JPH06186366A - Fuel assembly - Google Patents

Abstract

PURPOSE:To perform water level control efficiently in a water rod by providing a coolant supply port immediately below a delivery port thereby lowering static pressure at the position of the delivery port. CONSTITUTION:The fuel assembly comprises a plurality of fuel rods 1, an upper tie plate, a lower tie plate 3, a fuel spacer, and a channel box 5, with a water rod being provided in the center. The water rod has an ascending channel 61 and a descending channel 62. A coolant flow-in port 63 is opened at the lower end of the ascending channel 61 and a plurality of coolant delivery ports 64 are opened in the side face at the lower end part of the descending channel 62. A large number of coolant supply holes 31 are opened, in addition to the flow-in port 63, in the upper end face of the lower tie plate 3. One of them is opened immediately below the descending channel 62. In other words, central axis of the descending channel 62 is present in one supply hole 31 region. A line extending vertically downward from the delivery port 64 is also present in the region of the supply hole 31. This constitution increases flow rate of coolant in the vicinity of the delivery port 64 thus locally lowering the static pressure.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to a fuel assembly for a boiling water reactor.

[0002]

2. Description of the Related Art A conventional boiling water reactor fuel assembly is
It has a pipe (water rod) through which only cooling water flows in order to accelerate the deceleration of neutrons. The use of the water rod increases the number of hydrogen atoms with respect to the uranium atoms in the assembly, thereby increasing fission reactivity and effectively utilizing the nuclear fuel loaded in the core. However, in order to further improve the economical efficiency of nuclear fuel, it is better to change the number of hydrogen atoms as the fuel burns.

Japanese Unexamined Patent Publication No. 63-73187 shows one method. That is, a resistor (for example, a lower tie plate) is provided at the lower end of the fuel assembly, and a coolant ascending passage having a coolant inlet opening at a region below the resistor is connected to the coolant ascending passage. The water rod is provided with a coolant descending channel having a coolant discharge port opened in a region above the resistor. When the flow rate of the coolant passing through the core is reduced, the water rod is filled with steam, and when the flow rate is increased, the amount of steam in the water rod is significantly reduced, so that the reactivity can be increased at the end of the fuel cycle.

[0004]

According to the water rod described in JP-A-63-73187, the reactivity can be controlled by the flow rate of the coolant. The operating principle of the water rod is a balance between the pressure loss of the coolant before and after the resistor (the static pressure difference between the coolant inlet and the discharge port of the water rod) and the head pressure drop in the water rod. Therefore, the greater the pressure difference between the coolant inlet port and the outlet port, the higher the water level in the water rod. Moreover, the water level does not become lower than the position of the discharge port. Therefore, in order to increase the control range of the water level, it is preferable to provide the coolant discharge port as close to the resistor as possible and at a position where the static pressure is low.

When using a water rod in a conventional fuel assembly, it is convenient to use a lower tie plate that supports the lower end of the fuel rod as the resistor. The upper surface of the lower tie plate is provided with a large number of coolant supply holes that guide the coolant into the assembly. However, immediately above the upper end surface, the flow is disturbed due to the rapid expansion of the flow path, and the static pressure is not necessarily low at the discharge port position of the water rod.

An object of the present invention is to provide a method for reducing the static pressure at the discharge port position.

[0007]

The central axis of the coolant descending passage of the water rod is located inside the opening area of one of the plurality of coolant supply holes provided on the upper end surface of the lower tie plate. To be there. Alternatively, among one or a plurality of coolant discharge ports that open at the lower end of the coolant descending flow path, a straight line that is vertically downward from at least one of the discharge ports is the upper end surface of the lower tie plate or the resistor. One of the plurality of coolant supply holes provided in the inside of the opening portion region.

[0008]

By providing the coolant supply hole directly below the discharge port, the flow velocity of the coolant at the position of the discharge port has a maximum value within the cross section of the assembly. That is, the dynamic pressure is maximum and the static pressure is minimum at the discharge port position.

[0009]

EXAMPLE FIG. 1 shows a vertical cross section of a fuel assembly which is an example of the present invention. Multiple fuel rods 1, upper tie plate 2,
A water rod 6 is provided at the center of the fuel assembly including the lower tie plate 3, the fuel spacer 4, and the channel box 5. The water rod 6 includes an ascending channel 61 and a descending channel 6.
2, the lower tie plate 3 is used as a resistor, and a coolant inlet 63 is formed at the lower end of the ascending passage 61 and a plurality of coolant outlets 64 are formed at the lower end side surfaces of the descending passage 62. In addition to the coolant inlet port 63 of the water rod 6, a large number of coolant supply holes 31 are opened on the upper end surface of the lower tie plate 3. In this embodiment, the coolant ascending channel 61 and the descending channel 62 are connected at the upper end, but a concentric double tube structure may be used. During operation of the nuclear reactor, a boiling liquid level is formed inside the ascending flow path 61, and the liquid level rises and falls with a change in the core flow rate, whereby the reactivity is controlled.

FIG. 2 shows the upper end surface of the lower tie plate 3. Further, FIG. 3 shows an xx cross section of FIG. 2. Figure 2
As described above, in the lower tie plate 3, a large number of coolant supply holes 3 are provided between the fuel rod holding holes 32 that hold the lower ends of the fuel rods.
There are 1A, 31B, 31C, 31D, one of them 3
1D ′ opens directly below the descending flow path 62 indicated by the broken line. That is, when viewed in the cross section of the aggregate, the descending flow path 62
The central axis of is in the open area of the supply hole 31D '. At the same time, in the present embodiment, a straight line vertically downward from the discharge port 64 provided on the side surface of the lower end portion of the descending flow path 62 is also included in the opening portion region of the coolant supply hole 31D '. As a result, the coolant flow velocity is high near the discharge port 64, and the static pressure locally decreases.

[0011]

According to the present invention, the water level in the water rod can be controlled more efficiently.

[Brief description of drawings]

FIG. 1 is a vertical cross section of a fuel assembly according to an embodiment of the present invention.

2 is a top view of a lower tie plate of the fuel assembly of FIG.

FIG. 3 is a sectional view of a lower portion of the fuel assembly shown in FIG.

[Explanation of symbols]

1 ... Fuel rod, 2 ... Upper tie plate, 3 ... Lower tie plate, 4 ... Fuel spacer, 5 ... Channel box, 6
... Water rod, 31 ... Coolant supply hole, 32 ... Fuel rod holding hole, 61 ... Coolant upflow channel, 62 ... Coolant downflow channel, 6
3 ... Coolant inlet, 64 ... Coolant outlet.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Akihito Orii 7-2 Omika-cho, Hitachi-shi, Ibaraki Hitachi Energy Research Laboratory (72) Inventor Koji Nishida 7-2 Omika-cho, Hitachi-shi, Ibaraki No. 1 in the Energy Research Laboratory of Hitachi, Ltd. (72) Takuji Nagayoshi Inventor, Takuji Nagayoshi 7-21, Omika-cho, Hitachi City, Ibaraki Inside the Energy Research Laboratory of Hitachi, Ltd.

Claims (2)

[Claims] 1. An upper tie plate, a lower tie plate, a plurality of fuel rods whose both ends are held by the upper and lower tie plates, a fuel spacer for holding a distance between the fuel rods, and a lower end. A coolant ascending passage having a coolant inlet opening below the resistor provided in the portion, and a coolant outlet opening in a region above the resistor and connected to the ascending passage. In a fuel assembly including a water rod having a coolant descending passage and a channel box surrounding these, a central axis of the coolant descending passage is provided on an upper end surface of the lower tie plate or the resistor. A fuel assembly, wherein the fuel assembly is inside one of the plurality of coolant supply holes. 2. An upper tie plate, a lower tie plate, a plurality of fuel rods whose both ends are held by the upper and lower tie plates, a fuel spacer for holding a distance between the fuel rods, and a lower end. A coolant ascending passage having a coolant inlet opening below the resistor provided in the portion, and a coolant outlet opening in a region above the resistor and connected to the ascending passage. In a fuel assembly including a water rod having a coolant descending passage and a channel box surrounding the water rod, at least one or a plurality of the coolant outlets opening at the lower end of the coolant descending passage. A straight line vertically downward from one of the discharge ports is located inside the opening area of one of the plurality of coolant supply holes provided in the upper end surface of the lower tie plate or the resistor. The fuel assembly according to claim.