JP2019163945A - Fuel assembly and core of light water reactor loaded with fuel assembly - Google Patents

Abstract

To provide a fuel assembly capable of improving a minor actinide combustion efficiency while reducing the maximum line output density in a light water reactor having hardened a neutron spectrum during operation, and a core of the light water reactor in which the fuel assembly is loaded.SOLUTION: A fuel assembly 1 in which a plurality of fuel rods 3 are arranged in a triangular fashion in a square channel box 2 in a cross-section and which is loaded into a reactor core 22 of a light water reactor has a high deceleration region 7 having a high deceleration effect on neutrons due to light water, a low deceleration region 8 having a low reduction effect on neutrons due to light water. An average weight ratio of the minor actinide in heave metals obtained by adding uranium, plutonium, and minor actinide in the fuel rod 3 arranged in the high deceleration region 7 is larger than the average weight ratio of the minor actinide in heave metals obtained by adding uranium, plutonium, and minor actinide in the fuel rod 3 arranged in the low deceleration region 8.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a fuel assembly and a core of a light water reactor loaded with the fuel assembly.

A light water reactor (hereinafter referred to as a reduced-speed light water reactor) in which the neutron spectrum is hardened by arranging the fuel rods in the fuel assembly densely in a triangular lattice in a light water reactor that uses currently operating light water as a moderator and coolant has been proposed. Yes.
Low-speed light water reactors are loaded with plutonium and minor actinides (MA), which are super uranium nuclides (TRU), similar to fast reactors that use liquid sodium as a coolant, and burn TRU. TRU combustion furnace.
However, in the reduced-speed light water reactor, there is a problem that the output of the fuel rod increases and the maximum linear power density increases at a location where the light water as the moderator in the fuel assembly lattice is unevenly distributed. On the other hand, reduced-speed light water reactors are required to efficiently burn minor actinides that are difficult to burn in TRU.

  As a technique for reducing the maximum linear power density in such a reduced-speed light water reactor, for example, a technique described in Patent Document 1 has been proposed. In Patent Document 1, the power generated in the outermost fuel depletion is reduced by lowering the concentration of fissile plutonium in the outermost fuel rod having a low fuel rod filling density, a large coolant volume and a large proportion of low energy neutrons. A configuration that suppresses the peak and reduces the maximum linear power density is disclosed.

JP 2000-19280 A

However, in the configuration described in Patent Document 1, the ratio of depleted uranium increases because the outermost peripheral fuel rod is made of low plutonium enriched fuel, and the conversion of minor actinides from depleted uranium increases, resulting in minor actinides. The combustion efficiency is reduced.
Therefore, the present invention provides a fuel assembly capable of improving the minor actinide combustion efficiency while reducing the maximum linear power density in a light water reactor in which the neutron spectrum is hardened during operation, and a core of a light water reactor loaded with the fuel assembly.

In order to solve the above problems, a fuel assembly according to the present invention is a fuel assembly in which a plurality of fuel rods are densely arranged in a channel box having a square cross section and loaded in a square lattice pattern in a light water reactor core. The fuel assembly has a high deceleration region having a large deceleration effect on neutrons caused by light water and a reduced speed region having a small deceleration effect on neutrons caused by light water, and uranium and plutonium of fuel rods arranged in the high deceleration region The average weight ratio of the minor actinides in the heavy metal combined with the minor actinides is larger than the average weight ratio of the minor actinides in the heavy metal combined with uranium, plutonium and minor actinides of the fuel rods arranged in the reduction speed region. It is characterized by that.
The core of the light water reactor according to the present invention is a core of a light water reactor in which a plurality of fuel assemblies are loaded in a square lattice shape, and the fuel assemblies are triangularly packed in a channel box having a square cross section. A plurality of fuel rods arranged, a high deceleration region having a large deceleration effect on neutrons by light water, and a reduced speed region having a small deceleration effect on neutrons by light water, and when the fuel assembly is loaded on the core, Fuel rod uranium, plutonium, and minor actinides in heavy metals, which are added to uranium, plutonium, and minor actinides in the high-speed deceleration region, add up to uranium, plutonium, and minor actinides in the heavy metal. It is characterized by being greater than the average weight fraction of minor actinides in the combined heavy metals.

According to the present invention, it is possible to provide a fuel assembly capable of improving the minor actinide combustion efficiency while reducing the maximum linear power density in a light water reactor in which the neutron spectrum is hardened during operation, and a light water reactor core loaded with the fuel assembly. It becomes possible.
Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

It is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly of Example 1 according to one embodiment of the present invention. FIG. 2 is a cross-sectional view (horizontal cross-sectional view) of a core of a light water reactor in which a plurality of fuel assemblies shown in FIG. 1 are loaded. It is a schematic block diagram of the improved light water reactor which has a core shown in FIG. FIG. 2 is a cross-sectional view (horizontal cross-sectional view) of a fuel assembly showing a high deceleration region and a reduced speed region in the fuel assembly shown in FIG. 1. FIG. 5 is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly of Example 1 showing the TRU enrichment for each fuel rod and the weight ratio of minor actinides in heavy metals in the fuel assembly shown in FIG. 4. FIG. 5 is a cross-sectional view (horizontal cross-sectional view) of a fuel assembly of a comparative example showing the TRU enrichment for each fuel rod and the weight ratio of minor actinides in heavy metals in the fuel assembly shown in FIG. 4. It is a graph which shows the change of the neutron infinite image magnification with respect to the burnup of the fuel assembly shown to FIG. 5 and FIG. It is a graph which shows the change of the combustion efficiency of the minor actinide with respect to the burnup of the fuel assembly shown to FIG. 5 and FIG. It is a graph which shows the change of the neutron reaction cross section with respect to the neutron energy of Am-241 which is a typical minor actinide. It is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly of Example 2 according to another embodiment of the present invention. 5, 6, and 10 are graphs showing changes in the combustion efficiency of minor actinides with respect to the burnup of the fuel assembly.

In this specification, the fuel assembly according to the present invention and the light water reactor to which the core of the light water reactor loaded with the fuel assembly is applied include a recirculation pump and allows light water (cooling water) to flow outside the reactor pressure vessel as a coolant. A normal boiling water reactor (BWR) that circulates cooling water by flowing it back into the downcomer in the reactor pressure vessel, and an internal pump that improves cooling water circulation in the reactor pressure vessel Boiling Water Reactor (ABWR), a natural circulation system for cooling water by Chimney, and a highly economical simplified boiling water that eliminates the need for a recirculation pump in BWR and an internal pump in ABWR Type Reactor (Economic Simulated Boiling Wa) er Reactor (ESBWR), including a pressurized water reactor (PWR) in which the cooling water is high-temperature high-pressure water that does not boil over the entire core, and this high-temperature high-pressure water is sent to a steam generator to generate steam by heat exchange. . Hereinafter, ABWR will be described as an example of a light water reactor having a core in which a fuel assembly according to the present invention is loaded.
Embodiments of the present invention will be described below with reference to the drawings.

  Hereinafter, in the present embodiment, an improved light water reactor (ABWR) in which 872 fuel assemblies are loaded on the core will be described, but the present invention is not limited to this. Is applicable to other light water reactors loaded with a square lattice fuel assembly.

First, an improved light water reactor (ABWR) will be described as an example of a light water reactor. FIG. 3 is a schematic configuration diagram of an improved light water reactor (ABWR). As shown in FIG. 3, an improved light water reactor 20 having a core in which a fuel assembly (described later in detail) of this embodiment is loaded is provided with a cylindrical core shroud 26 in a reactor pressure vessel 21, and the core. A reactor core 22 loaded with a plurality of fuel assemblies (not shown) is installed in the shroud 26. Further, in the reactor pressure vessel 21, a shroud head 30 covering the core 22, a steam separator 28 attached to the shroud head 30 and extending upward, and steam drying disposed above the steam separator 28. A container 29 is provided.
An upper grid plate 24 is disposed in the core shroud 26 below the shroud head 30, is attached to the core shroud 26, and is positioned at the upper end of the core 22. A core support plate 23 is disposed in the core shroud 26 at the lower end of the core 22 and is installed in the core shroud 26. A plurality of fuel support fittings 25 are installed on the core support plate 23.
In addition, a control rod guide tube 32 is provided in the reactor pressure vessel 21 so that a plurality of cross-shaped control rods (not shown) can be inserted into the core 22 in order to control the nuclear reaction of the fuel assembly. It has been. The control rod drive mechanism 33 is provided in a control rod drive mechanism housing (not shown) installed below the bottom of the reactor pressure vessel 21, and the control rod is connected to the control rod drive mechanism 33.

  A plurality of internal pumps 31 are installed in the lower mirror 34, which is the bottom of the reactor pressure vessel 21, so as to penetrate the reactor pressure vessel 21 from below. The plurality of internal pumps 31 are outside the outermost peripheral portion of the plurality of control rod guide tubes 32 and are annularly spaced from each other at a predetermined interval, and a plurality of internal pumps 31 are arranged. Thereby, the internal pump 31 does not interfere with the control rod guide tube 32 or the like. An impeller of each internal pump 31 is positioned in an annular downcomer 27 formed between the cylindrical core shroud 26 and the inner surface of the reactor pressure vessel 21. Water (cooling water) that is a coolant in the reactor pressure vessel 21 is supplied to the core 22 from the lower mirror 34 side via the downcomer 27 by the impellers of the internal pumps 31. The cooling water flowing into the reactor core 22 is heated by a nuclear reaction of a fuel assembly (not shown), becomes a gas-liquid two-phase flow, and flows into the steam-water separator 28. The gas-liquid two-phase flow flowing through the steam separator 28 is separated into steam (gas phase) containing moisture and water (liquid phase), and the liquid phase again falls to the downcomer 27 as cooling water. On the other hand, the steam (gas phase) is introduced into the steam dryer 29 and moisture is removed, and then supplied to the turbine (not shown) via the main steam pipe 35. Cooling water that flows into the reactor pressure vessel 21 from the water supply pipe 36 via a condenser or the like flows (drops) downward in the downcomer 27. Thus, the internal pump 31 forcibly circulates cooling water to the core 22 in order to efficiently cool the heat generated in the core 22.

Next, the structure of the fuel assembly and the core 22 of the present embodiment loaded in the core 22 will be described. FIG. 1 is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly of this embodiment, and FIG. 2 is a cross-sectional view (horizontal cross-section) of a core of a light water reactor in which a plurality of fuel assemblies shown in FIG. 1 are loaded. Figure).
As shown in FIG. 1, the fuel assembly 1 has a densely arranged triangular arrangement of fuel rods 3 with 243 outer diameters of 7.2 mm and a gap of 1.5 mm inside a channel box 2 having a square cross section (horizontal cross section). is doing. Each fuel rod 3 has upper and lower tie plates at the upper and lower end tie plates (not shown), and a fuel spacer (not shown) that separates the middle of the fuel rod 3 at a constant interval in the axial direction. Is retained. The fuel rod 3 is a cladding tube (not shown) of pellets (not shown) of mixed oxide (hereinafter referred to as MOX fuel) enriched with superuranium nuclides containing plutonium containing fission plutonium and minor actinides in degraded uranium. ). A gap water region 4 that is saturated water and a cross-shaped control rod (a cross-shaped control rod) 5 are inserted outside the channel box 2. The upper half of the cross-shaped control rod 5 is provided with a follower portion made of carbon, which is a substance having a lower speed reduction ability than light water that is cooling water. Further, on the opposite side of the cruciform control rod 5 (the side facing the cruciform control rod 5 with the fuel assembly 1 in between), a water exclusion plate filled with carbon, which is a substance having a lower deceleration ability than light water that is cooling water 6 is installed. With the above configuration, a reduced-speed light water reactor is realized in which the cooling water flowing through the fuel assembly 1 is reduced and the neutron spectrum in the reactor core 22 is hardened (shifted to the high energy side).
2 is a cross-sectional view (horizontal cross-sectional view) of the core 22 of an improved light water reactor (ABWR) in which a plurality of fuel assemblies 1 shown in FIG. 1 are loaded. As shown in FIG. 2, 872 fuel assemblies 1 are loaded on the core 22 in a square lattice pattern, except for a plurality of fuel assemblies 1 arranged on the outermost periphery. The fuel assembly 1 is loaded on the core 22 so as to surround the cross-shaped control rod 5. The cross-sectional view (horizontal cross-sectional view) of the fuel assembly 1 shown in FIG. 1 shows one fuel assembly 1 among the four fuel assemblies.

  Next, the operation of this embodiment will be described with reference to FIGS. FIG. 4 is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly showing the high deceleration region and the reduction speed region in the fuel assembly 1 shown in FIG. That is, it is a diagram in which the inner region of the channel box 2 is divided by the deceleration effect of light water. The region where the neutron decelerated by the gap water region 4 flows, and the region where the deceleration effect by the light water is large due to the gap between the fuel rod 3 and the channel box 2 is defined as the high deceleration region 7. A region where the difference is small is defined as a reduction speed region 8. Here, the boundary 9 between the high deceleration region 7 and the reduced speed region 8 is, for example, a square channel in the horizontal cross section of the fuel assembly 1 in a state where the improved light water reactor (ABWR) is operated at the rated power. Position corresponding to the length (distance 10) of the mean free path of hydrogen (mean free path considering the hydrogen transport effect) from each inner surface (each side) of the box 2 to the inside of the fuel assembly 1 Set to Here, the reason why the distance 10 from the inner surface of the channel box 2 to the boundary 9 is the mean free path considering the transport effect in hydrogen that decelerates neutrons is that neutrons collide with hydrogen atoms in light water. This is because the speed is slowed down and attention is paid to the mean free path of hydrogen. In the present embodiment, as shown in Table 1 below, for example, the above-described distance 10 is 1.95 cm.

As shown in Table 1, the hydrogen scattering micro cross section is 30.27 × 10 ⁻²⁴ cm ² , the hydrogen capture micro cross section is 0.33 × 10 ⁻²⁴ cm ² , and the hydrogen number density ( Saturated water) is 4.9 × 10 ²² pcs / cm ³ , so the mean free path considering the hydrogen transport effect is 1.95 cm.
As shown in FIG. 4, the fuel rods 3 on the boundary 9 are assigned to the high speed reduction region 7 or the reduction speed region 8 based on the occupied area in the horizontal cross section of the fuel rod 3. In other words, if the occupied area in the high deceleration region 7 is larger than the occupied area in the reduced speed region 8, the fuel rod 3 is determined as the fuel rod 3 disposed in the high deceleration region 7.

FIG. 5 is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly 1 of this embodiment showing the TRU enrichment for each fuel rod and the weight ratio of minor actinides in heavy metals in the fuel assembly shown in FIG. Minor actinides are intensively loaded in the high deceleration area 7. Specifically, as shown in FIG. 5, 12 fuel rods 3a having a TRU enrichment of 63.6 wt% and a minor actinide weight ratio in the heavy metal of 37.6 wt% are provided only in the high deceleration region 7. Has been placed. In addition, 21 fuel rods 3b having a TRU enrichment of 73.3 wt% and a minor actinide weight ratio in the heavy metal of 38.8 wt% are arranged only in the high deceleration region 7, and the TRU enrichment is Twenty-four fuel rods 3c having a weight ratio of 80.5 wt% of minor actinides in heavy metals of 40.5 wt% are arranged only in the high deceleration region 7. In addition, 49 fuel rods 3d having a TRU enrichment of 60.9 wt% and a minor actinide weight ratio in the heavy metal of 0.0 wt% are arranged in the high speed reduction region 7 and 42 in the reduction speed region 8. ing. 95 fuel rods 3e having a TRU enrichment of 74.2 wt% and a minor actinide weight ratio in the heavy metal of 0.0 wt% are arranged only in the reduction speed region 8.
Therefore, the weight ratio of the minor actinides in the average heavy metal of the fuel rods (fuel rods 3a to 3d) arranged in the high deceleration region 7 is 22.1 wt%, and the fuel rods (fuel rods 3d and 3d) in the reduction speed region 8 The weight ratio of minor actinides in the heavy metal of the fuel rod 3e) is 0%. Therefore, the weight ratio of the minor actinides in the average heavy metal of the fuel rods arranged in the high deceleration region 7 is larger than the weight proportion of the minor actinides in the heavy metals of the fuel rods arranged in the reduction speed region 8. The local power peaking in FIG. 5 is 1.3 with zero burnup.

6 is a cross-sectional view (horizontal cross-sectional view) of a fuel assembly 1 of a comparative example showing the TRU enrichment for each fuel rod and the weight ratio of minor actinides in heavy metals in the fuel assembly shown in FIG. A fuel assembly as a comparative example in which local output peaking is reduced by the TRU enrichment distribution is shown. As shown in FIG. 6, only one fuel rod 3f in which the TRU enrichment is 6.7 wt% and the weight ratio of minor actinides in the heavy metal is 0.9 wt% is disposed only in the high deceleration region 7. Nine fuel rods 3g having an enrichment of 11.7 wt% and a minor actinide weight ratio in the heavy metal of 1.5 wt% are arranged only in the high deceleration region 7. Further, five fuel rods 3h having a TRU enrichment degree of 26.7 wt% and a minor actinide weight ratio in the heavy metal of 3.5 wt% are arranged only in the high deceleration region 7, and the TRU enrichment degree is 36. Fifteen fuel rods 3i having a weight ratio of 7 wt% of minor actinides in heavy metals of 4.8 wt% are arranged only in the high deceleration region 7. Twenty-seven fuel rods 3j having a TRU enrichment of 51.7 wt% and a minor actinide weight ratio in the heavy metal of 6.8 wt% are disposed only in the high deceleration region 7, and the TRU enrichment is 66.7 wt%. The fuel rods 3k in which the weight ratio of the minor actinides in the heavy metal is 8.7 wt% are arranged in 49 in the high speed reduction region 7 and 42 in the reduction speed region 8, and the TRU enrichment is 83.2 wt%. There are 95 fuel rods 3m in which the weight ratio of minor actinides in heavy metals is 10.9 wt% in the reduction speed region 8 only.
Therefore, in the comparative example, in order to reduce local output peaking, the enrichment of TRUs in the heavy metal of the fuel rods (fuel rods 3f to 3k) arranged in the high deceleration region 7 is reduced. The weight ratio of the minor actinides in the heavy metal of the fuel rods (fuel rods 3f to 3k) to be arranged is 6.7 wt%, and the heavy metals of the fuel rods (fuel rods 3k and 3m) arranged in the reduction speed region 8 The weight ratio of the minor actinide is 10.2 wt. Therefore, in the comparative example, the weight ratio of the minor actinides in the average heavy metal of the fuel rods arranged in the high deceleration region 7 is larger than the weight ratio of the minor actinides in the heavy metals of the fuel rods arranged in the reduction speed region 8. small. In addition, the local output peaking in FIG.

  FIG. 7 is a graph showing the change of the neutron infinite image magnification with respect to the burnup of the fuel assemblies shown in FIGS. 5 and 6, and FIG. 8 is a minor actinide with respect to the burnup of the fuel assemblies shown in FIGS. It is a graph which shows the change of the combustion efficiency of. 7 and 8 show the simulation results. In the simulation, the neutron infinite multiplication factor, local power peaking, and the weight ratio of minor actinides in heavy metals are VMONT (Monte Carlo Fuel Assembly Combustion Characteristic Analysis Code). ) Was used. VMONT is, for example, the literature 1 (Harumi Maruyama, et al.:A Vectorized Monte Carlo Method with Pseudo-SCATtering for Neutron Transport Analysis:.... Proc Int Topl Mtg Advances in Reactor Physics, Mathematics and Computation, Paris, France, April 27-30, 1987, 3, 1791-1800 (1987)), or reference 2 (Yuichi Morimoto, et al .: Neutral Analysis Code for Fuel Assessed Vectors). e Carlo Method: N. Sci. & Eng., 103, 351 (1989)).

The curve a in FIG. 7 shows the combustion change at the neutron infinite multiplication factor in the fuel assembly cross section of FIG. 5 of this embodiment, and the curve b shows the change accompanying the combustion at the neutron infinite multiplication factor in the fuel assembly cross section of FIG. Is shown. The curve a and the curve b almost coincide with each other. When the reactor is loaded, FIG. 5 and FIG. 6 become critical at the same burnup degree. Curve a in FIG. 8 shows the combustion change of the minor actinide combustion efficiency of the fuel assembly cross section of FIG. 5 of this embodiment, and curve b shows the combustion change of the minor actinide combustion efficiency of the fuel assembly cross section of FIG. 6 of the comparative example. ing. The minor actinide combustion efficiency is obtained by the following equation (1).
Minor actinide combustion efficiency (%) with burn-up a = (M ₀ −Ma) / M ₀ × 100 (1)
Here, M ₀ is the fuel assembly average minor actinide weight at the time of loading, and Ma is the fuel assembly average minor actinide weight of the burnup a.

FIG. 9 is a graph showing changes in the neutron reaction cross section with respect to the neutron energy of Am-241 which is a typical minor actinide. In the graph shown in the upper part of FIG. 9, the horizontal axis represents neutron energy, and the vertical axis represents the neutron reaction cross section. In addition, specific values are shown in a table in the lower part of FIG. Regarding the relationship between fission and capture, the minor actinide Am-241 captures neutrons and causes fission to burn the minor actinide Am-241. . Therefore, only capturing neutrons is irrelevant to the combustion efficiency of Am-241, which is a minor actinide, and Am-241, which is a minor actinide, is burned by fission by the captured neutrons.
In FIG. 9, neutron energy is increased by increasing the average weight ratio of Am-241, which is a minor actinide in the heavy metal of the fuel rod 3 arranged in the high deceleration region 7, by the configuration of the fuel assembly of the present embodiment. In the low region (high deceleration region 7), the fission cross section is large. In the high deceleration region 7, the capture cross section is larger by two orders of magnitude than the fission cross section. However, the absolute value of the fission cross section has a sufficient size, so that the minor actinide Am-241 It becomes possible to achieve an improvement in combustion efficiency. In other words, since the minor actinide Am-241 has a larger capture cross section (capture) as the neutron energy is smaller, the minor actinide Am-241 is loaded into the high deceleration region 7 where the neutron energy is low. The actinide Am-241 can be burned while reducing local power peaking by neutron capture.

As described above, according to the present embodiment, the fuel assembly capable of improving the minor actinide combustion efficiency while reducing the maximum linear power density in the light water reactor in which the neutron spectrum is cured during operation, and the core of the light water reactor loaded with the fuel assembly. Can be provided.
Further, according to the present embodiment, a fuel assembly capable of improving the combustion efficiency of minor actinides while reducing local power peaking in a light water reactor in which the neutron spectrum is hardened during operation, and a light water reactor loaded with the fuel assembly It is possible to realize a core of

  FIG. 10 is a cross-sectional view (horizontal cross-sectional view) of the fuel assembly of the second embodiment according to another embodiment of the present invention. The present embodiment is different from the first embodiment in that minor actinides are loaded only on the fuel rods 3n arranged on the outermost periphery in the cross section (horizontal section) of the fuel assembly. The same components as those in the first embodiment are denoted by the same reference numerals.

As shown in FIG. 10, in the fuel assembly 1 of the present embodiment, only the fuel rods 3n arranged on the outermost periphery have a high average weight ratio of minor actinides in heavy metals. This is because the neutron is decelerated most in the outermost peripheral part. FIG. 11 is a graph showing the change in the combustion efficiency of minor actinides with respect to the burnup of the fuel assembly in FIGS. 5, 6, and 10. Curve a in FIG. 11 shows the combustion change of the minor actinide combustion efficiency of the fuel assembly cross section of FIG. 5 of the first embodiment, and curve b of the minor actinide combustion efficiency of the fuel assembly cross section of FIG. Curve c shows the combustion change of the minor actinide combustion efficiency in the cross section of the fuel assembly in FIG. 10 of this embodiment. As shown by the curve c in FIG. 11, it can be seen that the minor actinides can be burned efficiently also in this embodiment.
According to the present embodiment, similar to the first embodiment, minor actinides can be burned efficiently.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described.

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly 2 ... Channel box 3, 3a-3n ... Fuel rod 4 ... Gap water area 5 ... Cross-shaped control rod 6 ... Water exclusion plate 7 ... High deceleration area 8 ... Reduction speed area 9 ... High deceleration area and reduction Boundary 20 of the high speed region ... Improved light water reactor 21 ... Reactor pressure vessel 22 ... Core 23 ... Core support plate 24 ... Upper lattice plate 25 ... Fuel support fitting 26 ... Core shroud 27 ... Downcomer 28 ... Steam separator 29 ... Steam drying 30 ... Shroud head 31 ... Internal pump 32 ... Control rod guide tube 33 ... Control rod drive mechanism 34 ... Lower mirror 35 ... Main steam pipe 36 ... Water supply pipe

Claims (8)

A fuel assembly in which a plurality of fuel rods are densely arranged in a triangular shape in a channel box having a square cross section, and loaded into a square lattice in the core of a light water reactor,
The fuel assembly has a high deceleration region with a large decelerating effect on light water neutrons and a reduced speed region with a small decelerating effect on light water neutrons, and uranium, plutonium and minor actinides of fuel rods arranged in the high decelerating region The average weight ratio of the minor actinides in the combined heavy metal is greater than the average weight ratio of the minor actinides in the heavy metal combined with uranium, plutonium and minor actinides of the fuel rods arranged in the reduction speed region. Fuel assembly. The fuel assembly according to claim 1, wherein
The boundary between the high speed reduction region and the reduction speed region is a state where it is operated at a rated output, and in the horizontal section, it has a hydrogen transport effect from the inner surface of the square channel box to the inside of the fuel assembly. A fuel assembly, characterized in that the fuel assembly is set at a position corresponding to the length of the mean free path considered. The fuel assembly according to claim 1, wherein
The fuel assembly arranged in the high deceleration region is a fuel assembly arranged on the outermost periphery in a horizontal section of the fuel assembly. The fuel assembly according to claim 2, wherein
The fuel rods arranged at the position between the high deceleration region and the reduced speed region are arranged on the fuel rods arranged in the high deceleration region or the reduced speed region based on the occupied area in the horizontal section. A fuel assembly characterized by being determined as a fuel rod. A core of a light water reactor in which a plurality of fuel assemblies are loaded in a square lattice shape,
The fuel assembly is
A plurality of fuel rods arranged in a triangular dense arrangement in a channel box having a square cross section;
A uranium of fuel rods arranged in the high deceleration region when the fuel assembly is loaded into the core, having a high deceleration region with a large deceleration effect on light beam neutrons and a reduced speed region with a small deceleration effect on light water neutrons. The average weight ratio of minor actinides in heavy metals combined with plutonium and minor actinides is the average weight ratio of minor actinides in heavy metals combined with uranium, plutonium and minor actinides in fuel rods arranged in the reduction speed region A light water reactor core characterized by a larger size. In the core of the light water reactor according to claim 5,
The boundary between the high speed reduction region and the reduction speed region is a state where it is operated at a rated output, and in the horizontal section, it has a hydrogen transport effect from the inner surface of the square channel box to the inside of the fuel assembly. A core of a light water reactor, which is set at a position corresponding to the length of the mean free path considered. In the core of the light water reactor according to claim 5,
The fuel rod disposed in the high deceleration region is a fuel rod disposed on the outermost periphery in a horizontal cross section of the fuel assembly. In the core of the light water reactor according to claim 6,
The fuel rods arranged at the position between the high deceleration region and the reduced speed region are arranged on the fuel rods arranged in the high deceleration region or the reduced speed region based on the occupied area in the horizontal section. The core of a light water reactor, characterized in that it is determined as a fuel rod.

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