JP2012112768A - Nuclear fuel assembly and reactor core using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To raise response sensitivity of a neutron detector which senses a state of water in a spectral shift rod while assuring reactivity gain by spectral shift effect.SOLUTION: In a nuclear fuel assembly 11B including: a plurality of fuel rods 1 arranged in a square lattice shape; two round spectral shift rods 3 which can form steam pools at the top parts and control a water level inside by reactor core flow rate; a channel box 2 which surrounds the circumference of the plurality of fuel rods 1, a center-of-gravity position G of the spectral shift rods 3 is on a line connecting the center B of a control rod 20 and the center C of a fuel lattice, the spectral shift rods 3 are decentered and arranged on the side of a neutron detector 30 so that eccentricity Xwhich is defined by the following formula becomes √2. X=d/p, here, d is distance from the center C of the fuel lattice to the center-of-gravity position G of the spectral shift rod 3, and p is distance between the centers of the fuel rods 1.

Description

  The present invention relates to a nuclear fuel assembly loaded in a boiling water reactor and a core using the same, and more particularly to a nuclear fuel assembly including a spectrum shift rod and a core using the nuclear fuel assembly.

  In order to economically use fuel in a boiling water reactor, a nuclear fuel assembly having a spectrum shift rod (hereinafter also simply referred to as SSR) has been developed. This SSR is a kind of water rod, and a water level can be provided inside by forming a steam reservoir in the upper part. The water level in the SSR can be varied by controlling the core flow rate. Conventionally, one or a plurality of SSRs arranged in a nuclear fuel assembly are arranged such that the center of gravity of the SSR is located at the center of the cross section of the nuclear fuel assembly (e.g., Patent Document 1).

  By utilizing the feature that the internal water level can be changed by the core flow rate, the fuel in the fuel assembly can be used efficiently. That is, at the initial stage of the operation cycle, the water level in the SSR is set low by lowering the core flow rate, and steam is accumulated at the top of the SSR. Thereby, plutonium is accumulated in the region in the fuel rod corresponding to the steam region above the water level in the SSR. Thereafter, at the end of the operation cycle, the core flow rate is increased to raise the water level inside the SSR, and finally the interior of the SSR is filled. Since the accumulated plutonium burns by raising the water level inside the SSR, the output of the upper part of the fuel rod in which the plutonium is accumulated tends to increase. By performing the spectrum shift operation in this way, as a result of burning the accumulated plutonium, fuel can be used efficiently and a gain in reactivity can be obtained (spectrum shift effect).

  The water level in the SSR is related to the amount of plutonium accumulated and the amount of plutonium consumed when the SSR becomes full, and ultimately affects the power distribution of the core at the end of the operation cycle. Therefore, in order to accurately monitor the power distribution of the core, it is desirable to accurately estimate the water level in the SSR.

  By the way, when the water level in the SSR is set low, there is a difference in the neutron moderation effect due to the difference in water density between the vapor region above the water level and the liquid region below. That is, the neutron moderation effect in the upper vapor region is smaller than the neutron moderation effect in the lower liquid region. For this reason, the axial distribution of the neutron flux measured by the neutron detector of the in-core nuclear instrumentation system is affected by the water level in the SSR.

  Therefore, it is considered that the water level in the SSR can be estimated from the position where a change in the axial direction is observed in the neutron flux measurement value of the neutron detector.

  As one of such methods, the water level in the SSR is calculated from the difference between the power distribution obtained by the thermal hydraulic calculation process including the water level calculation in the SSR and the power distribution in the axial direction of the core measured by the neutron detector. A correction method has been proposed (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 5-232273 JP 2009-236727 A

  As described above, when the water level in the SSR is estimated using the axial power distribution of the core measured by the neutron detector, the response sensitivity of the neutron detector affects the estimation accuracy of the water level in the SSR. . If the estimation accuracy of the water level in the SSR is low, the amount of plutonium accumulated in the fuel rod and the output accompanying combustion of plutonium after raising the water level in the SSR cannot be accurately predicted. For this reason, there is a problem that margins for various thermal limit values cannot be accurately grasped and the core monitoring performance is deteriorated.

  Therefore, in order to perform the spectrum shift operation without deteriorating the core monitoring performance, it is necessary to increase the response sensitivity of the neutron detector as much as possible. The magnitude of the influence of the state change of the cooling water (moderator) in the SSR on the neutron detector depends on the distance between the SSR and the neutron detector. That is, the shorter the distance between the SSR and the neutron detector, the higher the response sensitivity of the neutron detector. However, when the SSR is brought close to the neutron detector, there is a problem that the reactivity gain due to the SSR is lowered. On the other hand, conventionally, the position of the SSR in the nuclear fuel assembly has not been studied from the viewpoint of estimating the water level in the SSR based on the measurement result of the neutron detector.

  Therefore, an object of the present invention is to improve the response sensitivity of a neutron detector that senses the state of water in a spectrum shift rod while ensuring a reactivity gain due to the spectrum shift effect.

  In the present invention, the spectral shift rod of the fuel assembly is eccentric to the neutron detection side within a range where the spectral shift effect can be obtained.

According to a first aspect of the invention,
A plurality of fuel rods arranged in a square lattice, one or a plurality of spectral shift rods capable of forming a steam reservoir at the top and controlling the internal water level by the core flow rate, and the outer circumferences of the plurality of fuel rods A nuclear fuel assembly comprising: a spectral box, and a position of the center of gravity of the horizontal section of the spectral shift rod is on a line connecting the center of the control rod and the center of the fuel lattice, and the reactivity gain by the spectral shift rod A nuclear fuel assembly is provided in which the spectral shift rod is eccentrically arranged on the opposite side of the control rod with respect to the center of the fuel lattice within a range in which is positive.

According to a second aspect of the invention,
A core of a boiling water reactor comprising a plurality of fuel assemblies, comprising: a neutron detector disposed in the core; and the fuel assembly according to the first aspect adjacent to the neutron detector. A boiling water reactor core is provided.

  In the nuclear fuel assembly according to the present invention, the position of the center of gravity of the horizontal section of the spectrum shift rod is on the line connecting the center of the control rod and the center of the fuel lattice, and within the range where the reactivity gain by the spectrum shift rod is positive. A spectral shift rod is eccentrically arranged on the opposite side of the control rod with respect to the center of the fuel grid. For example, the spectral shift rod is eccentrically arranged on the neutron detector side so that the amount of eccentricity becomes √2 or √2 / 2. This is equivalent to decentering the spectral shift rod by one row from the conventional position toward the neutron detector with the fuel rod arrangement. Thereby, the response sensitivity of the neutron detector which senses the state of the water in a spectrum shift rod can be improved, ensuring the reactivity gain by a spectrum shift effect.

  By improving the response sensitivity of the neutron detector, the accuracy of estimating the water level in the spectrum shift rod is improved. As a result, the amount of plutonium accumulated in the fuel rods and the consumption of plutonium after raising the water level in the SSR can be accurately predicted, and the margin for various thermal limit values can be accurately estimated. Therefore, the monitoring performance of the core can be improved.

  Further, in the core of the boiling water reactor according to the present invention, the nuclear fuel assembly according to the present invention in which the spectrum shift rod is eccentrically arranged is used as the nuclear fuel assembly surrounding the neutron detector, and other nuclear fuel assemblies are used. A nuclear fuel assembly in which reactivity gain is prioritized and a spectral shift rod is arranged near the center of the fuel lattice is used. With the nuclear fuel assembly according to the present invention in which the spectrum shift rod is eccentrically arranged, it is possible to accurately grasp the amount of plutonium accumulated, and by laying this, the amount of plutonium accumulated in the entire core can be accurately grasped. Can do. On the other hand, plutonium can be effectively accumulated by the conventional nuclear fuel assembly in which the reactivity gain of the spectrum shift rod is prioritized.

  Thereby, the neutron detector response sensitivity can be improved, the core monitoring performance can be improved, and a high reactivity gain can be obtained at the end of the operation cycle.

(A) And (c) is a cross-sectional view of the comparative nuclear fuel assembly, (b) is a cross-sectional view of the nuclear fuel assembly according to the first embodiment of the present invention. It is a figure which shows the reactivity gain with respect to an eccentric position, and a neutron detector response sensitivity. (A) And (c) is a cross-sectional view of the nuclear fuel assembly for a comparison, (b) is a cross-sectional view of the nuclear fuel assembly which concerns on the 2nd Embodiment of this invention. It is a figure which shows the reactivity gain with respect to an eccentric position, and a neutron detector response sensitivity. (A) And (c) is a cross-sectional view of the nuclear fuel assembly for a comparison, (b) is a cross-sectional view of the nuclear fuel assembly which concerns on the 3rd Embodiment of this invention. It is a figure which shows the reactivity gain with respect to an eccentric position, and a neutron detector response sensitivity. It is a transverse cross section of the core concerning a 4th embodiment of the present invention.

  Hereinafter, four embodiments of the present invention will be described with reference to the drawings. The first to third embodiments relate to nuclear fuel assemblies, and the fourth embodiment relates to the core of a boiling water reactor having those nuclear fuel assemblies. In addition, in each figure, the component which has an equivalent function is attached | subjected the same code | symbol, and detailed description of the component of the same code | symbol is not repeated.

(First embodiment)
The nuclear fuel assembly according to the first embodiment will be described. 1A, 1B, and 1C show cross-sectional views of the nuclear fuel assemblies 11A, 11B, and 11C, respectively. In each figure, a cross-sectional view of the cross-shaped control rod 20 for controlling the output of the nuclear fuel assembly and the neutron detector 30 for detecting the neutron flux is also shown.

  As shown in FIGS. 1A, 1B, and 1C, each of the nuclear fuel assemblies 11A, 11B, and 11C includes a plurality of fuel rods 1 arranged in a 10-row, 10-column square lattice pattern, A channel box 2 surrounding these fuel rods 1 and two round spectrum shift rods 3 are provided. One spectrum shift rod 3 occupies four lattice positions (for two rows and two columns) in the square lattice in which the fuel rod 1 is disposed. That is, one spectrum shift rod 3 is provided by replacing four fuel rods 1.

  The nuclear fuel assembly according to the present embodiment is a nuclear fuel assembly 11B shown in FIG. 1B, and the nuclear fuel assemblies 11A and 11C are for comparison. That is, as can be seen from FIGS. 1 (a) and 1 (b), the nuclear fuel assembly 11B according to this embodiment is equivalent to one fuel rod array from the center of gravity position of the spectrum shift rod 3 in the nuclear fuel assembly 11A. Only the position of the center of gravity of the spectrum shift rod 3 is decentered toward the neutron detector 30.

ここで、Ｇ：重心位置、Ａ_ｉ：ｉ番目のスペクトルシフトロッドの断面積、Ｒ_ｉ：ｉ番目のスペクトルシフトロッドの中心位置の位置ベクトル、ｎ：燃料格子内にあるスペクトルシフトロッドの総数である。位置ベクトルは、燃料格子の中心（原点）から中性子検出器の方向に正を取ったｘ軸と、ｘ軸と原点で垂直に交わるｙ軸とで張られるｘ−ｙ平面におけるベクトルである。通常、スペクトルシフトロッドはｘ軸に関して対称に配置されるため、重心位置はｘ軸上にある。 The “center of gravity position” is defined by equation (1) and is treated as an index of the average position of the spectrum shift rod in the fuel lattice.

Where G: center of gravity position, A _i : cross-sectional area of i-th spectrum shift rod, R _i : position vector of center position of i-th spectrum shift rod, n: total number of spectrum shift rods in the fuel lattice is there. The position vector is a vector in the xy plane stretched by the x axis that is positive in the direction of the neutron detector from the center (origin) of the fuel lattice and the y axis that intersects the x axis and the origin perpendicularly. Usually, since the spectral shift rod is arranged symmetrically with respect to the x-axis, the position of the center of gravity is on the x-axis.

  In FIG. 1A, FIG. 1B, and FIG. 1C, only one nuclear fuel assembly is shown for one control rod 20 in order to show the relative positional relationship. Normally, four nuclear fuel assemblies are arranged around one control rod 20.

  As shown in FIGS. 1A, 1B, and 1C, the neutron detector 30 includes a center C of a lattice (hereinafter referred to as a fuel lattice) appearing in a cross section of the nuclear fuel assembly, and a control rod 20. Is provided near the corner of the nuclear fuel assembly opposite to the control rod 20. The neutron detector 30 uses a fission counter tube in which a fission material such as uranium 235 is applied to an electrode and an ionized gas is sealed in the tube. An in-core probe (TIP) and a local power range monitor (LPRM) are used. The LPRM is an instrumentation system which is arranged at predetermined intervals along the axial direction and has a function of monitoring a local neutron flux level in the core and an alarm function. The TIP is an instrumentation system that is provided so as to be movable in the axial direction and has a function of calibrating LPRM and a function of measuring an axial output distribution.

  The difference between the nuclear fuel assemblies 11A, 11B, and 11C is the position of the spectrum shift rod 3, as can be seen from FIGS. 1 (a), (b), and (c). That is, the gravity center position G of the two spectrum shift rods 3 included in the nuclear fuel assembly 11A coincides with the center C of the fuel lattice. On the other hand, the gravity center position G of the spectrum shift rod 3 in the nuclear fuel assemblies 11B and 11C is on a straight line connecting the center C of the fuel lattice and the neutron detector 30, and is eccentric to the neutron detector 30 along this straight line. In the position.

  Here, an eccentric position is defined to indicate the position of the spectral shift rod with respect to the neutron detector. This eccentric position is the center-of-gravity position G of the spectral shift rod on the axis where the center C of the fuel grid is the origin and the direction from the center C to the neutron detector 30 is positive. Is expressed as a unit.

In order to show the degree of eccentricity of the spectrum shift rod from the center of the fuel lattice, the amount of eccentricity X _dec is defined by equation (2).
X _dec = d / p (2)
Here, d is the distance from the center C of the fuel lattice to the center of gravity of the spectrum shift rod, and p is the distance between the centers of the fuel rods.

  The eccentric positions of the spectrum shift rod 3 in the nuclear fuel assemblies 11A, 11B, and 11C are 0, √2, and 2√2 [pitch], respectively. In this way, the eccentric position is shifted by + √2 [pitch] each time the gravity center position G of the SSR is brought closer to the neutron detector 30 side by one column from the center C of the fuel lattice.

Next, the influence of the eccentric position on the spectral shift effect and the neutron detector response sensitivity will be described. Figure 2 is a nuclear fuel assembly 11A, for each eccentric position of 11B and 11C, shows a neutron detector response sensitivity S and reactivity gain _{G r.}

The neutron detector response sensitivity S is defined by equation (3).
S = (Z ₁ −Z ₀ ) / Z ₀ × 100 [%] (3)
Here, Z ₁ is the amount of change in the uranium 235 fission cross section at the position of the neutron detector before and after the rise of the water level in the SSR when the SSR is arranged at a predetermined position of the fuel lattice, and Z ₀ : This is the amount of change in the uranium 235 fission cross section at the neutron detector position before and after the rise of the water level in the SSR when the SSR is arranged at the reference position.

The reactivity gain _Gr is defined by equation (4).
G _r = k ₁ −k ₀ (4)
Here, k ₁ : Infinite multiplication factor after raising the water level in the SSR when the SSR is arranged at a predetermined position of the fuel lattice, k ₀ : Water having the same shape as the SSR at the reference position of the fuel lattice It is an infinite multiplication factor at the end of the cycle when a rod (full water) is arranged.

  In the case of this embodiment, the reference position in the definition of the neutron detector response sensitivity and the reactivity gain is the position of the eccentric position 0.

  As shown in FIG. 2, the greater the eccentric position, that is, the more the position of the center of gravity of the spectrum shift rod 3 is decentered toward the neutron detector 30, the better the neutron detector response sensitivity, while the reactivity gain decreases. To do. The reactivity gain is reversed between positive and negative in the eccentric position between √2 and 2√2. This is because the spectral shift rod 3 is excessively decentered toward the neutron detector 30, so that the decrease in reactivity due to the decrease in water near the center of the fuel lattice is more than the increase in reactivity due to the spectral shift effect. This is because it has grown.

  In the nuclear fuel assembly 11B according to the present embodiment, since the spectrum shift rod is arranged at the eccentric position √2, as shown in FIG. 2, the reactivity gain is positive, and the neutron detector response sensitivity is This is an improvement over the conventional nuclear fuel assembly 11A in which the spectrum shift rod is arranged at the eccentric position 0.

In the example of FIG. 1, two round spectrum shift rods are arranged. However, the spectrum shift rod is not limited to this, and can be arranged so that the eccentric amount X _dec becomes zero by replacement with a fuel rod. As long as it is a simple spectral shift rod, its shape and number are arbitrary.

(Second Embodiment)
Next, a second embodiment will be described. One of the differences between the second embodiment and the first embodiment is the number and shape of spectrum shift rods. In other words, the two spectral shift rods in the first embodiment are round and arranged, whereas in the present embodiment, only one spectral shift rod is arranged in a square shape.

  3A, 3B, and 3C are cross-sectional views of the nuclear fuel assemblies 12A, 12B, and 12C, respectively. In each figure, a cross-sectional view of the control rod 20 for controlling the output of the nuclear fuel assembly and the neutron detector 30 for detecting the neutron flux are also shown.

  As shown in FIGS. 3A, 3B, and 3C, each of the nuclear fuel assemblies 12A, 12B, and 12C includes a plurality of fuel rods 1 arranged in a 10-row, 10-column square lattice pattern, A channel box 2 surrounding these fuel rods 1 and one rectangular spectrum shift rod 4 are provided. One spectrum shift rod 4 occupies nine lattice positions (for three rows and three columns) among the square lattices on which the fuel rods 1 are arranged. That is, one spectrum shift rod 4 is provided by replacing nine fuel rods 1.

  A nuclear fuel assembly 12B shown in FIG. 3B is a nuclear fuel assembly according to this embodiment, and the nuclear fuel assemblies 12A and 12C are for comparison. That is, as can be seen from FIGS. 3 (a) and 3 (b), the nuclear fuel assembly 12B according to the present embodiment is equivalent to one fuel rod array from the center of gravity of the spectrum shift rod 4 in the nuclear fuel assembly 12A. Only the position of the center of gravity of the spectrum shift rod 4 is decentered toward the neutron detector 30.

  The difference between the nuclear fuel assemblies 12A, 12B and 12C is the position of the spectrum shift rod 4 as can be seen from FIGS. 3 (a), (b) and (c). Specifically, as can be seen from FIG. 3A, the spectral shift rod 4 of the nuclear fuel assembly 12A is disposed at the eccentric position −√2 / 2. The eccentric positions of the spectrum shift rod 4 in the nuclear fuel assemblies 12B and 12C are √2 / 2 and 3√2 / 2, respectively.

  Since the fuel rod arrangement number is 10 (even number), the center of gravity position G of the spectrum shift rod 4 cannot be arranged so as to coincide with the center C of the fuel lattice of the nuclear fuel assembly 12A. That is, the spectrum shift rod 4 cannot be arranged at the eccentric position 0.

  Next, the influence of the eccentric position on the spectral shift effect and the neutron detector will be described. FIG. 4 shows the neutron detector response sensitivity and reactivity gain for each eccentric position of the nuclear fuel assemblies 12A, 12B, and 12C. The definitions of the neutron detector response sensitivity and the reactivity gain in the present embodiment are the same as in the first embodiment, but the reference position in the definition is the position of the eccentric position −√2 / 2.

  As shown in FIG. 4, the neutron detector response sensitivity is improved as the spectrum shift rod 4 is decentered toward the neutron detector 30 side. On the other hand, the reactivity gain decreases, and the eccentric position reverses between positive and negative between √2 / 2 and 3√2 / 2. This is because when the spectrum shift rod 4 is decentered too far toward the neutron detector 30, the decrease in reactivity due to the decrease in water near the center of the fuel lattice is more than the increase in reactivity due to the spectrum shift effect. This is because it has grown.

  In the nuclear fuel assembly 12B according to the present embodiment, since the spectrum shift rod is arranged at the eccentric position √2 / 2, the reactivity gain is positive and the neutron detector response sensitivity as shown in FIG. Is improved as compared with the conventional nuclear fuel assembly 12A in which the spectrum shift rod is arranged at the eccentric position −√2 / 2.

In the example of FIG. 3, one rectangular spectral shift rod is arranged. However, the spectral shift rod is not limited to this, and is arranged so that the eccentric amount X _dec becomes zero by replacement with the fuel rod. Any spectral shift rod that cannot be used is included in this embodiment.

(Third embodiment)
Next, a third embodiment will be described. One of the differences between the third embodiment and the second embodiment is the number of fuel rods arranged. That is, the number of fuel rods arranged in the nuclear fuel assembly according to the second embodiment is 10, whereas the number of fuel rods arranged in the nuclear fuel assembly according to this embodiment is 11.

  FIGS. 5A, 5B, and 5C are cross-sectional views of the nuclear fuel assemblies 13A, 13B, and 13C, respectively. In each figure, a cross-sectional view of the control rod 20 for controlling the output of the nuclear fuel assembly and the neutron detector 30 for detecting the neutron flux are also shown.

  As shown in FIGS. 5A, 5B, and 5C, each of the nuclear fuel assemblies 13A, 13B, and 13C includes a plurality of fuel rods 1 arranged in a square grid of 11 rows and 11 columns, A channel box 2 surrounding these fuel rods 1 and one rectangular spectrum shift rod 5 are provided. One spectrum shift rod 5 occupies nine lattice positions (for three rows and three columns) among the square lattices on which the fuel rods 1 are arranged. That is, one spectrum shift rod 5 is provided by replacing nine fuel rods 1.

  A nuclear fuel assembly 13B shown in FIG. 5B is a nuclear fuel assembly according to this embodiment, and the nuclear fuel assemblies 13A and 13C are for comparison. That is, as can be seen from FIGS. 5 (a) and 5 (b), the nuclear fuel assembly 13B according to this embodiment is equivalent to one fuel rod array from the center of gravity position of the spectrum shift rod 5 in the nuclear fuel assembly 13A. Only the position of the center of gravity of the spectrum shift rod 5 is decentered toward the neutron detector 30.

  The difference between the nuclear fuel assemblies 13A, 13B, and 13C is the position of the spectrum shift rod 5 as can be seen from FIGS. 5 (a), (b), and (c). Specifically, as can be seen from FIG. 5A, the nuclear fuel assembly 13 </ b> A is disposed at the eccentric position 0 of the spectrum shift rod 5. Further, the eccentric positions of the spectrum shift rod 5 in the nuclear fuel assemblies 13B and 13C are √2 and 2√2, respectively.

  Next, the influence of the eccentric position on the spectral shift effect and the neutron detector will be described. FIG. 6 shows neutron detector response sensitivity and reactivity gain for each eccentric position of the nuclear fuel assemblies 13A, 13B, and 13C. The definitions of the neutron detector response sensitivity and reactivity gain in this embodiment are the same as in the first embodiment, but the reference position in the definition is the position of the eccentric position 0.

  As shown in FIG. 6, the neutron detector response sensitivity is improved as the spectrum shift rod 5 is decentered toward the neutron detector 30 side. On the other hand, the reactivity gain decreases, and the eccentric position reverses between positive and negative between √2 and 2√2. This is because the spectral shift rod 5 is excessively decentered toward the neutron detector 30, so that the decrease in reactivity due to the decrease in water near the center of the fuel lattice is more than the increase in reactivity due to the spectral shift effect. This is because it has grown.

  In the nuclear fuel assembly 13B according to the present embodiment, since the spectrum shift rod is arranged at the eccentric position √2, as shown in FIG. 6, the reactivity gain is positive, and the neutron detector response sensitivity is This is an improvement over the conventional nuclear fuel assembly 13A in which the spectrum shift rod is disposed at the eccentric position 0.

  Heretofore, three embodiments according to the nuclear fuel assembly have been described. According to the first to third embodiments, the response sensitivity of the neutron detector 30 can be improved while ensuring the reactivity gain. That is, a higher reactivity can be obtained by spectral shift operation than the conventional nuclear fuel assembly having a water rod arranged at the center of the fuel lattice, and the response sensitivity of the neutron detector can be improved. . By improving the response sensitivity of the neutron detector, it is possible to accurately grasp margins for various thermal limit values and improve the core monitoring performance.

As is apparent from FIG. 5, the present embodiment can be arranged so that the eccentric amount X _dec becomes 0 by replacement with a fuel rod, as in the first embodiment shown in FIG. 1. Spectrum shift rod. That is, in the embodiment in which the spectrum shift rod is eccentric to the neutron detector side so that the eccentric amount X _dec is √2, the number of fuel rods included in the nuclear fuel assembly is as shown in FIGS. 1 and 5. It is not limited by the shape of the spectrum shift rod, but it is only whether or not the eccentric amount X _dec becomes 0 by replacement with the fuel rod. This also applies to the second embodiment in which the spectrum shift rod is eccentric toward the neutron detector so that the eccentric amount X _dec is √2 / 2.

(Fourth embodiment)
Next, a core of a boiling water reactor according to the fourth embodiment will be described.

  FIG. 7 shows a cross section of the core of the boiling water reactor according to this embodiment. As shown in FIG. 7, the plurality of nuclear fuel assemblies 14 are arranged symmetrically with respect to the straight lines L1 and L2 in the horizontal cross section of the core.

  As shown in FIG. 7, the control rod 20 is arranged so that it can be inserted into and removed from the gap in the center of the nuclear fuel assembly group 16 having a square cross section composed of four nuclear fuel assemblies 14.

  The neutron detectors 30 are arranged in consideration of the symmetry of the core so that the number of the neutron detectors 30 is reduced. That is, the neutron detector 30 is arranged so that it can represent not only the neutron flux at the position where it is actually arranged, but also the neutron flux at a position symmetrical with respect to the arrangement position and the straight lines L1 and L2. For example, the detection result of the neutron flux of the neutron detector 30A shown in FIG. 7 is representative not only of the neutron flux at the position of the neutron detector 30A but also the detection result of the neutron flux at the position symmetric with respect to the straight line L1 (symmetric position S). is doing. Therefore, the neutron detector 30A is also a neutron detector representing a symmetrical position. As can be seen from FIG. 7, the same applies to the other neutron detectors 30.

  In the core according to the present embodiment, for at least one neutron detector, the nuclear fuel assemblies 11B, 12B, and 13B described in the first to third embodiments are used as the nuclear fuel assemblies adjacent to the neutron detector. Either one is used. In this case, in order to correctly grasp the neutron flux, the nuclear fuel assembly adjacent to the neutron detector and the symmetric position with respect to the straight lines L1 and L2 has the same configuration as the nuclear fuel assembly adjacent to the neutron detector. It is necessary to arrange a nuclear fuel assembly. For example, as shown in FIG. 7, when the nuclear fuel assembly 11B is disposed at a position adjacent to the neutron detector 30A, the nuclear fuel assembly 11B is also disposed at a position symmetric with respect to the straight line L1.

  In the vicinity of the outer periphery of the core, neutron leakage is large and the output is low, so that the contribution of the nuclear fuel assembly to the entire core is small. For this reason, it is considered that high detection accuracy is not necessarily required for the neutron detectors near the outer periphery of the core. Therefore, in the vicinity of the outer periphery of the core, in order to give priority to the reactivity gain, a nuclear fuel assembly in which the spectrum shift rod is arranged in the vicinity of the center of the fuel lattice is arranged. It is preferable to arrange the nuclear fuel assemblies according to the first to third embodiments in an area where high neutron flux detection accuracy is required.

  As described above, according to this embodiment, it is possible to improve the neutron detector response sensitivity and improve the core monitoring performance while maintaining the reactivity gain as high as possible as the whole core.

  Heretofore, the four embodiments according to the present invention have been described. In the first to third embodiments, the combination of various shapes and numbers of spectrum shift rods and atomic fuel assemblies having different numbers of fuel rods has been described. In any case, the spectrum shift rod is arranged at the center of the fuel lattice. Is eccentrically arranged in the range from which the reactivity gain becomes positive in the direction of the neutron detector. That is, in a superordinate concept, the present invention is such that the position of the center of gravity of the horizontal cross section of the spectrum shift rod (including both the single and plural cases) is on a line connecting the center of the control rod and the center of the fuel grid, The spectral shift rod is eccentrically arranged on the side opposite to the control rod with respect to the center of the fuel lattice within a range in which the reactivity gain by the spectral shift rod is positive.

  Based on the above description, those skilled in the art may be able to conceive additional effects and various modifications of the present invention, but the aspects of the present invention are not limited to the individual embodiments described above. . Various additions, modifications, and partial deletions can be made without departing from the concept and spirit of the present invention derived from the contents defined in the claims and equivalents thereof.

1 Fuel rod 2 Channel box 3, 4, 5 Spectrum shift rod (SSR)
11A, 11B, 11C Nuclear fuel assemblies 12A, 12B, 12C Nuclear fuel assemblies 13A, 13B, 13C Nuclear fuel assemblies 14 Nuclear fuel assemblies 16 Nuclear fuel assemblies 20 Control rods 30, 30A Neutron detector B Control rods Center C of fuel grid G Center of gravity position S of spectral shift rod Symmetric position

Claims (5)

A plurality of fuel rods arranged in a square lattice, one or a plurality of spectral shift rods capable of forming a steam reservoir at the top and controlling the internal water level by the core flow rate, and the outer circumferences of the plurality of fuel rods A nuclear fuel assembly comprising:
The position of the center of gravity of the horizontal section of the spectrum shift rod is on the line connecting the center of the control rod and the center of the fuel grid, and the spectrum shift rod is positioned within the range in which the reactivity gain by the spectrum shift rod is positive. A nuclear fuel assembly characterized by being arranged eccentrically on the opposite side to the control rod with respect to the center of the fuel. When the eccentricity amount X _dec of the spectral shift rod is defined as the following equation, a spectral shift rod that can be arranged so that the eccentricity amount becomes 0 by replacement with the fuel rod, the eccentricity amount is √2 The fuel assembly according to claim 1, wherein the fuel assembly is arranged so as to be eccentric.
X _dec = d / p
Here, d is the distance from the center of the fuel lattice to the center of gravity of the spectrum shift rod, and p is the distance between the centers of the fuel rods. When the eccentricity amount X _dec of the spectrum shift rod is defined as the following equation, a spectral shift rod that cannot be arranged so that the eccentricity amount becomes 0 by replacement with the fuel rod, the eccentricity amount is 2. The fuel assembly according to claim 1, wherein the fuel assembly is arranged eccentrically so that √2 / 2.
X _dec = d / p
Here, d is the distance from the center of the fuel lattice to the center of gravity of the spectrum shift rod, and p is the distance between the centers of the fuel rods. A boiling water reactor core comprising a plurality of fuel assemblies,
A neutron detector disposed in the core;
A fuel assembly according to any one of claims 1 to 3, which is adjacent to the neutron detector.   The plurality of fuel assemblies are arranged symmetrically with respect to a straight line in a horizontal section of the core, and the fuel assemblies adjacent to the neutron detector and symmetrical with respect to the straight line are any one of claims 1 to 3. The core of a boiling water reactor according to claim 4, wherein the core is a fuel assembly according to claim 1.

Images