JPH0712973A - Fuel assembly and reactor core - Google Patents

Abstract

(57) [Summary] [Structure] In a fuel assembly provided with a water rod whose water level in a pipe is variable, a plurality of burnable poison elements having different neutron absorption cross-sections are used as burnable poisons of a fuel rod 2 containing burnable poisons. , In the region of the upper half of the fuel rod with burnable poison, neutron absorption cross-sectional area is arranged relatively more than the region of the lower half of the burnable poison element such as boron, the neutron absorption cross-section is relatively small, in the region of the lower half, Placing more burnable poisonous elements such as gadolinium, which has a relatively larger neutron absorption cross section, than in the upper half region. [Effect] The output distribution in the vertical direction of the fuel rods and the output distribution in the horizontal direction at the end of the operation cycle can be flattened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a boiling water reactor fuel assembly and a core loaded with the fuel assembly.

[0002]

2. Description of the Related Art A conventional boiling water reactor fuel assembly is
As described in JP-A-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 in the fuel assembly can be flattened and the number of hydrogen atoms with respect to uranium atoms can be increased, so that the nuclear fuel loaded in the core can be effectively utilized.

However, in order to further effectively utilize the nuclear fuel material, it is effective to adjust the neutron energy by changing the number of hydrogen atoms in the core along with the combustion of the nuclear fuel material.

JP-A-57-125390 and JP-A-57-125391
The gazette discloses that a slow neutron absorbing water push rod and a medium speed neutron absorbing rod made of stainless steel having a reactivity value higher than that of the water push rod are provided, and the insertion amount of these water push rods into the core is controlled to control the inside of the core. JP-A-61-38589 discloses that the amount of cooling water in the water rod is adjusted.
It is described that a fuel rod having a low concentration of 35 is installed and the change in the void amount in the water rod before and after the disappearance of the uranium 235 of the fuel rod is utilized. In addition, JP-A-63-73187
Japanese Patent Publication proposes a water rod (hereinafter, referred to as a spectrum shift type water rod) capable of changing the moderator liquid level height in the pipe by adjusting the core cooling water flow rate without a moving part. . In the spectrum shift type water rod, the liquid level height in the water rod tube changes 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, water, which is a coolant, has a neutron moderating effect, so
The rise of the liquid level in the water rod tube increases the reactivity of the reactor core. Therefore, by increasing the core coolant flow rate to raise the liquid level in the water rod tube, it is possible to compensate for the loss of reactivity caused by the combustion of the fissile material. 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, the excess reactivity of the reactor is suppressed and the reactivity is maintained at an appropriate level, so the core flow rate is reduced and the liquid level in the water rod tube is kept at a low position. .
Therefore, in the upper region of the fuel, the void fraction in the water rod tube is 100%, so the deceleration of neutrons is not sufficient,
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. Therefore, it is possible to effectively utilize the nuclear fuel material.

[0005]

In the conventional water rod,
Because the neutron moderating effect of water in the water rod is great,
The reactivity of the fuel around the water rod is high, and the output there is relatively high compared to other places. In order to suppress it, it is appropriate to place a burnable poison near the water rod and increase the neutron absorption there. Gadolinia (oxide of gadolinium) is mainly used as the burnable poison. Since gadolinium has a remarkably large absorption cross section for thermal neutrons, it is characterized in that the excess reactivity can be controlled with a small mixing amount.

The structure of the spectrum-shifting water rod is different from that of the conventional water rod, which causes some problems. That is, first of all, in a boiling water reactor in which bubbles are generated in the coolant, the water density differs due to the difference in the amount of bubbles in the upper and lower parts of the core axial direction, but the lower part is a liquid layer and the upper part is a gas layer. When the configured spectrum shift type rod is adopted, the difference is further increased.

Secondly, there is a change in the liquid level that the liquid level in the water rod rises from the beginning of the cycle to the end of the cycle. Combustible poisonous substances, such as gadolinia, which have large absorption, have a large burning rate with time, and are almost completely burned at the end of the operation cycle. At the end of combustion, the liquid level reaches the upper end of the spectrum shift type rod, so the neutron energy spectrum becomes soft only around the water rod in the upper part of the core where the vapor rate is high, and burnable poisons are burned to completion. In some situations, the power around the water rod is locally higher than at other locations above the core (second issue). An object of the present invention is to solve the above problems 1 and 2 that occur when a spectrum-shifting water rod is used.

[0008]

The fuel assembly of the present invention comprises:
Using multiple burnable poison elements with different neutron absorption cross sections as burnable poisons of burnable poison-bearing fuel rods, boron with a relatively small neutron absorption cross-section in the upper half region of burnable poison-bearing fuel rods. More combustible poison elements such as gadolinium, which has a relatively large neutron absorption cross section, are arranged in the lower half area more than in the upper half area. Arrange and configure. Further, in the burnable poison-bearing fuel rods arranged in a position adjacent to the spectrum shift type water rod, the burnable poison-bearing fuel rod number is larger than the burnable poison-bearing fuel rod number consisting of one burnable poisonous element. To be configured.

The reactor core of the present invention is constructed by loading at least one type of fuel assembly.

[0010]

In the assembly of the present invention, a gadolinia or the like having a large thermal neutron absorption cross section is arranged at the bottom of the fuel rod containing the burnable poison, and the thermal neutron absorption cross section is about 100 times less than that of gadolinium-157. By disposing -10 or the like on the upper part of the fuel rod containing burnable poison, it is possible to increase the absorption amount of thermal neutrons in the axial lower part and suppress the increase of the output distribution there (the solution to the first problem). .

Further, burnable poisonous elements, such as boron, which have a relatively small absorption cross-section, have a small burning rate and therefore remain until the end of the operation cycle. The output peaking near the spectrum-shifted water rod in the upper axial direction, which tends to increase at the end of combustion, can be suppressed by the thermal neutron absorption effect of the remaining combustible poison element (solution of the second problem).

[0012]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 shows a first embodiment of the present invention. In the drawing, (a) is a view of a radial cross section of the fuel assembly, and (b) is an axial configuration of the fuel rod in (a). Reference numeral 1 is a fuel rod filled with a nuclear fuel substance, 2 is a burnable poison-bearing fuel rod to which a burnable poison is added, and 3 is a spectrum shift type 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 18, of which 8 are adjacent to the water rod. The upper part of the burnable poisoned fuel rod adjacent to the water rod contains natural boron as a burnable poison and the lower part contains gadolinia. The neutron energy spectrum is soft, and the gadolinia with a large neutron absorption cross section is placed in the lower part of the axial direction where the thermal neutron flux is large, thereby flattening the axial power distribution. Further, since boron remains unburned at the end of the operation cycle, it is possible to absorb the thermal neutron flux that increases due to the neutron moderating effect of the water reaching the upper end of the water rod, and to flatten the output.

[0014]

According to the present invention, the output distribution in the vertical direction of the fuel assembly and the horizontal output distribution at the end of the operation cycle can be flattened.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a fuel assembly according to an embodiment of the present invention.

[Explanation of symbols]

1 ... Fuel rod, 2 ... Fuel rod containing burnable poison, 3 ... Water rod.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoko Ishibashi 7-2-1, Omika-cho, Hitachi-shi, Ibaraki Hitachi Energy Research Laboratory

Claims (3)

[Claims] 1. A plurality of fuel rods filled with a nuclear fuel, a plurality of burnable poison-added fuel rods containing a burnable poison, and a liquid level height in a pipe depending on a flow rate of a coolant flowing into an assembly. In the fuel assembly composed of a water rod that can be formed, the burnable poison of the burnable poison-containing fuel rod has a first burnable poison element and an absorption cross section for thermal neutrons smaller than the first burnable poison. Consisting of two burnable poison elements, the average concentration of the first burnable poison in the region of the axial upper half is less than or equal to the average concentration of the first burnable poison in the lower half, the axial direction A fuel assembly, wherein an average concentration of the second burnable poison in the upper half region is equal to or higher than an average concentration of the second burnable poison in the lower half region. 2. The burnable poison-bearing fuel rod adjacent to the water rod, wherein the number of burnable poison-bearing fuel rods containing the second burnable poisonous element is the second burnable poisonous rod. A fuel assembly that contains more burnable poison-containing fuel rods that does not contain toxic elements. 3. The reactor core according to claim 1, wherein the fuel assembly is at least integrally loaded.