JPS6186676A - Hollow control rod nuclear reactor - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a boiling water nuclear reactor, and particularly to a nuclear reactor equipped with a control rod having a structure suitable for adjusting power distribution and improving fuel economy.

[Background of the invention]

In a boiling water reactor, since the void distribution is maintained in the axial direction as shown by curve 1 in FIG. 2, skewing occurs in the power distribution shown by curve 2 in the same figure. In the lower part of the core, which is the inlet of the cooler, the coolant is in a subcooled state, and as the coolant rises into the core, it enters the subcooled boiling and saturated boiling regions, and in the upper part of the core, where it simmers at the exit of the coolant. The void volume ratio reaches around 70%.

As a result, neutrons heat up more in the lower core than in the upper core, and in boiling water reactors, the power peak position skews toward the lower core. To improve this characteristic,
Conventional boiling water reactors have taken the following measures.

1. Use of shallow control rods (Figure 3) From below the core 1
Control rod 3 inserted shallowly at the output peak position around /4 (
shallow control rod). This method of flattening the power distribution using the shallow control rods 3 locally controls the lower part of the reactor core to flatten the power distribution. As shown in Figure 3,
In the fuel assembly surrounding the control rods, the output is significantly suppressed compared to the upper part, and the progress of burnup is greatly delayed. Therefore, after a certain period of combustion, when the shallow control rod is pulled out due to the next control rod pattern change, etc.
The output peak is concentrated there. In order to suppress this output peak, another shallow supply rod is required, which tends to create a vicious cycle. Thus, power distribution control methods using shallow control rods tend to produce strong lower peaks in a small number of fuel assemblies.

2. Use of axial reactivity differential fuel (Figure 4) Divide the fuel assembly into multiple regions in the axial direction (generally, two regions divided near the center height of the core) and use fuel with the same temperature and void ratio. Below, a fuel 4 in which the infinite neutron multiplication factor in the upper part is higher than the infinite multiplication factor in the lower part is used. By taking such measures,
It is possible to alleviate the skewing of the power distribution toward the lower part of the core that occurs due to the void distribution in the axial direction, and to obtain a flat power distribution. A typical method for creating reactivity differences in the upper and lower axial directions is to vary the average uranium enrichment in the plane perpendicular to the axis of the fuel assembly between the upper and lower parts of the fuel assembly. Since the upper and lower uranium enrichment distributions take advantage of the difference between the upper and lower contents of uranium-235, the effect of controlling the upper and lower reactivity can be maintained even after combustion.

As mentioned above, measures have been taken to correct the skewing of the power distribution caused by the void distribution specific to boiling water reactors. Measures to improve fuel economy have also been called for.

3. Flow rate control spectrum shift operation (Figures 5 and 6) In boiling water reactors, when the void fraction increases, the neutron energy spectrum deteriorates and the amount of plutonium produced increases. This is because, among the isotopes of uranium, uranium-238 absorbs high-energy fast neutrons and is converted into plutonium through a neutron capture reaction. On the other hand, uranium-235 and plutonium-239 produced by the above reaction undergo nuclear fission efficiently using low-energy thermal neutrons.

Reducing the core flow rate increases the amount of voids in the core.

Therefore, by lowering the core flow rate to store plutonium from the W period to the middle period of operation, increasing the core flow rate at the end of continuous operation to collapse the voids, softening the neutron spectrum, and burning the accumulated plutonium, the extraction burnup can be increased. Yes, but with spectral shift due to flow control,
When the flow h1 decreases, the heat removal ability decreases and there is no margin for the thermal limit value, so there are restrictions on flow rate control and the amount of change in voids is limited.

Related to this type of technology, Japanese Patent Application Laid-Open No. 53-178
91 and JP-A-53-17893.

These technologies propose the provision of a gas plenum section to alleviate the thermal influence of the strong neutron absorber on the fuel and to extend the life of the control rods; It has not yet been possible to actively and satisfactorily perform spectrum shift control.

[Purpose of the invention]

The purpose of the present invention is to eliminate the above-mentioned drawbacks, and more specifically, to provide a nuclear reactor equipped with a control rod having a structure that is effective in flattening the power distribution and improving fuel economy. It is.

[Summary of the Invention] The principle characteristics of the present invention are based on four main elements in the control of a boiling water reactor: people, surplus reactivity control B, axial power distribution control C1, radial power distribution control D1, spectrum shift. Hollow control rods are used to perform all operations effectively. A hollow control rod is a control rod in which the inside of the control rod is hollow, that is, the inside of the control rod is in the same state as a void in the moderator.

Conventionally, the surplus reactivity of A is controlled by using control rods filled with boron, a strong neutron absorbing substance, and fuel rods doped with gadolinia, a burnable poison that is also a strong neutron absorbing substance, to maintain the operating cycle. The reactivity multiplication rate is controlled from the beginning of the cycle to near the end.

In contrast, in the present invention, by inserting hollow control rods into the reactor core, the neutron energy spectrum of the reactor core is hardened, the number of nuclear fission reactions is reduced, the reactivity is suppressed, and the surplus neutrons are transferred to plutonium. We have adopted a method that is useful for generation.

B's +111 direction power distribution control controls the power peaking at the bottom of the core caused by the axial distribution of voids.
This is suppressed by shallow control rods or controlled by the reactivity distribution such as the enrichment of the fuel itself.In contrast, in the present invention, the voids, which are the original cause of skewing of the axial power distribution, are suppressed. This is to make the axial distribution uniform. In other words, a hollow control rod is inserted from the bottom of the core, and the hollow part is used as a void to effectively generate voids in the bottom of the core, thereby making the axial void distribution (moderator density) uniform (Figure 1). ,.

Conventionally, the radial power distribution control of C has adopted a method of inserting a boron-filled 1111 bar at a position adjacent to an aggregate with a high output. In this method, the power of adjacent control rod assemblies becomes too low and combustion is delayed, resulting in a large so-called power or burnup mismatch. For this reason, the control rod pattern or fuel loading pattern had to be planned so that the fuel would burn uniformly.

In the present invention, it is possible to control the local neutron energy spectrum of the reactor core by operating the hollow control rods adjacent to each sting assembly. Therefore, it is possible to harden the spectrum and reduce the output around the aggregates with high output, and conversely, soften the spectrum and increase the output around the aggregates with low output, allowing operation with a uniform power distribution throughout the core. is possible.

Is D's spectrum shift operation conventionally cycle-based? lJJ
From υJ to the middle stage, the coolant flow rate is lowered, the void ratio is increased, the neutron energy spectrum is hardened, and plutonium is accumulated. At the end of the cycle, the coolant flow rate is increased, the void fraction is reduced, the energy spectrum is softened, and the reactivity is increased.

In the present invention, the hollow control rods can significantly change the void ratio in the reactor core. At this time, the hollow control rods only change the effective void ratio, and the void state inside the channel box does not change significantly. , the heat transfer properties of the fuel are
Almost no change.

At the end of the cycle, the hollow control rods are withdrawn and the accumulated plutonium reactivity is effectively used to obtain a large reactivity gain.

The basic features of the present invention can be summarized as follows.

(1) Nuclear characteristics of thermal neutron reactors [When the energy spectrum of neutrons is about 100%, more fission occurs and the reactivity of the core increases; conversely, when the spectrum is hard, less fission occurs, and the core As the reactivity decreases,
The most effective method is so-called spectrum shift operation, which utilizes the principle that plutonium accumulates due to neutron absorption by uranium-238.

Conventionally, in spectrum shift operation, the void ratio has been changed by controlling the flow rate, but this method has an effect on thermal characteristics, so the range of variation in the void ratio is limited.

By using the hollow control rod of the present invention in spectrum shift operation, the void ratio of the reactor core can be changed significantly and the energy spectrum of neutrons can be changed significantly because there is no effect on the thermal characteristics inside the channel box. . Therefore, the gain in reactivity due to the spectral shift can be greatly increased.

(2) Reactors ■] Control rods filled with boron, which is a strong neutron absorbing substance, have traditionally been used to control the reactivity or power distribution. The idea is to absorb excess neutrons into so-called poisonous substances and annihilate them.

The present invention has the idea of controlling these surplus reactivity or power distribution and simultaneously producing new fuel (plutonium), and maximizes the spectrum shift operation in (1).

(3) Since the axial power distribution is skewed due to the axial void distribution within the reactor, shallow control rods and axial reactivity distribution fuel have been conventionally used to flatten this.

The present invention adopts the concept of flattening the axial void distribution itself, which is the cause of this skewing of the output distribution.

[Embodiments of the invention]

The present invention will now be described in detail with reference to Examples.

The first embodiment has a hollow part and a conventional boron-filled part 1.
(This is an example in which 1141 mountain frames are used together and each frame is arranged alternately in the reactor core.

FIG. 7 shows the arrangement of the control rods of this embodiment in a horizontal core analysis plane. In the figure, 6 represents the fuel assembly housed in the channel box, 7 represents the position of the boron-containing cruciform control rod, and 8 represents the position of the cruciform hollow control rod of the present invention.

FIG. 2 shows the system of this embodiment and how to use the nine bars.

That is, during normal J1 rotation, the excess reactants in the reactor core, axial power distribution, and radial power distribution are controlled by adjusting the insertion ratio of hollow control rods, and plutonium is accumulated at the same time. The reason why the 9 boron-containing control rods are left in place as in the past is to obtain the required reaction characteristics during an emergency stop (scram).

FIG. 7 shows the scram completion state, in which the boron-containing control rods are fully inserted and the hollow control rods are fully withdrawn. Since the hollow control rod can control the excess reactivity by 1 to 2 inches, it is possible to sufficiently control the excess reactivity using the hollow control rod when combined with gadolinia, as in the past.

The reactivity gain resulting from this is about 3 inches Δ, and the burnup gain is about 3000 Mwd/.

In this case, the following combinations with fuel are possible:

(1) Hollow control rod partial insertion + upper and lower uniform fuel (11) Hollow control rod fully inserted + upper and lower two region fuel In the former case, the power distribution is flattened and a spectrum shift effect is obtained at the same time.
In the latter, axial reactivity distribution fuel is used for power distribution control.

The latter is a method that also expects a spectrum shift effect in the upper part of the core, and is a more preferable method from the perspective of effectively utilizing the plutonium accumulated in the upper part of the core when the hollow control rods are withdrawn at the end of the cycle.

Next, another embodiment of the present invention will be explained using FIG. 1O, FIG. 11, and FIG. 12. In this embodiment, a hollow control rod ffi'11;
The arrangement of till 1nll 4:'n 9 is shown in a horizontal section of the reactor core, and the arrangement is the same as the conventional one.

Figure 11 shows the real b1! The operation method of the control rod of example i is shown.

During normal operation, only the hollow portions of the control rods 9 are inserted into the reactor core. In this case, if further reactivity control is necessary, control 1
Of course, the boron part of the fist 9 is also inserted.

FIG. 12 shows a scrum state. The hollow part of the control rod 9 protrudes above the core, and the boron part is completely inserted into the core.

In this example, in order to effectively control surplus reactivity, power distribution, and spectrum: In order to fully control skewing,
The hollow area in the control rod is required to be approximately 1/4 or more of the effective length of the control rod at the upper end of the control rod.

〔Effect of the invention〕

According to the present invention, in a boiling water reactor, by making the inside of the control rod hollow, the energy spectrum of neutrons at the control rod insertion part is hardened, and a control method is adopted to reduce the reactivity, thereby reducing excess reaction. By controlling the power distribution and power distribution, and at the same time accumulating plutonium, it is possible to obtain sufficient spectral shift reactivity gain.

In addition, the axial void distribution unique to boiling water reactors is flattened by hollow control rods inserted from the bottom of the core, homogenizing the core condition and achieving flattening of the axial and radial power distribution. .

[Brief explanation of the drawing]

Figure 1 is a diagram showing the distribution of voids and power when a part of the hollow (2) rod according to the present invention is inserted, Figure 2 is a diagram showing the distribution of voids and power in a conventional nuclear reactor, and Figure 3 is a diagram showing the distribution of voids and power in a conventional nuclear reactor. The diagram is/
A diagram showing the distribution of voids and output when a Yarrow control cape is inserted, Figure 4 shows the distribution of voids and output when using the upper and lower two area fuel cans, Figure 5 shows the distribution of voids and output at low flow rate. FIG. 6 is a diagram showing the distribution of voids and output at high flow rates, and FIG. 7 is a diagram showing the arrangement of the control O1l when the hollow control rod according to the present invention is used in combination with a conventional control rod. , FIG. 8 is a diagram showing the embodiment shown in FIG. 7 during normal operation, FIG. 9 is a diagram showing the embodiment shown in FIG. 7 during scram, and FIG. Figure 11 showing the arrangement of control rods in
Fig. 1θ is a diagram showing the embodiment during normal operation, and Fig. 12 is a diagram showing the embodiment in Fig. 1O during scram. l...Curve of void distribution, 2...Curve of output distribution,
3... Control rod containing boron, 4... Upper and lower two area fuel, 5
...Hollow control rod, 6...Fuel assembly, 7...Boron-containing control rod position, 8...Hollow control rod position, 9...
A control rod with a hollow section at the top end.

Claims (1)

[Claims] 1. A hollow control rod atom in a boiling water nuclear reactor equipped with a cross-shaped control rod that is inserted and withdrawn from below the reactor core, characterized in that the inside of the cross-shaped control rod is hollow. Furnace. 2. A hollow control rod nuclear reactor according to claim 1, characterized in that a hollow portion is provided at the upper end of the control rod containing a strong neutron absorber. 3. A hollow control rod nuclear reactor according to claim 1 or 2, characterized in that hollow control rods and control rods containing strong neutron absorbers are arranged alternately in the radial direction of the reactor core. 4. A hollow control rod nuclear reactor according to any one of the above claims, characterized in that the hollow control rod is combined with a fuel assembly having a uniform enrichment in the axial direction. 5. A hollow control rod nuclear reactor according to any one of claims 1 to 3, characterized in that the hollow control rods are combined with fuel assemblies having enrichments distributed in the axial direction. 6. A hollow control rod nuclear reactor according to claim 1 or 2, characterized in that the hollow portion is approximately 1/4 or more of the effective length of the control rod.