JP6726596B2 - Fuel assembly and core of boiling water reactor loaded with it - Google Patents

Description

TECHNICAL FIELD The present invention relates to a fuel assembly and a core of a boiling water reactor for loading the fuel assembly.

In a boiling water reactor currently loaded with fuel assemblies and cruciform control rods, the fuel rods in the fuel assembly are arranged in a dense triangular lattice and voids are generated in the channel box during operation. Has proposed a boiling water reactor having a hardened neutron spectrum (hereinafter referred to as a reduced-speed spectrum boiling water reactor).
The low-moderation spectrum boiling water reactor can be a high conversion reactor in which a fuel region loaded with plutonium fuel and a blanket region loaded with depleted uranium fuel are arranged, similarly to a fast reactor using liquid sodium as a coolant.
However, due to the hardening of the neutron spectrum in the reduced-rate spectral boiling water reactor, the reactivity that is input when the core flow rate of the coolant water (cooling water) decreases and the void ratio in the channel box rises There is a problem that the negative absolute value of a certain void reactivity coefficient decreases. It is necessary to keep the void reactivity coefficient negative for safe operation of the reactor.
As a technique for reducing the void reactivity coefficient in such a reduced-speed spectrum boiling water reactor, for example, a technique described in Patent Document 1 has been proposed. In Patent Document 1, a low-concentration region for arranging a low plutonium-rich fuel in the blanket region is provided, and a neutron absorber is arranged in this blanket region to increase the neutron absorption amount when the void fraction increases, The void reactivity coefficient is made more negative. Specifically, a configuration is disclosed in which, in a low-concentration region within a cross section (horizontal section) of a fuel assembly, concentrated boron having an increased concentration of boron 10 (B10) is dispersed and arranged as a neutron absorber. ..

Japanese Patent Laid-Open No. 2000-241582

However, in the configuration described in Patent Document 1, the neutron absorber arranged in the blanket region (low concentration region) of the low plutonium-enriched fuel absorbs neutrons not only when the void ratio increases but also during normal operation. Therefore, it is necessary to increase the plutonium enrichment in order to increase the amount of fissile material to make the core of a boiling water reactor critical, and there is a concern that neutron economic efficiency will deteriorate. Therefore, in Patent Document 1, the blanket region of the low plutonium-enriched fuel in which the neutron absorber is arranged should be shortened in the axial direction, and this makes the improvement of the void reactivity coefficient insufficient.
Therefore, the present invention improves the void reactivity coefficient in a boiling water reactor in which the neutron spectrum is hardened during operation, and a fuel assembly capable of improving the safety of the reactor and a boiling water atom loaded with the fuel assembly. Provides the core of the furnace.

In order to solve the above problems, a fuel assembly according to the present invention is a fuel assembly loaded in a tetragonal lattice on a core of boiling water type atoms, and the fuel assembly has a core operated at a rated output. With a high-power fuel region and a low-power blanket region in the axial direction in a state where the core is operating at the rated power, the total of uranium nuclide and transuranium nuclide in the channel box of the fuel assembly The ratio of the number density of hydrogen to the number density of hydrogen is 0.6 or more and 2.1 or less, and the low-power blanket region of the fuel assembly having a burnup of 0 occupies the uranium nuclide and the transuranium nuclide of the fuel. The transuranic nuclide is enriched in depleted uranium oxide such that plutonium 239 is contained in an amount of 0.1 wt% or more and 1.0 wt% or less.
Further, the core of the boiling water reactor according to the present invention is the core of the boiling water reactor in which a plurality of fuel assemblies are loaded in a square lattice, and the fuel assembly has a rated output. Uranium nuclides and transuranium in the channel box of the fuel assembly with the fuel region of high power and the blanket region of low power in the axial direction in a state where the core is operated at a rated power. The ratio of the number density of hydrogen to the total number density of nuclides is 0.6 or more and 2.1 or less, and the low-power blanket region of the fuel assembly with a burnup of zero is the uranium nuclide and transuranium of the fuel. The transuranic nuclide is enriched in depleted uranium oxide such that plutonium 239 in the nuclide is 0.1 wt% or more and 1.0 wt% or less.

According to the present invention, the void reactivity coefficient in a boiling water reactor whose neutron spectrum is hardened during operation is improved, and a fuel assembly capable of improving the safety of the reactor and a boiling water atom loaded with the fuel assembly can be improved. It becomes possible to provide the core of the furnace.
Problems, configurations, and effects other than those described above will be clarified by the following description of the embodiments.

FIG. 1 is a schematic configuration in the axial direction of a fuel assembly of Example 1 according to an embodiment of the present invention and an AA sectional arrow view (transverse sectional view) of the fuel assembly. FIG. 2 is a horizontal cross-sectional view (horizontal cross-sectional view) of a core of a boiling water reactor in which a plurality of fuel assemblies shown in FIG. 1 are loaded. FIG. 3 is a schematic configuration diagram of an improved boiling water reactor having the core shown in FIG. 2. It is a graph which shows the change of a void reactivity coefficient with respect to burnup in the cross section (horizontal section) of the 1st internal blanket field where H/HM is 1.8. It is a graph which shows change of a void reactivity coefficient with respect to burnup in the cross section (horizontal section) of the 2nd internal blanket field whose H/HM is 1.2. It is a graph which shows the change of a void reactivity coefficient with respect to H/HM in the cross section (horizontal section) of the 2nd internal blanket field of burnup zero. It is a graph which shows the change of a void reactivity coefficient with respect to H/HM in the cross section (horizontal section) of the 2nd internal blanket field of burnup 15GWd/t. It is a graph which shows change of a void reactivity coefficient with respect to burnup in the cross section (horizontal section) of the 2nd internal blanket field where H/HM is 0.6. It is a graph which shows the change of the void reactivity coefficient of a boiling water reactor with respect to the plutonium 239 enrichment of a 1st internal blanket area|region and a 2nd internal blanket area. FIG. 6 is a horizontal cross-sectional view (horizontal cross-sectional view) of a fuel assembly of Example 2 according to another example of the present invention. It is a graph which shows change of a void reactivity coefficient with respect to burnup in a cross section (horizontal section) of the 2nd internal blanket field where H/HM is 1.8.

In the present specification, the boiling water reactor to which the core of the fuel assembly according to the present invention and the boiling water reactor for loading the fuel assembly is applied is equipped with a recirculation pump, and water (cooling water) is used as a coolant. A normal boiling water reactor (BWR) that circulates cooling water by flowing it outside the reactor pressure vessel and then again flowing it into a downcomer inside the reactor pressure vessel. By using an improved boiling water reactor (ABWR) that circulates in the reactor pressure vessel and a natural circulation system of cooling water by chimney, a recirculation pump in BWR and an internal pump in ABWR are not required. It includes a high-economy simplified boiling water reactor (ESBWR) and the like. In the following, an ABWR will be described as an example of a boiling water reactor having a core loaded with a fuel assembly according to the present invention.
Embodiments of the present invention will be described below with reference to the drawings.

Hereinafter, in the present embodiment, an improved boiling water nuclear reactor (ABWR) in which 872 fuel assemblies are loaded in the core will be described. However, as described above, the fuel assembly according to the present invention has a cross section. The present invention can also be applied to other boiling water reactors in which the number of fuel assemblies loaded with cross-shaped control rods (cross-shaped control rods) is different.

First, an improved boiling water reactor (ABWR) will be described as an example of the boiling water reactor. FIG. 3 is a schematic configuration diagram of an improved boiling water nuclear reactor (ABWR). As shown in FIG. 3, an improved boiling water reactor 20 including a core loaded with a fuel assembly (described later in detail) of the present embodiment has a cylindrical core shroud 26 inside a reactor pressure vessel 21. A core 22 provided with a plurality of fuel assemblies (not shown) is installed in the core 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 arranged 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 located at the upper end of the core 22. The core support plate 23 is located 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.
Further, in the reactor pressure vessel 21, a control rod guide pipe 32 is provided which allows a plurality of control rods (not shown) having a cross-shaped cross section to be inserted into the core 22 for controlling the nuclear reaction of the fuel assembly. Has been. A control rod drive mechanism housing (not shown) installed below the bottom of the reactor pressure vessel 21 is provided with a control rod drive mechanism 33, and the control rods are 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 arranged outside the outermost peripheral portion of the plurality of control rod guide pipes 32, are annularly spaced from each other at a predetermined interval, and are arranged in plurality. As a result, the internal pump 31 does not interfere with the control rod guide pipe 32 or the like. The impeller of each internal pump 31 is positioned within an annular downcomer 27 formed between the cylindrical core shroud 26 and the inner surface of the reactor pressure vessel 21. Water (cooling water) as a coolant in the reactor pressure vessel 21 is supplied from the lower mirror 34 side to the reactor core 22 via the downcomers 27 by the impellers of the internal pumps 31. The cooling water flowing into the core 22 is heated by a nuclear reaction of a fuel assembly (not shown) to become a gas-liquid two-phase flow, and flows into the steam separator 28. The gas-liquid two-phase flow that passes through the water-water separator 28 is separated into steam (gas phase) containing moisture and water (liquid phase), and the liquid phase again drops to the downcomer 27 as cooling water. On the other hand, the steam (gas phase) is introduced into the steam dryer 29 to remove moisture, and then supplied to the turbine (not shown) via the main steam pipe 35. The cooling water flowing into the reactor pressure vessel 21 through the water supply pipe 36 via the condenser or the like flows (falls) downward in the downcomer 27. In this way, the internal pump 31 forcibly circulates the 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 schematic configuration in the axial direction of the fuel assembly of this embodiment and a sectional view (cross-sectional view) taken along the line AA of the fuel assembly, and FIG. 2 shows the fuel assembly shown in FIG. It is a cross-sectional view (horizontal cross-sectional view) of a core of a boiling water reactor loaded with a plurality of bodies.
As shown in the right diagram of FIG. 1, the fuel assembly 1 includes a channel box 2 having a square cross section (horizontal cross section) with 243 outer diameters of 7.2 mm and a gap of 1.5 mm. They are densely arranged. The upper and lower ends of each fuel rod 3 are an upper tie plate and a lower tie plate (not shown), and a fuel spacer (not shown) is provided in the middle of the fuel rod 3 so as to be axially spaced at regular intervals. Is held. A gap water region 4 of saturated water and a cross-shaped control rod (cross-shaped control rod) 5 are inserted outside the channel box 2. The upper half of the cross-shaped control 5 is provided with a follower portion made of carbon, which is a substance having a moderating ability smaller than that of light water which is cooling water. On the opposite side of the cross-shaped control rod 5 (the side facing the cross-shaped control rod 5 with the fuel assembly 1 in between), a water exclusion plate in which carbon, which is a substance having a moderating ability smaller than light water that is cooling water, is enclosed. 6 is installed. With the above configuration, the cooling water flowing in the fuel assembly 1 is reduced and the neutron spectrum in the core 22 is hardened (shifted to the high energy side) to realize the reduced-speed spectrum boiling water reactor. ..

As shown in the left diagram of FIG. 1, the fuel assembly 1 has an active fuel length of 180 cm, and has a high-power fuel region 12 having an output in the axial direction which is equal to or higher than the average linear power density of the core 22 during the rated power operation. The low power upper blanket 11 having a power equal to or lower than the average linear power density of the core 22, the first inner blanket region 13, and the second inner blanket region 14 are alternately arranged. In other words, the first internal blanket region 13 is provided on the upstream side (the lower side in the axial direction of the fuel assembly 1) along the flow direction of the cooling water, and the downstream side is provided along the flow direction of the cooling water (on the downstream side). A second inner blanket region 14 is arranged above the fuel assembly 1 in the axial direction) via the fuel region 12. The fuel rod 3 includes a cladding tube (not shown) of pellets (not shown) of depleted uranium oxide or mixed oxide (hereinafter referred to as MOX fuel) enriched with transuranium nuclide containing fission plutonium in depleted uranium. ) Is filled.

FIG. 2 is a horizontal cross-sectional view (horizontal cross-sectional view) of a core 22 of an improved boiling water reactor (ABWR) loaded with a plurality of the fuel assemblies 1 shown in FIG. As shown in FIG. 2, 872 fuel assemblies 1 are loaded in the core 22 in a square lattice shape. Except for the plurality of fuel assemblies 1 arranged at the outermost periphery, four fuel assemblies 1 adjacent to each other are provided. The fuel assembly 1 is loaded in the core 22 so as to surround the cross-shaped control rod 5. The horizontal cross-sectional view (horizontal cross-sectional view) of the fuel assembly 1 shown in the right side of FIG. 1 above shows one of the four fuel assemblies. As shown in the right diagram of FIG. 1, the two sides connecting at one corner forming the channel box 2 having a square cross section (horizontal cross section) are slightly separated from the two blades of the cross-shaped control rod 5. Are loaded in the core 22 so as to face each other. On the other hand, the two sides connected at the other corner located diagonally with respect to the one corner have a slight gap with the two sides forming the water removal plate 6 having an L-shaped cross section (horizontal section). The core 22 is loaded so as to face each other.

Next, the operation of the fuel assembly 1 of this embodiment shown in FIG. 1 will be described with reference to FIGS. 4 to 9. FIG. 4 shows the average void fraction at the rated output in the channel box 2 in the first internal blanket region 13 shown in the left diagram of FIG. 1 in which the cross section shown in the right diagram of FIG. 1 is burned in a two-dimensional system. The change in the void reactivity coefficient with respect to the burnup is shown. The ratio of the number density of hydrogen to the total number density of the uranium nuclide and the transuranium nuclide in the channel box 2 at the burnup of zero when operating at the rated output of the first internal blanket region 13 (hereinafter referred to as H/HM Referred to as) is 1.8. In FIG. 4, the curve a shows the change in the void reactivity coefficient with respect to the burnup of depleted uranium oxide that is usually used in blankets. Further, the curve b shows the change in the void reactivity coefficient with respect to the burnup of the MOX fuel having the fissile plutonium enrichment of 0.4 wt% and the plutonium 239 enrichment of 0.3 wt %. The curve c shows the change in the void reactivity coefficient with respect to the burnup of the MOX fuel having a fissile plutonium enrichment of 1.1 wt% and a plutonium 239 enrichment of 0.9 wt %. Here, the fissile plutonium enrichment is the weight ratio of fissile plutonium (the total of plutonium 239 and plutonium 241) to the total weight of uranium nuclides and transuranium nuclides, and the plutonium 239 enrichment is It is the weight ratio of plutonium 239 to the total weight of uranium and transuranium nuclides.

FIG. 5 shows the second internal blanket region 14 shown in the left side of FIG. 1 in which the average void fraction at the rated output in the channel box 2 is burned in a two-dimensional system in the cross section shown in the right side of FIG. The change in the void reactivity coefficient with respect to the burnup is shown. When operating at the rated output of the second internal blanket region 14, the H/HM in the channel box 2 at the burnup of zero is 1.2. In FIG. 5, the curve a shows the change in the void reactivity coefficient with respect to the burnup of depleted uranium oxide that is usually used for blankets. Further, the curve b shows the change in the void reactivity coefficient with respect to the burnup of the MOX fuel having the composition of the fissionable plutonium enrichment of 0.4 wt% and the plutonium 239 enrichment of 0.3 wt% at the burnup of zero. Showing. The curve c shows the change in the void reactivity coefficient with respect to the burnup of the MOX fuel having a composition of fissile plutonium enrichment of 1.1 wt% and plutonium 239 enrichment of 0.9 wt% at zero burnup. There is.

During operation at the rated output, water having a cooling and neutron moderating function (cooling water) flows between the fuel rods 3 from the lower part to the upper part of the fuel assembly 1 shown in the left diagram of FIG. The H/HM in the channel box 2 of the first inner blanket area 13 is larger than the H/HM in the channel box 2 of the second inner blanket area 14. The curve a using the depleted uranium oxide as a blanket has a large void reactivity coefficient at the time when the burnup is zero, and suddenly becomes negative with combustion. On the other hand, as shown in FIGS. Curve b of a low enrichment MOX fuel (MOX fuel with a fissionable plutonium enrichment of 0.4 wt% and plutonium 239 enrichment of 0.3 wt%) and a low enrichment MOX fuel (fissile plutonium enrichment Curve 1.1% of the MOX fuel having a plutonium 239 enrichment of 0.9 wt%), the change in the void reactivity coefficient due to combustion is small. In the first internal blanket region 13 (FIG. 4) and the second internal blanket region 14 (FIG. 5), the fission and capture of plutonium 239 greatly contribute to the void reactivity coefficient. The curve a using the depleted uranium oxide as a blanket has a burnup of zero and the plutonium 239 has zero, and the formation of the plutonium 239 makes the void reactivity coefficient negative. On the other hand, since the low enrichment MOX fuels of the curves b and c contain plutonium 239 from the early stage of combustion, the void reactivity coefficient is more negative than the curve a using depleted uranium oxide as a blanket. Plutonium 239 accumulates even when burned with MOX fuel, but at the same time, the neutron spectrum becomes harder (shifts to the higher energy side), and the plutonium 240 that contributes to the positive void reactivity coefficient accumulates, so the void reactivity coefficient is increased. Is gradual. As H/HM in the channel box 2 becomes smaller and the neutron spectrum becomes harder, the contribution of the nuclide that makes the void reactivity coefficient to the positive side becomes larger. Therefore, in FIG. 5, the curve of plutonium 239 enrichment 0.9 wt% is shown. Although the void coefficient of c is more positive than that of the curve b with the plutonium 239 enrichment of 0.3%, it is more negative than the curve a using depleted uranium oxide as a blanket in the early stage of combustion.

FIG. 6 shows the void reactivity coefficient at zero burnup when H/HM in the channel box 2 is changed from 0.6 to 2.1 in the horizontal cross section (horizontal cross section) of the second internal blanket region 14. ing. That is, it is a graph showing changes in the void reactivity coefficient with respect to H/HM in the cross section (horizontal section) of the second internal blanket region 14 where the burnup is zero. FIG. 7 is a void reactivity coefficient at a burnup of 15 GWd/t when H/HM in the channel box 2 is changed from 0.6 to 2.1 in the horizontal cross section (horizontal cross section) of the second internal blanket region 14. Is shown. That is, it is a graph showing changes in the void reactivity coefficient with respect to H/HM in the horizontal cross section (horizontal cross section) of the second internal blanket region 14 having a burnup of 15 GWd/t. The burnup of 15 GWd/t is the average burnup of the second internal blanket region 14 at the end of the operation cycle. Curves a in FIGS. 6 and 7 are depleted uranium oxides commonly used in blankets at zero burnup, and curves b are fissionable plutonium enrichment of 0.4 wt% at zero burnup. MOX fuel with a composition of 239 enrichment of 0.3 wt%, curve c shows fissionable plutonium enrichment of 1.1 wt% at burnup of zero and MOX composition of plutonium 239 enrichment of 0.9 wt% It is fuel. In FIG. 6 in which the burnup is zero, the curve a used as a blanket of depleted uranium oxide has no void accumulation of plutonium 239. Is more negative. On the other hand, in FIG. 7 in which combustion progressed, the curve c of the low enrichment MOX fuel has a void from the curve a using the depleted uranium oxide as a blanket in the entire range of H/HM from 0.6 to 2.1. The reactivity coefficient deteriorates.

In order to make H/HM in the channel box 2 of the inner blanket region (first inner blanket region 13, second inner blanket region 14) larger than 2.1, not only the void ratio is reduced, but also the fuel rod 3 is thinned. It is necessary to increase the diameter and increase the area of the cooling water, which deteriorates the conversion ratio. On the other hand, when H/HM in the channel box 2 is made smaller than 0.6 and the neutron spectrum is made harder, the MOX fuel is depleted uranium except at the point where burnup is as close to zero as possible, that is, immediately after combustion. The void reactivity coefficient is worse than that of oxide.

FIG. 8 shows the average void fraction in the channel box 2 at the rated output when the H/HM in the channel box 2 at the burnup of zero is 0.6 for the transverse section (horizontal section) of the second internal blanket region 14. 1 shows changes in the void reactivity coefficient with respect to burnup when the cross section shown in the right side of FIG. 1 is burned in a two-dimensional system. That is, it is a graph showing the change in the void reactivity coefficient with respect to the burnup in the cross section (horizontal section) of the second internal blanket region 14 where H/HM is 0.6. In FIG. 8, curve a is the depleted uranium oxide normally used for blankets at zero burnup, curve b is a fissionable plutonium enrichment of 0.4 wt% at zero burnup, and plutonium 239 enrichment. MOX fuel with a composition of 0.3 wt% enrichment, curve c is a MOX fuel with a fissionable plutonium enrichment of 1.1 wt% at zero burnup and a composition of plutonium 239 enrichment of 0.9 wt%. is there. As shown in FIG. 8, when H/HM in the channel box 2 is made smaller than 0.6 to make the neutron spectrum harder (shifted to the high energy side), the enrichment becomes low when the burnup progresses slightly from zero. It was found that the MOX fuels (curve b and curve c) have a worse void reactivity coefficient than the depleted uranium oxide (curve a).

FIG. 9 is a graph showing changes in the void reactivity coefficient of a boiling water reactor with respect to the plutonium 239 enrichment in the first inner blanket region and the second inner blanket region. That is, FIG. 9 shows the fuel shown in the left diagram of FIG. 1 when the first internal blanket region 13 and the second internal blanket region 14 shown in the left diagram of FIG. 1 are changed from depleted uranium oxide to low enrichment MOX. 3 is a graph in which the void reactivity coefficient of the core 22 of FIG. 2 loaded with the assembly 1 is plotted against the enrichment degree of plutonium 239 having a low enrichment degree MOX. The upper blanket 11 shown on the left side of FIG. 1 is loaded with depleted uranium oxide, and the fuel region 12 is loaded with highly enriched MOX fuel having a fissionable plutonium enrichment of 26 wt %. The core 22 has a heat output of 3926 MW and a take-out burnup of 45 GWd/t. The point where the plutonium 239 enrichment is zero indicates the case where a deteriorated uranium oxide is used as a conventional blanket (hereinafter referred to as a deteriorated uranium oxide blanket). As shown in FIG. 9, by loading a low enrichment MOX of plutonium 239 enrichment up to 1.0 wt% in the inner blanket region (first inner blanket region 13, second inner blanket region 14), the conventional depleted uranium oxide The void reactivity coefficient could be made more negative than the blanket. As shown by the curve a in FIG. 9, the void reactivity coefficient became the minimum of −11 pcm/% void when the plutonium 239 enrichment was 0.3 wt %.

As described above, in the present embodiment, the fuel assembly 1 has the high-power fuel region 12 and the low-power output in the axial direction (along the direction of the flow of the cooling water) while the core 22 is operating at the rated output. Fuel region (first internal blanket region 13, second internal blanket region 14). When the core 22 is operating at the rated output, the ratio (H/HM) of the number density of hydrogen to the total number density of the uranium nuclide and the transuranium nuclide in the channel box 2 of 1 of the fuel assembly is The low-power fuel region (first internal blanket region 13, second internal blanket region 14) of the fuel assembly 1 having a burnup of 0.6 or more and 2.0 or less and a burnup of zero is the uranium nuclide of the fuel and the The transuranic nuclide is enriched with depleted uranium oxide so that the fissile plutonium (plutonium 239) in the uranium nuclide becomes 0.1 wt% or more and 1.0 wt% or less. As a result, it is possible to realize the fuel assembly 1 capable of improving the void reactivity coefficient in the boiling water reactor in which the neutron spectrum is hardened during operation and improving the safety of the reactor. Further, by loading the above-described fuel assembly 1 into the core 22, it is possible to improve the void reactivity coefficient in the boiling water reactor in which the neutron spectrum is hardened during the operation and to improve the safety of the reactor. A water reactor can be realized.

According to the present embodiment, the boiling reactivity of the fuel assembly and the fuel assembly that can improve the safety of the reactor by improving the void reactivity coefficient in the boiling water reactor in which the neutron spectrum is hardened during operation are improved. It becomes possible to realize the core of a nuclear reactor.

FIG. 10 is a cross-sectional view (horizontal cross-sectional view) of a fuel assembly of Example 2 according to another embodiment of the present invention, and FIG. 11 shows a second internal blanket region of H/HM of 1.8. It is a graph which shows change of a void reactivity coefficient with respect to burnup in a cross section (horizontal section). In the present embodiment, in the cross section (horizontal section) of the fuel assembly, the low enrichment MOX fuel is filled only in the inner blanket region of the fuel rod arranged in the outermost periphery, and the fuel rods arranged in the outermost periphery are filled. It differs from the first embodiment in that it is configured to fill the depleted uranium oxide in the inner blanket region of the fuel rods other than the above. The same components as those in the first embodiment are designated by the same reference numerals, and the duplicated description of the first embodiment will be omitted below.

FIG. 10 corresponds to the cross-sectional view (horizontal cross section) of the fuel assembly 1 shown in the left diagram of FIG. 1 in the above-described first embodiment. As shown in FIGS. 6 and 7 in Example 1 described above, the larger the H/HM, the more negative the void reactivity coefficient of the low enrichment MOX fuel becomes with respect to the depleted uranium oxide. Since the gap water region 4 in FIG. 10 is saturated water, the fuel rods 3a arranged at the outermost circumference (placed closest to the channel box 2) in FIG. 10 are higher in H/ than other fuel rods 3b. HM is large. Therefore, when the low enrichment MOX fuel is filled only in the fuel rods 3a arranged at the outermost periphery in the first inner blanket region 13, and when the first inner blanket region 13 of the fuel rods 3b is filled with depleted uranium oxide. FIG. 11 is a diagram showing the combustion change of the void reactivity coefficient in the same manner as FIG. 4 in the above-described first embodiment. That is, for the first internal blanket region 13 of the fuel assembly 1, the average void fraction at the rated output in the channel box 2 is the void reaction with respect to the burnup when the cross section shown in FIG. 10 is burned in a two-dimensional system. The change in the frequency coefficient is shown. Here, H/HM in the channel box 2 at a burnup of zero is 1.8.

In FIG. 11, a curve a shows the change in the void reactivity coefficient with respect to the burnup of depleted uranium oxide that is usually used for blankets. The curve b is a void with respect to burnup when all the fuel rods 3a and 3b have a fissionable plutonium enrichment of 0.4 wt% and MOX fuel with a plutonium 239 enrichment of 0.3 wt%. The change in reactivity coefficient is shown. These curves a and b are the same as the curves a and b shown in FIG. 4 in the above-described first embodiment. A curve c shows that the fuel rods 3a arranged at the outermost periphery shown in FIG. 11 are filled with a low enrichment MOX fuel having a plutonium 239 enrichment of 1.7 wt% and the fuel rods 3b are filled with depleted uranium oxide. 2 shows the void reactivity coefficient of the horizontal cross section (horizontal cross section) in which the cross section average plutonium 239 enrichment is the same as the curve b (0.3 wt %). It was confirmed that the curve b and the curve c have the same cross-sectional average plutonium 239 enrichment of 0.3 wt %, but the curve c can have a void reactivity coefficient more negative than that of the curve b. It has been confirmed that the same effect can be obtained in the second inner blanket region 14.

The void reactivity coefficient of the core 22 loaded with the fuel assembly 1 in which the fuel rod configuration of the curve c of FIG. 11 is arranged in the core 22 with the heat output of 3926 MW and the take-out burnup of 45 GWd/t shown in FIG. 2 is −15 pcm/% void Then, the void reactivity coefficient was more negative than that of the above-described configuration of Example 1.

As described above, according to the present embodiment, in addition to the effect of the first embodiment, the void reactivity coefficient can be set to the negative side as compared with the configuration of the first embodiment, and the void reactivity coefficient is further improved. It becomes possible.

It should be noted that the present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the configurations described. Further, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and to add the configuration of another embodiment to the configuration of one embodiment.

DESCRIPTION OF SYMBOLS 1... Fuel assembly 2... Channel box 3, 3a, 3b... Fuel rod 4... Gap water area 5... Cross control rod 6... Water exclusion plate 11... Upper part Blanket 12... Fuel region 13... First inner blanket region 14... Second inner blanket region 20... Improved boiling 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 dryer 30 ・・・Shroud head 31...Internal pump 32...Control rod guide pipe 33...Control rod drive mechanism 34...Bottom mirror 35...Main steam pipe 36...Water supply pipe

Claims (10)

A fuel assembly loaded in a tetragonal lattice on a boiling water atom core,
The fuel assembly includes a high-power fuel region and a low-power blanket region in the axial direction in a state where the core is operated at a rated power,
The ratio of the number density of hydrogen to the total number density of the uranium nuclide and the transuranium nuclide in the channel box of the fuel assembly is 0.6 or more and 2.1 or less in the state where the core is operated at the rated output. In the low-power blanket region of the fuel assembly with zero burnup, the plutonium 239 occupying the uranium nuclide and the transuranium nuclide of the fuel is 0.1 wt% or more and 1.0 wt% or less A fuel assembly characterized by being enriched in depleted uranium oxide. The fuel assembly according to claim 1, wherein
The low power blanket area is
A first internal blanket region disposed axially downward and upstream in the direction of the flow of cooling water flowing into the fuel assembly;
A second internal blanket region disposed axially on the upper side and on the downstream side in the direction of the flow of the cooling water flowing into the fuel assembly;
The fuel assembly, wherein the second inner blanket region is disposed above the first inner blanket region through the high-power fuel region. The fuel assembly according to claim 2,
A high power fuel region disposed below the first internal blanket region and a high power fuel region disposed above the second internal blanket region;
The active fuel length, which is the length from the lower end of the high-power fuel region disposed below the first internal blanket region to the upper end of the high-power fuel region disposed above the second internal blanket region. The fuel assembly has a size of 180 cm. The fuel assembly according to claim 1, wherein
A plurality of fuel rods arranged in a triangular close-packed manner in the channel box,
The plurality of fuel rods are composed of a first fuel rod arranged in the outermost periphery and a second fuel rod arranged in a region excluding the outermost periphery, and have a low output of plutonium 239 among uranium nuclides and transuranium nuclides. The transuranic nuclide is enriched in the depleted uranium oxide only in the first fuel rod so that the average ratio in the cross section of the blanket region is 0.1 wt% or more and 1.0 wt% or less, and the second uranium nuclide is enriched in the second fuel rod. A fuel assembly in which the fuel rods are filled only with depleted uranium oxide. The fuel assembly according to claim 4,
The low power blanket area is
A first internal blanket region disposed axially downward and upstream in the direction of the flow of cooling water flowing into the fuel assembly;
A second internal blanket region disposed axially on the upper side and on the downstream side in the direction of the flow of the cooling water flowing into the fuel assembly;
The fuel assembly, wherein the second inner blanket region is disposed above the first inner blanket region through the high-power fuel region. A core of a boiling water reactor in which a plurality of fuel assemblies are loaded in a square lattice,
The fuel assembly includes a high-power fuel region and a low-power blanket region in the axial direction in a state where the core is operated at a rated power,
The ratio of the number density of hydrogen to the total number density of the uranium nuclide and the transuranium nuclide in the channel box of the fuel assembly is 0.6 or more and 2.1 or less in the state where the core is operated at the rated output. In the low-power blanket region of the fuel assembly with zero burnup, the plutonium 239 occupying the uranium nuclide and the transuranium nuclide of the fuel is 0.1 wt% or more and 1.0 wt% or less A boiling water reactor core characterized by being enriched in depleted uranium oxide. The core of the boiling water reactor according to claim 6,
The low power blanket area is
A first internal blanket region disposed axially downward and upstream in the direction of the flow of cooling water flowing into the fuel assembly;
A second internal blanket region disposed axially on the upper side and on the downstream side in the direction of the flow of the cooling water flowing into the fuel assembly;
The core of a boiling water nuclear reactor, wherein the second inner blanket region is disposed above the first inner blanket region through the high-power fuel region. The core of the boiling water reactor according to claim 7,
A high power fuel region disposed below the first internal blanket region and a high power fuel region disposed above the second internal blanket region;
The active fuel length, which is the length from the lower end of the high-power fuel region disposed below the first internal blanket region to the upper end of the high-power fuel region disposed above the second internal blanket region. A core of a boiling water reactor characterized by having a diameter of 180 cm. The core of the boiling water reactor according to claim 6,
A plurality of fuel rods arranged in a triangular close-packed manner in the channel box,
The plurality of fuel rods are composed of a first fuel rod arranged in the outermost periphery and a second fuel rod arranged in a region excluding the outermost periphery, and have a low output of plutonium 239 among uranium and transuranium nuclides. The transuranic nuclide is enriched in the depleted uranium oxide only in the first fuel rod so that the average ratio in the cross section of the blanket region is 0.1 wt% or more and 1.0 wt% or less, and the second uranium nuclide is enriched in the second fuel rod. The boiling water reactor core is characterized in that the fuel rods are filled only with depleted uranium oxide. The core of the boiling water reactor according to claim 9,
The low power blanket area is
A first internal blanket region disposed axially downward and upstream in the direction of the flow of cooling water flowing into the fuel assembly;
A second internal blanket region disposed axially on the upper side and on the downstream side in the direction of the flow of the cooling water flowing into the fuel assembly;
The core of a boiling water nuclear reactor, wherein the second inner blanket region is disposed above the first inner blanket region through the high-power fuel region.

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