JPH01250789A - Boiling water reactor and its operating method - Google Patents

Abstract

PURPOSE:To improve profitability by preparing a means for narrowing opening area in a cooling water flow-in passage of a water channel and setting the opening area so as to produce steam voids at the time of low core cooling water flow of low output operation. CONSTITUTION:A float valve 105d of an inverse trigonal pyramid is set on a lower end plug 105a of a water channel or a large-sized water rod 105A to form by cooling water quantity upward and downward movably. Further, the valve 105d is set at the full line position of the order of output of about 65% and core cooling water flow of about 40%. Operation is performed by following load and the regulation of the output together with load follow-up is performed by core cooling water flow control, voids are positively produced in the channel 105A by the action of the float valve 105d, and spectral shift effect is produced at the time of low output and low flow. Therefore, fuel can economically be burned.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a boiling water nuclear reactor and its operating method,
More specifically, 9 is equipped with an improved large-diameter water channel (or large-diameter water rod) to obtain a high spectral shift effect during low power/low flow operation.
The present invention relates to a boiling water reactor loaded with a ×9 type fuel assembly, load follow-up operation through flow rate control, and spectrum shift operation in combination with load follow-up operation.

[Prior art] Most of the boiling water reactors currently in practical use in Japan are of the type (8X8-2 ) fuel assembly is adopted.

An example of this conventional 8×8 type fuel assembly is shown in FIG. 7, for example.

Moreover, FIG. 6 shows 1/4 of the core of a boiling water reactor, and the remaining 3/4. The reactor core is shown in Figure 1.
In Figure 0, which is arranged symmetrically about the ■ axis and J axis of the /4 core, 107 is a cross-shaped control rod containing a neutron absorber, and 101 is a fuel assembly. Fuel assembly 10
1, as shown in FIG. 7, a total of 62 wood fuel rods 104 numbered 1 to 62, and two quarter rods 105C that do not contain fuel material inside and are hollow and allow cooling water to flow through them are arranged in an 8×8 structure. They are arranged in a square grid, surrounded by a Zircaloy channel box 106, and loaded into the reactor adjacent to the control rods 107. During power operation of the nuclear reactor, cooling water flows into the water rod 105C from the bottom and flows to the top without creating voids.

Next, the operation of a boiling water reactor will be explained.

In the operation of boiling water reactors, by periodically replacing the fuel tank, the surplus reactivity necessary to maintain continuous operation of 1 to 2 years is provided, and the reactivity is maintained except at the end of the cycle. Therefore, it is necessary to suppress excess reactivity by using a burnable poison represented by gadolinia mixed in the control rods 107 and part of the fuel rods 104. At the same time, it is important that excess reactivity can be suppressed by reducing the flow rate of core cooling water (core flow rate) and increasing the void ratio. It provides the basic principles of controlled spectrum shift operation.

Literature: G.C.Hopkins; “BWR5pectral 5hift-GEAP-
25391 (1981).

General Electric, Co.

Spectrum shift operation means consciously changing the neutron spectrum (spectrum shift).

This is an operating method that promotes the capture of 238 U neutrons and increases the amount of plutonium produced, and is effective in improving the economic efficiency of nuclear fuel. However, in boiling water reactors, as shown in the above literature, , is known to be realized by core flow control.

Here, regarding the spectrum shift operation by core flow rate control (hereinafter referred to as flow rate control spectrum shift operation) of this conventionally known boiling water reactor, we will explain the change in the void ratio of cooling water depending on the core flow rate while referring to the attached drawings. This will be explained using the relationship between , and surplus reactivity.

FIG. 8 is an output/flow rate map showing the locus of operating output and flow rate of a boiling water reactor. As shown, the normal operating range of the reactor is the rated power flow control curve 202.10
It is in the region surrounded by the 0% flow rate curve 204 and the limit curve 207 for preventing cavitation of the recirculation pump. Note that 201 is an expanded operating range (core flow rate control range) at rated power, and 206 is a natural circulation curve.

In order to perform the flow rate control spectrum shift operation, the rated output flow rate control curve 202 is expanded to the high output side 203 and the 100% flow rate curve 204 is expanded to the high flow rate side 205. That is,
In the early and middle stages of the operation cycle, the core flow rate control range 201 is operated at the lower limit (the left end of 201, about 90% flow rate), and at the end of the operation cycle when the surplus reactivity has decreased, the upper limit of the core flow rate control range 201 (the right end of 201, Approximately 105
% flow rate) in 8 lines. According to this operating method, the void ratio increases in the early and middle stages of the cycle, and the moderator (
The neutron spectrum shifts to the higher energy side, as a result of which the neutron spectrum shifts to the higher energy side.
The neutron absorption of 23♂U, which is a fuel parent material, increases and Pu
The conversion ratio to will be higher. Furthermore, if the flow rate is maximized at the end of the cycle, the void fraction will be reduced and the neutron spectrum will be reversely shifted to the lower energy side, and the fission rate of the produced Pu will increase, making it possible to increase the surplus reactivity.

As is clear from the above explanation, the flow rate control is smooth.

The effect of torque shift operation, that is, the increase in conversion ratio and the improvement in fuel economy, is at the upper limit of the core flow control range.
It is expressed by the magnitude of change in void ratio between the lower limit.

[Problems to be Solved by the Invention] However, in the flow control spectrum shift operation of a boiling water reactor as described above, the possible control range of the core flow rate during constant and rated power operation is about 90 to 105%. limited to a relatively narrow range. As pointed out in the above literature, if the flow rate is significantly reduced, the burnout margin (MCPR) will be reduced, and if the flow rate is significantly increased, the capacity of the recirculation pump may be exceeded or the core internal structure may be damaged. This is because the flow rate is restricted due to the risk of vibration. Therefore, it is inevitable to limit the control range of the core flow rate during rated power operation.

In the conventional method, the change in the amount of moderator, that is, the change in steam void volume fraction due to flow rate control in the range of approximately 90% to 105%, is only 5% (core average), and the spectral shift effect is also small. There is a point.

This invention was made in view of the above problems, and its purpose is to obtain a highly economical boiling water reactor that exhibits a high spectral shift effect during low output and low flow rate operation. Furthermore, it is an object of the present invention to provide an operating method in which a high spectral shift effect can be obtained by adjusting the output by controlling the core flow rate when performing load following operation of this boiling water reactor. Further, it is also part of the object of the present invention to achieve the maximum spectrum shift effect by performing this load following operation in combination with spectrum shift operation.

[Means for Solving the Problems] In order to achieve the above object, the boiling water reactor of the present invention is a boiling water reactor whose output can be adjusted by controlling the flow rate of core cooling water. It has a fuel assembly in which the multiple fuel rods in the center of the fuel assembly are replaced with square large-diameter water channels or round large-diameter quarter rods that allow cooling water to flow inside from the bottom to the top. A means is provided in the cooling water inflow path at the bottom of these water channels or water rods to narrow the opening area of the cooling water inflow path in response to a decrease in the core cooling water flow rate.

In addition, in this method of operating a boiling water reactor, when controlling the output during load following operation by controlling the flow rate of core coolant, the inside of the water channel or water rod is connected to the cooling water inside the core during rated power operation. Near the rated flow rate,
The cooling water is filled with cooling water without generating steam voids, and at low flow rates of cooling water in the core during low-power operation, steam voids can occur at around these rated flow rates and at the prescribed cooling water in the core. The opening area of the cooling water inflow path is set by the means in accordance with the water flow rate.

In this case, in order to further improve fuel economy, in the output range near the rated output of the load following operation, the core coolant flow rate at the beginning and middle of the operation cycle is set to approximately 90% of the rated flow rate, and the core coolant flow rate at the end of the operation cycle is reduced to approximately 90% of the rated flow rate. Approximately 90 to 10% of the rated flow rate, making the material flow rate approximately 105% of the rated flow rate.
Spectral shift operation by controlling the core coolant flow rate in the range of 5% may also be performed.

[Function] As mentioned above, since the control range of the core cooling water flow rate during rated power operation is unavoidably restricted (approximately 90 to 105%), the present invention exhibits a high spectral effect during low power and low flow rate operation. The aim is to create a boiling water reactor. Hereinafter, the effects of the present invention will be explained.

The fuel assembly loaded in the boiling water nuclear reactor of the present invention is of a type including a large-diameter overchannel or a large-diameter quarterrod in the east direction of the fuel element. In the cooling water inlet passage at the bottom of this large-diameter overchannel or large-diameter quarter rod (represented by water rod in the following description of this column), the cooling water inlet passage Means for narrowing the opening area is provided. Therefore, when the core cooling water flow rate decreases, the opening area of the cooling water inflow path into the water rod becomes narrower, and void spaces are more likely to occur in the quarter rod, so that it is possible to exhibit a spectrum shift effect. In other words, by actively creating voids within the water rod at low flow rates,
It can promote the capture of 2SU neutrons in the fuel and increase the amount of plutonium produced.

The reason why the fuel assembly of the present invention is a 9x9 type fuel assembly is because MC
This is to improve PR as much as possible and to secure margin for flow rate control operation. This means that the 9x9 type fuel assembly is different from the conventional 8x9 type fuel assembly.
This is based on the fact that since the heat transfer area is increased compared to the x8 type fuel assembly, the heat load per unit area is reduced and MCPR is improved.

Furthermore, the reason why this 9×9 type fuel assembly is equipped with a large-diameter water rod is that the basic function of the present invention is to actively eliminate voids within the water rod during low flow rate operation as described above. This is because the spectrum shift effect is obtained by generating the water rod, and the larger the flow path area of the water rod, the greater the effect.

Next, a method of operating a boiling water reactor according to the present invention will be explained.

According to the present invention, the load following operation is performed, and the output adjustment for the load following operation is performed by flow rate control. that time,
The means provided in the cooling water inlet at the lower part of the quarter rod sets the opening area of the cooling water inlet in accordance with the core cooling water flow rate. It is done as follows.

In the vicinity of the rated flow rate of cooling water in the core during rated power operation, the water rod is set so that the cooling water flows into grooves without creating void spaces within the water rod, similar to conventional water rods.

On the other hand, at a predetermined low flow rate of cooling water in the core during low power operation, the setting is made so that a void space can be generated.

In this case, a spectral shift effect occurs during predetermined low flow rate operation. That is, a void space is generated within the water rod at a predetermined low flow rate, and the amount of plutonium produced increases. Furthermore, since the void space disappears near the rated flow rate, the amount of plutonium produced decreases.

Therefore, it is possible to burn fuel economically while performing load following operation.

Furthermore, when operating near the rated output in the load following operation,
It is also possible to perform spectrum shift operation by controlling the flow rate. In this case, the flow rate control range for spectrum shift operation is approximately 90 to 1
The range is 0.05%. Therefore, the spectrum shift effect obtained by the spectrum shift operation alone is similar to that of the conventional method.

However, since a spectrum shift effect can also be obtained by the load following operation, a high spectrum shift effect can be expected as a result by performing both operations together.

[Example] As an example of the present invention, Table 1 shows the main design specifications of a 9x9 type fuel assembly loaded in a boiling water nuclear reactor of the present invention.

The embodiment shown in column A of Table 1 is a 9x9 fuel assembly with one large-diameter square water channel;
The example shown in the column is a 9x9 fuel assembly with two large diameter round quarter rods.

The cross-sectional views of the 9×9 type fuel assemblies shown in columns A and B are shown in FIGS. 1 and 2, respectively. 1st
In the figures and FIG. 2, reference numerals 103 and 104 shown in the figures are similar to the conventional 8×8 type fuel assembly shown in FIG. 7 of the prior art described above, except that the dimensions and shape are different. Shows the components.

Column C of Table 1 shows the main design specifications of a conventional 8x8 fuel assembly for comparison.

As is clear from Table 1, the 9x9 type fuel assembly adopted in the present invention has a larger heat transfer area than the conventional 8x8 type fuel assembly. channel 1
The cross-sectional area of 05A or water rod 105B is 3 to 5
It has about twice the size.

The reason for adopting the 9×9 type fuel assembly equipped with these large-diameter channels 105A or water rods 105B is as described in the section of the effect above.

Next, the lower structure of the water channel 105A (or water rod 105B) will be explained.

FIG. 3 is an internal explanatory diagram of a 9×9 type fuel assembly (corresponding to the one shown in column A of Table 1 and FIG. 1) of the present invention. It is shown with most of it removed.

In the 9×9 type fuel assembly shown in FIG.
This is the format replaced by water channel 105A.
The lower structure (means for narrowing the opening area of the cooling water inflow path in response to a decrease in the core cooling water flow rate) is shown in cross-sectional view in FIG.

In FIGS. 3 and 4, cooling water flows in from holes 105c provided at several locations in the circumferential direction of the lower end plug 105a of the water channel 105A, and is provided in an inverted triangular shape on the inner peripheral surface of the lower end plug 105a. The water passes through the gap between the inner wall of the valve chamber 105f and the inverted triangular float valve 105d, rises in the water channel 105A, and flows out from the water channel outlet 105b. The float valve 105d can move up and down eight times depending on the flow rate of the core cooling water, and a stopper (locking means) 105e for restricting its upward movement is fixed by welding or the like above the valve chamber 105f. The stoppers 105e are provided at only two or three locations radially so as not to obstruct the flow of cooling water passing through the gap between the float valve 105d and the lower end plug 105a.

In Figure 4, the float valve 105d is shown by a solid line and a broken line.
%, the flow rate flowing into the water channel 105A is at the position of the broken line.

Set to . Since the pressure difference ΔP that drives the flow rate of the water channel 105A strongly depends on the core cooling water flow rate, when the flow rate flowing into the water channel 105A decreases, the float valve 105d lowers, but the output is 65%.
, set so that the solid line is at the core cooling water flow rate of about 40%.

In this embodiment, the local pressure drop coefficient that determines the flow resistance at the portion where the float valve 105d is located is
When 5d is at the lower limit (solid line), it is about eight times as much as when it is at the upper limit (dotted line).

Next, water channel 105A by this lower structure
Or, the origin of the void space in the quarter rod 105B will be explained.

Water channel 105A or water rod 10
The pressure difference that drives the flow rate of 5B (referred to as a water rod in the car in the following description) is defined as ΔP. This ΔP is expressed by the following equation.

△p=p, -P2 ・ ・ ・ (1) or ΔP=ΔP1+△P, +△Pw ・・・(2) However, P
,: Pressure at the water rod inlet inch channel (inside the channel box and outside the quarter rod).

P2: Pressure in the inch channel at the water rod outlet ΔpH: Pressure drop at the water rod inlet ΔPb: Pressure drop at the water rod outlet ΔPw: Pressure drop at the straight pipe section of the water rod In the above equation (2), 62w indicates a small flow velocity within the water rod. Therefore, ignoring frictional pressure loss and acceleration pressure loss, it can be approximated as 8P, = △ph = ρL (3).

However, ΔPh: Hydrostatic head L: Straight pipe length of water rod (to be precise,
Distance between coolant inlet and outlet) ρ: Average coolant density of Niraota Lorado Zarani ρ is (
2) From equations (3), it can be expressed by the following equation.

ρ=1/L(8P-(ΔP1 +ΔPb))・
(4) Here, ρ is an average value including the volume of voids, if any.

On the other hand, when the flow rate is low, the inch channel pressure P1 at the water rod inlet becomes small, and the quarter rod flow rate drive pressure difference ΔP also becomes small. Referring to equation (4), at this time, the sum of pressure losses at the water rod inlet and outlet, △P,
It can be seen that if +ΔPb is set to a sufficiently large value, ρ can be made small. As ρ decreases, steam is generated in the quarter rod due to gamma ray overheating, neutron moderation, and heat transfer from the water rod wall, resulting in the existence of a void space. Sum of pressure losses ΔP at water rod inlet and outlet
, +ΔPb can be set to a sufficiently large value by setting the pressure drop ΔP at the quarter rod inlet, that is, by setting the float valve.

In addition, in the conventional fuel assembly, in the normal operating range (area surrounded by lines 202, 204, and 207 shown in Fig. 8 above), on the condition that no voids are generated in the water rod, the inside of the water rod is Since the configuration was such that the cooling water is filled with cooling water, even during low output/low flow operation, the rate at which the sum of the pressure drops at the water rod inlet and outlet, ΔP, +ΔPb, decreases is smaller than the rate at which the flow drive pressure difference ΔP decreases. is large, making it difficult for void spaces to occur, and even if they did occur, the amount would not be sufficient to obtain a practical spectral shift effect.

Next, a method of operating a boiling water reactor according to the present invention will be explained.

In the operating method of the present invention, load following operation is performed, but
Adjustment of output according to load following is performed by core cooling water flow rate control. In this case, the lower structure of the water rod causes a spectral shift effect at low power and low flow rates.

Therefore, fuel can be burned economically while performing load following operation. It should be noted that the output adjustment pattern of typical daily load following operation is the so-called t4h-th
-8h-1h method. That is, after holding 100% rated output for 14 hours, lower output, e.g. 65% of rated output.
%, maintain that low output for 8 hours, and then return to 100% output in 1 hour, repeating this pattern.

Furthermore, during the load following operation, there is also a method of simultaneously performing the flow rate control spectrum shift operation of the prior art when operating near the rated output. In this case, in addition to the spectral shift effect of the conventional flow control spectral shift operation, the spectral shift effect of the load following operation described above can be obtained, and a high spectral shift effect can be achieved.

Next, the flow rate in the water rod and the generation of voids, which are important constituent factors of the present invention, will be explained with reference to the output/flow rate map shown in FIG.

FIG. 5 shows the trajectory of the operating output and flow rate during spectrum shift operation and load following operation by flow control of the boiling water reactor of the present invention, and also shows the void volume fraction. In this case, 92 fuel assemblies employed in the present invention are loaded on the outermost periphery of the core and 672 on the inner periphery, making the total number of fuel assemblies loaded in the entire core 764 (672+92). .

In FIG. 5, 301 is a flow rate control spectrum shift operation with rated output, and 302 is a load following operation with flow rate control (output 100 to 65%, flow rate).

0 to 40%). Further, 303 indicates the volume average in the water rod of voids generated in the water rod for 672 fuel assemblies on the inner circumference side of the core, and 304 for 92 fuel assemblies on the outermost circumference of the core.

Referring to the void ratio 303 of the fuel assembly in the inner periphery of the core,
At low flow rates, the void ratio is approximately 25%, but at rated flow rates, it is slightly over 5%.

On the other hand, the void ratio 304 of the fuel assembly at the outermost periphery of the core is 2 to 3% larger than the void ratio 303 at the inner periphery of the core over the entire range of the core cooling water flow rate. This is because the power decreases at the outermost periphery of the core, so cooling water flows into the fuel assembly through an orifice (not shown, but a core component attached to the fuel support installed below the lower tie plate 108 in Figure 3). This is because the entire flow rate is controlled to be low. Therefore, in the configuration of the present invention, an increase in the void ratio at the outermost periphery is unavoidable. However, the number of loaded fuel assemblies at the outermost periphery is approximately 10 in the entire core.
%, the effect of the present invention can be obtained with about 90% of the fuel assemblies loaded on the inner circumferential side.

In addition, 305 in FIG. 5 is a change in the void ratio 303 of the fuel assembly on the inner periphery of the core, converted into an inch channel void ratio. The conversion formula is as follows.

αwXAv/At However, α: Equivalent void ratio in the inch channel converted from Nira Ota Rado void ratio 6w: Average void ratio in the water rod Aw: Flow passage area in the water rod A1: Flow passage area in the in-channel (water As is clear from this characteristic curve 305, in the present invention, the effective void ratio increases at low flow rates, and by performing flow rate control load following operation, approximately 5% of the inflow is reduced. A spectral shift effect corresponding to a change in the void ratio within the channel can be obtained. This is a significant effect considering that the effect of conventional flow rate control spectrum shift operation is approximately 5% at most. This is because when performing flow rate control load following operation in the boiling water reactor of the present invention, if conventional flow rate control spectrum shift operation is also performed, the combined effect of both operations will result in a maximum void rate change of 10%. This is because a corresponding spectral shift effect is achieved.

For reference, FIG. 5 also shows quarterrod void ratio changes 306 and 307 when 764 conventional 9×9 fuel assemblies are loaded into the core in the same manner as above. 306 is 3 for the 672 fuel assemblies on the inner circumferential side.
07 shows the outermost fuel assembly 92. Note that the conventional 9X9 type fuel assembly here has the same structure as that shown in columns A and B of Table 1, except that it does not have a lower structure like the present invention.

In the flow rate control range of about 40 to 105% of the rated flow rate, the void ratio change 306 on the inner peripheral side is only about 5% change in the void ratio in the water rod, and is about three times or more larger in the present invention.

[Effects of the Invention] Since the present invention is configured as described above, it produces effects as described below.

Means for narrowing the opening area of the cooling water inlet in response to a decrease in the core cooling water flow rate, in the cooling water inlet at the bottom of the large diameter water channel (or large diameter water rod) of the fuel assembly loaded in the reactor core. By using this method to appropriately set the opening area of the cooling water inflow path, voids can be actively generated within the water channel (or 6-water rod) during low flow rate operation, producing a spectrum shift effect. is possible.

Further, as described above, this boiling water reactor is effective in low flow rate operation, and therefore is particularly suitable for flow rate control load following operation that accompanies low flow rate operation. In this case, by setting the opening area of the cooling water inlet passage so that a void is generated in the water channel (or quarter rod) at a predetermined low flow rate, the spectral shift effect can be achieved while performing load following operation. can get.

Furthermore, by performing the flow rate control load following operation and the flow rate control spectrum shift operation in combination, the maximum spectrum shift effect can be obtained and an excellent operation method with high fuel economy can be achieved.

[Brief explanation of the drawing]

Figures 1 and 2 are cross-sectional views of a 9x9 fuel assembly to be loaded into the boiling water reactor of the present invention, and Figure 3 is the interior of the 9x9 fuel assembly to be loaded into the boiling water reactor of the present invention. Explanatory diagram,
FIG. 4 is a sectional view showing the lower structure of the water channel (or water rod) used in the fuel assembly shown in FIGS. 1 to 3, and FIG. Diagram showing the characteristics of output, flow rate, and void ratio. Figure 6 is a cross-sectional view showing 1/4 of the core of a boiling water reactor. Figure 7
The figure is a sectional view of a conventional 8x8 type fuel assembly. FIG. 8 is a diagram showing flow control characteristics of a boiling water reactor. 105A...Water channel 105B...Water rod 105a...Lower end plug 105b...Water channel outlet 105c...
Population 105d...Float valve 105e...Stopper 105f...Valve chamber Note that the same reference numerals in each figure indicate the same or equivalent parts. Representative Patent Attorney Tadashi Sato 1st Table 104: Fuel rod 105A: Large diameter tube channel 106: Channel box 104: Fuel rod 105B: Large diameter quarter rod 106: Channel box 101: Fuel assembly 107: Control rod No. 3 No. 4 Fig. 105a Lower end plug 105c: Inlet + 05d + Float valve 105e Stopper 105f Valve chamber ~ 106 101: Fuel assembly 104: Fuel rod 105c: Water rod 106: Channel port, box 107: Control rod N 8 Figure core flow rate Cm)

Claims (1)

[Claims] 1. In a boiling water reactor whose output can be adjusted by controlling the flow rate of core cooling water, a plurality of fuel rods in the center of a 9x9 square lattice array are cooled internally from the bottom to the top. The fuel assembly is replaced with a rectangular large-diameter water channel or a round large-diameter water rod that allows water to flow, and the core cooling water A boiling water nuclear reactor characterized by comprising means for narrowing an opening area of the cooling water inflow path in accordance with a decrease in flow rate. 2. In the operation of the boiling water reactor according to claim 1, when the output during load following operation is controlled by core coolant flow rate control, the inside of the water channel or water rod is inside the core during rated power operation. Near the rated flow rate of cooling water,
The cooling water is filled with cooling water without generating steam voids, and at low flow rates of cooling water in the core during low-power operation, steam voids can occur at around these rated flow rates and at the prescribed cooling water in the core. A method for operating a boiling water reactor, characterized in that the opening area of the cooling water inflow path is set by the means in accordance with the water flow rate. 3. In the output range near the rated output of the load following operation, the core coolant flow rate at the beginning and middle of the operation cycle is approximately 90% of the rated flow rate, and the core coolant flow rate at the end of the operation cycle is approximately 105% of the rated flow rate. 3. The method of operating a boiling water reactor according to claim 2, further comprising performing spectrum shift operation by controlling the flow rate of core coolant within a range of about 90 to 105% of the rated flow rate.