JP2000039488A - Fuel aggregate for nuclear reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance a mean enrichment, by setting an uranium enrichment of a fuel rod with combustible poison to a value lower than a maximum enrichment of an aggregate of a partial rod and higher than the other rod with poison, and using a low enrichment poison rod adjacent to a water rod. SOLUTION: A fuel aggregate of 9×9 lattice array is used as an example. Large size water rods 3 are disposed in an area of 7 fuel rods at a center of the aggregate, and four types of uranium enrichments of fuel rods are used. The enrichment is 4.9 wt.% in the highest fuel rod. First gadolinium-filled fuel rod is next higher, and a second gadolinium-filled fuel rod is further next higher. The rod disposed in an outermost peripheral corner area of the array is the lowest. The rods of 60% or more except the outermost peripheral corner position and 16 gadolinium-filled rods are set to the highest enrichment, and 75% of the gadolinium-filled rods is next higher enrichment of the highest. The gadolinium-filled rods adjacent to the water rods 3 are set to low enrichment second gadolinium-filled fuel rods.

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 more particularly, to a fuel rod containing a burnable poison mixed with uranium using uranium having a maximum enrichment limit as a fuel. The present invention relates to a technique for increasing the average enrichment of a fuel assembly in a used fuel assembly.

[0002]

2. Description of the Related Art A fuel assembly loaded in a boiling water reactor has a plurality of fuel rods in which fuel pellets containing fissile material are arranged in a grid, and a plurality of spacers, an upper tie plate and a lower type. The bundles at the rate are housed in a square tubular channel box. Since fissile material decreases with the generation of energy, it has a high reactivity in the early stage of combustion when the amount of fissile material is large. Boiling water reactors use a burnable poison-filled fuel rod that contains pellets containing fissile materials and other burnable poisons as well as fissionable materials in the fuel to suppress the high reactivity in the early stage of combustion, thereby increasing the reactivity of the core. Is adjusted to some extent.

[0003] The reactor must be designed to satisfy thermal limiting conditions such as linear power density and critical power ratio for safe operation. In order to improve local power peaking and axial power distribution, fuel is required. It is necessary to adjust the fuel rod enrichment distribution and burnable poison distribution in the assembly.

It is effective to reduce the fuel cycle cost by increasing the burnup of the fuel in order to improve the economic efficiency of the nuclear power plant, and to reduce the number of spent fuel bodies generated by increasing the burnup. In recent years, there has been a demand for higher burnup of fuel. To increase the burnup of the fuel, it is necessary to increase the fuel enrichment to obtain a high reactivity.

There is a limitation that currently operating fuel production equipment cannot produce fuel pellets having a uranium enrichment of 5.0 wt% or more. Also, from the viewpoint of criticality safety, it is desirable to keep the maximum uranium enrichment low to some extent.

Japanese Unexamined Patent Application Publication No. 10-115690 discloses that under the restriction that the maximum uranium enrichment is less than 5.0 wt%, the enrichment distribution and gadolinia for increasing the average enrichment to increase the take-out burnup above 45 GWd / t. Limits on enrichment of entrained fuel rods are described.

In Japanese Patent Application Laid-Open No. 7-151883, at least one fuel rod having the highest enrichment is disposed at the outermost periphery of the fuel assembly lattice array, and is disposed at a position excluding the water rod and the outermost periphery. Describes a method for obtaining a fuel assembly having a high average enrichment under the restriction that the maximum uranium enrichment is 5.0 wt% or less.

[0008]

As described above, in order to increase the average enrichment of a fuel assembly under the condition that the maximum enrichment of uranium has an upper limit, it is necessary to suppress local output peaking and reduce the fuel containing burnable poison. Bar output suppression is a problem.

Although gadolinia is used as a burnable poison in the fuel of a boiling water reactor, the fuel rod containing gadolinia has a lower thermal conductivity than a fuel rod containing only uranium. In order to ensure the above, in the fuel design of the boiling water reactor, the enrichment of the fuel rod containing gadolinia is set so that the output of the fuel rod containing gadolinia is lower than the output of the fuel rod containing only uranium.

[0010] Generally, the reactivity suppression period by the burnable poison is one cycle from the start of the operation of the reactor to the stop thereof. For high burnup fuel assemblies, it is necessary to increase the number of fuel rods containing burnable poisons corresponding to the increase in reactivity due to the increase in the average enrichment of the fuel assemblies, and to increase the concentration of burnable poisons corresponding to the extension of one cycle operation period. It is.

[0011] The decrease in thermal conductivity due to the addition of gadolinia is as follows.
Depending on the concentration of gadolinia mixed, the output of the fuel rod containing gadolinia rises due to the uranium enrichment of the fuel rod containing gadolinia and the thermal neutron flux distribution at the position where the fuel rod containing gadolinia is located. Therefore, setting the enrichment of gadolinia-containing fuel rods is a problem in increasing the average enrichment of fuel assemblies under the constraint of the highest enriched uranium.

Japanese Patent Application Laid-Open No. 10-115690 discloses a method of increasing the average enrichment of a fuel assembly under the restriction of the enrichment of fuel pellets and increasing the output burn-up of more than 45 GWd / t. Although an enrichment distribution method focusing on the local output peaking of the fuel assembly has been proposed, no study has been conducted on the output of burnable poison-containing fuel rods focusing on the thermal neutron spectrum of the cross section of the fuel assembly. The type of burnable poisoned fuel rod enrichment is used.

Further, the technique described in Japanese Patent Application Laid-Open No. 7-151883 discloses that a burnable poison is contained in a position where neutron deceleration is relatively unlikely to occur, except for a position adjacent to the outermost peripheral portion of a fuel assembly and a water rod. This shows a method of solving the problem of the fuel rod temperature rise of the burnable poison-containing fuel rod even when the uranium enrichment is maximized by arranging the fuel rods. Because the burnable poison in the burnable poison-containing fuel rods remains unburned, the reactivity is suppressed until the time when the reactivity suppression effect of the burnable poison is not expected, and the reactivity decreases.
From the viewpoint of fuel economy, it is desirable that the burnable poison-bearing fuel rod be disposed at a position where the thermal neutron flux distribution is relatively high, so as to eliminate the influence of the reduction of reactivity due to unburned fuel.

[0014] An object of the present invention is to provide a fuel assembly which does not cause a problem in thermal soundness such as a rise in temperature and an internal pressure of a fuel rod containing a burnable poison when there is an upper limit on the maximum enrichment of uranium that can be used. It is an object of the present invention to provide a fuel assembly which is effective for improving the fuel economy by increasing the average uranium enrichment of the fuel and improving the fuel burnup.

[0015]

In the boiling water reactor fuel, a flow path of water as a moderator is partitioned by a channel box, and an inner region surrounded by the channel box is generated by heat generated from fuel rods. Water containing voids flows, and water not containing voids flows outside the channel box. For this reason, the thermal neutron flux is high near the channel box, and low in the center of the fuel assembly where the water density is relatively low due to voids. The water rod located at the center of the fuel assembly allows the non-boiling water to flow through the central region of the low thermal neutron flux, thereby increasing the thermal neutron flux in the central portion and flattening the thermal neutron flux distribution of the fuel assembly. Has the function of effectively burning the fuel.

A fuel assembly compatible with high burnup requires more moderator because the neutron spectrum becomes harder due to the increase in uranium enrichment. For this reason, a larger water rod is required, but as the water rod becomes larger, the thermal neutron flux distribution around the water rod becomes very high.

In a fuel assembly having an average enrichment of 4.0 wt% or more, gadolinia-containing fuel rods containing gadolinia, which is a burnable poison, account for 20% or more of the total number of fuel rods. It is not desirable to dispose gadolinia-containing fuel rods in the outermost layer of the fuel assembly lattice array, particularly the portion facing the insertion position of the control rods, because it has an effect on the reactivity suppression effect of the control rods. In addition, in order to reduce reactivity loss due to unburned gadolinia, the gadolinia-containing fuel rods are arranged at relatively high positions of the thermal neutron flux, and the gadolinia-containing fuel rods shield each other from the thermal neutron flux. It is desirable not to take an arrangement (an arrangement in which fuel rods containing gadolinia are densely packed).

In consideration of the above points, when gadolinia-containing fuel rods are arranged in a fuel assembly corresponding to high burnup,
The gadolinia-containing fuel rod will be placed around the water rod with a relatively high thermal neutron flux. However, gadolinia-filled fuel rods located closer to the water rods produce higher output than gadolinia-filled fuel rods located elsewhere due to the height of the thermal neutron flux distribution.

An object of the present invention described above is to provide a fuel assembly having a large water rod at a position where the center of gravity of the water rod coincides with the center of the fuel assembly lattice array. At least two or more types of burnable poison-containing fuel rods having different uranium enrichment mixed with 0 wt% or more of burnable poisons are provided, and the first burnable poison having the highest uranium enrichment among the burnable poison-containing fuel rods is provided. The uranium enrichment of the entrained fuel rod is lower than the uranium enrichment of the highest enriched fuel rod in the fuel assembly, and the uranium enrichment of the second burnable poisoned fuel rod is reduced to the first burnable poisoned fuel rod. All of the burnable poisoned fuel rods which are lower than the uranium enrichment and are vertically and horizontally adjacent to the water rods in the fuel rod lattice arrangement are achieved by the second burnable poisoned fuel rods.

[0020]

(Embodiment 1) A first embodiment of the present invention will be described with reference to FIGS. FIG. 1A is a cross-sectional view of a fuel assembly according to a first embodiment of the present invention, and FIG. 1B is a cross-sectional view of FIG. It shows the relationship. In the figure, 1 is a channel box, 2 is a uranium-only fuel rod, 3 is a water rod,
Reference numeral 4 denotes a gadolinia-containing fuel rod obtained by adding gadolinia, which is a burnable poison, to uranium. The fuel assembly of this embodiment has a 9 × 9 lattice array, and two large water rods are arranged in a fuel rod lattice area for seven fuel rods at the center of the fuel assembly. In this embodiment, four types of fuel rods with uranium enrichment are used. The fuel rod U3 has the highest uranium enrichment in the fuel assembly and the maximum uranium pellet enrichment is less than 5.0 wt%. It is 4.9 wt%, which is expected to be the highest enrichment that can be produced in consideration of production crossover.

The enrichment of the fuel rod indicated by G1, which is the first gadolinia-containing fuel rod, has the highest uranium enrichment next to the fuel rod having the highest enrichment in the fuel assembly, and the second gadolinia-containing fuel rod indicated by G2 is: The next highest concentration. The U1 fuel rod located in the outermost corner region of the fuel rod lattice array, which is the position of the highest thermal neutron flux in the fuel assembly, is set at the lowest level to suppress local power peaking. Also, U1
The uranium enrichment is also kept low because the fuel rod U2 adjacent to the fuel cell also produces very high local power peaking.

In this embodiment, 60% of the corner positions of the outermost periphery of the fuel rod lattice array and 60% other than the fuel rods containing 16 gadolinia are used.
Since the above fuel rods are the fuel rods with the highest enrichment, and 75% of the fuel rods containing gadolinia are the fuel rods with the highest uranium enrichment next to the fuel rod with the highest enrichment,
High average fuel assembly enrichment. Furthermore, by using the second burnable poison-containing fuel rod with low enrichment as the burnable poison-containing fuel rod adjacent to the large water rod, the thermal integrity of the gadolinia-containing fuel rod can be ensured over the fuel life. .

(Embodiment 2) FIG. 3 shows a second embodiment of the present invention. In this embodiment, a large square-tube-shaped water rod having a fuel rod lattice area for nine fuel rods is disposed at the center of the fuel assembly. In this embodiment, since the water rod area is large, the thermal neutron flux at the position of the fuel rod adjacent to the water rod becomes very high, and the thermal neutron flux distribution in the fuel assembly is more flat than in the first embodiment of the present invention. However, local output peaking can be reduced.

In the present embodiment, as in the first embodiment of the present invention, the maximum enrichment that can be taken into account in view of the production intersection under the constraint that the maximum uranium enrichment is less than 5.0 wt% is 4.9 wt% and is indicated by U3. The enrichment of the fuel rod is 4.9 wt%. In the present embodiment, the second layer corner position inside the first layer from the outermost periphery of the fuel rod lattice array ((2, 2) of the coordinates where the upper left is (1, 1) in the cross section of the fuel assembly of FIG. 2A) (The position and its symmetric position) the gadolinia-containing fuel rods are arranged. This position is a position where the thermal neutron flux is very high, similar to the position adjacent to the water rod.
The second gadolinia-containing fuel rod G2 having a lower enrichment than that of the second gadolinia-containing fuel rod G1 is arranged. This ensures the thermal integrity of all gadolinia-filled fuel rods.

Further, by arranging the gadolinia-containing fuel rods at the corner positions of the second layer, the local output peaking of the fuel rods U1 and U2 at the outermost corner positions can be suppressed, so that the uranium enrichment of U1 and U2 can be reduced. It is possible to increase the fuel assembly average enrichment even when the number of gadolinia-containing fuel rods having a low enrichment is increased.

(Embodiment 3) FIG. 4 shows a third embodiment of the present invention. FIG. 3A shows a cross-sectional view of a fuel assembly and a fuel rod enrichment distribution, and FIG. 3B shows an axial enrichment distribution of the fuel rod shown in FIG. In the figure of (b), the symbols written in alphabetical letters represent the enrichment of the fuel rods.
The magnitude relation is shown in the lower right of the figure.

In a boiling water reactor, water as a moderator flows from the lower part of the core toward the upper part, and the upper region of the fuel assembly located at the upper part of the core has a void (bubble in the moderator) ratio in the lower region. This has the effect of lowering the water density and reducing the thermal neutron flux. In consideration of this axial distribution, in order to increase the water distribution in the upper part of the fuel assembly having a high void ratio, a fuel rod having a partial length shown in FIG.

As in the first and second embodiments of the present invention, this embodiment also uses a uranium enrichment having a maximum enrichment of 4.9 wt%.

This embodiment is similar to the second embodiment of the present invention.
Since gadolinia-containing fuel rods are arranged at the corner positions of the second layer from the outermost layer of the fuel rod lattice array, U1, U
The local output peaking can be suppressed even when the fuel rod enrichment of No. 2 is increased.

In this embodiment, in addition to the gadolinia-containing fuel rods G1 and G2, a gadolinia-containing fuel rod G3 is provided in order to reduce the axial output peaking at the lower part of the fuel assembly at the beginning of combustion. Has increased the number of fuel rods containing gadolinia. The gadolinia concentration of the gadolinia-containing fuel rods of G1 and G2 indicates that the excess reactivity of the fuel assembly is 1
Although 4.5 wt% is set so as to suppress the cycle period, the gadolinium concentration of G3 provided to suppress the output peaking at the lower part in the axial direction at the beginning of combustion is 2.0 wt%.

FIG. 5 is a graph showing the relationship between the gadolinium concentration for maintaining the thermal integrity of the gadolinia-containing fuel rod adjacent to the water rod and the uranium enrichment. Since the gadolinia concentration of the gadolinia-containing fuel rod G3 provided for suppressing the axial output is lower than 3.0 wt%, the G2 gadolinia-containing fuel rod G2 can secure thermal integrity without lowering the enrichment as compared with the gadolinia-containing fuel rod G3. it can.

[0032]

According to the present invention, in a fuel assembly of a boiling water reactor using uranium as a fuel, even if the maximum value of the uranium enrichment is restricted, the thermal integrity of the fuel is ensured. A fuel assembly with a high average enrichment can be provided.

[Brief description of the drawings]

FIGS. 1A and 1B are a cross-sectional view showing a fuel assembly according to a first embodiment of the present invention and a layout of fuel rods in FIG. 1A.

FIG. 2 is a longitudinal sectional view showing the fuel assembly of the first embodiment.

3 (a) and 3 (b) are a cross-sectional view showing a second embodiment of the present invention and a layout of fuel rods in FIG. 3 (a).

FIGS. 4A and 4B are a cross-sectional view showing a fuel assembly according to a third embodiment of the present invention and an axial concentration distribution characteristic diagram of a fuel rod shown in FIG. 4A.

FIG. 5 is a graph showing a relationship between gadolinia concentration and uranium enrichment for maintaining thermal integrity of a gadolinia-containing fuel rod adjacent to a water rod.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Channel box, 2 ... Uranium fuel rod, 3 ... Water rod, 4 ... Fuel rod containing burnable poison, 5 ... Partial length fuel rod,
6 ... spacer, 7 ... lower tie plate, 8 ... upper tie plate.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G21C 3/32 E

Claims (6)

[Claims] 1. A plurality of fuel rods containing uranium pellets obtained by sintering uranium as a nuclear fuel substance and at least one large water rod through which non-boiling water flows through a spacer, an upper tie plate, and a lower tie plate. In a fuel assembly bundled in a channel box at intervals with a lattice shape, water rods are provided at the center of the fuel assembly with a fuel rod lattice area of seven or more fuel rods centered on the center of the fuel assembly lattice array. Among the plurality of fuel rods, there are at least two or more types of fuel rods containing uranium enriched with at least two types of burnable poisons in which uranium is mixed with at least 3.0 wt% of burnable poisons. The uranium enrichment of the first burnable poisoned fuel rod with the highest uranium enrichment of the toxic enriched fuel rods is lower than the uranium enrichment of the highest enriched fuel rod in the fuel assembly The uranium enrichment of the second burnable poisoned fuel rod is lower than the uranium enrichment of the first burnable poisoned fuel rod, and the burnable poisoned fuel vertically and horizontally adjacent to the water rods in the fuel rod grid arrangement. A fuel assembly for a nuclear reactor, wherein all the rods are the second burnable poisoned fuel rod. 2. A plurality of fuel rods containing uranium pellets obtained by sintering uranium as a nuclear fuel substance and at least one large water rod through which non-boiling water flows through a spacer, an upper tie plate, and a lower tie plate. In a fuel assembly stored in a channel box bundled in a grid at intervals, the water rods are placed at the center of the fuel assembly so that the center of gravity is at the center of the fuel assembly grid array and at least 7 fuel rods are provided at the center of the fuel assembly. There is a fuel rod containing at least two types of burnable poisons having different uranium enrichment in which uranium is mixed with burnable poisons of not less than 3.0 wt% among a plurality of fuel rods, the fuel rods being arranged with a fuel rod lattice region. The first burnable poisoned fuel rod with the highest uranium enrichment among the burnable poisonous fuel rods has a lower uranium enrichment than the highest enriched fuel rod in the fuel assembly and the second burnable Uranium enrichment of the poison containing fuel rods is lower than the enrichment of the first burnable poison containing fuel rod, the outermost layer closest to the channel box in a fuel rod lattice arrangement,
A second burnable poison-containing fuel rod is used for the burnable poison-containing fuel rod located at a corner position inside the outermost layer and at a position adjacent to the water rod in the vertical and horizontal directions, and is disposed at other positions. A fuel assembly for a nuclear reactor, wherein the first burnable poison-containing fuel rods are all provided with the first burnable poison-containing fuel rods. 3. The fuel assembly according to claim 1 or 2, wherein the burnable poison-containing fuel rod has a burnable poison of 3.0 watts.
The first burnable poison-containing fuel rod has an average uranium enrichment of the first burnable poison-containing fuel rod, and the second burnable poison-containing fuel rod has an average uranium enrichment of the second burnable poisonous fuel rod. A fuel assembly for a nuclear reactor, wherein the average uranium enrichment of the burnable poisonous pellets is greater than the average uranium enrichment. 4. The fuel assembly according to any one of claims 1 to 3, wherein the plurality of fuel rods filled with uranium pellets as nuclear fuel have at least two or more fuel rods having different lengths. A fuel assembly for a nuclear reactor. 5. The fuel assembly according to claim 1, wherein a water rod serving as a non-boiling water flow path has a square cross section. . 6. The fuel assembly according to any one of claims 1 to 3, wherein two or more water rods serving as non-boiling water passages are arranged in the fuel assembly. A fuel assembly for a nuclear reactor, wherein one center of gravity for a plurality of arranged water rods coincides with the center of the fuel assembly lattice array.