JPS6140587A - Fuel rod for boiling-water type reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to fuel rods used in boiling water nuclear reactors.

[Technical background of the invention and its problems]

A fuel rod used in a conventional boiling water nuclear reactor will be explained with reference to FIG. Here, FIG. 4 shows a longitudinal sectional view of a fuel rod for a boiling water reactor. The fuel rod 1 is constructed such that a large number of fuel pellets 3 are stacked in a cylindrical cladding tube 2, and the cladding tube 2 is sealed.

Further, an upper plenum 6 is formed in the upper part of the cladding tube 2, and an upper plenum spring 7 for fixing the fuel pellets 3 is disposed in the upper plenum. In the above configuration, the role of the cladding tube 2 is to hold the fuel pellets 3.
The purpose is to prevent a reaction between the fuel pellets 3 and the coolant in the reactor, and to prevent radioactive fission products released from the fuel pellets 3 from being mixed into the coolant. Therefore, in the unlikely event that the cladding tube 2 is damaged during reactor operation, fission products will mix into the coolant and the radioactivity level of the coolant will increase.
There is a risk that the plant will eventually stop operating. Zircaloy, which is normally used for the cladding tube 2, is a metal that has excellent corrosion resistance against water and steam, has a small neutron absorption cross section, and has sufficient ductility even under radiation irradiation. but,
It has been found that cracks occur in the cladding tube 2 when the output of the nuclear reactor is suddenly increased. Since this crack shows a brittle fracture with almost no plastic deformation, corrosive fission products (
It has been determined that this is stress corrosion cracking of Zircaloy caused mainly by iodine). Cladding tube 2 causes stress corrosion cracking
The tensile stress acting on the fuel pellet 3 is based on the mechanical interaction between the fuel pellet 3 and the cladding tube 2. That is, fuel pellet 3
Since the coefficient of thermal expansion of
A large tensile stress will act on the cladding tube 2. This tensile stress increases as the temperature rise in the fuel pellet 3 due to the increase in output increases. Further, during the power increase, corrosive fission products were rapidly released from the fuel pellet 3, causing stress corrosion cracking.

[Purpose of the invention]

The purpose of the invention is to reduce the temperature rise of the fuel pellet that occurs during the increase in the output of a nuclear reactor, thereby reducing the tensile stress acting on the N tube and improving the integrity of the fuel rod.

[Summary of the invention]

In the present invention, fuel pellets are loaded in a stacked manner in a cladding tube,
After helium gas is sealed in the cladding tube, the pressure of the helium gas sealed in the boiling water reactor fuel rod is made by sealing both the upper and lower ends of the cladding tube with an upper end plug and a lower end plug. A fuel rod for a boiling water reactor characterized by having a pressure of 0.01 atmosphere or less.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 1 to 3. The fuel rod showing an embodiment of the present invention has the same structure as the conventional fuel rod shown in FIG. 4, but the pressure of helium gas sealed inside the cladding tube is set to 0.01 atmosphere or less. . When the helium gas sealing pressure is reduced in this manner, the gap heat transfer coefficient in the gap between the fuel pellet and the cladding tube becomes smaller, and the temperature of the fuel pellet becomes higher. Here the first
The figure assumes that the only gas existing in the cladding tube is helium, without considering the release of fission product gas, and the helium gas sealing.
A characteristic diagram of the fuel rod showing the relationship between the output and the temperature of the fuel pellet when the input pressure is changed is shown. In Figure 1, curve A
5 shows the case where the conventional helium filling pressure is 1 atm, curve B shows the case where the sail is 1 atm, and curve C shows the case where the sail is 0.01 atm.

As shown in Figure 1, as the helium filling pressure decreases,
Although the temperature of the fuel pellet increases, this temperature difference becomes smaller as the output increases to a certain degree.

Next, Fig. 2 shows a fuel rod characteristic diagram showing the relationship between the output and the temperature of the fuel pellet when fission product gas is generated during the increase in the reactor's output. This shows the case where the conventional helium filling pressure is 1 atm, and curve E
shows 0.1 atm, curve FF1O, old atmospheric pressure.

As shown in Figure 2, when the fission product gas is released from the fuel pellet, the gap heat transfer coefficient between the fuel pellet and the cladding becomes poor due to the poor thermal conductivity of the fission product gas, and the fuel pellet Temperature rises rapidly. Only 17
, when the gas composition within the cladding tube comes to be dominated by fission product gas, the difference due to the initial helium filling pressure becomes almost invisible. Indicated in FIG. 2 is ΔT1. Δ
T2. ΔT8 is 5 from the output 'e 100 W/CrfL
This is the range of temperature increase in the fuel pellet that occurs when the temperature is increased to 00W/crIL. As the initial helium filling pressure is lowered, the temperature rise width becomes smaller as ΔT+>ΔT2>ΔT3. This decrease in temperature rise means that
This leads to a reduction in the deformation of the fuel pellet due to thermal expansion, and as a result, the tensile stress generated in the cladding due to the interaction between the fuel pellet and the cladding can be reduced.

Here, FIG. 3 shows a characteristic diagram of the fuel rod showing the relationship between the helium filling pressure and the temperature rise width of the fuel pellet.

As shown in FIG. 3, by setting the helium filling pressure to 0.01 atmosphere or less, the temperature rise in the fuel pellet is rapidly reduced 4, and the tensile stress generated in the cladding tube can be reduced.

As mentioned above, by reducing the sealing pressure of helium gas in the cladding tube to 0.01 atmosphere or less, the stress generated in the cladding tube when the reactor output increases is reduced, and the integrity of the fuel is accordingly improved. can be improved.

〔Effect of the invention〕

According to the present invention, since the pressure of helium gas sealed in the fuel rods is set to 0.01 atmosphere or less, it is possible to reduce the stress generated in the cladding tube when the output of the reactor increases.
In turn, the integrity of the fuel rods can be improved.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram of fuel rods that compares the relationship between power output and temperature of fuel pellets between conventional and inventive fuel rods, and Figure 2 shows the relationship between the output and temperature of fuel pellets when fission product gas is released during the increase in reactor output. A characteristic diagram of a fuel rod showing the relationship between output and temperature of fuel pellets. Figure 3 is a characteristic diagram of a fuel rod showing the relationship between helium filling pressure and temperature rise range of fuel pellets. Figure 4 is a diagram of a conventional boiling water type fuel rod. FIG. 2 is a longitudinal cross-sectional view showing a conventional example of a fuel rod for a nuclear reactor. 1. Fuel rod 2. Cladding tube 3. Fuel pellet 4. Upper end plug 5. Lower both end plugs 6. Upper plenum 7. Upper plenum spring agent, patent attorney Noriyuki Chika (and 1 others) name) Figure 4

Claims (1)

[Claims] A boiling water type atom that is made by loading fuel pellets in a layered manner into a cladding tube, sealing helium gas into the cladding tube, and then sealing both the upper and lower ends of the cladding tube with an upper end plug and a lower end plug. A fuel rod for a boiling water reactor, characterized in that the helium gas is sealed at a pressure of 0.01 atmosphere or less.