JPH01248091A - Fuel rod for boiling water nuclear reactor - Google Patents

Abstract

PURPOSE:To improve a mechanical integrity of a fuel rod, by providing an intermediate end plug at a plenum zone, providing a plenum spring between the intermediate end plug and a fuel pellet and making a space between the intermediate end plug and an upper end plug to be an empty space. CONSTITUTION:An intermediate end plug 2 is located at an axial position 100mm from an upper end of a fuel pellet 3 and a plenum spring of around 100mm length is inserted between the upper end of the fuel pellet 3 and the intermediate end plug 2. A space above the intermediate end plug 2 is an empty space 6 and to a cladding tube of around 250mm length up to the upper end plug 5, a thin wall cladding tube 8 is adopted. In addition, a cladding tube at a space part 4 is a thin walled one an even with the thickness t is of an order of 0.4-0.5mm, it is well endurable to a mechanical condition under a high burn-up situation and its pressure can be diminished with a rate of 10-20% even at an end of fuel life, compared to a conventional fuel, consequently it is very effective for making a fuel to be a high burn-up one.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel rod for a boiling water reactor (BWR) with an improved plenum structure.

[Prior Art] As shown in Fig. 4, the gas plenum located at the upper end of the fuel rod is designed to prevent the movement of the fuel pellets 3 during transportation and to reduce the difference in thermal expansion between the pellets and the cladding tube 7 during use in the furnace. A plenum spring 4 is inserted for absorption. In addition,
5 is an upper end plug.

Normally, BWR fuel has a plenum part of about 350 mm at the upper end of the fuel rod, which has a total length of about 4 m. Approximately 20 cm is provided for fuel. A plenum spring is usually inserted into this plenum portion over its entire length, and the spring volume occupies several percent of the plenum portion volume.

[Problems to be Solved by the Invention] Recently, efforts have been made to increase the burnup of fuel in order to extend the operating period of nuclear reactors and improve fuel economy. BW
For R fuel, improvements such as replacing some of the fuel rods in the center of the assembly with water rods or replacing them with one large water rod have been made for the purpose of improving neutron economy due to higher burnup. It is being said.

However, since the amount of uranium loaded in the aggregate is restricted, the effect of improving the water/uranium ratio by the large diameter water rod may be reduced.

In view of the above, the present invention aims to improve the structure of the plenum section located at the upper end of the fuel section, and at the same time secures the necessary amount of uranium and at the same time reduces F P (F
The purpose of this project is to suppress the rise in internal pressure of fuel rods due to increased gas emissions, improve the mechanical integrity of fuel rods, and provide fuel rods that can cope with lower fuel pressure loss as necessary. There is.

[Means for Solving the Problems] A specific example of the fuel rod of the present invention, which has been made to achieve the above objectives, will be described with reference to FIG. 1.

A hollow intermediate end plug 2 is provided in the plenum portion 1 of the fuel rod, a plenum spring 4 is provided between the intermediate end plug 2 and the upper end of the fuel pellet 3, and a space 6 is provided between the intermediate end plug 2 and the upper end plug 5. The fuel rod has a plenum structure. Note that 7 is a cladding tube.

[Effect] This will be explained using FIG.

Since the area from the intermediate end plug 2 to the upper end plug 5 is a mere space 6 and is a non-heat generating portion, oxidation and corrosion of the cladding tube 7 do not occur as much as in the heat generating portion. Therefore, it is sufficient to have mechanical strength to withstand the increase in internal pressure due to the release of F--P gas, and it is possible to make the cladding tube thinner than the thickness of the cladding tube in the fuel portion.

Furthermore, in fuels with an increasing number of fuel rods such as 9X9 type fuel, the exit density per unit length of the fuel rods decreases, so the FP
Designs can be made to reduce outgassing, and the plenum volume can also be reduced.

In this case, it is also possible to reduce the outer diameter of the cladding tube 7 in the upper space, which reduces the surface area of the coolant two-phase flow section, which is expected to reduce fuel pressure loss and improve channel stability. can.

Although the amount of uranium is likely to be restricted due to improvements in the shape of water rods to achieve higher burnup, the adoption of the present invention will enable designs to extend the effective length of fuel rods, improving design flexibility. .

[Example] (Example 1) Fig. 2 shows Example 1 of the present invention in which the structure of the plenum part was improved for the purpose of reducing the increase in internal pressure due to the increase in the amount of FP gas released due to the increase in fuel burnup. .

In the same figure, approximately 100 mm from the upper end of the fuel pellet 3
The intermediate end plug 2 is installed at the axial position of the fuel pellet 3, and a plenum spring 4 having a length of about 100 mm is inserted between the upper end of the fuel pellet 3 and the lower end of the intermediate end plug 2. A space 6 is provided above the intermediate end plug 2, and a thin-walled cladding tube 8 is used for the approximately 250 mm long cladding tube up to the upper end plug 5.

The cladding tube in the space 4 is thin, and its thickness t is 0.4.
It is possible to obtain sufficient mechanical strength even with a thickness of ~0.5 mm under high burnup conditions, and in this case, the effective volume of the plenum can be increased by more than 4 cm' compared to the conventional design. , even at the end of its life compared to conventional fuels.
The internal pressure can be lowered by 1.5%, which is extremely effective for high burn-up of fuel.

(Example 2) For fuel rods with low linear power density such as 9-input 9-type fuel, it is more effective to lower the pressure drop of the fuel than to reduce the increase in internal pressure. A second embodiment of the invention will be described with reference to FIG.

By reducing the outer diameter of the cladding tube above the intermediate end plug 2 to 7 to 9 mm, the surface area of the portion that comes into contact with the two-phase fluid can be reduced, thereby reducing the pressure loss of the assembly.

If the outer diameter of the fuel portion of the cladding tube is 11.0 mm and the outer diameter of the upper plenum space 6 is approximately 9 mm, the aggregate pressure drop will be 1 if the axial length of the narrow diameter portion is 250 mm.
．． It decreases by 5-2%. In addition, this lower pressure loss also improves channel stability, with a width reduction ratio of 0.03 to 0.
．． It becomes stable around 05.

[Effects of the Invention] BWR fuel of the present invention, (1) A hollow intermediate end plug is provided in the plenum part, and a short plenum spring is provided between the fuel pellet and the intermediate end plug, so that the upper end plug and the intermediate end plug are separated. It becomes possible to secure an effective plenum volume by creating a space between the two.

(2) The cladding tube in the upper space can be made thinner.

Furthermore, it becomes possible to lower the internal pressure, increasing the mechanical integrity of the fuel rods under high burnup.

(3) The diameter of the upper space can be reduced depending on the linear power density of the fuel rods, and the pressure loss of the assembly can also be reduced.

(4) Channel stability can be expected due to the above-mentioned low pressure loss.

etc.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal cross-sectional view of a main part of a specific example of the fuel rod of the present invention, Fig.
Fig. 3 shows Example 1 of the fuel rod (principal part) of the present invention, respectively.
．． FIG. 3 is a sectional view of main parts of Example 2. FIG. 4 is a sectional view of a main part of a conventional example. 1. Plenum part, 2. Middle end plug, 3. Fuel pellet, 4. Plenum spring, 5. Upper end plug, 6.
・Space part, 7...Claying tube. 8.Thin-walled cladding tube.

Claims (1)

[Claims] 1. In a fuel rod for a boiling water reactor, an intermediate end plug is provided in the plenum part, a plenum spring is provided between the intermediate end plug and the upper end of the fuel pellet, and a connection between the intermediate end plug and the upper end plug is provided. A fuel rod for a boiling water reactor with a plenum structure. 2. A fuel rod for a boiling water nuclear reactor having a plenum structure according to claim 1, wherein the wall thickness of the cladding tube in the space portion of the plenum structure is made smaller than the wall thickness of the cladding tube in the fuel portion. 3. A fuel rod for a boiling water nuclear reactor having a plenum structure according to claim 1, wherein the outer diameter of the cladding tube in the space portion of the plenum structure is smaller than the outer diameter of the cladding tube in the fuel portion.