JPS61207988A - Nuclear fuel rod - 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 nuclear fuel rods used in boiling water nuclear reactors.

[Technical background of the invention]

Conventionally, the nuclear fuel rods used in boiling water reactors are
It is configured as shown in the figure. Here, FIG. 4 shows a longitudinal cross-sectional view of a conventional nuclear fuel rod.

As shown in FIG. 4, nuclear fuel #1 contains cylindrical uranium dioxide sintered pellets (hereinafter referred to as UO) inside the cladding tube 2.

Beret. ) 3 is loaded, and an upper end plug 4 and a lower end plug 5 are welded to the upper and lower ends of the cladding tube 2. This lower end plug 5 has a small diameter shank portion 6 at its lower portion. An upper plenum 7 is formed in the upper part of the nuclear fuel rod 1, and a plenum spring 8 and a getter 9 are arranged within this upper plenum. Also,
The inner space of the nuclear fuel rod 1 is filled with helium gas. The cladding tube 2 is a thin-walled tube made of zirconium alloy, such as Zircaloy-2 or Zircaloy-4, because of its excellent corrosion resistance and low neutron absorption cut-off 5EvR. Similarly, for the upper end plug 4 and the lower end plug 5 for sealing both ends of the cladding tube 2, zirconylum alloy is used from the viewpoint of ease of welding and the above-mentioned properties.

[Problems with background technology]

In the above configuration, since the lower end plug 5 faces the lower end surface of the UO and pellet, heat is more easily transmitted and the temperature is more likely to rise than in a cladding tube where heat is transferred from the UO and pellet via the filling gas. . Therefore, a temperature difference occurs between the cladding tube and the lower end plug, and the lower end plug exhibits a temperature difference as shown in FIG. Here, FIG. 5 is a characteristic diagram showing the temperature distribution in the cross-sectional direction of the lower end plug. As shown in Figure 5, the center of the upper end plug was hotter than the surrounding area that was in contact with the cladding tube.This temperature difference caused the lower end plug and the cladding tube to heat up. A difference in expansion occurs, causing deformation as shown in FIG. 7 from the shape at the time of manufacture as shown in FIG. 6. Here, Fig. 6 shows a longitudinal section of the joint between the lower end plug and the cladding tube during manufacture, and Figure 7 shows the joint between the lower end plug and the cladding tube when a difference due to thermal expansion occurs. A vertical cross-sectional view is shown. As shown in FIG. 7, when a thermal expansion difference occurs, thermal stress is generated at the joint between the lower end plug and the cladding tube. This difference in thermal expansion is accompanied by fluctuations, especially when the output of the nuclear reactor changes, and there is a risk that the fatigue life will be shortened and the cladding tube will break due to fluctuations in thermal stress due to fluctuations in thermal expansion. Therefore, there was a natural limit to the width of the reactor's output change and the number of cycles.

[Purpose of the invention]

The present invention is to provide a nuclear fuel rod that alleviates the thermal stress generated at the conventional lower end plug and cladding joint, increases fatigue life, and enables a wider range of nuclear reactor output fluctuations. With the goal.

[Summary of the invention]

The present invention provides a nuclear fuel rod in which a plurality of uranium dioxide sintered pellets are loaded into a zirconium alloy manifold tube, and the upper and lower ends of the cladding tube are sealed with zirconium alloy upper and lower end plugs, A hole is bored in the lower end plug in the axial direction from the inside of the fuel rod, and this hole C2 is made of a heat conductive member made of a material having higher thermal conductivity than the zirconium alloy that is the structural material of the lower end plug. A nuclear fuel rod is characterized in that it is formed by inserting a.

[Embodiments of the invention]

A first embodiment of the present invention will be described below with reference to FIGS. 1 and 2. Here, FIG. 1 shows a longitudinal sectional view of a nuclear fuel rod according to a first embodiment of the present invention. Note that the same parts as in FIG. 4 are designated by the same reference numerals, and explanations of their configurations will be omitted. In FIG. 1, a hole 11 is axially bored in the center of a lower end plug 10 that is welded to the lower part of a cladding tube 2 made of Zircaloy alloy. This hole 11 is drilled up to the shank portion 12 or below, and has a gap in the hole 11 to the extent that it does not come into close contact with the side surface of the hole 11. A thermally conductive member 13 made of a high quality material (for example copper, aluminum, etc.) is inserted. The thermal conductivity of this zirconium alloy is based on the "Behavior of Light Water Reactor Fuel" (Nw
According to page 108 of N-hNsENNo., 13), it is shown as the first equation.

K = 7.51 +2.09 mowing O-2・T -1,45
Xl0-'T"+ 7.67 xto"-T-3・
...(1) However, K-cothermal conductivity (W/rf1・K)T
: Temperature (K) In the above configuration, the thermal conductivity member provided in the lower end plug has better thermal conductivity than the conventional lower end plug. Furthermore, at least up to the shank portion, there is a gap between the inner surface of the hole of the lower end plug and the side surface of the thermally conductive member,
Because they are not in close contact, the heat transfer between the heat conductive member and the lower end plug can be considered similar to the heat transfer between the UO, pellet 2, and the cladding tube, and the way of heat transfer is different from that of Zircaloy alloy. is getting worse. Therefore, in a thermally conductive member, the heat flux is large and the temperature tends to be high, but since heat is mainly transmitted in the axial direction, the second
The temperature distribution in the cross section along the line B-B in the figure is as shown in FIG. As shown in Figure 2, near the center, that is, in the heat conduction member, the temperature is higher than before, but the temperature drops rapidly between the side of the hole in the bottom end plug and the side of the heat conduction member, and the diameter at the bottom end plug decreases. The temperature gradient in this direction is smaller than that of the conventional example shown in FIG.

In this way, the difference between the average temperature of the cladding tube and the average temperature of the lower end plug can be made smaller than before, so the heat generated is proportional to the difference between the average temperature of the cladding tube and the average temperature of the lower end plug. Stress is also reduced. Therefore, thermal strain is also reduced, resulting in less fatigue damage than in the past. Furthermore, it will be able to withstand larger fluctuations in reactor output and the number of fluctuations in output than conventional reactors.

Next, a second embodiment of the present invention will be described with reference to FIG.

Here, FIG. 3 shows a longitudinal sectional view of a fuel rod according to a second embodiment of the present invention. Note that the same parts as in FIG. 1 are designated by the same reference numerals, and explanations of their configurations will be omitted. In FIG. 3, Zircaloy alloy Ii! A hole 21 is bored in the center of the lower end plug 20 welded to the lower part of the cladding tube 2 in the axial direction.

The inner diameter of this hole 21 gradually becomes narrower downward, and a heat conductive member 22 is inserted into this hole 21 . The thermally conductive member 22 is made of a material having higher thermal conductivity than the zirconium alloy that is the structural material of the lower end plug 20, and has a cross-sectional diameter that gradually increases downwardly.

With the above configuration, heat is transmitted more quickly in the axial direction than in the conventional lower end plug. On the other hand, there is a gap between the heat conductive member and the inner surface of the hole of the lower end plug at least up to the shearing part,
In addition, since the gap is reduced in the axial direction, not only the radial temperature gradient but also the axial temperature gradient in the lower end plug is alleviated.

Therefore, the temperature gradient of the lower end plug becomes smaller than when a cylindrical heat conductive member is used.

In this way, when the temperature gradient becomes smaller, the temperature difference between the cladding tube and the lower end plug becomes smaller, and the thermal stress proportional thereto also becomes smaller. Furthermore, since the strain caused by thermal stress is also reduced, fatigue damage is also reduced. Therefore, it is possible to make larger changes in the core output and the number of changes in the output than in the past. Furthermore,
In this embodiment, as an additional effect, the plenum volume can be made larger than before, so that the pressure increase due to the fission product gas in the internal space can be alleviated compared to before.

〔Effect of the invention〕

In the nuclear fuel rod according to the present invention, a hole is formed in the lower end plug, and a heat conductive member is inserted into the hole, so that the difference in average temperature between the cladding tube and the lower end plug can be reduced. Thermal stress generated in the end plug can be alleviated.

[Brief explanation of drawings]

-A characteristic diagram showing the temperature distribution in the B direction; FIG. 3 is a vertical cross-sectional view of a nuclear fuel rod according to the second embodiment of the present invention; FIG. 4 is a characteristic diagram showing the temperature distribution; FIG. FIG. 7 is a vertical cross-sectional view showing the joint between the lower end plug and the cladding tube during reactor power output. 1... Nuclear fuel #2... Cladding tube 3... UO,
Bellet 4... Upper end plug 5.10.20... Lower end plug 11.21... Hole 13.22... Heat conduction member representative Patent attorney Noriyuki Chika (and 1 other person) 1st Figure 3 Figure 6 Figure 7

Claims (2)

[Claims] (1) A nuclear fuel rod in which a plurality of sintered uranium dioxide pellets are loaded into a zirconium alloy cladding tube, and the upper and lower ends of the cladding tube are sealed with zirconium alloy upper and lower end plugs. A hole is bored in the end plug in the axial direction from the inside of the fuel rod, and a thermally conductive member made of a material having higher thermal conductivity than zirconium alloy, which is a structural material of the lower end plug, is inserted into this hole. A nuclear fuel rod characterized by comprising: (2) The nuclear fuel rod according to claim 1, wherein the heat conductive member is made of at least one member of copper or aluminum.