JPH03592B2 - - Google Patents

Description

[Detailed Description of the Invention] (1) Technical Field of the Invention The present invention relates to boiling water nuclear reactors, and particularly to new fuel assemblies loaded into the reactor core during fuel exchange. (2) Prior Art This will be explained with reference to FIGS. 1 to 9. A fuel rod 1 is shown in FIGS. 1 and 2. The fuel rods 1 are constructed by accommodating fuel pellets 3 in a fuel cladding tube 2 made of zirconium alloy, and sealing the upper and lower ends with an upper end plug 4 and a lower end plug 5. The fuel pellets 3 used are sintered uranium oxide powder. A gas plenum 6 is formed in the upper part of the fuel cladding tube 2, and the fission product gas is configured to accumulate in the gas plenum 6. Further, a plenum spring 7 is housed in this gas plenum 6, and this plenum spring 7
The fuel pellets 3 are pressed downward, and movement of these fuel pellets 3 in the axial direction is restricted. As shown in Figs. 3 and 4, these fuel rods 1 ... and water rods 8 ... are bundled into a total of 64 grids in 8 rows and 8 columns. The fuel assemblies 10 are accommodated in channel boxes 9 and constitute fuel assemblies 10 . The coolant mainly flows into the channel boxes 9 from the lower tie plates 11 of the fuel assemblies 10 and flows upward. Also, some of the coolant flows through the gaps between the fuel assemblies 10 from below to above. As shown in FIG. 4, four of these fuel assemblies 10 are arranged around a control rod 12 having a cross-shaped cross section to form a unit grid 13 . These unit lattices 13 are arranged in a lattice shape as shown in FIG. 5, forming a core 14 whose planar shape is nearly circular. In addition, each square in FIG. 5 indicates one unit cell 13 ..., and the cross within the unit cell 13... indicates the control rod 12.... When a boiling water reactor is loaded with fuel assemblies 10 into the reactor core, it is normally operated continuously at the rated thermal output for 9 to 12 months, and for at least 6 months. After this period, that is, one cycle of combustion, the reactor is stopped and the burned fuel assembly 10 is
... is removed, a new fuel assembly is loaded, and the next cycle is fired. In this fuel exchange, not all of the fuel assemblies 10 ... are replaced, but 1/3 to 1/4 of the fuel assemblies 10 ... are replaced at one time.
Therefore, the fuel assembly 10 is used for 3 to 4 cycles. Then, the infinite multiplication factor K∞ of the fuel assembly 10 ... decreases as combustion progresses, and as a result, the core 1
The effective multiplication factor Keff of all 4 also decreases. In order to enable rated operation through each cycle described above, the effective multiplication factor Keff of the entire core 14 must be 1.0 or more even at the end of each cycle.
Keff−1.0 is called surplus reactivity K _ex . And the uranium 235 of the new fuel assembly 10 ...
The average enrichment e is the core 14 during one cycle.
The surplus reactivity K _ex is designed to a value that can be maintained positive. By the way, the infinite multiplication factor K∞ of the fuel assembly 10 ...
decreases as combustion progresses, so if the average enrichment of uranium 235 in the new fuel assembly 10 is set so that the surplus reactivity K _ex of the core 14 is positive even at the end of the cycle, then at the beginning of the cycle The infinite multiplication factor K∞ becomes too high, the surplus reactivity K _ex of the reactor core 14 becomes excessive in the early stage of the cycle, and the shutdown margin for safely stopping the reactor decreases, reducing safety. For this reason, burnable poison is generally mixed into the fuel pellets 3. Gadolinia Gd ₂ O ₃ is generally used as this burnable poison. This burnable poison absorbs neutrons and suppresses reactivity, and as combustion progresses, the neutron absorption capacity decreases .
As shown by the solid line in Figure 6, the infinite multiplication factor K∞ of ... is relatively low at the beginning of combustion, increases as combustion progresses, and becomes infinite when combustion progresses to a certain extent and the neutron absorption capacity of the burnable poison is exhausted. The multiplication factor K∞ decreases again. Then, the period R during which this infinite multiplication factor K∞ increases
The amount of burnable poison mixed in is set so that the period of one cycle is equal, and the new fuel assembly 1
The increase in the infinite multiplication factor K∞ of 0... compensates for the decrease in the infinite multiplication factor K∞ of other old fuel assemblies 10..., so that the excess reactivity K _ex of the entire core 14 becomes approximately as shown in FIG. I try to keep it constant. Incidentally, the period of one cycle may be required to be extended due to changes in power demand, etc. The period of one cycle can be extended by increasing the number of new fuel assemblies 10 loaded during fuel exchange. However, as the combustion progresses in this new fuel assembly 10 , the infinite multiplication factor K∞ increases. Therefore, if the number of loaded new fuel assemblies 10 is increased more than planned, as combustion progresses, the excess reactivity K _ex of the core 14 will increase as shown by the solid line in Figure 8, and at the end of the cycle This causes a problem in which the excess reactivity K _ex becomes excessive and the stop margin becomes small. For this reason, a so-called two-stream system is adopted in which a fuel assembly A containing a small amount of burnable poison and a fuel assembly B containing a large amount of burnable poison are prepared in advance. If there is no change in the period of the next cycle, the planned number of A fuel assemblies are loaded as new fuel assemblies, and the reactor core 14 reacts as planned, as shown by the dashed line in Figure 8. Maintain the temperature approximately constant. Furthermore, if the period of the next cycle is extended, more B fuel assemblies are loaded than the planned number. Since this B fuel assembly has a high content of burnable poisons, the effect of suppressing the reactivity is large, and the change in the surplus reactivity K _ex of the core 14 is as shown by the broken line in Figure 8, and the surplus reactivity at the end of the cycle is The increase in reactivity K _ex is suppressed to a low level, and the necessary shutdown margin is secured. (3) Problems with the prior art The conventional two-stream type fuel assemblies A and B only have different contents of burnable poisons. Therefore, the infinite multiplication factor K for the progress of combustion
The change in ∞ is the characteristic shown by the solid line in FIG. 9 for the A fuel assembly and the broken line in FIG. 9 for the B fuel assembly. Then, B in the period R during which the infinite multiplication factor K∞ increases
The characteristics of the fuel assembly are simply the characteristics of the A fuel assembly shifted in parallel. Therefore, the characteristics of the change in the surplus reactivity of the reactor core when more B fuel assemblies than the planned number are loaded into the reactor core are shown by the solid line (A) in Figure 8.
When more fuel assemblies are loaded than the planned number), it is simply a parallel movement of the fuel assemblies. Therefore, as is clear from FIG. 8, there is a large change in the surplus reactivity between the beginning of the cycle and the end of the cycle. The range of change in this surplus reactivity increases as the number of loaded bodies increases. Therefore, the number of loaded objects cannot be changed much more than the planned number, and the cycle period can only be changed by ±1 month. In addition, since the surplus reactivity of the core decreases in the early stage of the cycle, the number of control rods inserted into the core decreases in the early stage of the cycle, and the degree of freedom in adjusting the core output and the power distribution within the core decreases. Furthermore, the infinite multiplication factor K∞ of the B fuel assembly loaded in a larger number than the planned number reaches its maximum at the end of the cycle. Therefore, at the beginning of the next cycle, there will be a large number of fuel assemblies with the maximum infinite multiplication factor, which will have a negative impact on the shutdown margin of the reactor in the next cycle. (4) Purpose of the invention The purpose of the invention is to increase the range of changes in the period of one cycle and to reduce fluctuations in the surplus reactivity of the reactor core during the cycle even when the period of one cycle is changed. (5) Structure of the Invention The present invention provides a two-stream type fuel assembly in which the average enrichment rate of uranium 235 of the B fuel assembly, the number of fuel rods containing burnable poison, and the amount of burnable poison per fuel rod are determined. The maximum concentration and the average concentration of burnable poison in the entire fuel assembly are both higher than those in fuel assembly A. Therefore, if A fuel assemblies and B fuel assemblies with different average enrichments of uranium-235 are loaded as a new fuel assembly at an arbitrary ratio, the total number of new fuel assemblies as a whole can be increased without changing the number of new fuel assemblies. The average value of the uranium-235 average enrichment can be changed, and the range of change in the period of one cycle can be increased. In addition, the number of fuel rods containing burnable poisons in fuel assembly B, the maximum concentration of burnable poisons per fuel rod, and the average concentration of burnable poisons per fuel assembly should be made larger than in fuel assembly A. If these A fuel assemblies and B fuel assemblies are loaded into the reactor core as new fuel assemblies, the influence on the change characteristics of the excess reactivity of the reactor core will be equal for A fuel assemblies and B fuel assemblies. This reduces fluctuations in the excess reactivity of the reactor core. (6) One embodiment of the invention This will be explained with reference to FIGS. 10 to 18. 10 and 11 show fuel rods 101 . The fuel rods 101 are constructed by housing fuel pellets 103 in a fuel cladding tube 102 made of zirconium alloy, and having their upper and lower ends sealed with an upper end plug 104 and a lower end plug 105. The fuel pellets 103 are made by sintering uranium oxide powder. A gas plenum 106 is formed in the upper part of the fuel cladding tube 102, and the fission product gas is configured to accumulate in the gas plenum 106. Further, a plenum spring 107 is housed within the gas plenum 106, and the plenum spring 107 presses the fuel pellets 103 downward to restrict movement of the fuel pellets 103 in the axial direction. As shown in FIGS. 12 and 13, these fuel rods 101 ... and water rods 108 ... are bundled into a grid of 64 in total in 8 rows and 8 columns.
...and a bundle of water sticks 108...is a channel box 10
9... constitutes a fuel assembly 110 .... The coolant mainly flows from the lower tie plate 111 of the fuel assembly 110 to the channel box 109.
...flows inward and upward. Also, some of the coolant flows through the gaps between the fuel assemblies 10 from below to above. As shown in FIG.
The bodies are arranged to form a unit cell 113. These unit lattices 113 are arranged in a lattice shape as shown in FIG. 14, forming a core 114 whose planar shape is nearly circular. In addition, each square in FIG. 14 indicates one unit cell 113 ..., and the cross inside each unit cell 113 ... indicates the control rod 112.
...is shown. In the core 114 , A fuel assemblies 110 ... and B fuel assemblies 110 ... are loaded as new fuel assemblies at the time of fuel exchange.
Fuel assembly 110 ... and B fuel assembly 110 ...
is loaded with 1/4 to 1/3 of the total number of bodies. These A fuel assemblies 110 ... and B fuel assemblies 110 ... differ in the average enrichment of uranium 235 and the content of burnable poison, that is, gadolinia. In other words, uranium 235 of A fuel assembly 110 ...
Fuel rod 1 containing gadolinia with average enrichment e _A
The number of fuel rods 101... is N _A , the maximum gadolinia concentration per fuel rod 101 is ρ _naxA weight%, the average concentration of gadolinia per fuel assembly is _A weight%, the number of fuel rods 101 with the maximum gadolinia concentration is N
(ρ _naxA ), the average enrichment of uranium 235 in fuel assembly B 110 is e _B , the number of gadolinia-containing fuel rods 101 is N _B , and the maximum gadolinia concentration per fuel rod 101 is ρ _naxB weight%, average gadolinia concentration per fuel assembly _B weight%,
The number of fuel rods 101 with the maximum gadolinia concentration is N
(When ρ _naxB , e _A ＜ e _B ………(1) N _A ＜ N _B ………(2) ρ _naxA ＜ ρ _naxB ………(3) _A ＜ _B ………(4) N (ρ _naxA )≦N(ρ _naxB ) (5).In addition, the above-mentioned gadolinia is mixed over the entire length of the fuel rod 101 .
In some cases, the fuel rods 101 are contained in only a portion of the fuel rods 101, and in such cases, the number of fuel rods 101 is determined by the ratio of the length of the portion where gadolinia is mixed and the total length of the fuel rods 101 . For example, 1/ of the total length
0.5 if gadolinia is mixed in part 2
Calculate it as a book. Furthermore, the above values are set in the following relationship. N _B −N _A = (e _B −e _A )／C ₁ ………(6) 0.25≦C ₁ ≦0.35 ………(6)′ ρ _naxB −ρ _naxA = (e _B −e _A )C ₂ ………(7) 5≦C ₂ ≦6 ………(7)′ Then, the A fuel assembly 110 … and the B fuel assembly 110 … are separated at a predetermined ratio corresponding to the period of the next cycle. It is loaded as a new fuel assembly. For example, if the period of the next cycle is to be the maximum, only the B fuel assemblies 110 ... are loaded as new fuel assemblies, and if the period of the next cycle is to be the minimum, only the A fuel assemblies 110 ... are loaded as new fuel assemblies. When the fuel assemblies are loaded as fuel assemblies and the period of the next cycle is between the maximum and minimum periods, the ratio between the A fuel assemblies 110 and the B fuel assemblies 110 is set corresponding to this period. Note that in either case, the total number of new fuel assemblies loaded as new fuel assemblies is constant. The minimum cycle period is 9 months, and the maximum is 9 months.
A fuel assembly 110 ... and B fuel assembly 110 when the cycle period is scheduled to last 12 months, and when the minimum cycle period is 12 months and the maximum cycle period is 15 months.
Examples of the specifications of ... are shown in the table below. [Table] Next, the operation of this embodiment will be explained. First, at the time of fuel exchange, the A fuel assemblies 110 ... and the B fuel assemblies 110 ... are loaded into the reactor core 114 as new fuel assemblies. In this case, the ratio of the A fuel assemblies 110 ... and the B fuel assemblies 110 ... is determined in accordance with the scheduled period of the next cycle. That is, A fuel assembly 110
Since the average uranium 235 enrichment e _A of ... is low, and the average uranium 235 concentration e _B of the B fuel assembly 110 ... is high, it is possible to change the surplus reactivity of the core 114 by changing these ratios. It is possible to change the number of days that the reactor core 114 can be continuously operated. For example, Figure 15 shows that when 200 new fuel assemblies are loaded in an 800 MW class boiling water reactor with a total of 548 fuel assemblies loaded in the core, B fuel assemblies among these 200 It shows the relationship between the body proportion (%) and the number of days that can be operated continuously at full output. Therefore, the next cycle period is 12 months, i.e.
If the period is 360 days, the proportion of this B fuel assembly 110 should be approximately 83%, and in this way, operation for this period is possible and the surplus reactivity of the core 114 is not increased unnecessarily. There's nothing to do. and,
In this case, unlike the conventional method, there is no need to change the total number of new fuel assemblies, so the range of change in cycle period can be increased. In addition, for B fuel assembly 110 , which has a high average enrichment of uranium-235, the number of gadolinia-containing fuel rods, the maximum gadolinia concentration per fuel rod, the average gadolinia concentration per fuel assembly, etc. Since the fuel assembly 110 is larger than the fuel assembly A 110 ... and the fuel assembly B 1.
The difference in infinite multiplication factor due to the difference in the average enrichment of uranium 235 of 10... is compensated for. Therefore, when these A fuel assemblies 110 ... and B fuel assemblies 110 ... are loaded into the reactor core 114 , the influence that A fuel assemblies 110 ..., B fuel assemblies 110 ... have on the surplus reactivity of the reactor core 114 is They are approximately equal to each other, and even if the ratio of the B fuel assemblies 110 is changed, the change characteristics of the surplus reactivity of the reactor core 114 will not change much, and the degree of freedom in controlling the output of the reactor core 114 and the reactor shutdown margin will not be impaired. . This effect will be specifically explained below. In other words, the influence on the surplus reactivity of the reactor core 114 is
In order to make them equal to each other, it is first necessary to make the infinite multiplication factors of the A fuel assembly 110 and the B fuel assembly 110 equal at the initial stage of combustion. When gadolinia is used as a burnable poison, such characteristics can be obtained by satisfying the above formulas (2), (6), and (6)'. Gadolinium in gadolinia is a strong neutron absorber, and even naturally occurring gadolinium has a neutron absorption capacity of about 49,000 burns. Therefore, the concentration of gadolinia in fuel rod 101 is approximately 2.0.
When the concentration exceeds % by weight, the neutron absorption ability of this gadolinia reaches saturation, and even at a concentration higher than this, the neutron absorption ability hardly changes. The concentration of gadolinia mixed in is generally 2.0% by weight or more. Therefore, in the early stage of combustion, the effect of suppressing the infinite multiplication factor of the fuel assembly due to gadolinia mixing is not proportional to the concentration of gadolinia, but is proportional to the number of fuel rods 101 containing gadolinia. The effect of suppressing this infinite multiplication factor K∞ is approximately 2.5%ΔK∞ per fuel rod containing gadolinia.
3.2%ΔK∞. In addition, the infinite multiplication factor suppressing effect of gadolinia is also affected by the uranium-235 enrichment e of the fuel, and in the normally used range of e = 2.7 to 3.5 wt%, it is approximately 9 to 10% ΔK∞ per 1% of e. be. Therefore, in the early stage of combustion, the A fuel assembly 110
Average enrichment of uranium 235 e _A and B fuel assemblies 11
The difference in the average enrichment e _B of uranium 235 of 0... Δe = e _B −e The difference in infinite multiplication factor due to _A [ΔK∞(Δe)] is ΔK if the number of fuel rods 101 containing gadolinia is equal. ∞(Δe)=Δe·t(t=9 to 10). And this difference in infinite multiplication factor [ΔK∞
(Δe)] by the difference ΔN in the number of fuel rods 101 containing gadolinia, ΔN=ΔK∞(Δe)/S (S=2.5 to 3.2). Therefore, ΔN=Δe/(S/t) ΔN=(e _B −e _A )/C ₁ . Here, since C ₁ = S/t, C ₁ = 0.25 to 0.35, and the above formulas (6) and (6)' N _B - N _A = (e _B - e _A )/C ₁ ...... (6) 0.25≦C ₁ ≦0.35 ………(6)′ is derived. Therefore, if these equations (6) and (6)' are satisfied, the A fuel assembly 110 ...
and the B fuel assembly 110 can be made equal to each other. Furthermore, in order to equalize the effects of the A fuel assemblies 110 and B fuel assemblies 110 on the surplus reactivity of the core 114 , the maximum values of the infinite multiplication factors that increase as combustion progresses must also be made equal. . To make these maximum values equal, the above (3), (4),
This is done by satisfying equations (5), (7), and (7)'.
The reason will be explained below. First, it is assumed that the average gadolinia concentration _B of the B fuel assemblies 110 and the maximum gadolinia concentration ρ _naxB per fuel rod are both equal to those of the A fuel assemblies 110 . B fuel assembly 11 in this case
The change characteristics of the infinite multiplication factor of 0... are shown in FIG. 16 by a solid line. Note that the dashed-dotted line in Fig. 16 indicates fuel assembly A 1.
10... is the change characteristic of the infinite multiplication factor. and,
As mentioned above, by increasing the number N _B of the gadolinia-containing fuel rods 101 of the B fuel assemblies 110 , the A fuel assemblies 110 ...
and the B fuel assembly 110 can be made equal to each other. However, as the combustion progresses, the neutron absorption capacity of gadolinia decreases, so as the combustion progresses, the average uranium-235 enrichment e _A , e of the A fuel assembly 110 ... and the B fuel assembly 110 ... The difference in _B causes a difference in the infinite multiplication factor, and a difference in h ₁ occurs in the maximum value of the infinite multiplication factor. Then, after the burnup when this infinite multiplication factor reaches its maximum, that is, the effect of gadolinia disappears, uranium is lost due to the progress of burning.
The infinite multiplication factor has decreased due to the impairment of 235, but
The rate of decline is not significantly affected by the average enrichment of uranium-235. Therefore, the difference between the maximum values of this infinite multiplication factor
h ₁ does not change much after that, and the A fuel assembly 110 ... and the B fuel assembly 110 ... at any burnup.
The difference between the infinite multiplication factors h ₂ is approximately equal to the above h ₁ . Therefore, the A fuel assembly 110 ... and the B fuel assembly 110
...The burnup until reaching a certain infinite multiplication factor is l
There will be a delay. In the first place, the uranium 235 of the A fuel assembly 110 ... and the B fuel assembly 110 ...
The reason for the difference in average enrichment e _A and e _B is that by giving the B fuel assembly 110 such a burnup delay, the cycle period can be shortened without changing the total number of fuels loaded in the new combustion assembly. This delay l is a necessary characteristic because it increases the range of change. However, since the above-mentioned difference h ₁ between the maximum values of the infinite multiplication factors increases the fluctuation range of the surplus reactivity of the core 114 during one cycle, this h ₁ must be made small.
In order to reduce this h ₁ , A fuel assembly 1
This can be achieved by setting the average gadolinia concentrations _A and _B of the fuel assemblies 10 and 110 so that _A < _B (4). By the way, as the combustion progresses, the neutron absorption capacity of gadolinia decreases, and the neutron absorption capacity eventually disappears, but the period until this neutron absorption capacity disappears, that is, the duration of the neutron absorption capacity, is the maximum gadolinia per fuel rod. Proportional to concentration. Therefore, if the average concentration of gadolinia per fuel assembly is set as shown in equation (4) above, B fuel assembly 11
The duration of the neutron absorption ability of gadolinia in 0... becomes equal to that in A fuel assembly 110 ..., and B
The period during which the infinite multiplication factor of the fuel assemblies 110 ... reaches the maximum becomes equal to that of the A fuel assemblies 110 .... Since the B fuel assemblies 110 are loaded to extend the period of one cycle, the period until the infinite multiplication factor of the B fuel assemblies 110 reaches the maximum is the same as that of the A fuel assemblies 110. …
If it is equal to , even if one attempts to extend the period of one cycle , the infinite multiplication factor of both A fuel assembly 110... and B fuel assembly 110 ... will reach its maximum before reaching the end of this cycle, and after that, the infinite multiplication factor will decrease. As a result, the surplus reactivity of the core 114 decreases before reaching the end of the cycle, and the cycle period cannot be extended. Therefore, the period during which the infinite multiplication factor of this B fuel assembly 110 reaches its maximum is A.
It is necessary to delay the fuel assembly 110 by l. In order to have this delay of l, the maximum value of gadolinia concentration ρ _naxB per fuel rod 101 of B fuel assembly 110 is determined by
For ρ _naxA of 0..., ρ _naxA < ρ _naxB ......(3). In this case, in order to ensure the above effect, the number N (ρ _naxB ) of the fuel rods with the maximum gadolinia concentration in the B fuel assembly 110 ... is set to N (ρ _naxA ) of the A fuel assembly 110 ... It is preferable that N(ρ _naxA )≦N(ρ _naxB ) (5). Furthermore, specifically, the burnup is 2 GWd/T per 1% gadolinia concentration when the gadolinia concentration is 2 to 5% by weight during the duration of the neutron absorption ability of gadolinia. In addition, the burnup delay l due to the difference in the average enrichment e of uranium 235 is 3.2~
In the range of 4.0% by weight, the average enrichment difference of uranium-235 is 0.1
It is about 1.0 to 1.2 GWd/T per weight %. If the delay in the timing at which the infinite multiplication factor reaches its maximum due to the maximum concentration difference of gadolinia is ΔE ₁ , then ΔE ₁ =(ρ _naxB −ρ _naxA )·ff≈2.0. Further, the burnup difference ΔE ₂ corresponding to the difference in the average enrichment of uranium 235 is ΔE ₂ =(e _B −e _A )·gg=10 to 12. Therefore, in order to delay the time when the infinite multiplication factor reaches its maximum due to the difference in the maximum concentration of gadolinia by a delay l corresponding to the difference in the average enrichment of uranium-235, it is sufficient to set ΔE ₁ = ΔE ₂ , and therefore (ρ _naxB −ρ _naxA )・f=(e _B −e _A )・g Therefore, ρ _naxB −ρ _naxA = (e _B −e _A )・g/f = (e _B −e _A )・C ₂ ………( 7) becomes. Here, since C ₂ = g/f, 5≦C ₂ ≦6 ………(7)′. Therefore, if the above equations (3), (4), (5), (7), and (7)' are satisfied, the influence on the surplus reactivity of the core 114 can be determined by the A fuel assembly 110 ... and the B fuel assembly 110. It can be made equal to... Note that FIG. 17 shows the actual A fuel assembly 110.
... and the B fuel assembly 110 ... with respect to the progress of the burnup. The solid line in FIG .
10... is shown. As is clear from FIG. 17, the A fuel assembly 110 ... and the B fuel assembly 110 ...
The infinite multiplication factors of ... are approximately equal at the beginning of combustion, the maximum values are also approximately equal, and the change in the infinite multiplication factors is also B
The fuel assemblies 110 are uniformly delayed by the amount of burnup corresponding to the difference in the average enrichment of uranium-235. Therefore, if only the A fuel assemblies 110... are loaded into the core 114 as new fuel assemblies as shown in FIG. 18, the change characteristics of the surplus reactivity of the core 114 will become as shown by the solid line in FIG. 18, and Only the B fuel assembly 110 ... is used as a new fuel assembly in the core 114.
If the core 114 is loaded with the same amount of fuel, the change characteristics of the surplus reactivity in the core 114 will be as shown by the broken line in FIG. Therefore, by changing the ratio of the numbers of A fuel assemblies 110 ... and B fuel assemblies 110 ..., the continuous operation period, that is, the cycle period, can be changed to P ₁ , P without changing the total number of new fuel assemblies to be loaded. It can be changed arbitrarily between ₂ . In this case, A fuel assembly 110...
and B fuel assembly 110 ... have the same influence on the change characteristics of the surplus reactivity of the core 114, so the fluctuation of the surplus reactivity of the core 114 is extremely small in either case. (7) Effects of the invention In the present invention, the average enrichment of uranium-235 in the B fuel assembly is higher than that in the A fuel assembly, so the ratio of the number of A fuel assemblies and B fuel assemblies to be loaded as new fuel assemblies is reduced. By changing , the cycle period can be changed without changing the total number of new fuel assemblies. Therefore, problems associated with changes in the number of loaded new fuel assemblies do not occur, and the range of changes in the cycle period can be increased. In addition, in the B fuel assembly, the number of fuel rods containing burnable poison, the maximum concentration of burnable poison per fuel rod, and the average concentration of burnable poison per fuel assembly are each larger than in the A fuel assembly, and the uranium 235 The difference in characteristics due to the difference in average enrichment is compensated for, and the effects of these A fuel assemblies and B fuel assemblies on the fluctuation characteristics of the excess reactivity of the reactor core are made equal.
Therefore, even if the ratio of the number of loaded bodies between fuel assemblies A and B fuel assemblies changes, there is no difference in the fluctuation characteristics of the surplus reactivity of the core, and the fluctuations of the surplus reactivity of the core can be kept small at all times. This allows for stable core characteristics without compromising flexibility in core power control, etc.

[Brief explanation of drawings]

1 to 9 show conventional examples, FIG. 1 is a longitudinal sectional view of a fuel rod, FIG. 2 is a sectional view taken along the - line in FIG. 1, and FIG. 3 is a perspective view of a fuel assembly. Fourth
The figure is a plan view of the unit cell, Fig. 5 is a schematic plan view of the core, Fig. 6 is a diagram showing the change characteristics of the infinite multiplication factor of the fuel assembly, and Fig. 7 is the fluctuation of the excess reactivity of the core. Figure 8 is a diagram showing the fluctuation characteristics of the excess reactivity of the core in the conventional two-stream system, and Figure 9 is the diagram showing the fluctuation characteristics of the infinite multiplication factor of the fuel assembly in the conventional two-stream system. FIG. 10 to 18 show one embodiment of the present invention, FIG. 10 is a vertical sectional view of a fuel rod, FIG. 11 is a sectional view taken along the line XI-XI in FIG. 10, and FIG. 12 is a fuel rod. A perspective view of the aggregate, FIG. 13 is a plan view of the unit cell,
Figure 14 is a schematic plan view of the core, Figure 15 is B
A diagram showing the relationship between the loading ratio of fuel assemblies and the number of days of continuous operation. Figure 16 is a diagram explaining the characteristics of fuel assemblies A and B. Figure 17 is a diagram showing the relationship between fuel assemblies A and B. FIG. 18 is a diagram showing the variation characteristics of the infinite multiplication factor of the fuel assembly, and FIG. 18 is a diagram showing the variation characteristics of the surplus reactivity of the core when loaded with the A fuel assembly and the B fuel assembly. 101 ... Fuel rod, 103... Fuel pellet,
108...Water rod, 110 ...Fuel assembly, 112
...Control rod, 113 ...Unit cell, 114 ...Reactor core.

Claims (1)

[Claims] 1 A fuel assembly and uranium in the entire fuel assembly
235 The average enrichment, the number of fuel rods containing burnable poisons, the maximum concentration of burnable poisons per fuel rod, and the concentration of burnable poisons in the entire fuel assembly are each greater than the above A fuel assembly. B fuel assembly is loaded into the reactor core as a new fuel assembly during fuel exchange, and the infinite multiplication factors of A fuel assembly and B fuel assembly are made equal at the beginning of combustion, and increase as combustion progresses. A fuel assembly and B
A boiling water nuclear reactor characterized by equalizing the maximum values of the infinite multiplication factors of the fuel assemblies. 2. In order to equalize the infinite multiplication factors of fuel assembly A and fuel assembly B at the initial stage of combustion, gadolinia is used as the burnable poison, and the average enrichment of uranium 235 in fuel assembly A is set to e _A weight percent. , the number of fuel rods containing burnable poison is N _A , the average enrichment of uranium 235 in the B fuel assembly is e _B weight %, and the number of fuel rods containing burnable poison is N _B , N _B The boiling water nuclear reactor according to claim 1, characterized in that -N _A = (e _B -e _A )/C ₁ 0.25≦C ₁ ≦0.35. 3. In order to equalize the maximum values of the infinite multiplication factors of fuel assembly A and fuel assembly B, which increase as the combustion progresses, gadolinia is used as the burnable poison, and uranium 235 of fuel assembly A is used as the burnable poison. The average enrichment is e _A weight %, the maximum concentration of burnable poison per fuel rod is ρ _naxA weight %, the average enrichment of uranium 235 in the B fuel assembly is e _B weight %, one fuel rod When the maximum concentration of burnable poison per unit is ρ _naxB weight %, ρ _naxB −ρ _naxA = (e _B − e _A )/C ₂ 5≦C ₂ ≦6. Boiling water nuclear reactor according to scope 1 or 2.