JPS6026991B2 - Nuclear fuel elements and their manufacturing methods - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel element and a method for producing the same, and in particular to a zirconium alloy cladding tube that does not undergo stress corrosion cracking due to iodine, which is a corrosive fission product, and arm formation due to hydrogen gas. An object of the present invention is to provide a nuclear fuel element and a method for manufacturing the same.

Nuclear fuel elements for water reactors are made by filling a Zircaloy cladding tube with nuclear fuel material, such as condensed uranium dioxide pellets, and
This cladding tube has a structure in which helium gas is sealed inside and both ends are closed with end plugs.

The role of this cladding is to prevent reactions between the nuclear fuel material and the coolant in the reactor, and to prevent radioactive fission products from being released from the nuclear fuel element and becoming mixed in during cooling.
Therefore, in the unlikely event that the cladding tube is damaged during reactor operation, fission products may enter the coolant, increasing the radioactivity level of the coolant and ultimately interfering with plant operation. become. Typically, zirconium alloy cladding is a metal that has excellent corrosion resistance against water and steam, has a small neutron absorption cross section, and has sufficient ductility even under irradiation. However, it has become clear that if stress corrosion cracking or hydrogen arm formation occurs in Zircaloy cladding, it will break with much smaller strain than the designed value.

The tensile stress acting on the cladding that causes stress corrosion cracking in Zircaloy cladding is based on the mechanical interaction between the fuel pellet and the cladding. In other words, the thermal field expansion coefficient of uranium dioxide pellets is approximately twice as large as the thermal expansion coefficient of zirconium alloy cladding, and moreover, the average temperature of the fuel pellets is 12
Not only is it very high, on the order of 0.00, but also as the power of the reactor increases, cracks appear in the pellets, and the pellets rearrange to fill the gap between the pellets and the cladding. Therefore, the pellet and the cladding tube come into contact, and a tensile stress is applied to the cladding tube. In order to prevent this stress corrosion cracking, it is necessary to eliminate either the tensile stress acting on the cladding tube or the removal of iodine, which is a fission product. To remove iodine within the nuclear fuel element and prevent direct reaction between the cladding and iodine, an iodine getter that reacts more easily with iodine than the zirconium alloy cladding can be loaded into the nuclear fuel element, or a pellet and cladding can be loaded into the nuclear fuel element. One possible method is to provide a metal layer between the tube and the tube to create a barrier for iodine. As an iodine getter, one in which barium oxide is dispersed on a matrix made of stainless steel wire mesh or activated alumina has been proposed, for example, in Japanese Patent Publication No. 48-232.

Additionally, Samiko Shofun-6-6-Sho No. 3 also proposes interposing metals such as AI, Nb, and stainless steel between the uranium dioxide scale pellets and the cladding tube in order to prevent them from reacting directly. has been done. In addition, Samurai Kaisho 51-697
No. 92 uses copper, nickel, and iron on the inner surface of the cladding tube, Tokusekki No. 51-69795 uses silconium, and Tokkan Sho 51-69796 uses aluminum, chromium, and
JP-A-51-71497 proposes a nuclear fuel element having a chromium-containing oxidation-resistant alloy, a chromium coating, an aluminization coating, and a silicide coating, in which molybdenum and niobium are alloyed with a cladding tube.
Furthermore, in the publication of JOKAI No. 51-69794, the inner surface of the cladding tube is coated with chromium or chromium alloy, and a metal liner of stainless steel, copper, copper alloy, nickel, or nickel alloy is placed between the cladding tube and the fuel. A proposal has been made to establish a Further, JP-A-51-71498 discloses that a metal barrier of aluminum, copper, niobium, nickel, stainless steel, or iron is provided inside the cladding tube, and an inner layer is further formed by metallurgically bonding a zirconium alloy to the inside of the metal barrier. There have been proposals to establish a All of the conventional examples exemplified above have a metal layer provided inside the cladding tube to prevent the corrosive fission products from directly reacting with the cladding tube.

Disadvantages of providing such a metal layer within a nuclear fuel element include the following. First, the cost of manufacturing nuclear fuel elements increases. Second, the metal layer and the cladding may react to form a reactive layer that changes the properties of the cladding.
Thirdly, if the metal layer is locally destroyed, corrosion may concentrate on a portion of the cladding. Fourth, if the neutron absorption cross section of the metal layer is larger than that of zirconium, it gives rise to a neutron absorption potential in core design. On the other hand, it is known that hydrogen gas, which is the main cause of the formation of hydrogen arms in the Zircaloy cladding tube, is generated from moisture adsorbed on the fuel pellet during the production of the fuel shell. During reactor operation, water vapor reacts with the cladding and fuel pellets to liberate hydrogen, and the gas atmosphere in the fuel chamber changes from an oxidizing atmosphere to a reducing atmosphere. On the other hand, Zircaloy-clad tubes do not undergo hydrogenation in the presence of moisture and oxidizing agents such as oxygen above a certain level. One of the ways to prevent such hydrogen saponification is to use a hydrogen getter made of nickel-titanium-zirconium alloy.
This is proposed in Publication No. 953. However, the getter material is made of a chemically active alloy, and since the fuel chamber is filled with water vapor at the beginning of operation, the getter reacts with the water vapor, making the remaining atmosphere reducing, and causing the cladding to deteriorate. This method has the disadvantage that it increases the risk of hydrogenation and that the getter surface is covered with an oxide film, which reduces its hydrogen absorption ability. For these reasons, it is still desired to develop a nuclear fuel element equipped with a Zircaloy cladding that does not cause stress corrosion cracking and hydrogen arm formation. The present invention has been made in view of the above-mentioned needs, and provides a nuclear fuel element and a method for manufacturing the same that do not cause stress corrosion cracking and hydrogen arm formation in the Zircaloy cladding due to iodine, which is a radioactive fission product. is intended to provide.

In other words, this invention prevents stress corrosion cracking of the Zircaloy cladding due to iodine, a radioactive fission product, by creating an oxidizing atmosphere within the nuclear fuel element and increasing the partial pressure of oxygen in the atmosphere to 7ron or more. and a nuclear fuel element capable of preventing hydrogen verticalization due to hydrogen gas, and a method for producing the same. Note that the following methods can be used to adjust the oxygen partial pressure within the nuclear fuel element to the above value.
○) In a nuclear fuel element in which a zirconium alloy cladding tube 2 is filled with nuclear fuel material 1 as shown in FIG. When sealing the pond end of the cladding tube 2 with all 3 end keys in a chamber filled with helium,
The end plug is welded under conditions where a partial pressure of oxygen exists in the helium gas. In order to achieve the above oxygen partial pressure, a mixed gas of helium and oxygen mixed in a predetermined ratio may be introduced into the chamber, or helium and oxygen may be mixed in the chamber. You may let them. or,
Although the pressure of helium is usually 1 atm, it is possible to manufacture the above-mentioned nuclear fuel element even in helium at a pressure of 1 atm or higher. ■ As shown in Figure 1, a small capsule 6 pre-filled with oxygen is placed in the plenum part 4 of the nuclear fuel element, and the capsule 6 is destroyed by impact before being loaded into the reactor, or the nuclear fuel material is The heat generated by this causes the capsule 6 to rupture, and the oxygen partial pressure within the nuclear fuel element to exceed 40°C.

A suitable material for the capsule 6 is glass, which is weak against impact, or a metal with a low melting point. In a boiling water reactor, the temperature of the plenum part 4 is approximately 290q○, so if, for example, 67% lead general % tin with a melting point of 2760 is used as the capsule material, the capsule will melt and oxygen gas will be released into the nuclear fuel element. fills the interior space of. According to the nuclear fuel element manufactured by the above method, the atmosphere in the internal space of the nuclear fuel element becomes an oxygen atmosphere in which the oxygen partial pressure is higher than the pore orr, so that the zirconium alloy is made of iodine, which is a radioactive corrosive fission product. Stress corrosion cracking and hydrogen sulfidation of the cladding can be prevented. In order to clarify the effectiveness of the present invention, specific examples will be shown below.

Example 1 In order to investigate the effect of oxygen on stress corrosion cracking caused by iodine in a Zircaloy-2 cladding tube, the Zircaloy-2 cladding tube was placed in a Natsuzora capsule together with iodine and oxygen.
A core of an AI alloy having a high coefficient of thermal expansion was inserted into the cladding tube, and the temperature was maintained at 35°C and 0.00°C to apply stress.

FIG. 2 shows the relationship between the rupture time of the cladding tube and the partial pressure of oxygen sealed in the vacuum capsule. From FIG. 2, it was confirmed that stress corrosion cracking does not occur when the partial pressure of oxygen is 9 om or more. Example 2 In order to investigate the influence of oxygen partial pressure on the hydrogenation of a Zircaloy 2 cladding tube, a reaction was carried out at 300°C in an atmosphere of a mixture of pure hydrogen and oxygen at 1 atm.

In pure hydrogen, all Zr in Zircaloy 2 is destroyed in 8 hours.
becomes a hydride of ZrH2, while the oxygen partial pressure is h
Zircaloy-2 did not react with hydrogen at all even after 8 hours of reaction in hydrogen containing Prr. It is clear that the presence of more than nine tons of oxygen in the nuclear fuel element from the above examples prevents stress corrosion cracking of the Zircaloy cladding and hydrogen absorption by the radioactively corrosive fission product iodine.

[Brief explanation of the drawing]

Fig. 1 is a vertical cross-sectional view showing only the upper part of the nuclear fuel element according to the present invention, and Fig. 2 is a curve diagram showing the influence of oxygen partial pressure on the time for stress corrosion cracking to occur in the Zircaloy 2 cladding tube in Fig. 1. be. 1... fuel pellet, 2... cladding tube,
3...End plug, 4...Plenum part, 5...
...Spring, 6...Capsule filled with oxygen gas. Figure 1 Figure 2

Claims (1)

[Scope of Claims] 1. A nuclear fuel element in which a zirconium alloy cladding tube is filled with uranium oxide-based sintered pellets, characterized in that oxygen gas having an oxygen partial pressure of 7 torr or more is sealed in the nuclear fuel element. nuclear fuel elements. 2. Claim 1, characterized in that oxygen gas is sealed in a capsule arranged in a plenum part, and when the capsule is destroyed, the oxygen partial pressure in the plenum part becomes 7 torr or more. Nuclear fuel elements listed. 3. After sealing one end of the zirconium alloy cladding tube with an end plug, nuclear fuel material is placed in the cladding tube, and then the other end of the cladding tube is sealed with an end plug in a chamber filled with helium gas. At this time, the partial pressure of oxygen in helium gas is 7 torr.
1. A method for producing a nuclear fuel element, characterized by sealing it under conditions that exist at least r.