. It is a figure which shows the structure of the axial direction of the fuel rod of 2nd Embodiment of the MOX fuel assembly of this invention.

  Embodiments of a fuel assembly according to the present invention will be described below with reference to the drawings.

<First Embodiment>
A first embodiment of a fuel assembly according to the present invention will be described with reference to FIGS.
First, an example of the fuel assembly will be described with reference to FIGS. 1 is a partially cutaway front view of the MOX fuel assembly, FIG. 2 is a cross-sectional view taken along the line AA in FIG. 1, and FIG. 3 is a cross-sectional view taken along the line BB in FIG.

  1-3, the fuel assembly 1 is schematically constituted by a long fuel rod 2, a short fuel rod 3, an upper tie plate 4, a lower tie plate 5, a water rod 6, a channel box 7, a spacer 8, and the like. .

This fuel assembly 1 includes a normal long fuel rod 2 having a relatively long effective fuel length, and a short fuel rod 3 having a shorter effective fuel length than the long fuel rod 2 and lacking fuel in the upper region. The large-diameter water rods 6 are bundled in a square grid of 9 rows and 9 columns with a plurality of spacers 8 arranged in the axial direction to maintain the spacing between the fuel rods. Of the bundled fuel rods 2 and 3, the long fuel rod 2 is fixed to the lower tie plate 5 with the upper tie plate 4 and the lower tie plate 5 together with the external spring 9. ing. The bundled fuel rods 2 and 3 are surrounded by a channel box 7.
The two water rods 6 are circular in cross section and larger in diameter than the fuel rods 2 and 3, and are disposed diagonally in an area where seven fuel rods in the center of the fuel assembly 1 can be disposed. Further, two adjacent sides of the channel box 7 are arranged so that the control rods 10 having a cross-shaped cross section are close to each other. And in order to arrange | position the control rod 10 and the neutron detector instrumentation tube which is not shown in figure, the space | interval between each fuel assembly is expanded outside the channel box 7, and the circumference | surroundings are filled with cooling water. . The cooling water flows inside and outside the channel box 7 upward from below in FIG.

Next, details of the configuration of the long fuel rod 2 and the short fuel rod 3 will be described with reference to FIGS. 4 and 5.
FIG. 4 is a cross-sectional view showing the arrangement of the fuel rods of the first embodiment of the fuel assembly according to the present invention, and FIG. 5 is a diagram showing the gadolinia concentration distribution of the gadolinia-added uranium fuel rods.

  As shown in FIGS. 4 and 5, the long fuel rod 2 in the fuel assembly 1 is a long MOX fuel rod 11 filled with MOX fuel, which is a mixed oxide of uranium and plutonium, as the first long fuel rod. A long uranium fuel rod (long Gd fuel rod) 12 filled with uranium fuel in which gadolinia is added to a flammable absorbent as a second long fuel rod, and a flammable absorber as a third long fuel rod. It has a long uranium fuel rod (long Gd fuel rod) 13 filled with uranium fuel with gadolinia added to the material. The long fuel rods 2 include 52 first long fuel rods 11, eight second long fuel rods 12, and six third long fuel rods 13.

  The short fuel rods 3 are short uranium fuel rods (short Gd fuel rods) 14 filled with uranium fuel in which gadolinia is added to a combustible absorbent as first short fuel rods, and combustible absorption as second short fuel rods. Short uranium fuel rod (short Gd fuel rod) 15 filled with uranium fuel with gadolinia added to the material, and short MOX fuel filled with MOX fuel, which is a mixed oxide of uranium and plutonium, as the third short fuel rod It consists of a bar 16. Here, eight short fuel rods 3 are arranged at the corner of the second layer from the outermost periphery of the square lattice array and at the center of each side, and the first short fuel rod 14 and the second short fuel rod 15 are respectively arranged. Two and the third short fuel rods 16 are four.

  Natural uranium is used as the uranium base material for the uranium fuel of the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15, and gadolinia is added to the natural uranium as a flammable absorbent. Filled with sintered pellets of the mixture.

  As shown in FIG. 5, the fuel assembly 1 is divided into an upper upper region “B” and a lower lower region “C” with the upper end of the effective fuel length of the short fuel rod 3 as a boundary. Among these, the gadolinia concentration distribution of the fuel assembly of the present embodiment will be described below.

  As shown in FIG. 5, in the second long fuel rod 12, the lower region “C” having the same length as the effective fuel length of the short fuel rod 3 has two types of gadolinia concentrations e and c in the axial direction. The upper region “B” has a density b different from the gadolinia concentrations e and c of the lower region “C”. The third elongate fuel rod 13 has a uniform gadolinia concentration c in the lower region “C”, and the upper region “B” has a concentration b different from the gadolinia concentration c of the lower region “C”. ing. Here, the gadolinia concentrations b, c, e in the long Gd fuel rods 12, 13 are higher toward the lower part in the axial direction, and the relationship b <c <e is satisfied.

  Further, the first short fuel rod 14 has a uniform gadolinia concentration d in the lower region “C”. Further, the second short fuel rod 15 has, in the lower region “C”, a natural uranium region f to which no gadolinia is added and a natural uranium region d to which gadolinia is added from above in the axial direction. . Here, the gadolinia concentration d in the short Gd fuel rods 14, 15 is smaller than the minimum value b of the gadolinia concentrations b, c, e in the long Gd fuel rods 12, 13, and the gadolinia concentration d is d. <B relationship is satisfied.

  The long MOX fuel rods 11 and the short MOX fuel rods 16 are actually divided into four types of Pu enrichment degree a and are distributed in the axial direction, but details thereof are omitted in this embodiment. To do.

  A region f in FIG. 5 shows a region composed of only natural uranium fuel to which no gadolinia is added. In the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15, It has the effect of suppressing leakage.

In FIG. 4, the second long fuel rod 12 in the fuel assembly 1 is placed at a position adjacent to the short Gd fuel rods 14 and 15 arranged at the second corner portion of the 9 × 9 square lattice array. Book placed. Further, six third long fuel rods 13 are arranged at positions adjacent to the water rod 6 arranged in the central region in the square lattice array.
Two first short fuel rods 14 are arranged at positions on the control rod 10 side and the counter-control rod side (the side far from the control rod 10). Further, two second short fuel rods 15 are arranged in the second layer corner portion of a square lattice arrangement. Four short MOX fuel rods 16 are arranged at the center of each side of the second layer from the outermost periphery of the square lattice array. And the 1st elongate fuel rod 11 is arrange | positioned in the remaining position.

  Normally, in MOX fuel, it is effective to increase the Pu enrichment of MOX fuel rods and reduce the uranium enrichment of gadolinia-added uranium fuel rods in order to increase the Pu utilization rate. As a result, the output of the MOX fuel rods increases and the local output peaking of the fuel assembly tends to increase, which causes the thermal margin to deteriorate.

  Here, FIG. 6 shows the burnup dependence of the relationship between the gadolinia concentration of gadolinia-added uranium fuel rods and the number of gadolinia fuel rods and the reactivity (infinite multiplication factor).

6, (1) is when the gadolinia concentration and the number of gadolinia fuel rods in the gadolinia fuel rods are average, and (2) is when the number of gadolinia fuel rods is average and the gadolinia concentration is higher than the average ((2) in the figure). If ₊ indicated by) and thin (in the figure (2) _- indicated by), (3) if gadolinia concentration is higher than the average mean a number (in Fig. (3 in gadolinia fuel rods) less and indicated by ₊₎ If (in the figure (3) _- indicated by), 4 if gadolinia concentration in the gadolinia fuel rods is larger number darker than average (5) is infinite multiplication factor for the burnup of the case, there is no gadolinia fuel rods ( k _∞ ), respectively. The horizontal axis represents the burnup and the vertical axis represents the infinite multiplication factor.

  In FIG. 6, as indicated by (1) in FIG. 6, the infinite multiplication factor gradually increases as combustion progresses and gadolinia burns, reaches a peak when gadolinia burns out, and gradually increases after the peak is exceeded. Descend.

Further, as indicated by (1) and (2) ₊ in FIG. 6, the higher the gadolinia concentration, the higher the burn-up degree that gadolinia burns out (slower gadolinia burnout time), and an infinite multiplication factor (k _∞ ). The maximum value of can be reduced. Conversely, as shown in (1) and (2) _- , the lower the gadolinia concentration, the lower the degree of burning that gadolinia burns out (the earlier the time when gadolinia burns out), and the maximum infinite multiplication factor (k _∞ ). The value can be increased.
In the BWR core, the power distribution generally has a downward peak, so when the gadolinia concentration is made uniform in the axial direction, the lower gadolinia burns out quickly, and the lower peak in the middle of the cycle There is a tendency to become excessive. For this reason, the lower peak in the middle of the cycle can be suppressed by dividing the concentration distribution region in the axial direction and increasing the gadolinia concentration in the lower region.

Further, as indicated by (1) and (3) ₊ in FIG. 6, as the number of gadolinia-added uranium fuel rods increases, the surface area of the fuel rods increases and neutron absorption increases. Becomes lower. On the contrary, as shown in (1) and (3) _- , the smaller the number of gadolinia-added uranium fuel rods, the smaller the surface area of the fuel rods and the lower the neutron absorption. To do.
In the BWR core, the power distribution generally has a downward peak from the beginning of the cycle to the middle of the cycle. For this reason, the short peak in the early stage of combustion can be suppressed by filling the short fuel rods in the lower region with gadolinia-added uranium fuel and increasing the number of gadolinia-added uranium fuel rods in the lower region.

  Further, as shown in FIG. 6 (5), when there is no gadolinia fuel rod, the infinite multiplication factor monotonously decreases as the reactivity increases.

  Thus, by combining the increase / decrease in the number of gadolinia fuel rods and the increase / decrease in gadolinia concentration (including axial concentration distribution), it is possible to appropriately control the excess reactivity of the core and the axial power distribution. It becomes.

Therefore, four of the 74 fuel rods provided in the fuel assembly 1 are short Gd fuel rods 14 and 15, and 14 are long Gd fuel rods 12 and 13. Of the long Gd fuel rods 12 and 13, the second long fuel rod 12 has a concentration distribution divided into two regions in the lower region “C”, and further increases as the gadolinia concentration goes downward in the axial direction. It has been. The second short fuel rod 15 is filled with uranium fuel whose base material is natural uranium to which no gadolinia is added on the upper side of the effective fuel length, and uranium fuel with a low gadolinia concentration added on the lower side. Filled. Further, the gadolinia concentration added to the short Gd fuel rods 14 and 15 is kept lower than the minimum value of the long Gd fuel rods 12 and 13.
Thus, the number of gadolinia-added uranium fuel rods 13 to 15 in the lower region “C” of the fuel assembly 1 is set to the lower region “C” with respect to the number of gadolinia-added uranium fuel rods 12 and 13 in the upper region “B”. 2 (by the short Gd fuel rods 14) and 4 at the lower side of the lower region “C” (by the short Gd fuel rods 14 and 15).

That is, the cross section of the fuel assembly 1 is the upper cross section (the upper region “B” cross section without the short fuel rod 2), and the central cross section (the gadolinia of the second short fuel rod 15 in the lower region “C” is not added. Including the region), and the lower cross section (including the region where the gadolinia of the second short fuel rod 15 is added), the number of fuel rods added with the combustible absorbent is
The upper cross section <the central cross section <the lower cross section.

  FIG. 7 is a diagram showing a state of transition during the cycle of the core average axial power distribution when the gadolinia concentration of the second long fuel rod is one region in the axial direction, and FIG. 8 is a diagram showing the lower part of the fuel assembly. It is a figure which shows the mode of transition in the cycle of a core average axial direction power distribution when the gadolinia density | concentration of the 2nd elongate fuel rod in area | region "C" is made into 2 area | regions in an axial direction. 7 and 8, A is the core average axial power distribution at the beginning of the cycle, B is at the middle stage, and C is at the end stage.

As shown in FIG. 7, when the gadolinia concentration of the second long fuel rod in the lower region “C” is one region in the axial direction, the output in the lower axial direction is higher than that in the upper portion in the middle of the cycle (B in FIG. 7). About 40%, and a large deviation occurs in the axial output distribution at the end of the cycle (C in FIG. 7).
On the other hand, as shown in FIG. 8, by setting the gadolinia concentration of the second long fuel rod in the lower region “C” to be two regions in the axial direction, the lower portion in the axial direction in the middle of the cycle (B in FIG. 8). The output can be reduced, thereby making it possible to reduce the output difference between the lower part and the upper part, and to flatten the axial output even at the beginning of the cycle (A in FIG. 8) and at the end (C in FIG. 8).

  FIG. 9 shows the fuel assembly in which the gadolinia concentration of the second long fuel rod in the lower region “C” of the fuel assembly is two regions in the axial direction, and the gadolinia concentration of the second long fuel rod for comparison. The figure of the change during the driving cycle of the maximum linear power density in the fuel assembly with one region in the axial direction is shown. In FIG. 9, A is the maximum linear power density in the fuel assembly with two regions and B with one region.

  As shown in FIG. 9, the fuel assembly in which the gadolinia concentration of the second long fuel rod in the lower region “C” is one region in the axial direction (B in FIG. 9) has a high maximum linear power density in the middle of the cycle. The margin to the limit value is small. In contrast, in the case of a fuel assembly in which the gadolinia concentration of the second long fuel rod is set to two regions in the axial direction (A in FIG. 9), the maximum linear power density in the middle of the cycle is almost the same level as in the initial and final cycles. Therefore, the change in the axial output distribution during the operation cycle can be stabilized, and the thermal margin can be greatly improved.

  FIG. 10 shows the core average axis at the initial stage of the cycle in the fuel assembly in which the gadolinia concentration of the second short fuel rod in the lower region “C” is two regions in the axial direction and the fuel assembly which is one region for comparison. The state of the directional output distribution is shown. In FIG. 10, A is the maximum linear power density in the fuel assembly with two regions and B with one region.

  As shown in FIG. 10, by setting the gadolinia concentration of the second short fuel rod in the lower region “C” of the fuel assembly to two regions in the axial direction (A in FIG. 10), one region (B in FIG. 10). Compared with the case, the output of the intermediate portion in the axial direction at the beginning of the cycle can be increased, and the axial output distribution can be flattened. Therefore, the maximum line power density at the beginning of the cycle can be improved, and the thermal margin from the beginning of the cycle to the middle can be improved.

  With the above configuration, the above-described effects can be obtained. Accordingly, while using natural uranium for the gadolinia-added uranium fuel rod, the axial output distribution in the initial stage of the cycle is flattened and the downward strain output distribution in the middle of the cycle is flattened. be able to. Therefore, it becomes possible to flatten the axial power distribution in the long-term operating core and the MOX fuel loaded core where the influence of the lower strain output distribution at the beginning of the cycle and the lower strain of the power distribution becomes significant, and the reduction in thermal margin is improved. An improvement of about 10% can be achieved at the maximum linear power density.

<Second Embodiment>
A second embodiment of the fuel assembly of the present invention will be described with reference to FIG.

The fuel assembly in the second embodiment has substantially the same configuration as the fuel assembly of the first embodiment except for the long fuel rods 2, and details thereof are omitted.
FIG. 11 is a diagram showing a gadolinia concentration distribution of a gadolinia-added uranium fuel rod.

  As shown in FIG. 11, in the second embodiment of the fuel assembly 1 of the present invention, the long fuel rod 2 in the fuel assembly 1 includes a first long fuel rod 11 and a second long fuel rod 12. And the point which has the 3rd elongate fuel rod 13 is the same as 1st Embodiment.

  Among these, as shown in FIG. 11, the third long fuel rod 13 has a uniform gadolinia concentration c in the lower region “C” and two types of gadolinia concentrations in the axial direction in the upper region “B”. b and d. In the second long fuel rod 12, the lower region “C” has two types of gadolinia concentrations e and c in the axial direction, and the upper region “B” has a concentration different from the gadolinia concentration of the lower region “C”. b. Here, the gadolinia concentrations b, c, d, and e are higher toward the lower part in the axial direction, and the relationship d <b <c <e is satisfied.

  Also in the fuel assembly of the second embodiment, the obtained effect is almost the same as that of the fuel assembly of the first embodiment. Furthermore, in the fuel assembly of the present embodiment, the gadolinia concentration in the long Gd fuel rod 12 is divided into two regions in the upper region “B”, and the upper side concentration is reduced, thereby reducing the residual amount of gadolinia at the end of the cycle. Reduction can be achieved, gadolinia can be used effectively, and the cost can be reduced. Moreover, the reactivity of an axial upper part can also be raised and the effect of planarizing an axial output distribution more is also acquired.

  In the present embodiment, the gadolinia concentration d of the third long fuel rod 13 and the gadolinia concentration d of the short Gd fuel rods 14 and 15 are the same, but the gadolinia concentration of the short Gd fuel rods 14 and 15 is the same. May be smaller than the gadolinia concentration of the third long fuel rod 13.

<Third Embodiment>
In the first embodiment, natural uranium (uranium 235 concentration is about 0.7 wt%) is used as the uranium base material of uranium fuel to be filled in the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15. However, deteriorated uranium can be used for the uranium base material of this uranium fuel. Other configurations are substantially the same as those of the fuel assembly of the first embodiment, and details thereof are omitted.
Here, depleted uranium is a general term for uranium fuel obtained in the enrichment process for obtaining enriched uranium from natural uranium, and generally refers to uranium having a uranium 235 concentration of about 0.2 to 0.3 wt%. Show.

  In this embodiment, depleted uranium is used as the uranium base material of the uranium fuel to be filled in the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15, gadolinia is added to the depleted uranium, and this mixture is used. Filled with sintered pellets.

  In general, the lower the concentration of uranium 235 in the uranium fuel, the more severe the local output peaking in the fuel assembly and the smaller the thermal margin. However, under the condition that the operation cycle period length and the take-out average burnup are not large, this allowance is utilized, and the fuel rod added with the combustible absorbent in the fuel assembly 1 as in the first and second embodiments is used. It is possible to use depleted uranium in the uranium base material by configuring the number such that the upper cross section <the central cross section <the lower cross section.

  As described above, even in the fuel assembly according to the third embodiment in which deteriorated uranium is used as the uranium base material of the uranium fuel filled in the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15, the obtained effect is the first. This is almost the same as the case of the fuel assembly of the first embodiment. Further, by using deteriorated uranium as a uranium base material, the amount of uranium used per fuel assembly can be reduced as compared with the case of using enriched uranium, and fuel economy can be improved. In addition, in the process of manufacturing enriched uranium fuel used in light water cooled / light water slow boiling water reactors, conversion, enrichment and reconversion costs are incurred in the process of enriching uranium 235. When used as a material, the conversion cost and the enrichment cost are not required among the above-mentioned costs, and the manufacturing cost of the fuel assembly can be reduced.

  Also in the third embodiment, as in the second embodiment, the gadolinia concentration in the upper region “B” of the third elongated fuel rod 13 is divided into two in the axial direction, and the lower the upper side, the lower the upper side. The concentration can be set, and the same effect as in the second embodiment can be obtained.

<Fourth embodiment>
In the third embodiment, an example in which deteriorated uranium is used as the uranium base material of the uranium fuel to be filled in the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15 has been described. Recovered uranium can also be used as the fuel uranium base material. Other configurations are substantially the same as those of the fuel assembly of the first embodiment, and details thereof are omitted.
Here, recovered uranium is a general term for uranium fuel obtained by reprocessing spent fuel, and generally indicates uranium having a uranium 235 concentration of about 1 wt%.

  In the present embodiment, recovered uranium is used as the uranium base material of the uranium fuel to be filled in the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15, gadolinia is added to the recovered uranium, and this mixture is used. Filled with sintered pellets.

  Thus, even in the fuel assembly of the fourth embodiment that uses recovered uranium as the uranium base material of the uranium fuel to be filled in the long Gd fuel rods 12 and 13 and the short Gd fuel rods 14 and 15, the effect obtained is the first. This is almost the same as the case of the fuel assembly of the first embodiment. Further, by using the recovered uranium as the uranium base material, the amount of uranium used per fuel assembly can be reduced compared to the case of using enriched uranium, and the fuel economy can be improved.

  Also in the fourth embodiment, as in the second embodiment, the gadolinia concentration in the upper region “B” of the third elongated fuel rod 13 is divided into two in the axial direction, and the lower the upper side, the lower the upper side. The concentration can be set, and the same effect as in the second embodiment can be obtained.

<Others>
In each of the above-described embodiments, the fuel assemblies for boiling water reactors arranged in a 9-by-9 square lattice as the fuel assemblies 1 have been described as examples. However, the present invention is not limited to this, For example, the present invention can be applied to a fuel assembly for a boiling water reactor arranged in a square grid of 10 rows and 10 columns or more.

  Further, the length, number, shape, and arrangement of the long fuel rods 2, short fuel rods 3, and water rods 6 in the channel box 7 differ depending on the respective reactors. Therefore, the arrangement and number of fuel rods and water rods are different. Is not limited to the fuel assembly 1 shown in the present embodiment.

  6 illustrates gadolinia as an example of the combustible absorbent, but the same tendency as in FIG. 6 is exhibited in cases where other types of combustible absorbent are used.

  Furthermore, in the above-described embodiment, the lower region “C” of the second elongate fuel rod 12 (first and second embodiments) and the upper region “B” of the third elongate fuel rod 12 (second embodiment). However, the number of divisions is not limited to two and may be two or more.

1 ... Fuel assembly,
2 ... Long fuel rod,
3. Short fuel rods
4 ... Upper tie plate,
5 ... Lower tie plate,
6 ... Water rod,
7 ... Channel box,
8 ... Spacer,
9 ... External spring,
10 ... Control rod,
11 ... long MOX fuel rod (first long fuel rod),
12. Long uranium fuel rod (second long fuel rod, long Gd fuel rod),
13. Long uranium fuel rod (third long fuel rod, long Gd fuel rod),
14. Short uranium fuel rod (first short fuel rod, short Gd fuel rod),
15. Short uranium fuel rod (second short fuel rod, short Gd fuel rod),
16. Short MOX fuel rod (third short fuel rod).

Claims (3)

First and second long fuel rods having a long effective fuel length, and first and second short fuel rods having a fuel effective length shorter than these long fuel rods are arranged in a square lattice of 9 rows and 9 columns or more. A fuel assembly for an array of boiling water reactors,
A first elongate fuel rod filled with MOX fuel, which is a mixed oxide of uranium and plutonium,
In the region of the same length as the short fuel rod, it is divided into at least two or more, and the lower region is filled with uranium fuel to which a combustible absorbent having a higher concentration than the upper region is added. A second elongate fuel rod;
A first short fuel rod filled with uranium fuel with a combustible absorbent added to the entire effective fuel length;
A second short fuel rod filled with uranium fuel added with a flammable absorbent in the lower region of the effective fuel longitudinal axis and filled with uranium fuel not added with a flammable absorber in the upper region; ,
The concentration of the combustible absorbent added to the first short fuel rod and the second short fuel rod is not more than the minimum value of the concentration of the combustible absorbent added to the second long fuel rod. Characteristic fuel assembly. The fuel assembly according to claim 1, wherein
A third region that is divided into at least two or more in a region longer than the short fuel rod, and is filled with uranium fuel to which a combustible absorbent having a lower concentration than that of the lower region is added in the upper region. A fuel assembly, further comprising a long fuel rod. The fuel assembly according to claim 1 or 2,
The fuel assembly, wherein the base material of the uranium fuel in the long fuel rod and the short fuel rod is any of natural uranium, deteriorated uranium, and recovered uranium.

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