JPH02249995A - Fuel assembly - Google Patents

Abstract

PURPOSE:To greatly change a deceleration effect of a neutron by containing a burnable poison in at least 50% of an adjacent fuel rod of a water rod for changing the height of a liquid level of a coolant of the inside of a fuel assembly by a reactor core flow rate control. CONSTITUTION:A fuel assembly 15 is provided with plural fuel rods 11 filled with nuclear fuel and a water rod 7. In this rod 7, by controlling a reactor core coolant flow rate, a liquid level of the reactor core coolant in its tube is varied. Therefore, since a liquid level rises and falls in accordance with an increase and a decrease of its flow rate, a thermal neutron increases and decreases and the reactivity rises and falls. In this nuclear reactor, it is operated by lowering the liquid level in the first half of an operation cycle and heightening it gradually in the latter half. Therefore, when abnormality for increasing suddenly pressure in the nuclear reactor and a coolant flow rate is generated in the first half of the operation, and the liquid level rises suddenly and the thermal neutron increases, the reactivity rise is suppressed by absorbing the thermal neutron exceeding the increase thermal neutron of a burnable poison contained in a fuel rod 3 containing >=50% gadolinia being adjacent to the rod 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel assembly, and particularly to a fuel assembly suitable for application to a boiling water nuclear reactor to effectively utilize nuclear fuel material.

[Conventional technology]

The conventional boiling water reactor is disclosed in Japanese Patent Application Laid-Open No. 54-12138.9.
As described in the publication, a fuel assembly having a pipe (hereinafter referred to as a water rod) through which only cooling water flows is loaded in a reactor core in order to accelerate the moderation of neutrons. If such water rods are used, under the operating conditions of conventional boiling water reactors, the greater the number of hydrogen atoms relative to uranium atoms, the higher the degree of reactivity, and the more effective the nuclear fuel material loaded into the reactor core. Can be used.

However, in order to make more effective use of nuclear fuel materials,
It is better to change the number of hydrogen atoms in the reactor core as the nuclear fuel material burns.

Jikai %57-125390 Publication and JP-A-57-12
Publication No. 5391 shows one such method. In other words, these publications provide a slow neutron absorbing water rod and a medium speed neutron absorbing water rod made of stainless steel, which has a higher reactivity value than this water rod, and the introduction of these water rods into the reactor core. It states that the amount of cooling water in the core is adjusted by controlling the amount of water. Water depletion is a means of changing the number of hydrogen atoms in the reactor core. Increasing the amount of water push rods inserted into the reactor core will reduce the amount of cooling water in the core, and reducing this amount of insertion will increase the amount of cooling water in the core. The method described above is structurally and operationally complicated because different types of water push rods must be newly provided and the water push rods must be operated by a displacing means.

A fuel assembly using static means to solve this problem is disclosed in Japanese Patent Laid-Open No. 61-38589. In this publication, as a means to change the number of hydrogen atoms, a fuel rod with a low concentration of uranium-235 is installed in the water rod of a fuel assembly, and the amount of voids in the water rod is determined before and after the uranium-235 in the fuel rod disappears. It describes the use of change.

To explain voids here, voids are vapor bubbles that occur in cooling water, and the next term, void ratio, refers to the volume ratio of voids in the cooling water flow path.

In the above method, a fuel rod with a low enrichment degree is used as a heating element, so the structure is complicated and manufacturing is troublesome. Also,
Even if 30% of the coolant channel were applied to the cross section of the water rod, a 30% change in void ratio would only correspond to 9% (30% x 0.3) of the entire fuel assembly, and neutrons would The effect of changing the deceleration effect is not large.

Furthermore, as a method that does not require the provision of a new operating means such as a water push rod, there is a method of adjusting the flow rate of cooling water flowing through the reactor core (hereinafter referred to as core flow rate).

The core flow rate is reduced at the beginning of the fuel cycle, and increased midway through the fuel cycle.

In this method, even if the range of change in core flow rate is 80 to 120%, the range of change in void ratio will only be about 9% at most when looking at the entire fuel assembly, and the effect is not large.

Furthermore, Japanese Patent Application Laid-Open No. 63-73187 has a coolant ascending passage and a coolant descending passage, and by adjusting the core flow rate, the height of the moderator liquid level in the tube can be changed without a moving part. We are proposing a water rod that can do this. In a reactor core loaded with a fuel assembly that employs water rods, the neutron moderation effect can be greatly changed by controlling the core flow rate, making it very effective for efficiently burning nuclear fuel material.

[Problem to be solved by the invention]

In the above-mentioned prior art, means have been proposed for changing the neutron moderation effect to effectively utilize nuclear fuel materials. especially,
As shown in Japanese Unexamined Patent Publication No. 63-73187, the use of water rods that can change the height of the liquid level in the tube by controlling the core flow rate greatly changes the neutron moderation effect without any moving parts. It is advantageous in that it can be done. -On the other hand, in a reactor core loaded with a fuel assembly that employs this water rod, during a transient period when the core pressure and flow rate increase rapidly, the liquid level in the water rod rises rapidly.
There was a problem in that the neutron moderation effect rapidly increased and a large positive reactivity was injected into the reactor core, making it thermally severe.

The purpose of the present invention is to provide a fuel assembly that can significantly change the neutron moderation effect through core flow rate control, and that can suppress an increase in reactivity during transient periods when the core pressure and flow rate increase rapidly throughout the operating cycle. It is about providing.

[Means to solve the problem]

The above purpose is to reduce the internal liquid level height formed by a plurality of fuel rods filled with nuclear fuel and a portion of the coolant flowing into the fuel rods by controlling the flow rate of the coolant placed between the fuel rods. in a fuel assembly having variable water rods, at least 50% of the fuel rods adjacent to said water rods;
is achieved by a fuel assembly characterized in that it contains a burnable poison.

[Effect]

In the water rod according to the present invention, by controlling the core coolant flow rate, the coolant liquid level in the tube of the water rod is changed. That is, when the core coolant flow rate is increased to raise the liquid level, the number of thermal neutrons increases and the reactivity increases.

On the other hand, if the core coolant flow rate is reduced to lower the liquid level, the number of thermal neutrons will decrease and the reactivity will drop.

As mentioned above, a nuclear reactor that employs water rods that can change the neutron moderation effect by changing the height of the coolant liquid level in the tubes by controlling the core coolant flow rate is used to cool the core during the first half of the operation cycle. The core coolant flow rate is reduced to keep the coolant level in the water rod tubes low, and in the latter half of the operation cycle, the core coolant flow rate is gradually increased to lower the coolant level in the water rod tubes. It is driven high.

In a nuclear reactor operated in this manner, an abnormality occurs in which the pressure inside the reactor and the flow rate of the core coolant suddenly increase during the first half of operation, and the liquid level of the coolant in the water rod tube rises rapidly, causing thermal neutrons to rise. When increased, the poison in the burnable poison-containing fuel adjacent to the water rod absorbs more thermal neutrons than the increased thermal neutrons, making the amount of change in reactivity negative. Especially if more than 50% of the fuel rods adjacent to the water rods contain burnable poison.

Although the burnable poison decreases over time, it has the effect of making the amount of change in the reactivity of the core negative during abnormalities that occur in the first half of the operating cycle, suppressing the increase in reactivity. In addition, in the second half of the operation cycle, the reactor is operated with a high level of coolant in the water rod pipe.
Even if the liquid level rises during an abnormality, the reactivity does not become so large that it becomes a problem.

〔Example〕

Before giving a detailed explanation of the present invention, a detailed explanation of the present invention will be given. In the water rod adopted in the present invention, the liquid level height within the water rod tube is changed by controlling the core coolant flow rate. That is, the liquid level in the tube increases when the core coolant flow rate is increased, and decreases when it is decreased. on the other hand,
Since water, which is a coolant, has a neutron moderating effect, a rise 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 tubes, the loss of reactivity caused by the combustion of nuclear fuel material can be compensated for.

Therefore, in order to control the reactivity of the nuclear reactor core during power operation, there is no need to operate control rods that are inserted into the reactor core to control the reactivity. In other words, it becomes possible to operate only by controlling the core flow rate without inserting control rods. In addition, in the first half of the operation cycle, the excess reactivity of the reactor is suppressed,
To maintain reactivity at an appropriate level, the core flow rate is reduced to keep the liquid level in the water rods low. Therefore, in the upper region of the fuel, the void ratio in the water rod tube is 100%, so that the moderation of neutrons is not sufficient, and the conversion of uranium-238 to plutonium is promoted. In this way, in the first half of the operation cycle, plutonium is accumulated in the upper region of the fuel, and in the second half,
By increasing the core flow rate, the liquid level in the water rod tubes can be raised, increasing the neutron moderation effect and burning plutonium. That is, the effective use of nuclear fuel material is improved, and the extraction burnup can be increased. Although the water rod of the present invention has the above-mentioned features and effects, there is a problem in that the amount of change in reactivity becomes extremely large in abnormal situations where the pressure or flow rate changes rapidly. In other words, when an abnormality occurs, the liquid level in the water rod pipe rises rapidly, the degree of reactivity increases rapidly, and the temperature becomes extremely severe. As a countermeasure against this, it is effective to arrange fuel rods containing burnable poison adjacent to water rods. This is because a sudden increase in thermal neutrons near the water rod due to a rise in the liquid level within the water rod can be suppressed by absorption of thermal neutrons by the burnable poison. However, the burnable poison burns out by the end of the first cycle while the fuel stays in the reactor, so for fuel that has passed one cycle or more, simply place the fuel rod containing the burnable poison adjacent to the water rod. alone cannot suppress the increase in reactivity. Therefore, in order to suppress the increase in the reactivity of the reactor core, which contains fuel with various in-reactor stay cycle numbers,
The first cycle fuel must have the effect of canceling out the increase in reactivity of the fuel staying in the reactor for one cycle or more. It is an object of the present invention to provide an effective means for realizing this, and the basis thereof is shown in FIG. FIG. 10 shows the effect of arranging a fuel rod containing gadolinia, which is one of the burnable poisons, adjacent to a water rod. In this figure, if gadolinia-containing fuel rods occupy 8 or more of the 16 adjacent positions of the water rods, the amount of change in reactivity of the fuel in the first cycle when the liquid level in the water rond rapidly increases until the middle of the cycle. It is negative during . In other words, by arranging fuel rods containing gadolinia at 50% or more of the positions adjacent to the water rods, the amount of change in reactivity of the first cycle fuel when the liquid level in the water rods rises becomes negative until the middle of the cycle. The negative reactivity change cancels out some of the positive reactivity change of fuel that remains in the reactor for more than one cycle. Therefore, in order to suppress the increase in reactivity of the reactor core during abnormal situations where the pressure and flow rate increase until the middle of the cycle, it is effective to place fuel rods containing gadolinia at 50% or more of the positions adjacent to the water rods. . In addition, in the latter stages of the cycle, the core coolant flow rate is increased to maintain the liquid level in the water rod at a high position, so even if the liquid level rises during an abnormality, the reactivity changes only in the upper part of the fuel. Since the increase in reactivity in the reactor core is limited to only a portion of the reactor core, the increase in reactivity in the reactor core does not become large enough to become a problem. As stated above,
In order to suppress the increase in reactivity during abnormal times when pressure and flow rate increase throughout the cycle and improve transient characteristics, it is necessary to maintain the negative reactivity change when the liquid level of the first cycle fuel rises until the middle of the cycle. is necessary. This can be achieved by arranging gadolinia-containing fuel rods in 50% or more of the adjacent water rods, as shown in FIG.

That is, due to the effect of the negative reactivity change of the first cycle fuel, the increase in reactivity of the reactor core is suppressed as shown in the relationship between cycle incremental burnup and reactivity in FIG. Also,
As shown in the relationship between burnup and infinite multiplication factor in Figure 8, gadolinia is burned out at the end of the cycle, so for all fuels, a positive reactivity gain is obtained due to the rise in the liquid level in the water rod, and the extraction combustion degree can be increased.

A preferred embodiment of the present invention utilizing the principles described above, that is, a fuel assembly applied to a boiling water reactor will be described below.

A first embodiment of a fuel assembly according to the present invention is shown in FIG.

FIG. 1 shows a cross section of the fuel assembly 15, and the square lattice arrangement of the fuel rods 11 in the fuel assembly 15 is 9 rows and 9
It is a column. One water rod 7 is arranged in an area corresponding to the position of nine fuel rods at the center of the cross section, and 14 of the 16 fuel rods (8
8%) is the fuel rod 3 containing gadolinia, which contains gadolinia, which is a burnable poison. This water rod 7 changes the height of the internal coolant by controlling the core flow rate, and this coolant also acts as a moderator for neutrons.

It is a water rod that can change the neutron moderation effect. The water rod 7 is provided with a coolant ascending passage 1 and a coolant descending passage 2, and the positional relationship between these passages and the fuel rods is shown in FIG.

In FIG. 9, the lower end of the fuel rod 11 (corresponding to the gadolinia-containing fuel rod 3 in FIG. 1) is held by the lower tie plate 5, and the lower part of the water rod 7 is also held by the lower tie plate 5. .

The water rod 7 is a cylinder with a substantially square cross section and a closed top.
The coolant ascending flow path 1 has a coolant inlet 8 that opens in a region below the lower tie plate 5. A coolant downward flow path 2 that guides the coolant downward is provided along a set of ridge lines located diagonally to the water rod 7 having a substantially square cross section. Rising pipe descending pipe connecting passage 1 connected to coolant ascending passage 1 at its upper part
0, and its lower part has a coolant discharge port 9 opening in an area above the lower tie plate 5.

Moreover, the water rod shown in FIG. 11 may be arranged instead of the water rod shown in FIG. 9.

This water rod has a double tube structure with an inverted section of the coolant flow path in the upper region of the interior. The inside of the inner tube of this double-tube structure is the coolant upward flow path 1, and the gap between the inner tube and the outer tube is the coolant downward flow path 2. Above flow path 1
, 2, the configuration of this water rod is almost the same as that shown in FIG. 9, so a description thereof will be omitted.

A fuel assembly 20 according to a second embodiment of the present invention is shown in FIG. In this embodiment, two of the present inventions are applied to a square lattice arrangement of 10 rows and 10 columns, and the water rods are arranged in an area where 16 fuel rods in the center are removed. In this embodiment, fuel rods containing gadolinia are arranged at 80% of the positions adjacent to the water rods.

A fuel assembly 30 according to a third embodiment of the present invention is shown in FIG. This embodiment is another embodiment in which the present invention is applied to a square lattice arrangement of 10 rows and 10 columns, and the water rods are arranged in areas where nine fuel rods are removed from two locations.

The reasons why the first to third embodiments described above are particularly effective are as follows. In order to carry out the present invention, many fuel rods containing gadolinia must be placed adjacent to the water rods. However, if the total number of fuel rods containing gadolinia is increased too much, the reactivity of the reactor will become low and criticality will not be maintained. The first to third embodiments optimally balance the above two requirements.

The present invention can also be applied to next-generation light water reactor fuel. Fourth
The figure shows a fuel assembly 4o according to a fourth embodiment of the present invention, and FIG. 5 shows a fuel assembly 50 according to a fifth embodiment. In these embodiments, the present invention is applied to a fuel assembly with 14 rows and 14 columns. In these examples, gadolinia-containing fuel rods are arranged in 50% or more of the positions adjacent to the water rods, and the total number of gadolinia-containing fuel rods is adjusted to obtain the reactivity necessary to maintain the reactor criticality. The cross-sectional shape of the water rod is determined.

〔Effect of the invention〕

According to the present invention, the fuel assembly has a plurality of fuel rods and a water rod between the fuel rods, which can change the height of the internal coolant liquid level by controlling the core flow rate. Since at least 50% of the adjacent fuel rods are fuel rods containing burnable poison, the neutron moderation effect can be greatly changed, and the core pressure and coolant flow rate can increase abnormally throughout the operating cycle. It is possible to suppress an increase in 1 reactivity during a transient period, and has the effect of improving the thermal margin of the reactor core during a transient period.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a fuel assembly according to a first embodiment of the present invention;
FIG. 2 is a cross-sectional view of a fuel assembly according to a second embodiment of the present invention;
FIG. 3 is a cross-sectional view of a fuel assembly according to a third embodiment of the present invention;
FIG. 4 is a cross-sectional view of a fuel assembly according to a fourth preferred embodiment of the present invention;
FIG. 5 is a cross-sectional view of a fuel assembly according to a fifth embodiment of the present invention;
Figure 6 is a diagram showing the relationship between the ratio of the number of fuel rods containing burnable poison at adjacent positions to the water rod and the change in reactivity when the liquid level in the water rod suddenly rises, and Figure 7 is the relationship between cycle incremental burnup and reactivity. 8 is a diagram showing the relationship between burnup and infinite multiplication factor of the fuel assembly of the present invention, FIG. 9 is a structural diagram showing an example of a water rod, and FIG. 10 is a diagram showing the relationship between burnup and infinite multiplication factor of the fuel assembly of the present invention. Diagram showing the change in reactivity when the liquid level in the rondo rises rapidly, No. 11
The figure is a structural diagram of another example of the water rod. 1... Coolant upward flow path, 2... Coolant downward flow path. 3... Fuel rod containing gadolinia, 5... Lower tie plate, 7... Water rod, 8... Coolant inlet, 9...
・・Coolant discharge port, 11 ・・Fuel rod, 15, 20° 3
0.40.50...Fuel assembly.

Claims (1)

[Claims] 1. A plurality of fuel rods filled with nuclear fuel, and an internal liquid that is arranged between the fuel rods and is formed by a portion of the coolant flowing into the fuel rods by controlling the flow rate of the coolant. 1. A fuel assembly comprising a water rod whose surface height can be changed, characterized in that at least 50% of the fuel rods adjacent to the water rod contain a burnable poison. 2. The water rod communicates with a coolant ascending channel opened in a region below the lower type plate that holds the lower end of the fuel rod above the coolant ascending channel, and connects the coolant ascending channel. a coolant descending flow path that guides the coolant downward in the opposite direction to the flow direction of the coolant; and a cooling water discharge port in an area above the lower type plate as an outlet of the coolant descending flow path. The fuel assembly according to claim 1, characterized in that: 3. The fuel assembly according to claim 1 or 2, wherein the water rod is arranged at the center of a cross section of the fuel assembly. 4. The fuel according to claim 3, wherein the fuel rods are arranged in 9 rows and 9 columns, and the water rod is arranged in a region corresponding to the 9 fuel rods in the center of the arrangement. Aggregation. 5. The fuel according to claim 3, wherein the fuel rods are arranged in 10 rows and 10 columns, and the water rods are arranged in a region corresponding to 16 fuel rods in the center of the arrangement. Aggregation.