JPH0363038B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a nuclear fuel element, and more particularly to a fuel element suitable for corrosion resistance and wear resistance in high temperature sodium.

[Background of the invention]

The cladding tubes of nuclear fuel elements used in fast reactors are made of SUS316 and are in direct contact with high-temperature sodium. The cladding tube is activated by irradiation with neutrons in the core region. Furthermore, the cladding tube corrodes in the sodium because it is exposed to the high temperature of the sodium. Based on these factors, radioactive corrosion products are leached into the sodium. The eluted radioactive corrosion products are carried along the flow of sodium to the cooling system piping and deposited on the surfaces of piping and equipment (pumps, intermediate heat exchangers, check valves, etc.). These deposited radioactive corrosion products become a source of radiation exposure during equipment maintenance and inspection, and are therefore a major hindrance during maintenance. For this reason, methods are being considered to remove the radioactive corrosion products released into the sodium using a cold trap to purify the impurities in the sodium. However, these removal methods do not necessarily have sufficient removal efficiency, and some of the radioactive corrosion products released into the sodium inevitably settle on the piping and equipment of the cooling system before they can be removed. .

Now, in the reactor core, the coolant sodium flows through the fuel assembly at a high flow rate of 5 to 6 m/s, so it is conceivable that the fuel elements within the fuel assembly vibrate and come into contact with each other. At this time,
There is a possibility that the cladding will wear out to some extent.
Therefore, it is preferable to have a structure that can reduce wear even if there is contact.

As a structure for reducing wear, it is known from JP-A-54-36469 that a wear-resistant coating made of titanium carbide or the like is formed on the outer peripheral surface of the cladding tube of the fuel rod.

However, in this known example, anti-abrasion is achieved by reliably improving peeling due to the difference in thermal expansion between the cladding tube and the coating in a high-temperature environment and preventing the diffusion of radioactive corrosion products by actively repairing the radioactive corrosion products in sodium. There is no disclosure that it will be established in balance with the above.

[Purpose of the invention]

Accordingly, an object of the present invention is to provide a nuclear fuel element capable of reducing the amount of corrosion and wear, which is highly reliable even in a high-temperature environment, and has a configuration that enables prevention of diffusion of radioactive corrosion products.

[Summary of the invention]

The structure of the nuclear fuel element of the present invention consists of a cladding tube sealed at both ends and a plurality of fuel pellets filled in the cladding tube. It is a nuclear fuel element characterized by having a layer containing Ni or Mo mixed in a ceramic formed of one of these materials.This structure makes the cladding excellent in corrosion resistance and wear resistance. It is protected by ceramic, and since the ceramic contains metals such as Ni or Mo, it improves the difference in thermal expansion between the metallic cladding tube and that layer, and furthermore, the layer is protected from the cladding tube in sodium. In addition to acting as a diffusion barrier for radioactive corrosion products attempting to escape into the reactor core, the layer also adsorbs radioactive corrosion products in the sodium flowing through the reactor core, preventing radioactive corrosion products from dispersing outside the reactor core. .

[Embodiments of the invention]

Hereinafter, a nuclear fuel element that is an embodiment of the present invention will be described.

FIG. 1 shows a cross section of a fuel assembly loaded with nuclear fuel elements of this example. The fuel assembly is
Although not shown, a lower tie plate with an entrance nozzle, a lapper tube 1 and a nuclear fuel element 2
It is made up of The lower end of the nuclear fuel element 2 is attached to the lower tie plate. The lower end of the wrapper tube 1 is also attached to the upper part of the lower tie plate. A nuclear fuel element 2 is housed inside the wrapper tube 1. FIG. 2 shows the detailed structure of the nuclear fuel element 2. The nuclear fuel element 2 includes a fuel cladding tube 3
The inside thereof is filled with fuel pellets 4 and blanket pellets 5A and 5B.
Furthermore, a spring 6 is provided above the blanket pellet 5A within the fuel cladding tube 3.
A wire spacer 7 is wrapped around the outside of the fuel cladding tube 3 . The spring 6 is connected to the pellet 4,
5A and 5B are fixed so that they do not move up and down freely. The wire spacer 7 prevents adjacent nuclear fuel elements 2 within the wrapper tube 1 from coming into direct contact with each other. In the nuclear fuel element 2, as shown in FIG. 3, the outer surface of the fuel cladding tube 3 is coated with a ceramic layer 9.

Suitable materials for the ceramic layer 9 include SiC, TiN, and TiC, which have excellent wear resistance and corrosion resistance. In addition to this, a so-called cermet, which is a mixture of metal powder in the aforementioned ceramics, may also be used. Ion plating is an example of a method for coating the surface of a metal material such as a fuel cladding tube (SUS316) with ceramics. This method reduces the pressure inside the vacuum container.
A high voltage (several hundred to several thousand V) is applied in an argon gas atmosphere of 10 ^-3 to 10 ^-4 Torr, and ionized argon gas and evaporated ceramic molecules are incident on the metal surface together. be. As a result, a ceramic coating is formed on the surface of the metal material.

The adhesion between the ceramic coating and the metal material is
It varies depending on the degree of contamination on the surface of the metal material. The surface of the metal material is cleaned by sputtering argon ions or the like in advance to remove dirt caused by oil or the like on the metal surface before ion plating is applied. If the surface of the material to be coated is cleaned in this way,
The adhesion between the metal and the coated ceramic layer is improved, and it is possible to prevent the ceramic layer from peeling off in sodium.

The thickness of the coated ceramic layer 9 may be arbitrary, but a thickness of about several μm is sufficient. The thickness of the fuel cladding tube 3 is approximately 350 to 600 μm, and the thickness of the coated ceramic layer 9 is several percent of this thickness. Since the manufacturing accuracy of the fuel cladding tube 3 is approximately several tens of μm, the thickness of the ceramic layer 9 can be kept within the manufacturing error range of the fuel cladding tube 3. The coefficient of linear expansion of ceramics, for example, the coefficient of linear expansion of TiC, is 7.5 to 8 × 10 ^-6 /℃, and the coefficient of linear expansion of the fuel cladding tube 3 (15 × in the case of SUS316)
10 ^-6 /℃). Because of the coating, when a fast reactor is operated with nuclear fuel elements 2 loaded into the reactor core, the temperature increases, so there is a concern that the ceramic layer 9 may peel off from the surface of the fuel cladding tube 3. However, since the thickness of the coated ceramic layer 9 is as thin as several μm, even if the fuel cladding tube 3 expands or contracts due to thermal expansion or the like, the ceramic layer 9 will not peel off from the surface of the fuel cladding tube 3. As mentioned above, the thermal expansion coefficient of ceramics is TiC or NbC.
This can be improved by adding Ni or Mo to either of them, that is, by making them a cermet. Furthermore, in terms of thermal conductivity, the ceramic layer 9 has almost no difference from SUS316, which is generally used as a cladding material. Fuel cladding tube 3
The temperature difference between the inner and outer surfaces is about several tens of degrees Celsius, and the thickness of the ceramic layer 9 is several μm, so the ceramic layer 9 does not become a resistance to heat transfer.

As described above, by coating the surface of the fuel cladding tube 3 with ceramics, etc., the material of the fuel cladding tube 3 does not come into direct contact with high-temperature sodium, so corrosion of the fuel cladding tube 3 is prevented. can. This can prevent radioactive corrosion products from eluting into the sodium from the fuel cladding tube 3, which has been activated by neutron irradiation, and the amount of radioactive corrosion products deposited on the surfaces of cooling system piping and equipment. This makes it possible to reduce the Furthermore, since the ceramic layer 9 has not only anti-corrosion properties but also anti-wear properties, the amount of wear and tear on the fuel cladding tube 3 due to contact between the nuclear fuel elements 2 in the fuel assembly or between the nuclear fuel elements 2 and the wire spacer 7 can be reduced. As a result, the integrity of the fuel cladding tube 3 can be improved, and nuclear fission products due to fuel damage or the like can be prevented from eluting into the sodium or transferring to the gas phase. Furthermore, if cermets containing Ni, Mo, etc. added to ceramics were used as a coating material, not only would heat conduction and coefficient of thermal expansion be improved, but impurities in sodium would become radioactive in the reactor core. Radioactive corrosion products such as ⁵⁸ Co, ⁶⁰ Co, ⁵⁴ Mn, etc. can be removed by adsorption. In other words, by using cermet, it is possible to prevent radioactive corrosion products from leaching into sodium, and the radioactive corrosion products present in sodium can be collected intensively on the surface of the fuel cladding tube. It is possible to achieve three effects at the same time: sex. Furthermore, by concentrating radioactive corrosion products in the fuel cladding,
In the treatment of radioactive waste, only the nuclear fuel elements need be treated, so that the treatment steps can be shortened and the dispersion of radioactivity can be minimized.

〔Effect of the invention〕

According to the present invention, since the layer covering the outer surface of the cladding tube of a nuclear fuel element is made of ceramic containing a metal that easily adsorbs radioactive corrosion products in sodium, the cladding tube has excellent corrosion resistance and wear resistance. The possibility of separation between the cladding and the cladding layer and the diffusion of radioactive corrosion products can be reduced, ensuring the integrity of the cladding and suppressing the diffusion of radioactive corrosion products, allowing This makes it possible to facilitate maintenance and simplify the radiation shielding structure.

[Brief explanation of drawings]

FIG. 1 is a sectional line of a fuel assembly, FIG. 2 is a vertical sectional view of a nuclear fuel element, and FIG. 3 is a partially enlarged view of the nuclear fuel element of FIG. 2. 2... Nuclear fuel element, 3... Fuel cladding tube, 4... Fuel pellet, 9... Ceramics layer.

Claims (1)

[Claims] 1. A nuclear fuel element consisting of a cladding tube sealed at both ends and a plurality of fuel pellets filled in the cladding tube, in which a ceramic film made of any one of SiC, TiN, and TiC is formed on the outer surface of the cladding tube. A nuclear fuel element characterized by having a layer in which Ni or Mo is mixed.