JP2003149383A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly capable of keeping integrity of a fuel rod without excessively increasing the internal pressure of the fuel rod even if it is burnt to high burnup, in a mixed-oxide fuel assembly having a 9×9 shape. SOLUTION: This application is related to the mixed-oxide fuel assembly composed by bundling, into a square lattice form of 9 rows by 9 columns, mixed- oxide fuel rods 2 each composed by filling multiple mixed-oxide pellets 6 formed by sintering a mixture of a plutonium oxide and a uranium oxide in a covering tube 7 and by sealing both its ends with end plugs 9 and 10, uranium oxide fuel rods 3 each composed by filling a uranium oxide alone or uranium oxide pellets 12 composed by mixing a neutron poisonous substance in a uranium oxide in a covering tube having the same shape as that of the fuel rod 2, and water rods. A helium gas of 0.3-0.5 MPa is enclosed in a free space inside the fuel rod 2 in manufacturing it, and a helium gas of around 1.0 MPa is enclosed in a free space inside the fuel rod 3 in manufacturing it.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used for a light water reactor, and more particularly to a fuel assembly including a mixed oxide fuel rod. 2. Description of the Related Art Fuel assemblies loaded in light water reactors are being developed for higher burnup in order to improve economic efficiency and reduce the number of spent fuel assemblies. Taking a fuel assembly (uranium oxide fuel assembly) using only uranium oxide fuel rods loaded in a light water reactor as an example, an 8 × 8 shape in which the fuel rods are arranged in 8 rows and 8 columns has recently been used. A 9 × 9 shape in which fuel rods are arranged in 9 rows and 9 columns is becoming mainstream. It has also been proposed to introduce a mixed oxide fuel assembly made from plutonium powder recovered by reprocessing spent fuel. [0003] In the future, as in the case of the uranium oxide fuel assembly, the burn-up of the mixed oxide fuel assembly is to be increased in order to improve the economy, and the shape of the fuel assembly is changed from the 8x8 shape to the 9x shape. It is expected to be changed to 9 shapes. [0004] In a fuel assembly loaded in a nuclear reactor, gaseous fission products such as xenon and krypton and helium gas are generated along with combustion (fission), and a part of these is generated from pellets in free space in a fuel rod. And the internal pressure of the fuel rod increases. From the viewpoint of the mechanical integrity of the fuel rods, the smaller the increase in internal pressure due to combustion, the better. Therefore, measures have been taken to suppress the increase in internal pressure. That is, since the release rate of fission products from the pellets tends to increase as the temperature of the pellets increases, helium gas having a large thermal conductivity is supplied to the fuel rods in advance in the fuel rods at the time of fuel rod production in order to suppress the rise in the pellet temperature. Attempts have been made to improve the heat transfer characteristics from the pellets to the reactor coolant by pressurizing and sealing the space. [0005] Since fission products have a lower thermal conductivity than helium gas, high burn-up fuel from which relatively large amounts of these are released is caused by mixing of the helium gas and fission products that have been sealed in advance. In order to suppress a decrease in the thermal conductivity of the mixed gas, the filling pressure of the helium gas is increased. However, if the filling pressure of the helium gas is excessively increased, the internal pressure of the fuel rod will increase due to the contribution of the helium gas itself. It is considered that there exists an optimum range depending on the size and shape of the. In a uranium oxide fuel assembly loaded in a boiling water reactor, the helium filling pressure of the uranium oxide fuel rod at atmospheric pressure, about 0.3 MPa
The pressure is gradually increased to about 0.5 MPa, and is about 1.0 MPa in the current 9 × 9 fuel assembly. [0007] Similarly to the uranium oxide fuel assembly, helium gas is pressurized and sealed in the mixed oxide fuel assembly. The helium filling pressure is about 0.5 MPa. On the other hand, it is known that mixed oxide pellets have a feature that the amount of helium gas generated by combustion is larger than that of uranium oxide pellets. Therefore, even if the mixed oxide fuel rods have the same combustion conditions as the uranium oxide fuel rods, the ratio of the released fission products and helium gas is different, so the mixed oxide fuel rods accumulate in the free space inside the fuel rods. The degree of decrease in the thermal conductivity of the gas is also different. Considering the characteristics of such mixed oxide fuel rods, a mixed oxide fuel assembly obtained by bundling both mixed oxide fuel rods and uranium oxide fuel rods depends on the ultimate burnup and the size and shape of the fuel rods. It is conceivable that the optimum range of the helium gas filling pressure at the time of manufacturing each fuel rod is different. SUMMARY OF THE INVENTION In view of the above, the present invention provides a 9 × 9 mixed oxide fuel assembly,
It is an object of the present invention to provide a fuel assembly that can maintain fuel rod integrity without excessively increasing the fuel rod internal pressure even when the fuel rod is burned to a high burnup. SUMMARY OF THE INVENTION The present invention provides a uranium oxide fuel comprising a plurality of uranium oxide pellets obtained by sintering uranium oxide alone or by mixing a neutron poison with uranium oxide in a cladding tube. A fuel assembly for a light water reactor having rods and mixed oxide fuel rods in which a number of mixed oxide pellets obtained by sintering a mixture of plutonium oxide and uranium oxide are loaded and arranged in a cladding tube in 9 rows and 9 columns. The helium gas sealing pressure in the cladding tube during the production of the mixed oxide fuel rod is 0.3 to 0.5 MPa, and the helium gas sealing pressure in the cladding tube during the production of the uranium oxide fuel rod is approximately It is characterized by being at 1 MPa. That is, as described above, in the current uranium oxide fuel assembly having a 9 × 9 shape, the helium gas filling pressure at the time of manufacturing the uranium oxide fuel rod is about 1 MPa, and the mixed oxide fuel assembly For the uranium oxide fuel rods inside, the same filling pressure of about 1 MPa is set as the optimum condition. On the other hand, as for the mixed oxide fuel rods in the mixed oxide fuel assembly, the amount of helium gas released during combustion is larger in the mixed oxide pellets than in the uranium oxide pellets. Even under the same combustion conditions as the fuel rod, the degree of decrease in the thermal conductivity of the mixed gas of helium gas and fission products accumulated in the free space inside the fuel rod is small. For this reason, in the mixed oxide fuel rod, even if the helium gas pressure sealed during production is lower than that of the uranium oxide fuel rod, the heat transfer coefficient of the mixed gas accumulated in the free space in the fuel rod during combustion is uranium oxide. Since it is possible to maintain the same level as that of the fuel rod, it is possible to suppress an increase in the amount of fission products released due to an increase in the pellet temperature and to prevent an increase in the fuel rod internal pressure. Further, as the helium gas filling pressure at the time of manufacturing is reduced as much as possible, the contribution of the helium gas itself to the fuel rod internal pressure can be reduced. As described above, by setting the helium gas filling pressure of the mixed oxide fuel rod to be lower than that of the uranium oxide fuel rod, the fuel rod in which the internal pressure of the fuel rod does not excessively increase even when the fuel is burned to a high burnup level. Can be realized,
In the present invention, this optimum value is set to 0.3 to 0.5 MPa. Embodiments of the present invention will be described below. FIG. 1 is a cross-sectional view of a mixed oxide fuel assembly according to the present invention. The mixed oxide fuel assembly 1 includes a mixed oxide fuel rod 2 in which a large number of mixed oxide pellets obtained by sintering a mixture of plutonium oxide and uranium oxide are placed in a cladding tube, uranium oxide alone or uranium oxide. Uranium oxide fuel rods 3 and water rods 4 each of which is loaded with a number of uranium oxide pellets mixed and sintered with neutron poisons in a cladding tube are mounted on a fuel spacer (not shown), an upper tie plate and a lower tie plate in 9 rows and 9 lines. They are bundled in a square lattice of rows and surrounded by a rectangular cylindrical channel box 5. FIG. 2 is a longitudinal sectional view of the mixed oxide fuel rod 2. The mixed oxide pellets 6 obtained by sintering a mixture of plutonium oxide and uranium oxide are coated with a cylindrical cladding tube 7 made of a zirconium alloy. The upper and lower ends of the cladding tube 7 are welded and sealed by an upper end plug 9 and a lower end plug 10 with the upper part being held down by a spring 8. Sometimes helium gas 11 is sealed at a pressure of 0.3 to 0.5 MPa. The heat generated in the mixed oxide pellets 6 is transmitted to the cladding tube 7 via the gas existing in the free space in the fuel rod, and furthermore, the reactor (not shown) existing around the cladding tube 7. Conveyed to the coolant. FIG. 3 is a longitudinal sectional view of the uranium oxide fuel rod 3. The cladding tube 7 has the same material and the same shape as the mixed oxide fuel rod 2.
The fuel rod is composed of an upper end plug 9 and a lower end plug 10, and a plurality of uranium oxide pellets 12 and springs 8, which are obtained by mixing uranium oxide with a neutron poison and sintering, are loaded in the fuel rod. Helium gas 11 is pressurized and sealed at 1.0 MPa in the free space in the fuel rod during manufacture.
Uranium oxide pellets 12 in uranium oxide fuel rods 3
May be obtained by sintering uranium oxide depending on the design. FIG. 4 is a diagram showing a change in the fuel rod internal pressure at the end of combustion when the helium gas filling pressure during the production of the mixed oxide fuel rod is changed. In this case, the ultimate burnup of the mixed oxide fuel assembly is 45 GWd / t. As can be seen from FIG. 4, in the mixed oxide fuel rod, when the helium gas filling pressure is low (0.2 MPa or less), the fuel rod is released due to fission products released during combustion. The heat transfer coefficient of the gas in the free space decreases, which increases the temperature of the pellet and further promotes the release of fission product gas, resulting in a high fuel rod internal pressure at the end of combustion. On the other hand, when the helium gas filling pressure at the time of production is high (0.6 MPa or more), the contribution of the filled helium gas itself to the fuel rod internal pressure increases, and in this case also, the fuel rod internal pressure at the end of combustion increases. . As described above, the optimal condition for reducing the internal pressure of the fuel rod at the end of combustion is that the helium gas filling pressure at the time of manufacture is 0.3 to 0.5 MPa. Accordingly, it is possible to prevent the internal pressure of the mixed oxide fuel rod at the end of combustion from increasing and keep the internal pressure low, thereby improving the mechanical integrity of the fuel rod. As described above, according to the present invention, in a mixed oxide fuel assembly having a 9 × 9 shape, the pressure of helium gas filling during the production of mixed oxide fuel rods and uranium oxide fuel rods. By optimizing the fuel rods, the fuel rod internal pressure can be maintained without excessively increasing the fuel rod internal pressure even when the fuel rods are burned to a high burnup.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a mixed oxide fuel assembly. FIG. 2 is a longitudinal sectional view of a mixed oxide fuel rod. FIG. 3 is a longitudinal sectional view of a uranium oxide fuel rod. FIG. 4 shows the relationship between the initial helium gas filling pressure and the fuel rod internal pressure at the end of combustion in a mixed oxide fuel rod. [Description of Signs] 1 Mixed oxide fuel assembly 2 Mixed oxide fuel rod 3 Uranium oxide fuel rod 4 Water rod 5 Channel box 6 Mixed oxide pellet 7 Clad tube 8 Spring 9 Upper end plug 10 Lower end plug 11 Helium Gas 12 Uranium oxide pellet

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Tsutomu Hirose             2-3-1 Kawa, Yokosuka City, Kanagawa Prefecture             Global Nuclear Fue             Inside Le Japan (72) Inventor Hidetoshi Akiyama             2-3-1 Kawa, Yokosuka City, Kanagawa Prefecture             Global Nuclear Fue             Inside Le Japan

Claims (1)

Claims: 1. A uranium oxide fuel rod in which a plurality of uranium oxide pellets obtained by sintering uranium oxide alone or by mixing a neutron poison with uranium oxide are loaded in a cladding tube, and a plutonium oxide rod. A fuel assembly for a light water reactor, in which a plurality of mixed oxide pellets obtained by sintering a mixture of a material and uranium oxide loaded in a cladding tube and holding them in an array of 9 rows and 9 columns, The helium gas filling pressure in the cladding tube during the production of the oxide fuel rod is from 0.3 to 0.1.
A fuel assembly, wherein the pressure of helium gas in the cladding at the time of manufacturing the uranium oxide fuel rod is about 1 MPa.