JPH06102385A - Fuel assembly and reactor core - Google Patents

Abstract

PURPOSE:To reduce the output at the time of a low flow of a core by arranging burnable poisonous rods more than a half of the total burnable poisonous rods across one fuel rod to the water rods. CONSTITUTION:Twelve burnable poisonous fuel rods 2 corresponding to 75% of the total of 16 are arranged at positions across one fuel rod 1 against water rods 3. Since the poisonous fuel rods 2 are arranged across the fuel rods 1, they can be prevented from being affected by the neutron spectrum change due to the height change of the liquid level of the water rods 3. Many poisonous fuel rods 2 are arranged on the second layer from the outermost layer of a fuel assembly having high thermal neutron flux, and burnable poisons can be effectively burnt regardless of the height change of the liquid level in the water rods 3. When this fuel assembly is used to reduce the output at the time of a low flow of a core coolant, the output reducing effect can be improved by about 3%. When the spectrum shift operation is performed with this fuel assembly, the burnable poisons can be effectively burnt at the cycle initial stage when the liquid level height in the water rods 3 is made low for operation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor and a core loaded with the fuel assembly, and more particularly to a fuel assembly for increasing the output reduction effect when the core cooling water flow rate is reduced. Regarding the body

[0002]

2. Description of the Related Art A conventional boiling water reactor fuel assembly is
As described in Japanese Patent Laid-Open No. 54-121389, a fuel assembly having a pipe (hereinafter, referred to as a water rod) through which only cooling water flows in order to accelerate deceleration of neutrons is loaded in a core. There is. By adopting the water rod, the distribution of thermal neutron flux can be flattened, the number of hydrogen atoms with respect to uranium atoms can be increased, and the nuclear fuel loaded in the core can be effectively used.

However, in order to further effectively utilize the nuclear fuel material, it is effective to perform a spectrum shift operation in which the number of hydrogen atoms in the core is changed with the combustion of the nuclear fuel material.

JP-A-57-125390 and JP-A-57-12
No. 5391 gazette is provided with a medium speed neutron absorbing rod composed of a slow neutron absorbing water pushing rod and a stainless steel having a reactivity value higher than that of the water pushing rod, and controlling the amount of insertion of these water pushing rods into the core. JP-A-61-38589 discloses that a fuel rod having a low concentration of uranium 235 is installed in the water rod, and the water rod before and after the uranium 235 disappears in the fuel rod is disclosed. It describes that the change in the void amount inside is used. In addition, JP-A-63-73187
The publication proposes a water rod capable of changing the moderator liquid level height in the pipe without moving parts by adjusting the core cooling water flow rate. In this water rod, a change in the void ratio in the coolant due to a change in the core coolant flow rate is added to the change in the amount of moderator in the water rod tube, so that a large spectrum shift effect can be obtained. As a result, the take-out burnup is increased and the nuclear fuel material can be used more effectively. As a method of using a burnable poison-bearing fuel rod in a fuel assembly using a water rod that can change the liquid level height in the water rod tube depending on the core flow rate, the burnable poison-bearing fuel rod is adjacent to the water rod. A method of suppressing the reactivity when the void ratio in the water rod sharply decreases by arranging them in such a manner is disclosed in JP-A-1-162189 and JP-A-2-2.
It is described in Japanese Patent Publication No. 49995. This is because during a transient increase in core pressure and flow rate, the liquid level in the water rod rises sharply, the void fraction decreases, the neutron moderating effect increases sharply, and a large positive reactivity is injected into the core. In this case, the reactivity can be suppressed by the burnable poison disposed adjacent to the water rod.

[0005]

In the above-mentioned conventional water rod, the liquid level height in the water rod tube is changed by controlling the core coolant flow rate. That is, the liquid level in the pipe rises when the core coolant flow rate is increased, and falls when it is decreased. On the other hand, since water as a coolant has a neutron moderating effect, an increase in the liquid level in the water rod tube increases the reactivity of the reactor core. Therefore, by increasing the core coolant flow rate and raising the liquid level in the water rod tube, it is possible to compensate for the loss of reactivity caused by the combustion of the fission product.
Therefore, in order to control the reactivity of the reactor core during power operation, it is not necessary to insert a control rod that is inserted into the core to control the reactivity. That is, the operation can be performed only by controlling the core flow rate without inserting the control rod. Also, in the first half of the operation cycle, in order to suppress the excess reactivity of the reactor and maintain the reactivity at an appropriate level, it is necessary to reduce the core flow rate and keep the liquid level in the water rod pipe at a low position. Become. Therefore, in the upper region of the fuel, the void ratio in the water rod tube becomes 100%, so the moderation of neutrons is not sufficient and the conversion of uranium 238 to plutonium is promoted. Thus, in the first half of the operation cycle, plutonium is accumulated in the upper fuel region, and in the latter half, the liquid level in the water rod tube is increased by increasing the core flow rate to enhance the neutron moderating effect and burn plutonium. It is possible to effectively use the nuclear fuel material.

Further, as a method of using the water rod of the above-mentioned prior art, the inside of the water rod is filled with water during normal operation so that it functions like the conventional water rod, and when the core flow rate decreases, the liquid level in the water rod pipe rises. It is possible to improve the stability by reducing the ratio of the number of hydrogen atoms to the number of uranium atoms at the time of low core flow to increase the reactivity reduction effect and lowering the core output. .

However, as shown in FIG. 3, since the rod containing burnable poisons for suppressing the excess reaction at the initial stage of combustion is arranged in the fuel assembly, as shown in FIG. In the early stage of combustion, which has a large effect, the height of the liquid surface in the water rod decreases, and if the water rod becomes void, the proportion of neutrons absorbed by the burnable poison also decreases and the reactivity increases. There is. Therefore, as shown in FIG. 4, in the early stage of the cycle in which the fuel assembly containing the burnable poison exists, the effect of reducing the reactivity due to the decrease in the liquid level in the water rod at the time of the low core flow rate is canceled and becomes small. There is a problem that it ends up. An object of the present invention is to provide a fuel assembly using a water rod capable of changing the neutron moderating effect depending on the core flow rate, and capable of reducing the output at a low core flow rate.

[0008]

The above object is to provide a plurality of fuel rods filled with a nuclear fuel substance, a plurality of burnable poison-bearing rods, and a water rod capable of changing the liquid level in the tube depending on the core flow rate. In the fuel assembly of the present invention, it is achieved by arranging more than 50% of the total number of burnable poison-bearing rods in the burnable poison-bearing rod with one fuel rod sandwiched between the rod and the water rod. If two or more fuel rods are placed between them, the degree of freedom of the position for placing the burnable poison-bearing rods is reduced, and the control rods are affected or the burnable poison-bearing rods are adjacent to each other. Will be placed in position.

[0009]

The thermal neutron flux at the position of the fuel rod adjacent to the water rod greatly changes when the water rod is filled with water and when the water is drained, compared with other positions. For this reason,
The burnable poison-filled rod placed at that position has a large effect on the change in the absorption effect due to the change in the void ratio in the water rod,
The change is about three times as much as the fuel rods placed one fuel rod apart from the water rods. Therefore, by disposing more than half of the burnable poison-containing rods with the fuel rods sandwiched so as not to be directly adjacent to the water rod, as shown in FIG. 5, when the void ratio in the water rod changes. , The effect of increasing reactivity due to combustible poisons in the early stage of combustion is reduced.

[0010]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 shows a first embodiment of the present invention. In the figure, 1 is a fuel rod filled with a nuclear fuel substance, 2 is a fuel rod containing a burnable poison added with a burnable poison, and 3 is a water rod capable of changing the liquid level in the pipe depending on the flow rate of the coolant. The number of burnable poison-bearing fuel rods is 16, and 75% of the fuel rods, 12 rods, are arranged at positions where one fuel rod is sandwiched between water rods. In this way, by placing many of the burnable poison-bearing fuel rods so that they are not directly adjacent to the water rods, the burnable poison-bearing fuel rods are arranged so that the neutrons associated with changes in the liquid level of the water rods are It is possible to avoid the influence of the spectrum change. Further, in the fuel assembly of the present embodiment, most of the burnable poison-containing fuel rods are arranged in the second layer from the outermost layer of the assembly having a high thermal neutron flux, so that there is a change in the liquid level in the water rod. It can burn burnable poisons effectively. When this fuel assembly is used for reducing the output when the core coolant flow rate is low, the output reducing effect can be improved by about 3% as compared with the conventional fuel assembly. As a result, core stability is increased and the amount of fuel loaded can be increased, so that the number of spent fuel generators can be reduced. Further, when the spectrum shift operation is performed using this fuel assembly, in the early stage of the cycle in which the liquid level in the water rod is lowered, the burnable poison is present at the position adjacent to the hard water rod in the thermal neutron spectrum. Since it is not arranged, the burnable poison can be effectively burned. Therefore, the burnup can be extended, and the uranium saving effect can be improved by 1% as compared with the conventional aggregate.

FIG. 6 shows a second embodiment of the present invention. In the figure, 1 is a fuel rod filled with a nuclear fuel substance, 2 is a fuel rod containing burnable poisons, and 3 is a water rod whose liquid level in the pipe can be changed by the flow rate of the coolant. In the second embodiment, since the area of the water rod is as large as nine fuel rods, the change in the neutron spectrum due to the change in the water level in the water rod is further increased, and the spectrum shift effect can be increased. In the case of this embodiment, all the fuel rods containing burnable poisons are arranged at the position of the second layer from the water rod with one fuel rod sandwiched between them. Therefore, the burnable poisons due to the change in the liquid level in the water rod pipe are placed. Has almost no effect on the change in absorption. Therefore, the effect of changing the neutron spectrum at the early stage of combustion is sufficiently maintained even for a water rod having a large spectrum shift effect.

FIG. 7 shows a third embodiment of the present invention, which is a modification of the first embodiment. In this embodiment, the combustion assembly array is 1
It is a grid of 0 × 10, in the figure, 1 is a fuel rod filled with nuclear fuel, 2 is a fuel rod containing burnable poisons, and 3 is a water rod capable of changing the liquid level in the tube depending on the core coolant flow rate. . In this example, since the fuel assembly arrangement is increased, the degree of freedom in the position where the burnable poison-bearing fuel rod can be arranged is increased. In this embodiment, 70% of the fuel rods containing burnable poisons are arranged at a position sandwiching the water rod and one fuel rod.

FIG. 8 shows a fourth embodiment of the present invention. The fuel assembly arrangement is 14 × 14, where 1 is a fuel rod and 2 is a fuel rod.
Is a fuel rod containing burnable poison, 3 and 4 are water rods, and 5 is a channel box. Since it is a large grid, the number of water rods is eight. Of the eight water rods, five shown by 4 in the figure are water rods for spectrum shift operation in which the water level changes according to the combustion of the nuclear fuel material during rated operation, and the four shown by 3 in the figure are A water rod that works in the same way as a conventional water rod in which the pipe is full of water during rated operation, and the liquid level in the pipe decreases when the core flow rate decreases. The relationship between the liquid level in the water rod pipe and the increase amount of the power reduction effect at the time of low core flow rate hardly changes when the liquid level in the water rod becomes lower than a certain height. For this reason, with a water rod that is adjusted to change the liquid level in the pipe for spectrum shifting, the water level in the pipe is lowered during rated operation. It is difficult to have a great effect on reduction. In the present embodiment, by using two types of water rods, a fuel assembly that can effectively burn fuel by spectrum shift operation and that can also increase output reduction at low core coolant flow rate is provided. . Since 80% of fuel rods containing burnable poisons are placed not directly adjacent to the water rods with one fuel rod sandwiched between them, burnable poisons can reduce the output even when the core cooling water flow rate is low at the beginning of the cycle. Since the burnable poison is effectively burned during the rated operation without being hindered, the effective use of the nuclear fuel material can be promoted.

FIG. 9 shows a fifth embodiment of the present invention, in which the burnable poison-bearing fuel rod arrangement in the upper region of the fuel assembly is larger than half as shown in the first embodiment of the present invention. The burnable poison-bearing fuel rod is arranged not to be directly adjacent to the water rod, and the lower part of the assembly where non-boiling water exists in the water rod is the conventional burnable poison-bearing fuel rod. As a result, the liquid level in the water rod changes at the time of low flow rate of the core cooling water, and in the upper part of the core where it becomes a void state, due to the liquid level height change,
Since the influence of the burnable poison on the reactivity reduction effect is reduced, the fuel rod containing the burnable poison will be placed at the soft position of the thermal neutron spectrum in the lower part of the assembly where the water rod is non-boiling water. , The effect of suppressing the reactivity of the burnable poison in the early stage of the cycle can be maintained.

The burnable poisons added to the fuel rods containing burnable poisons in the above-mentioned embodiment include gadolinia, boron, dysprosium and the like.

[0017]

According to the present invention, the reactivity of the fuel when the core coolant flow rate is reduced can be effectively suppressed, and the effect of reducing the output can be increased by about 3%, and the stability of the core can be improved. . This makes it possible to increase the amount of fuel loaded into the core and reduce the number of spent fuel generators.

[Brief description of drawings]

FIG. 1 is a sectional view of a fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing an example of arrangement of burnable poisons in a conventional fuel assembly.

FIG. 3 is a characteristic diagram showing a combustion change of the reactivity change amount when the void ratio in the water rod changes from 0% (water) to 100% (steam) in the conventional fuel assembly.

FIG. 4 is a characteristic diagram showing the presence or absence of combustible poisons in the fuel loaded in the core at the beginning of the cycle.

FIG. 5 is a characteristic diagram showing the combustion change of the reactivity change amount when the void ratio in the water rod changes from 0% (water) to 100% (steam) in the fuel assembly of the present invention.

FIG. 6 is a sectional view showing a fuel assembly according to a second embodiment of the present invention.

FIG. 7 is a sectional view showing a fuel assembly according to a third embodiment of the present invention.

FIG. 8 is a sectional view showing a fuel assembly according to a fourth embodiment of the present invention.

FIG. 9 is a sectional view showing a fuel assembly according to a fifth embodiment of the present invention.

[Explanation of symbols]

1 ... Fuel rod, 2 ... Burnable poison containing rod, 3 ... Water rod, 4
… Water rod, 5… Channel box.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryoji Masumi 1168 Moriyama-cho, Hitachi-shi, Ibaraki Prefectural Energy Research Institute

Claims (5)

[Claims] 1. A plurality of fuel rods filled with nuclear fuel, a plurality of rods containing burnable poisons added with burnable poisons, and a part of a coolant flow in, and the liquid level in the pipe is increased by the flow rate of the coolant. In a fuel assembly composed of variable water rods, more than 50% of the total number of burnable poison-containing rods is arranged at a position where one fuel rod is sandwiched between the rod and the water rod. The fuel assembly is characterized by being 2. A fuel assembly in which there is a region in which the disposition of the burnable poison-bearing rods has an arrangement according to claim 1 in the axial upper half or more. 3. A nuclear reactor core in which the fuel assembly according to claim 1 or 2 is at least integrally loaded. 4. The fuel assembly according to claim 1, wherein the rod containing burnable poison is a fuel rod containing burnable poison added to a nuclear fuel. 5. The fuel assembly according to claim 1 or 2, wherein the burnable poison-containing rod is added with a burnable poison in a space between claddings of a fuel rod filled with nuclear fuel.