JPH032691A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To prevent a generation of voids in a water pipe completely and to keep a burning condition which has beem presumed at a designing phase, even under a low power and a low flow operation by arranging a solid moderator material at a part corresponding to an upper part of an effective fuel length of a fuel assembly. CONSTITUTION:A capsule (a container) 2g containing a solid moderator material (a zirconium hydride) 2f is fixed to a region corresponding to an upper part of an effective fuel length in a water channel 2A. An axial length of the solid moderator material 2f or the capsule 2g corresponds to around 15% of the effective fuel length. In a fuel assembly 10 being constituted in this way, a space beneath the container capsule 2g of the solid moderator material in the water channel 2A is unfailingly filled by not-boiling water even in an output condition of a 60% flow and a 70% output and therefore its burning condition is coincided with a presumed one at a designing phase and there is no possibility to degrade a reliability of a designing calculation.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a fuel assembly for a boiling water reactor (BWR), and more specifically, to a fuel assembly for a boiling water reactor (BWR), and more specifically, to a fuel assembly for a boiling water reactor (BWR). This invention relates to a fuel assembly that uses both cooling water and a solid moderator as a moderator.

[Prior Art] This application mainly deals with the flow rate of cooling water flowing through the water pipes of a BWR fuel assembly. First, we will discuss the configuration of the fuel rods, water pipes, and the entire fuel assembly of a conventional BWR fuel assembly. Second
This will be explained with reference to the figures.

<Fuel rod> Figure 2 (a) shows the configuration of the fuel rod.

The fuel rod 1 is constructed by filling a zircaloy cladding tube 1b with sintered uranium dioxide pellets 1a, and arranging a blennium spring 1C in the blennium region above the cladding tube 1b, and using an inert gas with good thermal conductivity ( helium), and the upper and lower ends of the cladding tube 1b are filled with an upper end plug 1d and a lower end plug 1e, respectively.
It is constructed by sealing and welding.

Some of these fuel rods 1 have upper and lower end plugs 1d.

1e is a connecting fuel rod with a threaded portion, and its lower threaded end plug is connected to the lower tie plate 4 (see Figure 2 (c)).
is coupled to a corresponding threaded lower end plug insertion hole. on the other hand,
After installing the rod spring, the threaded upper end plug passes through the end plug insertion hole of the upper tie plate 3 and is connected with a nut 7.

Other fuel rods are bundled between the upper and lower tie plates 3.4 by inserting the upper and lower end plugs into the lower tie plate end plug insertion holes.

<Water Pipe> Figure 2 (b) shows the configuration of a water pipe (in the illustrated example, a water tube).

Similar to the fuel rod 1, the Autororad 2B has upper and lower end plugs 2d and 2e welded to the upper and lower parts of the main body, and these end plugs 2d,
Unboiled water flows from the bottom to the top through holes 2a and 2b provided near holes 2e.

<Fuel assembly> FIG. 2(c) shows the overall schematic configuration of the fuel assembly.

The fuel assembly, generally designated by the reference numeral 20, has its outer periphery covered with a channel box 6, and the lower end of the channel box 6 is positioned below the upper surface of the lower tie plate 4.

Note that this channel box 6 is connected to the upper tie plate 3.
One of the posts 9 at the four corners of the top surface is fixed with screws to wood and channel fasteners (not shown). In addition, the fuel rod 1 and the autorod 2B are the fuel assembly 20.
The mutual spacing is maintained by spacers 5 provided in several stages in the axial direction.

In the fuel assembly 20 configured in this manner, the cooling water that has flowed in through the opening of the lower tie plate 4 is reduced except for the amount that leaks from the leak hole 4a of the lower tie plate 4 and the gap between the lower tie plate 4 and the channel box 6.
It flows from the bottom to the top of the fuel assembly 20 to remove heat generated by the nuclear reaction and also acts as a neutron moderator.

The reason why the above-mentioned Autororad 2B is used together as the cooling water flow path is as follows.

The horizontal power distribution within the fuel assembly 20 is not uniform. In other words, in the periphery of the aggregate where there is a large amount of unboiled cooling water, the neutron moderating effect is large (the thermal neutron flux is large), so the output increases, whereas in the central area of the aggregate where there is less boiling and liquid cooling water. output tends to be low. Therefore, the above-mentioned water rod F2B is placed in the center, and the cooling water flowing through it increases the neutron moderation effect, and the fuel rods in the center
The horizontal power distribution is made uniform by increasing the concentration of the power.

<Water pipe flow rate> Next, the flow rate (flow rate) of cooling water flowing through the water pipes will be described.
The explanation will be made using the example shown in B as an example of an autarorad.

It is desirable that the cooling water flowing through the water rod be in an unboiled state until it reaches the outlet, so that the heat of the nuclear reaction flowing through the water rod,
It is necessary to ensure a sufficient flow rate, taking into consideration the increase in enthalpy due to the heat generation of the autororad and the heat transfer from the outer surface of the autororad. Here, the rated operating condition (100% output, 100% flow rate) is usually selected as the reactor operating condition to be assumed when determining the autororad flow rate.

Further, the amount of water flowing through the two-wheeled roller is determined by the pressure loss during the inflow and outflow days of the two-wheeled roller, which will be described below.

The pressure loss △P during the inflow and outflow on the outside of the outer roller is: (1-1) Local pressure drop △PEL of the spacer (1-2) Friction pressure loss △pcr of the fuel rod, the outer surface of the outer roller, and the inner surface of the channel box (1-3 ) Accelerated pressure loss △P due to volume increase from liquid phase to vapor phase (1-4) Hydrostatic head during inflow and outflow outside the water rod △P0. .

There is a sum of △PE=△PEL+△Po, +△PEA+△PEI ((
I) becomes.

On the other hand, pressure loss △P during inflow and outflow inside the water rod
I is (2-1) Local pressure loss △PIL at the entrance/exit (2-2)
Frictional pressure loss on the inner surface of the water rod △ PIF (2-3) Hydrostatic head during inflow and outflow days △PIH (2-4) Accelerated pressure loss due to cooling water volume change during inflow and outflow days △PIA Sum of △PI = △PIL + △ PIF+△PIH+△PIA(I
I), and between formula (I) and (II), △PE=
The relationship △PI holds true.

[Problem to be solved by the invention] In recent years, as the share of nuclear power generation in total power generation has increased, it has become necessary to change the operation of nuclear power plants to daily load following operation that matches the day and night power demand. .

For the daily load follow-up operation of a BWR, recirculation flow rate control is generally adopted in which output is controlled by changing the core recirculation flow rate (by changing the density of cooling water that also serves as a neutron moderator). As an example of daily load following operation using this recirculation flow rate control, at a high output level of 95% and a low output level of 70%, the high output retention time is 14 hours, the low output retention time is 8 hours, and the output rises and falls for 1 hour each (so-called 1 hour). 4h-1h-8h-1h
Figure 3 shows the relationship between reactor output and flow rate when the method (method) is repeatedly implemented. In this case, when the flow rate is 60%, the output is 70%.

In such a low flow rate/low power operation, the above-mentioned pressure loss △P on the outside of the overlord becomes small, and the hydrostatic head when the inside of the overlord is filled with non-boiling water (this hydrostatic head is hereinafter referred to as " (referred to as "design static head"). In this case, the flow rate of cooling water flowing through the two-way roller is significantly reduced, the local pressure loss △P and frictional pressure loss △PIF at the two-way roller inlet and outlet become almost 0, the acceleration pressure drop △PIA is also small, and the static head △P and □ are large. become a part. That is, no matter how much you adjust the water rod shape or the local pressure drop characteristics of the cooling water inlet/outlet, it will have no effect, and the pressure loss outside the water rod will be balanced only by the hydrostatic head. The fact that the pressure drop ΔP on the outside of the overlord is smaller than the design hydrostatic head means that a steam section (void space) is generated above the overlord. If combustion continues under these conditions, the enrichment distribution and burnable poison (typically gadolinia) distribution of the fuel assembly, which is designed on the assumption that the quarterlot will always be filled with non-boiling water, will change. Changes due to combustion of fuel deviate from the design values, reducing the reliability of design calculations. In the worst case, a power distribution different from that predicted and calculated may occur, leading to the risk of burnout or damage to the fuel rods.

This tendency increases as the diameter of the two-way rotor is increased (or a large square tube or channel is adopted) with the aim of increasing the burn-up of the fuel.

Here, as an example of void generation in a 1107jKWe class BWR, we will explain the core output when the flow rate is lowered on the rated power-flow rate curve, and the voids that occupy the effective fuel length in the waterlot of the fuel assembly located around and inside the core. Figure 4 shows the proportions of the space. The fuel assemblies located around the reactor core are more restricted than the fuel located inside the core due to the flow rate or orifices attached to the fuel support, resulting in a smaller pressure drop outside the reactor and the generation of voids within the reactor. arise.

The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to prevent changes in iA contraction degree distribution and burnable poison distribution due to combustion even under low flow rate and low power operation. It is an object of the present invention to provide a fuel assembly for a BWR in which the reliability of design calculations such as power distribution is not impaired.

[Means for Solving the Problems] The BWR fuel assembly of the present invention has a plurality of fuel rods arranged in a square lattice arrangement, in which a plurality of fuel rods located approximately at the center of the arrangement are connected to one or more water pipes (e.g. The above objective is achieved by arranging a solid moderator inside the water tube corresponding to the upper part of the effective fuel length of the fuel assembly in a fuel assembly that is replaced with a 2-autolot or 2-autachannel. be.

In this case, the preferred axial length of the solid moderator is about 15% of the effective fuel length.

Alternatively, a configuration may be adopted in which a hole is provided that passes through the solid moderator in the vertical direction, and this through hole and through holes provided in the upper and lower end plugs of the water tube form a flow path that flows through the water tube.

Furthermore, the solid moderator is preferably zirconium hydride (ZrH2).

Note that the solid moderator is not limited to being directly disposed within the water pipe, but may be disposed within the water pipe via an annular container containing the solid moderator.

[Operations] As mentioned above, if the pressure drop ΔPE outside the water pipe is smaller than the design hydrostatic head, the generation of steam in the upper part of the water pipe is unavoidable.

Therefore, in the present invention, a solid moderator is disposed at a location corresponding to the location of the steam generation inside the water pipe. In other words, in the present invention, the position corresponding to the generation position of the conventional steam part inside the water pipe is occupied by the solid moderator, and the presence of the steam part is not allowed.

In the axial segment of the water pipe in which this solid moderator is installed, the composition inside the water pipe is always solid moderator and does not change with output or flow rate, which improves the reliability of design calculations. There is no.

Here, the low flow rate and low output operating conditions are 60% flow rate and 7
Assuming the condition of 0% output, as shown in Figure 4 above, the ratio of void space to the effective fuel length of the water tubes of the fuel assembly placed around the core is 11%, so the axial length of the solid moderator The length may be set to a length corresponding to about 15% of the effective length of the fuel.

In addition, a hole is provided that passes through the solid moderator up and down, and the water pipe is connected from this through hole, a through hole provided at the upper end plug of the water pipe (inlet hole), and a through hole provided at the lower end plug (exit hole). By forming a flow path that flows through the water pipe, the distance between the water pipe's exit holes becomes larger.
Since the pressure loss during that time is reduced, the area of non-boiling water in the water pipe can be increased.

As a material for the solid moderator, zirconium hydride is suitable because it is stable at low and high temperatures, absorbs few thermal neutrons, and has a moderating function.

Furthermore, when using a powdered solid moderator, if the solid moderator is arranged in a water pipe via an annular storage container,
It is possible to prevent solid moderator from flowing out.

In order to further clarify the features and advantages of the present invention, preferred embodiments will be described below with reference to the accompanying drawings.

[Example] FIG. 1 shows an example of the present invention. Here, a high burnup 9
An example in which the present invention is applied to a ×9 type fuel assembly will be shown.

In FIG. 1(A), the basic structure of the fuel assembly, generally designated by the reference numeral 10, is the same as that of the conventional fuel assembly described above, except for the over-channel 2A. Further, most of the fuel rod 1 is omitted from illustration so that the internal structure is clear.

The configuration of the automatic channel 2A, which is characteristic of the present invention, will be described below with reference to FIG. 1(b).

A capsule (container) 2g containing a solid moderator (zirconium hydride) 2f is fixed in a region corresponding to the upper part of the effective fuel length in the water channel 2A. The axial length of the solid moderator 2f to capsule 2g corresponds to about 15% of the effective fuel length.

In general, solid moderators have inferior moderating properties compared to water (cooling water), and also increase the amount of waste. Therefore, the shorter the length in the axial direction, the better. In this example, under the conditions of 60% flow rate and 70% output in the daily load follow-up operation shown in FIG. It is taken into consideration that the ratio of void space is 11%.

On the other hand, the shape of the side circumference of the capsule 2g is made to match the shape of the inner edge of the water channel 2A, so that no unnecessary space is created between the two.

In the fuel assembly 10 configured as described above, the space below the solid moderator storage capsule 2g in the feeder channel 2A is always filled with non-boiling water even under the above-mentioned 60% flow rate and 70% output conditions, for example. Since the conditions are satisfied, the combustion conditions are consistent with those assumed at the time of design, and the reliability of design calculations is not compromised.

In addition, in the illustrated example, the through hole 2 extends through the capsule 2g.
h, and the cooling water that has flowed through the through hole 2h flows out of the water channel 2A from the outlet hole 2b provided in the upper end plug 2d. As described above, this is to increase the pressure loss between the quarter channel person outlet passages 2a and 2b and to increase the non-boiling water area within the quarter channel 2A. Even if this configuration is adopted, the through hole 2h will become a void space under low flow rate and low output conditions, but since the size of the void space is small, the combustion state may differ greatly from the one assumed at the time of design. There isn't. Therefore, there is no risk of significantly impairing the reliability of design calculations.

In the above embodiment, quarter channel 2A is used as the water pipe.
Although an example is shown in which a water rod is used, a configuration using a water rod may also be used. Furthermore, the solid moderator 2f may be suitably solid-molded and placed directly in the water pipe, without being housed in the capsule 2g.

[Effects of the Invention] As explained above, according to the BWR fuel assembly of the present invention, by arranging the solid moderator in the part corresponding to the upper part of the effective fuel length in the water pipe, the void space in this part can be reduced. This prevents the generation of water, and the space within the water pipe below the solid moderator is always filled with non-boiling water. Therefore, even under low output and low flow rate operation, such as when low output is maintained during daily load follow-up operation, the combustion state assumed at the time of design is maintained, and the reliability of design calculations is not impaired.

Here, the axial length of the solid moderator is approximately 15% of the effective fuel length.
%, it is possible to completely prevent the occurrence of voids in water pipes even at the lowest flow rate of daily load follow-up operation that is normally assumed.

Furthermore, if a configuration is adopted in which a flow path is formed through the water pipe from a through hole provided in the solid moderator and an artificial outlet hole of the water pipe, the non-boiling water region within the water pipe can be enlarged.

Furthermore, a configuration may be adopted in which the powdered solid moderator is housed in a storage container and disposed within the water pipe.

[Brief explanation of the drawing]

FIG. 1(a) is a perspective view showing the internal structure of a fuel assembly for BWR according to an embodiment of the present invention, FIG. Figures (a) and (b). (c) are conventional fuel rods, two-way rods, and BW, respectively.
A vertical cross-sectional view showing the internal structure of the R fuel assembly, Figure 3 is a diagram showing the relationship between reactor output and core flow rate during daily load follow-up operation,
FIG. 4 is a diagram showing the void ratio of the space corresponding to the effective length of the fuel in the autororad on the rated output-flow rate curve. [Explanation of symbols of main parts] 1...Fuel rod, 2A...Water channel, 2a...Water channel inlet hole, Water channel outlet hole, ・...Solid moderator, capsule, through hole In each figure, the same reference numerals indicate the same or equivalent parts.

Claims (5)

[Claims] (1) In a fuel assembly in which a plurality of fuel rods in a square lattice arrangement, a plurality of fuel rods located approximately in the center of the arrangement are replaced with one or more water pipes, the effective fuel length of the fuel assembly is A fuel assembly for a boiling water reactor, characterized in that a solid moderator is disposed inside the water tube corresponding to the upper part. (2) The fuel assembly for a boiling water reactor according to claim 1, wherein the axial length of the solid moderator is about 15% of the effective length of the fuel. (3) A hole is provided in the solid moderator that penetrates vertically, and this through hole and a through hole provided in the upper and lower end plugs of the water pipe form a flow path that flows through the water pipe. The fuel assembly for a boiling water nuclear reactor according to claim 1 or 2. (4) The fuel assembly for a boiling water reactor according to any one of claims 1 to 3, wherein the solid moderator is zirconium hydride. (5) The boiling water nuclear reactor according to any one of claims 1 to 4, wherein the solid moderator is disposed within the water pipe via an annular container containing the solid moderator. Fuel assembly for use.