JPS6018792A - Fuel element for fast 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 [Field of Application of the Invention] The present invention relates to a fuel element for a fast reactor, and particularly to a fuel element for a fast reactor that can prevent damage to a fuel cladding tube that occurs during the progress of a nuclear fission reaction in a fast reactor. Concerning fuel elements.

[Background of the invention]

The fuel element used in a fast reactor is shown in FIG. It is filled with fuel pellets 3 mainly made of nuclear fuel. The fuel pellets 2 made of nuclear fuel enriched with fissile material are called core fuel pellets, and the fuel pellets 3 made of nuclear fuel mainly composed of fissile parent material are called blanket fuel pellets. Both fuel pellets have a cylindrical shape with a length of about 1 crn, as shown in FIG. 1(b). The diameter of the pellet is also slightly smaller than the inner diameter of the fuel cladding tube 1, so that the fuel pellet and
A gap 4 is created between the fuel cladding tubes.

Currently, sintered oxides such as 2311 [J, 240 p u, etc., which are fission-friendly materials, are mainly used for blanket fuel pellets, and reactor core fuel pellets are made from fission-friendly materials such as 2311 [J, 240 p u, etc.].
3 m [J, 240 p u together with fissile material at a weight ratio of about 15 yen + 141 p u, 139 p u
, an oxide sintered body containing ass O is used.

Blanket fuel pellets are usually placed above and below the core fuel pellet section (2). In the fuel pellet section of the reactor core, fissile material undergoes nuclear fission, releasing large amounts of thermal energy and several neutrons. Although the probability of nuclear fission occurring in the blanket fuel pellet part is small, 28m, which is a fission-friendly material,
[J, 240 p 11 absorbs neutrons released from the core fuel part, and a reaction occurs that changes the pH to fissile material +141. The generated thermal energy is removed by a coolant flowing outside the fuel cladding.

Nuclear fission reactions produce fissile products (FP) with high radioactivity. Volatile FP becomes gas and is accumulated in the gas plenum 5 shown in FIG. 1(a).

The role of the fuel cladding tube 1 is to prevent the fuel pellets from coming into contact with the coolant sodium, and to keep them in the FPP2O fuel element.
The goal is to prevent it from being released to the outside. Fuel cladding tube 1
When the fuel pellets are damaged, FPP2O release occurs and contact between the fuel pellets and the sodium occurs. The surface of the fuel pellets that participate in nuclear fission reaches a high temperature of about 90 Or or more, which may lead to boiling of the sodium, which may lead to a nuclear reactor accident.

Among nuclear fission products, cesium (Cs) has relatively high volatility and high mobility, which is known to cause the following problems. Nuclear fission reactions occur in large numbers in the core fuel pellets, so the amount of C8 produced is also large mainly in the core fuel pellets. In addition, since the core fuel pellet portion releases a large amount of energy due to nuclear fission, the temperature is very high, and the generated Cs is vaporized and released into the cladding tube of the fuel element. The released Cs passes through the gap 4 in FIG. 1(a) and begins to diffuse into the upper and lower blanket fuel portions. In the blanket fuel pellet section, the rate of nuclear fission reactions is small, so the pellet temperature is much lower than in the core fuel pellet. Therefore, C that has spread
The s gas adheres to the surface of the blanket fuel pellet in the form of solid metal O8 in the blanket fuel pellet portion.

Here, C5 and U O4 of blanket fuel pellets
It is known that a reaction expressed as 2C8+UO4→C82UO4 (cesium uranite) occurs between the two. Since the crystal density of C82UO4 produced by this reaction is small, the volume of the C82UO4 adhered portion becomes large, and a bulge as conceptually shown in FIG. 2 is generated.

The embossed part 7 by C32U04 is large near the boundary between the blanket fuel pellets and the core fuel pellets, and when the fuel element stays in the reactor for a long time, stress acts on the cladding tube, and this stress causes the cladding tube to deteriorate. The strain may eventually lead to failure of the fuel cladding.

[Purpose of the invention]

The object of the present invention is to obtain 12UO4 generated by a nuclear reaction.
An object of the present invention is to provide a fuel element for a fast reactor that can prevent damage to a fuel cladding tube caused by generation.

[Summary of the invention]

It has been experimentally revealed that the areas where Cso adhesion and csiUO4 are likely to form are the boundaries between the core fuel pellets and the blanket fuel pellets, and the gaps between the blanket fuel pellets. Focusing on this point, in the present invention, Cs*U04 is easily generated and therefore, the fuel cladding tube is easily damaged in the areas where Cs*U04 is easily generated,
By reducing the diameter of the blanket fuel pellet and widening the gap between it and the fuel cladding tube, the volumetric expansion due to the production of 082UO4 was absorbed. The present invention will be described in detail below using Examples.

[Embodiments of the invention]

FIG. 3 shows an embodiment of the present invention. FIG. 3(a) shows a blanket fuel pellet of the present invention, which is a conventional blanket fuel pellet in which corners along the circumference have been cut off. FIG. 3(b) shows a state in which the pellets of the present invention are filled into a cladding tube. The core fuel pellets will be the same as before. FIG. 4 shows experimental results of Cs concentration in the longitudinal direction of the fuel element. The locally observed peaks occur in the gaps between the blanket fuel pellets.

A graph modeling this is shown in FIG. 5, which shows that the Cs concentration in the pellet gaps is high. Therefore, it is thought that cladding tube breakage due to the formation of C82C04 is likely to occur in the gaps between the blanket fuel pellets. According to the embodiment shown in FIG. 3, as a result of the gap between the blanket fuel and the cladding becoming larger in the gap between the blanket fuels, damage to the cladding due to the generation of C82UO4 in this area is prevented.

According to FIG. 4, the concentration of Cs is a maximum of 3×10 −4 g/capacity 2 per inner surface 2 of the fuel cladding tube.

Here, Cs atomic weight M(Cs) 133 C82UO4 atomic weight M(C82UO4) 568C8
If the 2UO4 density ρ is 6.6 g/an3, the thickness S of the accumulated C 83 UO4 is. Therefore, if the corners of the blanket fuel pellets are shaved off by approximately 100 μm or more, this is sufficient as a measure to prevent damage to the cladding tube. Since the increase in the gap between the fuel pellet and the cladding tube due to cutting off the corners of the fuel pellet is small, the decrease in heat transfer coefficient due to the gap is small υ, and the effect on the fuel pellet temperature is small. Therefore, even if the corners of the core fuel pellets are shaved off at the boundary between the core fuel pellets and the blanket fuel pellets, there will be no significant change in the bulkiness of the core fuel pellets, and the effect of preventing cladding damage will be reduced. It can be further increased.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to prevent damage to the fuel cladding tube due to the formation of cesium uranite in the fuel element for a fast reactor.

[Brief explanation of the drawing]

Figure 1(a) is a sectional view of a fuel element for a fast reactor, Figure 1(b)
) is an external view of the pellet shape, and Figure 2 is CS 2 U 0
FIG. 3(a) is a partial vertical cross-sectional view of a fuel element according to a preferred embodiment of the present invention, and FIG. ) is a diagram showing the shape of the blanket fuel pellet, FIG. 4 is a characteristic diagram of actual measurement results showing the Cs concentration in the fuel element, and FIG.
It is a conceptual diagram that models the graph in the figure. 1... Fuel cladding tube, 2... Core fuel pellets, 3...
...Blanket fuel pellets, 4...Gap, 5
...gas plenum, 6...suffring, 7...C
82UO4 (cesium urantite). Agent Patent Attorney Akio Takahashi (0-) No. (η) 2nd Part (b) (δ) No. 4- Imperial Order ￥te Map Peυ・, P 7f Naoomi Agata

Claims (1)

[Claims] 1. A core fuel section in which a fuel cladding tube is filled with nuclear fuel pellets formed by forming nuclear fuel whose main raw material is fissile parent material enriched with fissile material into a cylindrical shape, and a nuclear fuel whose main raw material is fissile parent material. In a fast reactor fuel element having a blanket fuel part filled with nuclear fuel pellets formed into a cylindrical shape, the shape of the nuclear fuel pellet in the blanket fuel part is modified by cutting off the corners along the circumference of the nuclear fuel pellet. A fuel element for a fast reactor, characterized in that it has a shape.