JPS6336189A - Fuel aggregate for boiling water type reactor - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a fuel assembly for a boiling water source nuclear reactor, and in particular, to a fuel assembly for a boiling water source nuclear reactor.
The present invention relates to a fuel assembly suitable for ensuring a sufficient nuclear thermohydraulic stability margin.

[Prior art]

The flow-output operation map of the second cause Ks water source reactor is shown. Boiling water is started in the reactor along the natural circulation line and forced circulation line shown in solid lines in the figure. That is, during natural circulation operation, the control rod is gradually withdrawn, and when the control rod pattern of rated output is achieved (corresponding to point A in the figure), the pump is rotated and switched to forced circulation. However, the flow rate -
High output, low flow area of the output operation map (upper left of the diagram)
Since there is an unstable region in , there is a problem that the stability margin becomes small at point A where the natural circulation line is switched to the forced circulation line. Therefore, in conventional nuclear reactors, the third
As shown in the figure, an orifice 3 is provided in a fuel support fitting 2 at the port of the fuel assembly 10. FIG. 4 shows the effect of this orifice. The horizontal axis of the figure shows the flow rate, and the vertical axis shows the reduction ratio, which is an index of stability. As defined in Figure 5, this attenuation ratio γ is the ratio of adjacent amplitudes of oscillating quantities (e.g. coolant flow rate, output, neutron flux, etc.) (in the figure, the ratio of the first amplitude to the third amplitude). ).

X.

γ = □ X.

That is, when the width reduction ratio is 1 or more, the vibrating part width increases with the passage of time, making it unstable; when the width reduction ratio is 1 or less, the vibration amplitude is attenuated, making it stable.

γ ≧ 1 Unstable γ Ku 1 Stable By the way, the relationship between this width reduction ratio and the resistance coefficient of the orifice 3 (hereinafter referred to as the orifice coefficient) is as shown in Figure 4. There is a dodge in which the reduction ratio decreases and the stability margin increases. However, if an orifice with a large orifice coefficient is used, the stability margin increases under natural circulation conditions, but at the rated point, since this coefficient is large, flow resistance also increases and the flow rate decreases. Therefore, it is necessary to use a pump with a large pump capacity, which increases the cost. Therefore, orifices currently used in nuclear reactors have a width reduction ratio of about 0.9 at the natural circulation point. A related device of this type is fi, patent application No. 55-127247, but this has a movable part that reduces the orifice diameter during natural circulation, so it has the disadvantage of poor reliability.

[Problem that the invention seeks to solve]

As mentioned above, the conventional technology has problems such as high costs because it is necessary to use one pump with a large capacity in order to ensure sufficient stability margin at the natural circulation mound point, or the need for moving parts. There was a problem in that the reliability of the device was low.

An object of the present invention is to provide a fuel assembly for a boiling level nuclear reactor that increases the stability margin of the reactor, has a simple structure, is highly reliable, and can reduce costs.

[Means for solving problems]

The above purpose is to provide water rods in the fuel assembly with
A column having a circular or oval cross section that crosses the flow of the coolant flowing through the water rod is fixed near the coolant inlet of the water rod, so that the inside of the water rod is
This is achieved by filling the cooling water with liquid phase during rated operation and with two-phase flow during natural circulation operation.

[Effect]

In order to improve the reactivity of the fuel, the Okita water source reactor has a water rod placed in the center of the fuel assembly, and is designed so that single-phase (liquid phase) cooling water always flows through the water rod. has been done. However, the inventors have discovered that the need for single-phase cooling water in the water rod is only during rated operation, and that it is more convenient to have two-phase cooling water during natural circulation operation. Ta. That is, when the state inside the water rod becomes a two-phase flow, the conversion of fast neutrons generated in the reactor core into thermal neutrons is inhibited, and therefore the reactivity also decreases, resulting in a decrease in output. In natural circulation operation, the output is increased by withdrawing the control rod, but for the same position of the restraining rod, the output will be lower if the condition inside the water rod causes a two-phase flow. Therefore, if natural circulation operation and forced circulation are made into a two-phase flow state, the output at switching point A can be expected to decrease, and the forced circulation line moves downward as shown in Fig. 6. As a result, the switching point A moves away from the unstable region, so the stability margin increases.

Figures 7 and 8 show quantitative evaluations of this output drop and stability margin. As is clear from the figure, as the void fraction in the water rond increases, the conversion of thermal neutrons decreases, resulting in a decrease in output, and the reduction ratio of the stability index also decreases, increasing the stability margin. I understand that. Note that this analysis result is for the 8X8 fuel east combination currently in operation. The shape of the fuel rods is changed to Mli < L, and the number is increased to 9.
In the case of X9 bunker combinations, larger diameter water rods are expected to be used, and these effects will be further enhanced as the water rods become thicker.

From the above results, if it is possible to change the condition in the water rond to a two-phase flow during natural circulation operation and to make the condition in the water rond a phase flow at the rated point, the stability margin will increase. I understand that.

The present invention makes this possible.

That is, the above-mentioned thing becomes possible by providing the above-mentioned columnar body in the water rod as in the present invention.

Hereinafter, a case where the columnar body is a cylinder will be explained. (However, the same effect can be obtained by using an elliptical cylinder instead of a cylinder.

) The flow around the cylinder is as shown in Figures 9 and 10.
Depends on flow rate. In other words, at low flow velocities, the boundary layer on the cylinder surface becomes a laminar boundary, and the drag coefficient increases; however, at high flow velocities, the boundary layer on the surface shifts to a turbulent boundary layer, as shown in Figure 9. As shown, the drag coefficient decreases.

The present invention takes advantage of this property. That is, at the rated point where the flow rate is high, the boundary layer on the surface of the cylinder 6 is a turbulent boundary layer, and at the natural discharge point where the flow rate is low, it is made to correspond to a laminar boundary layer. Furthermore, if the resistance coefficient of the cylinder group 6 at the rated point is made to be approximately the same value as the resistance coefficient in the conventional water rond (using equation (3) described later), then at the rated point, the water a nod Inner state: Silver becomes single-phase water, so the reactivity is almost the same as conventional products, and it shows almost the same customization. On the other hand, during natural circulation at a low flow rate, the state around the cylinder becomes a laminar boundary layer, and the drag coefficient increases. Therefore, the flow rate inside the waterrond decreases, and γ from the fuel
Due to wire heating, voids occur, resulting in two-phase flow and hollowness. When the inside of Waterrond becomes a two-phase flow state, as mentioned above,
As the reactivity within the fuel assembly decreases, the output decreases, and the output also decreases when switching from natural circulation operation to forced (shield) operation.As a result, the stability margin increases and expensive pumps with large pump capacity are unnecessary. Therefore, the cost is reduced.

The phenomenon of flow around a cylinder described above is a result of an infinite system, so I? It is necessary to confirm that the same phenomenon occurs within the country as well. Therefore, an apparatus as shown in FIG. 11 was manufactured and an experiment was conducted. The measurement items are flow rate F, temperature T, and pressure drop ΔP in the trial and exchange parts.

The specimens in the test section used were one with - rows of cylinders and one with two rows of cylinders, as shown in Figs. 12(a) and (b). In the experiment, the pressure drop ΔP at that time was measured by changing the degree of the valve V to obtain a positive flow.

Based on those measured values, the following dimensionless quantities were calculated and organized.

Drag coefficient Cd Reynolds number Re where, UOBS: Flow velocity on the cylinder surface = F' / (AFLOW-AOBS) AFLOW
: Flow path area on the upstream side of the cylinder AOBS : Total area of the cylinder facing the flow direction DOBS : Total equivalent diameter of the cylinder g : Gravitational acceleration : Kinematic viscosity coefficient.

In addition, between the resistance coefficient of the test part and the drag coefficient Cd,
The following relationship holds true.

The actual results are shown in Figures 13 and 14. Figure 13 shows
- Unity of columns of cylinders, and Figure 14 shows the results for two columns of cylinders. Due to the transition of the boundary layer, when a certain Reynolds number is exceeded, the pressure loss coefficient rapidly decreases as shown by B in the figure. It can be seen that the width of the decrease is large for two rows of cylinders. According to the above experimental results, if an object with a smooth circular or elliptical surface is installed in a pipe, the boundary layer will change due to the flow velocity K, and the drag coefficient will change. I understand. Therefore, if an object with such a smooth surface is installed inside the water rod, the resistance coefficient will change between the rated point and the natural circulation point, and the resistance coefficient will change between the rated point and the natural circulation point. In a state of flow,
During forced circulation, a single phase of water can be created. As a result, during forced circulation, the inside of the water rod becomes a single phase of water, exhibiting the same characteristics as before.On the other hand, during natural circulation, the inside of the water rod becomes a two-phase flow state, reducing the reaction time and reaching switching point A. The stability margin increases because the output of

〔'Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention.

In this embodiment, a water rod 4 located at the center of the fuel assembly is projected to the lower side of a lower tie plate 5, its lower end is open, and a group of cylinders 6 spaced apart from each other are arranged inside the water rod near the lower end. It was arranged by Yokoe. The resistance coefficient for the flow rate of the cylinder group 6! The characteristics are shown in Figure 9, 1
As shown in FIGS. 3 and 14, the amount of water immersed is small at high flow rates, and large at low flow rates. Therefore, water rod 4
K υ, that the cylinder n6 was placed inside? During rated operation with a large amount of M, the resistance decreases and the flow rate in the water rod 4 increases. When the flow rate increases, the generation of voids within the water rod 4 is further suppressed, the interior of the water rod enters a level phase state, and exhibits the same characteristics as before.

On the other hand, during natural circulation with a low flow rate, resistance increases, so the flow rate in the water rod decreases (see Figure 15). As a result, voids are generated in the water rod due to the heating of γ1@, and the inside of the pipe becomes a two-phase flow state (first
between 6 and 6 times). When the inside of the water rod becomes a two-phase flow state, the reactivity decreases and the output also decreases, so the output at the switching point A between natural circulation operation and forced circulation operation also decreases, and the sixth
As can be seen from the figure, the stability margin increases. When the stability margin increases, a pump with a large pump capacity is not required to ensure the flow rate at the rated time, so costs can be reduced.

Further, in this embodiment, since the υ resistance coefficient is changed by self-control of the fluid, there are no moving parts, which has the advantage of being highly reliable and maintenance-free.

Further, according to this embodiment, the above-mentioned increase in stability margin can be used for spectrum operation.

In other words, if the output at switching point A, which changes from natural circulation operation to forced circulation operation, is the same as the current output, the forced circulation line will move in parallel upwards at K, as shown in Figure 17, and the rated output point will also change. Low flow f 41111 K move. Therefore, as shown in the figure, the range of flow rate control at rated output is expanded. Spectral operation utilizes the flow rate control range at the rated output, and as the flow rate control range widens, the effect of extending the burnup further (the effect of bringing the curve to the right in Figure 18) increases. It is refused to go up. That is, by widening the flow rate control range by one inch, it is possible to reduce the cost by about 100 million yen.This spectrum operation will be explained below using FIG. 18. Spectral operation is to harden the initial burnup K (neutron) spectrum, and to
This is a method of synthesizing plutonium-239, softening the spectrum at the end of the burnup, and burning the synthesized plutonium-239 at the beginning of the burnup, resulting in a higher burnup than K. Therefore, it is necessary to change the hardness of the spectrum between the early and late stages of burnup, and to do this, the core flow rate is generally changed during operation. In other words, at the early stage of burnup, the flow rate is lowered to increase the void ratio in the core, making the spectrum hard and sharp.
On the other hand, at the end of the combustion phase, the flow rate is increased to lower the void ratio in the furnace, softening the spectrum. Therefore, when the flow rate control range at the rated output is wide, the change in spectral hardness is also large, and the burnup can be further increased. In this example, the resistance coefficient during natural circulation is approximately twice as large as that at the rated value, so the void ratio in the water rod during natural circulation is approximately 20%, and the output is reduced by 80 degrees (fuel assembly (per body).

Therefore, if we calculate the flow rate reduction rate of the rated output from this 80% value, it will be about 5%, which is about 0.
It can be seen that the cost was reduced by 800 million yen.

Note that this analysis result is for the case of 8×8 fuel assembly, but in the case of 9×9ja combination, a large diameter water rod is used. This large diameter water rod requires about three times the flow path area of the water rod of the 8x8 fuel assembly, so
The cost reduction will be approximately 2.4 billion yen.

FIG. 19 shows another embodiment of the present invention. In this example,
The inlet orifice (see Fig. 3) conventionally provided in the fuel support fitting 2 is eliminated, and a net-like inlet orifice 8 is provided below the fuel assembly, and the water rod 4 in the center of the fuel assembly is inserted into the orifice 8. protrude to the bottom of the
The lower end of the water rod 4 is open, and cylinder groups 6 are arranged laterally within the water rod 4 near the lower end at intervals. With this structure, the pressure difference applied to both ends of the water rod 4 includes pressure loss at the inlet orifice, and therefore varies greatly between natural circulation and rated operation (see Table 1). As a result, the flow rate inside the water rod is determined by the pressure difference between the two ends, so the flow rate during natural circulation is significantly reduced, which increases the amount of voids inside the water rod and reduces the reactivity within the channel. Go down. Therefore, the output at the time of switching also decreases, and the stability margin increases significantly.

Table 1 Pressure loss unit (atmospheric pressure) In this example, since the mesh-shaped inlet orifice 8 is installed below the fuel assembly, the flow rate distribution in the radial direction within the fuel assembly is uniform, and there is a thermal margin. Move in the direction of increasing leeway in terms of degrees. Further, as in the embodiment described above, the increase in stability margin can also be used for spectrum operation.

FIG. 20 shows yet another embodiment of the present invention.

In this embodiment, as in the embodiment shown in FIG. It is arranged. With such a structure, as in the embodiment described in @, the resistance coefficient around the cylinder increases during natural circulation, so the flow rate in the water rod 4 decreases and voids occur. As a result, the reactivity decreases and the output at switching point A decreases, so the stability margin increases. Furthermore, in this embodiment, since a conical cylinder group 9 is provided at the tip of the lower end of the water rod 4, this serves as a guide when assembling the fuel assembly, making the installation of the water rod 4 easier. becomes easier,
Damage to the spacer 1 lower tie plate 5, etc. can be prevented. Further, as in the embodiment described above, the increase in the stability margin can also be used for spectrum operation.

FIG. 21 shows yet another embodiment of the present invention.

In this embodiment, the lower end of the water rod 4 in the center of the fuel assembly is closed, and the size of the inlet 1o on the side is made sufficiently large to eliminate resistance there.
In the vicinity of the fourth water rod, the flow rate in the water rod decreases and voids occur. When this is exhausted, the reactivity decreases and the output at the time of switching decreases, so the stability margin increases. Furthermore, in this embodiment, since the water rod 4 has the same shape as the conventional one and connects to the lower tie plate 5, it has the advantage that it can be easily backfitted to an existing plant.

FIG. 22 shows yet another embodiment of the present invention.

In this example, as in the example shown in Fig. @1, the water rod 4 in the center of the fuel assembly protrudes to the bottom of the lower tie plate, the lower end is open, and the inside of the water rod is shaped according to the flow velocity. Obstacles 12 whose values change are provided laterally at intervals. That is, when the flow velocity is high, it takes on a streamlined shape as shown in Figure (a), and when the flow velocity is low, it changes to a shape that becomes an obstacle to the flow as shown in Figure (b). Due to this change in shape, the resistance becomes small at the rated point where the flow rate is large, while the resistance becomes large during natural circulation where the flow meter is small, so that the same effect as in the previous embodiment is achieved.

Furthermore, in this embodiment, since the shape of the obstacle 12 can be changed significantly, in an extreme case, the inside of the water rod can be filled with only steam during natural circulation, and the effects of the present invention can be further increased.

〔Effect of the invention〕

According to the present invention, the state inside the water rod becomes a two-phase flow during natural circulation when it decreases, and the reactivity of the aggregate decreases, so the output when switching between natural circulation and forced circulation decreases, and there is a stability margin. This has the effect of increasing However, with this, there is no need to use a pump with a large pump capacity to maintain the flow rate at the rated time, and if the pump capacity is kept the same, the flow control range can be expanded by the increase in the stability margin. It becomes possible to use it for spectrum operation and increase the burnup.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing a flow rate-power operation map of a conventional boiling water source reactor, and Fig. 3 is a diagram showing an embodiment of the present invention.
Figures (a) and (b) are diagrams showing the conventional example near the inlet of the fuel assembly, Figure 4 is a diagram showing the relationship between the orifice coefficient and width reduction ratio, and Figure 5 (a) and (b) are diagrams showing the relationship between the orifice coefficient and width reduction ratio. Diagram showing the definition of width ratio,
FIG. 6 is a diagram showing a distribution-output operation map according to the present invention;
Figure 7 is a diagram showing the relationship between the void ratio in the water rod and the output per fuel assembly, Figure 2g8 is a diagram showing the relationship between the void ratio in the water rod and the width reduction ratio, and Figure 9 Figure 1 shows the relationship between Reynolds number and drag coefficient.
Figures 0 (a) and (b) are diagrams explaining the boundary layer transition around the cylinder, Figure 11 is a diagram explaining the coating +1, and Figure 12 (a) and (b) are diagrams showing the test specimen. Figure 13, 1st
Figure 4 is a diagram showing the experimental conclusion, Figures 15 and 16 are diagrams showing the relationship between the resistance coefficient, flow velocity, and void ratio in the water rod, and Figure 17 is a diagram showing the relationship between the resistance coefficient in the water rod and the flow rate according to another embodiment of the present invention. FIG. 18 is a diagram showing an output operation map, FIG. 18 is a diagram explaining spectrum operation, FIG. 19 is a diagram showing another embodiment of the present invention, and FIG.
0 is a diagram showing still another embodiment of the present invention, FIG. 21 is a diagram showing still another embodiment of the present invention, FIG. 22(a),
(b) is a diagram showing still another embodiment of the present invention. Explanation of symbols 1...Fuel Togane body 2...Fuel support fittings 3.
・・Orifice 4・Water rod 5・
...Lower tie plate 6...Cylinder group 7...Fuel! ! 8...Mesh-shaped orifice 9...Conical column group 10...Inflow port 12...Obstacle. Kota Tani ΩB □ 11. j 4.-Us-Tarod 6--Enbei-o group 5
-Lower %plate 7-・Fuel supply Fig. 2 Flow rate (%) Fig. 4 Flow rate (%) Fig. 6 Flow rate (%9 ) One row A joint specimen Two row A joint specimen Drag coefficient CD Figure 14 Comparison of Inols Figure 15 Figure 16) Relative value of redundancy coefficient Figure 17) Hazardous quantity (%) Figure 18 Burnup (GWD/l) ) Fig. 19 Fig. 20 Fig. 5--lower tie flute 9---Cylinder-shaped Entoou-kun ￥ Fig. 21-Cylindrical group Fig. 22 Rating) as a quantitative natural circulation circle

Claims (1)

[Claims] 1. A boiling water type atom characterized in that a columnar body having a circular or elliptical cross section that crosses the flow of coolant flowing through the water rod is fixed to the water rod near the coolant inlet of the water rod. Fuel assembly for reactor. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein the coolant inlet of the water rod is an open lower end of the water rod that projects downward from the lower tie plate. 3. The fuel assembly for a boiling water nuclear reactor according to claim 1, wherein the coolant inlet of the water rod is a sufficiently large opening provided on the side surface of the water rod, and the lower end of the water rod is closed. 4. The fuel assembly for a boiling water reactor according to claim 2 or 3, wherein the column is provided inside the water rod. 5. The fuel assembly for a boiling water nuclear reactor according to claim 2, wherein the column is provided in a conical arrangement at the lower end of the opening of the water rod. 6. Claims 2, 4, or 6, wherein the fuel assembly has a mesh-like orifice directly below the lower tie plate, and the open lower end of the water rod protrudes below the mesh-like orifice. The fuel assembly for a boiling water source nuclear reactor according to item 5.