JP4809151B2 - Light water reactor core and control rod - Google Patents

Description

    The present invention relates to a light water reactor core and a control rod, and in particular, in the height direction, an axial heterogeneous core in which an upper blanket region, an upper core region, an internal blanket region, a lower core region, and a lower blanket region are arranged from the top. It is related with the core and control rod of a light water reactor suitable for applying to the parfait type core which is.

    Non-Patent Document 1 describes the concept of a parfait-type core including five regions, an upper blanket region, an upper core region, an internal blanket region, a lower core region, and a lower blanket region, arranged in order from the top in the height direction. is doing. The upper core region and the lower core region contain fissile material. The parfait core is an axially inhomogeneous core having five regions in the axial direction.

  Further, Patent Document 1 and Patent Document 2 use a hexagonal fuel assembly in which fuel rods are arranged in a triangular dense arrangement so that the water-to-fuel volume ratio is 0.6 or less, so that the growth ratio is around 1.0 or The basic concept of a light water breeder reactor of 1.0 or higher is described. These patent documents describe the core of a light water breeding reactor adopting a parfait type core configuration as an example. A control rod applied to the core is also described. This core has blanket regions containing depleted uranium at the upper and lower ends, and a core region containing fissile plutonium is disposed between the blanket regions in the height direction. This core region is divided into three regions in the height direction. In the core region, a second region having a fissile plutonium enrichment of less than 6 wt% is arranged in the center in the height direction, and a first region having an fissile plutonium average enrichment of 6 wt% or more is arranged above the second region. Is done. The core region between the second region and the lower blanket region is a third region having a fissile plutonium enrichment of 6 wt% or more. The fuel assembly arranged in such a core has a hexagonal cross section. The control rod inserted between the fuel assemblies has three blades arranged at intervals of 120 degrees, and a follower portion having a lower speed reduction ability than that of light water is arranged at the tip portion.

  Patent Document 3 describes a control rod. This control rod has a concentration of boron 10 (B10), which is a neutron absorbing material, from an end portion (insertion end portion) inserted first into the core toward an end portion (drawing end portion) on the control rod drive side. It is gradually decreasing.

JP 2003-222694 A JP-A-8-21890 JP-A-10-293188 GA DUCAT, et al .: Evaluation of the Parfait blanket Concept For Fast breeder Reactors, COO-2250-5, MITNE-157, Massachusetts Institute of Technology, January (1974)

  The parfait-type core has a Puf breeding ratio defined by the ratio of fissile plutonium (hereinafter referred to as Puf) when the fuel assembly is removed from the core and Puf when the fuel assembly is loaded to the core to 1 or more or 1 It is a core that can be brought close to. In order to realize the Puf breeding ratio 1, the parfait-type core is different from the current BWR in the axial configuration.

  The parfait-type core has an upper blanket region, an inner blanket region, and a lower blanket region in the axial direction, an upper fuel region between the upper blanket region and the inner blanket region, and between the inner blanket region and the lower blanket region. Each lower fuel region is arranged. The parfait-type core is an axially inhomogeneous core. U-238 included in each blanket region captures neutrons and newly generates Pu-239 that is Puf. However, since neutron capture by U-238 is remarkable in a resonance region having a relatively large neutron energy, in order to improve the Puf breeding ratio, it is necessary to increase the neutron energy in the core, that is, to harden the neutron spectrum. . In order to harden the neutron spectrum, it is necessary to reduce light water in the core, which is the main cause of neutron energy reduction, as much as possible. However, since light water is both a moderator and a coolant, the parfait-type core must reduce the effective volume ratio of water to fuel while maintaining heat removal performance, that is, thermal margin. Therefore, in the parfait-type core, the fuel rod gap is made smaller than the current BWR as long as the heat removal performance can be maintained, the outputs of all the loaded fuel assemblies are matched as much as possible, and the thermal margin between the fuel assemblies is The rated core flow rate is reduced by reducing the variation of

  As a result of various studies on such a parfait core, the inventors have found a new problem that the thermal margin is reduced as will be described later.

  The objective of this invention is providing the core and control rod of a light water reactor which can increase a thermal margin more.

The feature of the present invention that achieves the above-described object is that the upper core region, the inner blanket region, and the lower core region are arranged in this order from the top in the axial direction, and the upper core region and the lower core region are fissile plutonium or fissile A reactor core region containing plutonium and actinide nuclides, and a plurality of control rods for adjusting the reactor power inserted into the reactor core region and having a neutron absorber containing region containing a neutron absorber,
The neutron absorber-containing region has an upper region and a lower region, and the ratio of the average concentration in the lower region to the average concentration of the neutron absorber in the upper region is in the range of greater than 0.26 and less than 1.0, With the control rod fully inserted into the core region, the boundary defining the upper region and the lower region is located between the upper end of the inner blanket region and the lower end of the inner blanket region.

  In the present invention, the ratio of the average concentration in the lower region to the average concentration of the neutron absorber in the upper region is in the range of more than 0.26 and less than 1.0, and defines the upper region and the lower region. Since the boundary is located between the upper end of the inner blanket region and the lower end of the inner blanket region, the first main point described later can be improved. With the improvement of the first point, the thermal margin of the core of the light water reactor can be increased.

  Preferably, the neutron absorber-containing region has a central region that serves as a boundary defining the upper region and the lower region, and the average concentration of the neutron absorber in the central region is lower than that of the lower region, It is desirable to make the length in the axial direction shorter than the length in the axial direction of the inner blanket region. The central region where the average concentration of the neutron absorber is lower than that of the lower region and the axial length of the central region is shorter than the axial length of the inner blanket region, Since it arrange | positions between, the below-mentioned 2nd main point can be improved. With the improvement of the second point, the thermal margin of the light water reactor core can be increased.

  Preferably, the plurality of control rods include a plurality of first control rods inserted into the inner region of the core region and a plurality of second control rods inserted into the outer region of the core region surrounding the inner region. The ratio of the concentration of the neutron absorber in the first control rod to the average concentration of the neutron absorber in the second control rod is preferably in the range of greater than 0.47 and less than 1.0. The ratio of the concentration of the neutron absorber in the first control rod to the average concentration of the neutron absorber in the second control rod is in the range of greater than 0.47 and less than 1.0. Can be improved. With the improvement of the third point, the thermal margin of the core of the light water reactor can be increased.

  As described above, the inventors have found a new problem that a thermal margin is reduced in a puffer type core that is an axially inhomogeneous core applied to a light water reactor, for example, a boiling water reactor (BWR). It was. Furthermore, the inventors have newly found that there are three factors that cause a decrease in the thermal margin. Three factors newly found by the inventors will be described in detail below.

  The first factor is that since the parfait-type core is an axially inhomogeneous core as shown in FIG. 7 (a), the power in the three blanket regions containing almost no fissile material is low, and the fission is relatively low. The power in each of the upper core region 1 and the lower core region 2 containing the active substance is increased (see FIG. 6). The three blanket areas are an upper blanket area 3, an inner blanket area 4 and a lower blanket area 5. In order to improve the thermal margin of the parfait-type core, it is essential to flatten the output in the upper core region 1 and the lower core region 2. However, in the parfait-type core, since the internal blanket region 4 exists between the upper core region 1 and the lower core region 2, the void ratios are greatly different between the upper core region 1 and the lower core region 2. An example of the void ratio distribution in the axial direction and the power distribution in the axial direction in the parfait-type core is shown in FIG. The average void rate in the lower core region 2 is about 55%, whereas the average void rate in the upper core region 1 is about 80%. Since the power output of the nuclear reactor depends on the void ratio, the difference in the void ratio between the upper core region 1 and the lower core region 2 causes distortion of the axial power distribution in the core, and the thermal margin decreases.

  The second factor is that the output of the upper core region 1 may increase in the middle of the operation cycle of the reactor due to the control rod being pulled out of the core. In BWR, in order to compensate for the lack of reactivity due to the burning of nuclear fuel material, an operation of gradually pulling out the control rod for adjusting the reactor power from the core is performed during the operation of the reactor. In BWR, structures such as steam separators and steam dryers are installed above the core, so the control rod is pulled down from the core by the control rod drive installed at the bottom of the reactor. It is. For this reason, in the parfait-type core, depending on the method of operating the control rod, a state in which the control rod is inserted into the lower core region 2 but pulled out from the upper core region 1 occurs in the middle of the reactor operating cycle. there is a possibility. When such a state occurs, the output difference between the upper core region 1 and the lower core region 2 increases, and the thermal margin decreases. One operation cycle in the nuclear reactor means a period from when the nuclear reactor is started to when the nuclear reactor is stopped for replacement of the fuel assembly in the core.

  The third factor is that there is a possibility that the flattening of the output distribution in the radial direction and the flattening of the output distribution in the axial direction are not compatible. In a parfait-type core, it is essential to flatten the power distribution in the radial direction in order to reduce the rated core flow rate. However, in the axially inhomogeneous fuel assembly loaded in the parfait-type core, the output ratio of the blanket part and the fuel part changes with the combustion of the nuclear fuel material, and the output ratio of the fuel part decreases with the combustion of the nuclear fuel material. It has the property to do. For this reason, even if the power distribution in the radial direction of the core is flattened, that is, the output of the fuel assembly is made uniform, the output of the fuel part of the fuel assembly at the initial stage of combustion is higher than that at the end of combustion, and there is a thermal margin. Decrease.

  In order to maximize the thermal margin of the parfait-type core, it is necessary to improve all the above three factors. However, by improving at least one of the above three factors, the thermal margin of the parfait-type core can be improved as compared with the prior art.

  The inventors have examined measures for improving the above three new factors of the parfait-type core, and have found new measures for improvement. The present invention, which contributes to the improvement of the thermal margin of the parfait-type core, that is, the axially non-homogeneous core of the light water reactor, has been made based on the improvement measures found by the inventors. Specific improvement measures will be explained in detail.

  As shown in FIG. 7B, the improvement measures newly found by the inventors with respect to the first factor are as follows. As shown in FIG. 7B, in the axial direction, two regions including the upper region 13 and the lower region 14 contain neutron absorber-containing regions ( B10-containing region), and use of the control rod 12 for adjusting the reactor power having the follower portion 8. The follower portion 8, the upper region 13, and the lower region 14 are arranged in this order from the insertion end portion of the control rod 12 toward the withdrawal end portion. By applying this control rod 12 to the axially inhomogeneous core shown in FIG. 7A, for example, a parfait-type core, the reduction of the thermal margin due to the first factor is improved, and the heat of the parfait-type core is improved. Increases the margin. The control rod 12 is inserted between the fuel assemblies loaded in the core region 6 of the parfait-type core. The follower portion 8 is made of a material that does not contain B10 and absorbs less neutrons. The follower portion 8 has a function of preventing the light water as a moderator from flowing between the fuel assemblies from which the control rods 12 have been pulled out and increasing the Puf growth ratio. It is desirable that the axial length of the follower portion 8 is a length from the lower end of the lower core region 2 to the upper end of the upper core region 1.

  Incidentally, the parfait-type core has an upper blanket region 3, an inner blanket region 4 and a lower blanket region 5 arranged in the height direction (axial direction) of the core as shown in FIG. The upper core region 1 and the lower core region 2. The upper core region 1 is disposed between the upper blanket region 3 and the inner blanket region 4. The lower core region 2 is disposed between the inner blanket region 4 and the lower blanket region 5. The upper blanket region 3, the upper core region 1, the inner blanket region 4, the lower core region 2 and the lower blanket region 5 constitute a core region 6. The upper core region 1 and the lower core region 2 contain at least Puf.

  The inventors examined the concentration of B10 in each of the upper region 13 and the lower region 14 of the control rod 12. As a result, when the ratio of the B10 average concentration in the lower region 14 to the average concentration of B10 in the upper region 13 was changed, the maximum linear power density changed as shown in FIG. 8 through the operation cycle of the reactor. This characteristic is obtained when the boundary between the upper region 13 and the lower region 14 is located in the middle of the inner blanket region 4 in the axial direction of the core. In FIG. 8, the ratio of the average B10 concentration of the lower region 14 to the average B10 concentration of the upper region 13 is 1 when the average B10 concentrations of the upper region 13 and the lower region 14 are equal. This is a conventional control rod in which the B10-containing region is one region in which the upper region 13 and the lower region 14 are not distinguished. According to the result shown in FIG. 8, in the parfait-type core, the ratio of the average B10 concentration of the lower region 14 to the average B10 concentration of the upper region 13 is set to a size within a range greater than 0.26 and less than 1.0. It was found that the maximum linear power density can be reduced than before.

  Next, the inventors examined the position of the boundary that defines the upper region 13 and the lower region 14. When the lower end of the lower blanket region 5 is used as a reference, the position of the upper end of the inner blanket region 4 in the axial direction of the core is x, and the position of the lower end of the inner blanket region 4 is y. When the upper end of the B10-containing region of the control rod 12 (upper end of the upper region 13) is 1, and the lower end of the B10-containing region (lower end of the lower region 14) is 0, the boundary position of the control rod 12 is defined as the lower blanket region. The maximum linear power density was determined by moving from the lower end of 5 to the upper end of the upper blanket region 3. With the above movement of the boundary position, the maximum linear power density throughout the reactor operation cycle changed as shown in FIG. Based on this result, the upper core region 1 and the lower core region 2 that define the upper region 13 and the lower region 14 in the control rod 12 contain Puf. The inventor has a new finding that the maximum linear power density can be reduced if the boundary position is between the upper end of the inner blanket region 4 and the lower end of the region 4, that is, within the inner blanket region 4 in the axial direction. Could get.

  Further, the first factor is that, in the axial direction shown in FIG. 7C, the neutron absorber containing region (B10 containing region) of the three regions of the upper region 9, the central region 10 and the lower region 11, and the follower portion 8 It has been found that this can also be improved by using the control rod 7 for adjusting the reactor power having From the insertion end portion of the control rod 7 to the withdrawal end portion, the follower portion 8, the upper region 9, the central region 10, and the lower region 11 are arranged in this order. By applying the control rod 7 to the axially inhomogeneous core shown in FIG. 7A, for example, a parfait-type core, the reduction of the thermal margin due to the first factor is improved, and the thermal efficiency of the parfait-type core is improved. The margin increases. That is, also in the control rod 7, when the central region 10 is located in the range from the lower end of the inner blanket region 4 to the upper end in the axial direction, the ratio of the concentration of the lower region 11 to the average B10 concentration of the upper region 10. It was found that the maximum linear power density can be reduced as compared with the conventional case and the thermal margin is increased by making the size within a range larger than 0.26 and smaller than 1.0.

  In the control rod 7, the central region 10 is a boundary that defines an upper region 13 and a lower region 14. The central region 10 is located in the range from the lower end to the upper end of the inner blanket region 4 in the axial direction, like the control rod 12, the boundary defining the upper region 13 and the lower region 14 is the inner blanket in the axial direction. This corresponds to being located in the range from the lower end of the region 4 to the upper end thereof.

  As shown in FIG. 7 (c), the improvement measures newly found by the inventors for the second factor are as follows. In the control rod 7, the lower region is located between the upper region 9 and the lower region 11 in the axial direction. The central region 10 having a lower density than the average B10 density of 11 is arranged. By applying the control rod 7 having the central region 10 whose average B10 concentration is lower than that in the lower region 11 to the axially non-homogeneous core shown in FIG. The reduction of the thermal margin due to this is improved, and the thermal margin of the parfait-type core is increased.

  The reason why the reduction in the thermal margin due to the second factor can be improved by providing the central region 10 having the above average B10 concentration will be described below. When the reactor power falls below the rated power after the reactor is started, the reactor power is maintained at the rated power by pulling out the control rod 7 from the core. By pulling out the control rod 7, the upper region 9 of the control rod 7 is pulled out from the upper core region 1, so that the output of the upper core region 1 increases. However, the central region 10 existing in the inner blanket region 4 is extracted from the inner blanket region 4 and inserted into the lower core region 2. For this reason, the output of the lower core area | region 2 increases.

  As described above, when the output of the upper core region 1 is increased by the operation of pulling out the control rod 7 having the central region 10 in the middle of the operation cycle of the reactor, the lower core region 2 of the central region 10 of the control rod 7 is increased. Is inserted, and the power of the lower core region 2 is increased. Therefore, the difference between the power in the upper core region 1 and the power in the lower core region 2 can be reduced in the middle of the operation cycle of the reactor. This leads to improvement of the second factor, and can increase the thermal margin of the core.

  The inventors examined the length of the central region 10 in the axial direction. FIG. 10 shows the examination results. That is, when the ratio of the length of the central region 10 to the axial length of the inner blanket region 4 was changed, the maximum linear power density changed as shown in FIG. 10 through the reactor operation cycle. . Based on this characteristic, the inventors set the ratio of the axial length of the central region 10 to the axial length of the inner blanket region 4 to a size within a range of more than 0 and less than 1. Thus, it has been found that the second factor can be improved and the thermal margin can be increased. The characteristics shown in FIG. 10 are obtained when the average B10 concentration in the central region 10 is 0%. When the average B10 concentration of the central region 10 is changed within a range smaller than the concentration of the lower region 14, the value of the lowest maximum linear power density with respect to the average B10 concentration of the central region 10 is the value in FIG. More than. However, within the range where the average B10 concentration of the central region 10 is smaller than that of the lower region 14, the ratio of the axial length of the central region 10 to the axial length of the inner blanket region 4 is less than zero. It has been found that by setting the size in a large range of less than 1, the second factor is improved and the thermal margin is increased.

An improvement measure newly found by the inventors with respect to the third factor is that a fuel assembly having a high burnup in the core (a fuel assembly having a long residence time in the core) is used for the first adjustment for reactor power adjustment. The control rods are adjacent to each other, and the second control rod for adjusting the reactor power is adjacent to a fuel assembly having a small burnup (a fuel assembly having a short stay in the core). This improvement uses two types of control rods for adjusting reactor power. Axially heterogeneous core shown in FIG. 7 (a), for example, the first control rod arranged in a central portion of the parfait-type core (inner region), arranging the second control rod to the outer periphery of the core (outer region). Also, the average B10 concentration of the first control rod is made smaller than that of the second control rod.

  The inventors examined the B10 concentration of each of the first control rod and the second control rod in more detail. As a result, when the ratio of the average B10 concentration of the first control rod to the average B10 concentration of the second control rod was changed, the maximum linear power density changed as shown in FIG. 11 through the operation cycle of the reactor. . This characteristic is obtained when the control rod 7 described above is used. Based on the results of FIG. 11, the ratio of the concentration of the first control rod to the average B10 concentration of the second control rod is set to a size within a range greater than 0.47 and less than 1.0. It was found that the power density can be further reduced. The maximum linear power density when the ratio of the average B10 concentration of the first control rod of the control rod 12 to the average B10 concentration of the second control rod of the control rod 12 is changed is the ratio of 0. It can be reduced when the size is in a range greater than 47 and less than 1.0.

  According to the present invention, the thermal margin of the core of the light water reactor having the internal blanket region can be increased.

  Examples of the present invention will be described below.

  A light water reactor core according to a preferred embodiment of the present invention, specifically, a parfait-type core that is an axially inhomogeneous core applied to a BWR will be described in detail with reference to the drawings.

  Before describing the parfait-type core of this embodiment, a BWR to which the parfait-type core is applied will be described with reference to FIG. The BWR has a reactor pressure vessel 15, and a parfait-type core 18, a steam / water separator 16 and a steam dryer 17 are installed in the reactor pressure vessel 8. Further, a plurality of control rod drive devices 19 are installed at the bottom of the reactor pressure vessel 15. The parfait-type core 18 has a core region 6 and a plurality of control rods 7A and 12A for adjusting the reactor power inserted into the core region 6. The reactor power is controlled by inserting or withdrawing the control rods 7A, 12A from the parfait-type core 18. The control rods 7A and 12A are driven by a motor-driven (or hydraulic pressure-driven) control rod drive device 19. The cooling water (light water) supplied to the parfait-type core 18 is heated by heat generated by the nuclear fission of Puf in the fuel assembly 30 and becomes steam. After the moisture is removed by the steam separator 16 and the steam dryer 17, the steam is sent to a turbine (not shown) through a main steam pipe 20 connected to the reactor pressure vessel 15, and the turbine ( Drive (not shown). Electric power is generated by the rotation of a generator connected to the turbine. The steam exhausted from the turbine is condensed by the condenser to become water. This water is returned to the reactor pressure vessel 15 by the water supply pipe 21 as water supply.

  The parfait-type core 18 includes a core region 6 (see FIG. 7A) configured by loading a plurality of fuel assemblies 30 shown in FIG. 1, and a plurality of control rods for adjusting the reactor power shown in FIG. 7A and 12A (see FIG. 3). The control rods 7A and 12A are disposed in the core region 6. Further, the parfait core 18 includes a plurality of control rods 23 inserted into the core region 6. As shown in FIG. 7 (a), the core region 6 of the parfait-type core 18 includes an upper blanket region 3, an upper core region 1, an inner blanket region 4, a lower core region 2, and an axially arranged portion from the top. A lower blanket region 5 is included. The fuel assembly 30 is a fuel assembly having a hexagonal cross section, and 720 bodies are loaded in the core region 6. The control rods 7A, 12A and 23 are control rods having three blades arranged in a Y-shaped cross section at intervals of 120 degrees. A total of 223 control rods 7A, 12A, and 23 are provided.

  The configuration of the control rods 7A, 12A, 23 will be described with reference to FIG. Each of these control rods has three blades 25 extending in three directions from a tie rod 24 located at the axis of the control rod. The angle between the adjacent blades 25 is 120 °. Each blade 25 has 17 neutron absorbing rods 27 arranged in a U-shaped sheath 26 attached to the tie rod 24. Each neutron absorber rod 27 is filled with a neutron absorber 28 containing B10 in a stainless steel cladding tube. The control rod 23 is fully extracted from the core during the operation of the reactor, and is fully inserted into the core when the operation of the reactor is stopped. The control rod 23 is a control rod for stopping the nuclear reactor, and is driven by a control rod drive device (not shown) driven by water pressure.

  The fuel assembly 30 constituting the parfait core 18 will be described. The upper blanket region 3, the upper core region 1, the inner blanket region 4, the lower core region 2, and the lower blanket region 5 of the parfait-type core 18 are constituted by a fuel assembly 30. As shown in FIG. 1, the fuel assembly 30 includes an upper blanket portion 33, an upper fuel portion 31, an inner blanket portion 34, a lower fuel portion 32, and a lower blanket portion 35 from the upper portion toward the lower portion. The upper blanket portion 33 has an axial length of 13 cm, the upper fuel portion 31 has an axial length of 22 cm, the inner blanket portion 34 has an axial length of 45 cm, and the lower fuel portion 32 has an axial length of The length in the axial direction of 22 cm and the lower blanket part 35 is 18 cm. The fuel assembly 30 has a plurality of fuel rods (not shown). Each fuel rod is filled with a large number of depleted uranium fuel pellets including depleted uranium at positions corresponding to the upper, inner, and lower blanket portions 33, 34, and 35. Each fuel rod is filled with a large number of MOX fuel pellets containing a mixture of depleted uranium, plutonium and actinoid nuclides at positions corresponding to the upper and lower fuel portions 31 and 32. The plutonium contains Puf. The upper fuel part 31 and the lower fuel part 32 have an average enrichment of Puf of 18 wt%. The enrichment of Puf in the inner blanket region 4 is 0 wt%. The fuel assembly 30 containing Puf having an average enrichment of 18 wt% is a new fuel assembly having a burnup of 0 GWd / t.

  The fuel pellets filled in the positions corresponding to the upper, inner and lower blanket portions 33, 34 and 35 may be composed of uranium including at least one of deteriorated uranium, natural uranium, depleted uranium, and low enriched uranium. In addition, the fuel pellets filled in the positions corresponding to the upper and lower fuel parts 31 and 32 enrich Puf and actinoid nuclides into uranium containing at least one of depleted uranium, natural uranium, depleted uranium, and low enriched uranium. The nuclear fuel material may be used. The latter fuel pellet can also be comprised of Puf with a nuclear fuel material enriched in uranium that includes at least one of depleted uranium, natural uranium, depleted uranium, and low enriched uranium.

  The upper blanket region 3 of the parfait-type core 18 is constituted by the upper blanket portion 33 of each fuel assembly 30. Similarly, the upper core region 1 is constituted by the upper fuel portion 31, the inner blanket region 4 is constituted by the inner blanket portion 34, the lower core region 2 is constituted by the lower fuel portion 32, and the lower blanket region 5 is the lower blanket. The unit 35 is configured.

  The parfait-type core 18 is divided in the radial direction into a central portion (inner region) and an outer peripheral portion (outer region) surrounding the central portion. The central portion is an area within one half of the radius of the parfait-type core 18, based on the axis of the parfait-type core 18. The parfait-type core 18 has a fuel assembly 30 with a low burnup and a large excess reactivity at the outer peripheral portion, a fuel assembly 30 with a high burnup and a low surplus reactivity at the center (the fuel loaded on the outer periphery). By loading a fuel assembly 30) having a longer residence time in the core than the assembly 30, the power distribution in the radial direction is flattened. For example, a new fuel assembly 30 with a burnup of 0 GWd / t and a fuel assembly 30 entering the second operation cycle are loaded on the outer periphery, and the operation cycle of the third cycle or more is loaded in the center. An entering fuel assembly 30 is loaded. As shown in FIG. 3, the control rod 7B is disposed on the outer peripheral portion, and the control rod 7A is disposed on the central portion. Of the 223 control rods, there are a total of 37 control rods 7 for adjusting the reactor power, 19 control rods 7A, and 18 control rods 7B.

  As shown in FIG. 1, the control rod 7A has a follower portion 8, an upper region 9A, a central region 10A, and a lower region 11A arranged in this order from the insertion end portion of the control rod 7A toward the withdrawal end portion. The follower portion 8 is made of carbon, which is a substance having a lower speed reduction ability than light water, and is provided in the sheath 26. The axial lengths of the upper region 9A, the central region 10A, and the lower region 11A are 36 cm, 33 cm, and 41 cm, respectively. Upper region 9A is filled with boron carbide having a B10 concentration of 90%, central region 10A is filled with boron carbide having a B10 concentration of 20%, and lower region 11A is filled with boron carbide having a B10 concentration of 50%. . The average B10 concentration in the axial direction of the upper region 9A, the central region 10A, and the lower region 11A, that is, the neutron absorber-containing region is about 54%. Each neutron absorber rod 27 disposed below the follower portion 8 contains boron carbide having a B10 concentration of 90%, 20%, and 50% at positions corresponding to the upper region 9A, the central region 10A, and the lower region 11A, respectively. Each is filled.

  As shown in FIG. 1, the control rod 12A has a follower portion 8, an upper region 13A, and a lower region 14A arranged in this order from the insertion end portion toward the withdrawal end portion. The axial lengths of the upper region 9B and the lower region 11B are 36 cm and 74 cm, respectively. The upper region 13A is filled with boron carbide having a B10 concentration of 80%, and the lower region 11B is filled with boron carbide having a B10 concentration of 70%. The average B10 concentration in the axial direction of the upper region 9A and the lower region 11A, that is, the neutron absorber-containing region is about 73%. Similarly to the control rod 7A, each neutron absorber rod 27 of the control rod 7B is filled with a corresponding B10 concentration boron carbide in the neutron absorber containing region.

  In the control rod 7A, the ratio of the concentration of the lower region 11A to the concentration of B10 in the upper region 9A is about 0.56, and the central region 10A is located in the range from the lower end of the internal blanket region 4 to the upper end thereof. Further, in the control rod 12A, the ratio of the concentration of the lower region 11A to the concentration of B10 in the upper region 9A is about 0.88, and the position of the boundary between the upper region 13A and the lower region 14A starts from the lower end of the inner blanket region 4. Located in the upper end range. For this reason, the parfait type core 18 can improve the above-mentioned first essential point by the above-described functions of the control rods 7A and 12A. In this embodiment, the thermal margin increases with the improvement of the first point.

  The ratio of the length of the central region 10A to the axial length of the inner blanket region 34 is 0.73, and the B10 concentration in the central region 10A is lower than that of the lower region 11A. The parfait-type core 18 having the control rod 7A including the central region 10A can improve the second main point described above. In this embodiment, the thermal margin increases with the improvement of the second point.

  In this embodiment, the ratio of the average concentration of the control rod 7A to the average B10 concentration in the axial direction of the control rod 12A is about 0.74. The parfait-type core 18 in which the control rod 12A is arranged at the outer peripheral portion and the control rod 7A is arranged at the central portion can improve the third main point described above. In this embodiment, the thermal margin increases with the improvement of the third point.

  As described above, the parfait-type core 18 that can improve the first, second, and third main points is as shown by a solid line in FIG. 5 in a period from the start of one operation cycle to the end of the operation cycle. The maximum linear power density of the fuel assembly can be significantly reduced. In this embodiment, the maximum linear power density can be reduced by about 6% as compared with the conventional parfait-type core indicated by the broken line. For this reason, the present embodiment can remarkably increase the thermal margin of the nuclear reactor. The control rods 7A and 12A used in this embodiment leave the follower portion 8 in the core at the end of the operation cycle, and the neutron absorbing material-containing region is pulled out below the core, that is, the lower blanket region 35.

  A conventional parfait reactor core in which a conventional control rod (for reactor power adjustment) having a uniform B10 concentration in the axial direction of the neutron absorber containing region is arranged has one operating cycle according to the state of the control rod being pulled out. During the period, the maximum linear power density of the fuel assembly changes greatly as shown by the broken line in FIG. However, in the parfait-type core 18 of the present embodiment using the control rods 7A and 12A, the variation range of the maximum linear power density can be remarkably reduced throughout one operation cycle.

  In this embodiment, the control rod 12A is arranged on the outer periphery of the parfait-type core 18, but the neutron absorber-containing region has three axial regions instead of the control rod 12A having two axial regions. It is also possible to arrange the control rod 7 on the outer periphery. The control rod 7 is capable of improving the first, second and third points (for example, the B10 concentration of the central region 10, the position and length of the central region 10 in the axial direction, and the average B10 concentration of the control rod 7A). The ratio of the concentration of the control rod 7 is satisfied.

  In the present embodiment, the control rod 7A may be arranged on the outer peripheral portion instead of the control rod 12A. As in the second embodiment, the parfait-type core of this example is configured by arranging control rods 7A, which are control rods for adjusting the reactor power, at the outer peripheral portion and the central portion of the parfait-type core. This parfait type core cannot improve the third point, but can improve the first and second points. The parfait-type core of this example can increase the thermal margin by improving the first and second main points.

  A core of a light water reactor according to another embodiment of the present invention, specifically, a parfait-type core applied to a BWR will be described in detail with reference to FIGS. The parfait-type core 18A of the present embodiment has a plurality of fuel assemblies 30 and control rods 12B and 23. The fuel assembly 30 has the same configuration as the fuel assembly 30 used in the first embodiment. The control rod 12B for adjusting the reactor power has an upper region 13B and a lower region 14B. The control rod 12B has a configuration in which the B10 concentration in the upper region 13B of the control rod 12A is changed to 90%. The B10 concentration in the lower region 14B is 70%. Other configurations of the control rod 12B are the same as those of the control rod 12A. The number of control rods 12B arranged in the parfait-type core 18A is 37.

  In this embodiment, in the control rod 12B, the ratio of the concentration of the lower region 14B to the concentration of B10 in the upper region 13B is about 0.78, and the position of the boundary between the upper region 13B and the lower region 14B is the position of the inner blanket region 4. It is located within the range from the lower end to the upper end. For this reason, the parfait type core 18A can improve the above-mentioned first essential point by the above-described functions of the control rod 12B. In this embodiment, the thermal margin increases with the improvement of the first point.

  As in the first embodiment, the parfait-type core 18A has a fuel assembly 30 with a small burnup and a large excess reactivity at the outer periphery in the radial direction, and a fuel with a large burnup and a small excess reactivity at the center. The output distribution in the radial direction is flattened by loading the assembly. The outer peripheral portion of the core in the present embodiment is the same region as the outer peripheral portion of the core of the first embodiment. In the parfait-type core 18A, the distortion of the power distribution in the radial direction is smaller than the distortion of the power distribution in the axial direction. For this reason, in the parfait-type core 18A, the control rod that satisfies the conditions obtained based on the characteristics shown in FIG. 11 is not disposed on the outer peripheral portion, but the thermal margin can be increased. The parfait-type core 18A of the present embodiment cannot improve the second and third main points.

  As described above, the parfait-type core 18A that can improve the first point is the maximum line of the fuel assembly in the period from the start of one operation cycle to the end of the operation cycle, as shown by the solid line in FIG. The power density can be reduced. In this embodiment, the maximum linear power density can be reduced by about 2% as compared with the conventional parfait-type core indicated by the broken line. For this reason, this embodiment can increase the thermal margin of the nuclear reactor. Further, in the parfait-type core 18 of the present embodiment using the control rod 12B, the variation range of the maximum linear power density throughout the period of one operation cycle is not as large as that of the first embodiment, but compared with the conventional parfait-type core. Can be reduced.

  A core of a light water reactor according to another embodiment of the present invention, specifically, a parfait-type core applied to a BWR will be specifically described with reference to FIGS. 15 and 16. The parfait-type core 18B of the present embodiment includes a plurality of fuel assemblies 30, control rods 12A and 12C for adjusting the reactor power, and a plurality of control rods 23 for stopping the reactor. The fuel assembly 30 has the same configuration as the fuel assembly 30 used in the first embodiment. The control rod 12A has the same configuration as the control rod 12A used in the first embodiment. The control rod 12C has an upper region 13C and a lower region 14C in the neutron absorber containing region. The B10 concentration in the upper region 13C is 90%, and the B10 concentration in the lower region 14C is 50%. The other structure of the control rod 12C is the same as that of the control rod 12A. The control rod 12C is disposed at the center of the parfait-type core 18B, and the control rod 12A is disposed at the outer periphery of the parfait-type core 18B. The outer peripheral portion of the core in the present embodiment is the same region as the outer peripheral portion of the core of the first embodiment. There are 19 control rods 12C and 18 control rods 12A. Incidentally, the number of control rods 23 is 186.

  In this embodiment, in the control rod 12C, the ratio of the concentration of the lower region 14C to the concentration of B10 in the upper region 13C is about 0.56, and the position of the boundary between the upper region 13B and the lower region 14B is the position of the inner blanket region 4. It is located in the range from the lower end to the upper end. Further, as described in the first embodiment, the control rod 12A has the former ratio of about 0.88, and the position of the latter boundary is located in the range from the lower end of the internal blanket region 4 to the upper end thereof. For this reason, the parfait type core 18B can improve the above-mentioned first essential point by the above-described functions of the control rods 12C and 12A. In this embodiment, the thermal margin increases with the improvement of the first point.

  The average B10 concentration in the axial direction of the control rod 12C is 63%, and the concentration of the control rod 12A is 73%. In this embodiment, the ratio of the concentration of the control rod 7A to the average B10 concentration in the axial direction of the control rod 12A is about 0.86. The parfait-type core 18B in which the control rod 12A is disposed on the outer peripheral portion and the control rod 12C is disposed on the central portion can improve the above-described third point. In this embodiment, the thermal margin increases with the improvement of the third point.

  As described above, the parfait-type core 18B that can improve the first and third main points has a fuel assembly as shown by a solid line in FIG. 17 in the period from the start of one operation cycle to the end of the operation cycle. The maximum line power density can be reduced. In this embodiment, the maximum linear power density can be reduced by about 3% as compared with the conventional parfait-type core indicated by a broken line. For this reason, this embodiment can increase the thermal margin of the nuclear reactor. In addition, the parfait-type core 18B of the present embodiment using the control rods 12C and 12A has a change width of the maximum linear power density throughout the period of one operation cycle, which is not as large as that of the first embodiment, but is the same as that of the conventional parfait-type core. Compared to, it can be reduced. The change width in the parfait-type core 18B is larger than the change width in the first embodiment, but is smaller than the change width in the second embodiment.

It is a block diagram of the fuel assembly loaded in the parfait type | mold core of Example 1 of this invention, and two types of control rods inserted in the core. It is a schematic block diagram of BWR to which the parfait type core of Example 1 is applied. It is a top view of the parfait type | mold core of Example 1 which is one preferable Example of this invention. FIG. 4 is an enlarged cross-sectional view of the control rod shown in FIGS. 1 and 3. It is a characteristic view which shows the change of the maximum linear power density in the driving cycle of Example 1. It is explanatory drawing which shows the power distribution and void ratio distribution in the axial direction of a parfait type | mold core. The control rod used as an improvement measure of the problem of the parfait-type core is shown, (a) is a schematic longitudinal cross-sectional view of the parfait-type core, (b) is a schematic structural diagram of the control rod in which the neutron absorber containing region is two regions, c) is a schematic structural diagram of a control rod having three neutron absorber-containing regions. FIG. 6 is a characteristic diagram showing the relationship between the ratio of the concentration in the lower region of the control rod to the average B10 concentration in the upper region of the control rod and the maximum linear power density. It is a characteristic view showing the relationship between the position of the boundary between the upper region and the lower region of the control rod in the core axis direction and the maximum linear power density. It is a characteristic view showing the relationship between the ratio of the length of the central region of the control rod to the axial length of the internal blanket region and the maximum linear power density. FIG. 4 is a characteristic diagram showing the relationship between the ratio of the concentration of control rods arranged in the inner region of the core to the average B10 concentration in the axial direction of control rods arranged in the outer region of the parfait-type core and the maximum linear power density. is there. It is a top view of the parfait type | mold core of Example 2 which is another Example of this invention. FIG. 6 is a configuration diagram of a fuel assembly loaded on a parfait-type core of Example 2 and control rods inserted into the core. It is a characteristic view which shows the change of the maximum linear power density in the driving cycle of Example 2. It is a top view of the parfait type | mold core of Example 3 which is another Example of this invention. FIG. 6 is a configuration diagram of a fuel assembly loaded in a parfait-type core of Example 3 and two types of control rods inserted into the core. It is a characteristic view which shows the change of the maximum linear power density in the driving | running cycle of Example 3.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Upper core area, 2 ... Lower core area, 3 ... Upper blanket area, 4 ... Internal blanket area, 5 ... Lower blanket area, 6 ... Core area, 7, 7A, 12, 12A, 12B, 12C, 23 ... Control Rod, 8 ... follower part, 9, 9A, 13, 13A, 13B, 13C ... upper region, 10, 10A ... central region, 11, 11A, 14, 14A, 14B, 14C ... lower region, 15 ... reactor pressure vessel , 18, 18A, 18B ... parfait core, 19 ... control rod drive, 25 ... blade, 26 ... sheath, 27 ... neutron absorber rod, 30 ... fuel assembly, 31 ... upper fuel part, 32 ... lower fuel part, 33 ... upper blanket part, 34 ... internal blanket part, 35 ... lower blanket part.

Claims (11)

In the axial direction, an upper blanket region, an upper core region, an inner blanket region, a lower core region, and a lower blanket region are arranged in this order from the upper part, and the core region in which the upper core region and the lower core region contain fissile plutonium. When,
A plurality of control rods for adjusting the reactor power inserted into the core region and having a neutron absorber containing region containing a neutron absorber, and
The neutron absorber-containing region has an upper region and a lower region, and the ratio of the average concentration in the lower region to the average concentration of the neutron absorber in the upper region is greater than 0.26 and less than 1.0. With the control rod fully inserted into the core region, the boundary defining the upper region and the lower region is located between the upper end of the inner blanket region and the lower end of the inner blanket region and it is,
The neutron absorber-containing region has a central region that serves as a boundary that defines the upper region and the lower region, and the average concentration of the neutron absorber in the central region is lower than the average concentration of the lower region And the axial length of the central region is shorter than the axial length of the internal blanket region . The plurality of control rods include a plurality of first control rods inserted into an inner region of the core region, and a plurality of second control rods inserted into an outer region of the core region surrounding the inner region. cage, wherein the ratio of the concentration in the second control rod the first control rod to the average concentration of the neutron absorber in the light water reactors according to claim 1 greater Ru near the range of less than 1.0 from 0.47 Core. In the axial direction, an upper blanket region, an upper core region, an inner blanket region, a lower core region, and a lower blanket region are arranged in this order from the upper part, and the core region in which the upper core region and the lower core region contain fissile plutonium. When,
A plurality of control rods for adjusting the reactor power inserted into the core region and having a neutron absorber containing region containing a neutron absorber, and
The neutron absorber-containing region has these regions arranged in the order of the upper region, the central region, and the lower region in the axial direction, and the average in the lower region with respect to the average concentration of the neutron absorber in the upper region The concentration ratio is in the range greater than 0.26 and less than 1.0;
The average concentration of the neutron absorber in the central region is lower than the average concentration in the lower region, the axial length of the central region is shorter than the axial length of the internal blanket region, A core of a light water reactor , wherein the central region is located between an upper end of the internal blanket region and a lower end of the internal blanket region in a state where the control rod is fully inserted into the core region . The plurality of control rods include a plurality of first control rods inserted into an inner region of the core region, and a plurality of second control rods inserted into an outer region of the core region surrounding the inner region. The ratio of the concentration in the first control rod to the average concentration of the neutron absorber in the second control rod is in the range of greater than 0.47 and less than 1.0 . Core. The core of a light water reactor according to any one of claims 1 to 4 , wherein the upper core region and the lower core region contain actinide nuclides . The said upper core area | region and the said lower core area | region contain the nuclear fuel which enriched the fissile plutonium in the uranium containing at least one of depleted uranium, natural uranium, depleted uranium, and low enriched uranium. Item 5. The light water reactor core according to any one of items 4 to 4 . The upper core region and the lower core region contain nuclear fuel enriched in uranium containing the fissile plutonium and actinide nuclides including at least one of depleted uranium, natural uranium, depleted uranium, and low enriched uranium. 5. The core of the light water reactor according to 5 . The core of a light water reactor according to any one of claims 1 to 7, wherein the neutron absorber is boron 10 . In the axial direction, the upper blanket region, the upper core region, the inner blanket region, the lower core region and the lower blanket region are arranged in this order from the top, and the upper core region and the lower core region are fissile plutonium, Or a control rod inserted into the core containing the fissile plutonium and actinoid nuclides and having a neutron absorber containing region containing a neutron absorber,
The neutron absorber-containing region has an upper region and a lower region, and the ratio of the average concentration in the lower region to the average concentration of the neutron absorber in the upper region is greater than 0.26 and less than 1.0. The boundary defining the upper region and the lower region is located between the upper end of the inner blanket region and the lower end of the inner blanket region, with the boundary fully inserted into the core,
The neutron absorber-containing region has a central region that serves as a boundary that defines the upper region and the lower region, and the average concentration of the neutron absorber in the central region is lower than the average concentration of the lower region. And the axial length of the central region is shorter than the axial length of the internal blanket region . In the axial direction, the upper blanket zone, an upper core region, inner blanket region, the lower core region and a lower blanket zone a core Ru Tei arranged from the top in this order, wherein the upper core region and the lower core region fissile plutonium Or a control rod inserted into the core containing the fissile plutonium and actinide nuclides and having a neutron absorber containing region containing a neutron absorber,
The neutron absorber-containing region has these regions arranged in the order of the upper region , the central region, and the lower region in the axial direction, and the average in the lower region with respect to the average concentration of the neutron absorber in the upper region The concentration ratio is in the range greater than 0.26 and less than 1.0;
The average concentration of the neutron absorber in the central region is lower than the average concentration in the lower region, the axial length of the central region is shorter than the axial length of the internal blanket region, The control rod , wherein the central region is located between an upper end of the inner blanket region and a lower end of the inner blanket region in a state of being fully inserted into the core . The control rod according to claim 9 or 10, wherein the neutron absorber is boron 10 .

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