JPS58165085A - Nuclear fuel element - 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 The present invention relates to nuclear fuel elements, and more particularly to mechanical interactions between sintered uranium dioxide pellets and zirconium alloy cladding.
The present invention relates to a nuclear fuel element consisting of fuel pellets capable of reducing PCMI.

Conventionally, sintered uranium dioxide (UO2) pellets have been used as fuel in nuclear fuel elements for light water reactors, and the tubes covering these fuel pellets have excellent corrosion resistance and high-temperature strength, and have a small neutron absorption cross section that is sufficient even under irradiation. Thin-walled tubes made of zirconium alloys (eg, Zircaloy-2, Zircaloy-4) are used because of their advantages, such as their ductility.

FIG. 1 is a longitudinal cross-sectional view of such a type of nuclear fuel element. In the figure, 1 is a zirconium alloy cladding tube, 2 is a uranium oxide fuel pellet, 3 is an upper end plug, 4 is a lower end plug,
5 is a space (plenum), 6 is a plenum spring, and 7 is a gap provided between the fuel pellet and the inner surface of the cladding tube. Furthermore, a spring may be provided between the lower end plug 4 and the fuel pellets 2. The internal space is filled with a filling gas such as helium (He).

The manufacturing process of this nuclear fuel element usually consists of the following steps.

First, the lower end plug 4 is TIG welded to one end of the cladding tube 1. Next, after inserting the fuel pellet 2 and spring 6, etc.
The whole is heated and dried in a vacuum to remove the adsorbed moisture. Finally, the upper end plug 3 is TIG-welded in helium, and at the same time, helium gas is sealed inside.

By the way, the fuel pellet 2 generates heat due to nuclear fission in the nuclear reactor, but since the uranium dioxide sintered body does not have good heat conduction, the temperature becomes high in the center, causing an uneven temperature distribution. Due to the thermal stress generated based on this temperature distribution and the thermal expansion of the fuel pellet 2, the fuel pellet 2 breaks irregularly. The gap 7 between the fuel pellet 2 and the cladding tube 1 is generally designed to be quite small, several hundred μm, in order to improve heat conduction, but if the phenomenon of expansion and deformation of the fuel pellet 2 as described above occurs, , the gap 7 decreases, and eventually the fuel pellets 2 and the cladding tube 1 come into contact, causing mechanical interaction (PCMI) and possibly breaking. The main purposes of the cladding are, first, to prevent chemical reactions between the nuclear fuel and the coolant, and second, to prevent the highly radioactive fission products from being released into the coolant. It is. Therefore, if the cladding tube is destroyed by PCMI, the coolant etc. will be contaminated with radioactivity, which may cause problems in the operational management of the nuclear reactor. Therefore, prevention of PCMI is strongly required for safe operation of nuclear reactors.

In order to alleviate this PCMI, attempts have been made to slow down the rate of increase in the output of the nuclear reactor or change the shape of the fuel pellets, but these efforts are not very effective.

The present invention has been made in view of the above-mentioned needs, and it is an object of the present invention to provide a nuclear fuel element that reduces mechanical interaction between fuel pellets and cladding.

That is, in a nuclear fuel element in which a zirconium alloy cladding tube is filled with sintered uranium dioxide pellets and sealed, the sintered uranium dioxide pellets to which niobium pentoxide or nichromium oxide is added are added to the central part of the fuel rod (preferably). is a nuclear fuel element characterized in that it is loaded in one-third portion).

By using uranium dioxide sintered pellets added with niobium pentoxide or chromic oxide as described above,
The following effects can be expected.

In other words, uranium dioxide pellets to which 0.1 to 1.5 wt% of niobium pentoxide or 0.1 to 1.0 wt% of chromic oxide have been added have a much higher creep rate than uranium dioxide pellets without additives. . An example of the relationship between the amount of niobium pentoxide added and the creep rate is shown in FIG. As can be seen from this figure, uranium dioxide pellets with 1.5 wt% or less of niobium pentoxide or 1.0 wt% or less of chromium oxide have a higher creep rate than uranium dioxide pellets with no additives. However, when the amount of niobium pentoxide added is 1.5 wt % or more or the amount of dichromium oxide is 1.0 wt % or more, the creep rate becomes lower than that of uranium dioxide pellets without additives. By arranging pellets with a high creep rate in the fuel rod where the output is high, that is, where PCMI is likely to occur, the pellets themselves will undergo creep deformation due to the stress generated when the pellets come into contact with the cladding.
Since the tensile stress on the cladding tube can be reduced, P
CMI will be eased. The neutron absorption cross section of niobium is 1.
The neutron absorption cross section of chromium is relatively small at 2.9 perns, so it does not pose much of a problem in terms of neutron economy, but niobium pentoxide or chromic oxide is added to all uranium dioxide pellets loaded into fuel rods. It is wasteful to do so in terms of increased manufacturing costs, and as mentioned above, it is necessary to use pellets with niobium pentoxide or chromic oxide added to areas where PCMI is likely to occur, that is, about one-third of the center of the fuel rod. It is also sufficiently effective to use currently used uranium dioxide pellets for other parts.

[Brief explanation of drawings]

The drawings are for explaining the present invention, and FIG. 1 is a longitudinal cross-sectional view of a nuclear fuel element, and FIG. 2 is a diagram showing the relationship between the amounts of niopium pentoxide and chromic oxide added and the creep rate. DESCRIPTION OF SYMBOLS 1...Zirconium alloy cladding tube, 2...Fuel pellets, 3, 4...End plug, 5...Plenum, 6...
plenum spring. Agent Patent Attorney Akio Takahashi Figure 2 Ol ,, 2 3 11'

Claims (1)

[Claims] 1. A nuclear fuel element in which a zirconium alloy cladding tube is filled with sintered uranium dioxide pellets and sealed, in which niobium pentoxide or chromic oxide is added to the uranium dioxide sintered pellets. A nuclear fuel element characterized by loading something into the center of a fuel rod. 2. A patent claim characterized in that the niobium pentoxide content of the uranium dioxide sintered pellets is 0.1 to 1.5 wt%, and the chromic oxide content is 0.1 to 1.0 wt%. Nuclear fuel elements according to scope 1.