JPS6335952B2 - - Google Patents

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 in particular to a fuel 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. Regarding elements.

[Background of the invention]

As shown in FIG. 1, the fuel element used in a fast reactor is a fuel cladding tube 1 made of stainless steel.
It is filled with fuel pellets 2 made of nuclear fuel enriched with fissile material, and fuel pellets 3 made of nuclear fuel mainly made of fissile parent material. 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 containing fissile parent material are called blanket fuel pellets. Both fuel pellets have a cylindrical shape with a length of about 1 cm, as shown in Figure 1b. The diameter of the pellet is slightly smaller than the inner diameter of the fuel cladding tube 1, so that a gap 4 is created between the fuel pellet and the fuel cladding tube.

Currently, sintered oxides such as ²³⁸ U and ²⁴⁰ Pu, which are fission-friendly materials, are mainly used for blanket fuel pellets, and core fuel pellets are made from fission-friendly materials such as 238 U and 240 Pu.
Along with ²³⁸ U and ²⁴⁰ Pu, an oxide sintered body containing fissile materials ²⁴¹ Pu, ²³⁹ Pu, and ²³⁵ U at a weight ratio of approximately 15% is used. Blanket fuel pellets are
They are usually placed above and below the core fuel pellet section. In the core fuel pellet section, 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 portion is small, the fission parent materials ²³⁸ U and ²⁴⁰ Pu absorb the neutrons released from the core fuel portion, causing fissile material to form.
A reaction occurs that changes it to ²⁴¹ Pu. The heat energy generated is removed by sodium coolant flowing outside the fuel cladding.

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

The role of the fuel cladding tube 1 is to prevent contact between the fuel pellets and the coolant sodium, and to keep the FP gas within the fuel element and prevent it from being released to the outside. If the fuel cladding tube 1 breaks, FP gas will be released and the fuel pellets will come into contact with sodium. The surface of the fuel pellets that undergo nuclear fission is approximately
The high temperature is over 900℃, which could lead to boiling of the sodium, which could 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 mostly in the core fuel pellets, so
The amount of Gs produced is also large mainly in the core fuel pellets. In addition, the core fuel pellets are extremely hot due to the large amount of energy released by nuclear fission, and the generated Cs is vaporized and released into the cladding tubes of the fuel elements. The released Cs is shown in Figure 1a.
The fuel begins to diffuse into the upper and lower blanket fuel sections through gap 4 of the fuel tank. In the blanket fuel pellet portion, the rate of nuclear fission reactions is small, so the pellet temperature is much lower than in the core fuel pellet. Therefore, the diffused Cs gas becomes solid metal Cs in the blanket fuel pellet portion and adheres to the surface of the blanket fuel pellet.
Here, Cs and blanket fuel pellets are
It is known that the reaction expressed as 2Cs + UO ₄ →Cs ₂ UO ₄ (cesium uranite) occurs with UO ₄ . Because the crystal density of Cs ₂ UO ₄ produced in this reaction is small, the volume of the cohesive part of Cs ₂ UO ₄ becomes large,
A rise as conceptually shown in FIG. 2 occurs.

This raised portion 7 due to Cs ₂ UO ₄ is large near the boundary between the blanket fuel pellet and the core fuel pellet, and as the stay time of the fuel element in the reactor becomes longer, stress acts on the cladding tube, and this stress causes the cladding tube to deteriorate. The strain may eventually lead to damage to the fuel cladding.

[Purpose of the invention]

The object of the present invention is to
An object of the present invention is to provide a fuel element for a fast reactor that can prevent damage to fuel cladding due to Cs ₂ UO ₄ production.

[Summary of the invention]

It has been experimentally revealed that the areas where Cs adhesion and Cs ₂ UO ₄ are likely to be formed 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,
By reducing the diameter of the blanket fuel pellet and widening the gap between it and the fuel cladding tube, Cs ₂ UO ₄ can be reduced in areas where Cs ₂ UO ₄ is likely to be generated, and therefore the fuel cladding is likely to be damaged. By generation,
Made to absorb volumetric expansion. The present invention will be explained in detail below using Examples.

[Embodiments of the invention]

FIG. 3 shows an embodiment of the present invention. FIG. 3a shows a blanket fuel pellet of the present invention, in which the corners along the circumference have been removed from the conventional blanket fuel pellet. FIG. 3b shows a cladding tube filled with pellets of the present invention. The core fuel pellets will be the same as before. FIG. 4 shows the 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, and shows that the Cs concentration in the pellet gaps is high. Therefore, it is considered that cladding tube failure due to the formation of Cs ₂ CO ₄ is likely to occur in the gaps between the blanket fuel pellets. According to the embodiment shown in FIG. 3, the gap between the blanket fuel and the cladding increases in the gap between the fuel and the cladding, thereby preventing damage to the cladding due to the formation of Cs ₂ UO ₄ in this area.

According to FIG. 4, the concentration of Cs is at most 3×10 ^-4 g/mm ² per 1 mm ² of the inner surface of the fuel cladding tube. Here, if Cs atomic weight M (Cs) 133 Cs ₂ UO ₄ atomic weight M (Cs ₂ UO ₄ ) 568 Cs ₂ UO ₄ density ρ 6.6 g/cm ³ , the thickness s of accumulated Cs ₂ UO ₄ is s =568 133×2×3×10 ^-4 6.6×10 ^-3 =0.097mm. 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 the heat transfer coefficient due to the gap is small, 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 temperature of the core fuel pellets, which will further enhance the effect of preventing cladding failure. be able to.

〔Effect of the invention〕

As described above, according to the present invention, damage to the fuel cladding tube due to the formation of cesium uranite in the fuel element for a fast reactor can be prevented.

[Brief explanation of drawings]

Figure 1a is a cross-sectional view of a fuel element for a fast reactor, Figure 1b is an external view of a fuel pellet, Figure 2 is a conceptual diagram showing swelling of a cladding tube due to Cs ₂ UO ₄ production, and Figure 3
Figure a is a partial vertical sectional view of a fuel element according to a preferred embodiment of the present invention, Figure 3b is a diagram showing the shape of the blanket fuel pellet shown in Figure 3a, and Figure 4 is a diagram showing the inside of the fuel element. Figure 5 is a conceptual diagram that models the graph in Figure 4. 1... Fuel cladding tube, 2... Core fuel pellets,
3... Blanket fuel pellet, 4... Gap, 5... Gas plenum, 6... Spring, 7
...Cs ₂ UO ₄ (cesium urantite).

Claims (1)

[Claims] 1. A core fuel section filled with nuclear fuel pellets made by molding nuclear fuel whose main raw material is a fissile parent material enriched with fissile material into a cylindrical shape in a fuel cladding tube, and a nuclear fuel whose main raw material is a fissile parent material enriched with fissile 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 a shape in which corners along the circumference of the nuclear fuel pellet are shaved off. A fuel element for fast reactors, characterized by: