JPS5958387A - Cladding tube of 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 improvements in cladding tubes for nuclear fuel elements.

In nuclear reactors that are currently being constructed, manufactured, and operated, nuclear fuel material is usually enclosed in a cladding tube that has excellent corrosion resistance, nonreactivity, and thermal conductivity.

Then, these fuel elements are assembled in a lattice shape at regular intervals in channels through which coolant flows, forming fuel assemblies, and a suitable number of these fuel assemblies are combined to form a fission chain reaction capable of nuclear fission reactions. They are combined to form a reactor core, which is housed in a reactor vessel through which coolant flows.

Therefore, cladding is installed for several purposes:
Its primary purpose is to prevent contact and chemical reactions between the nuclear fuel and the coolant or moderator. The second purpose is
To prevent radioactive fission products, which are partially gaseous, from escaping from the fuel into the coolant or moderator, or into both the coolant and moderator if both are present. be. Common cladding materials include stainless steel, zirconium and its alloys. Failure of these claddings, i.e., loss of leak-tightness, can result in contamination of the cladding material or moderator and its associated systems with radioactive long-lived products to the extent that plant operation is impaired. There is a risk. Zirconium and its alloys are excellent cladding materials under uterine conditions. The reason is that zirconium and its alloys have a small neutron absorption cross section, and
This is because it is strong, ductile, extremely stable, and non-reactive in the presence of steam, which is commonly used as a nuclear reactor cooling material and moderator at temperatures below r.

However, the operation of fuel elements has revealed problems in which the interaction of the nuclear fuel, the cladding, and the fission products produced during the fission reaction results in brittle cracking of the cladding. This undesirable condition is believed to be facilitated by localized mechanical stress due to fuel-cladding differential expansion. This phenomenon is called stress corrosion cracking. Currently, nuclear reactor operating methods must be restricted in order to avoid stress corrosion cracking due to the release of fission products and mechanical interactions. In order to ensure the efficient operation and safety of the nuclear reactor, this stress g negative control f
There is an urgent need to establish a method to prevent L.

The present invention was made in view of the above situation, and it is an object of the present invention to provide four complete cladding tubes for nuclear fuel elements that can prevent stress corrosion cracking caused by fission products without reducing strength.

The cladding tube of the nuclear fuel element of the present invention does not have a nuclear fuel material loaded inside the cladding tube having a multilayer structure, and the cladding tube has the following characteristics:
The innermost tube is formed from a material having a lower strength than the outer tubes, and the innermost tube undergoes plastic deformation beyond its yield point before the nuclear fuel elements are assembled into a bundle. The cladding tube is raised up and an internal pressure having a thickness below the yield point is applied to the outer tube, so that a compressive force is applied to the right inside of the ift of the cladding tube.

In general, this occurs when stress constraints, material, environment, and stress conditions are satisfied. Therefore, this! One of the main factors, for example, stress corrosion cracking can be prevented if the applied stress is reduced. And stress corrosion cracking occurs under tensile stress, but not under compressive stress. ! , If compressive stress is applied to the material in advance, tensile stress will be applied to the material, f? L"45, it becomes difficult to shift from compressive stress to tension. Based on this fact, in the embodiment of the present invention, the strength of the material constituting the innermost tube is higher than the strength of the outer tube. It has a multi-tube structure lower than that of the metal clad tube, and by applying internal pressure to the innermost tube, compressive stress is applied to the innermost tube to prevent stress corrosion cracking.

FIG. 1 shows embodiments of the nuclear fuel element sheathing 7 to 7 of the present invention, and as shown in FIG. In a double layer like 2,
The stress-strain curve when internal pressure is applied is schematically drawn as shown in Figure 2. Figure 2 is a stress-strain curve diagram with strain on the horizontal axis and stress on the vertical axis.
Curve 4 of D is the outer cladding tube, ABI! J-curve 3 shows the respective stress-strain curve of the inner cladding. In such a cladding tube, after applying an internal pressure of such a magnitude that the inner tube U undergoes plastic deformation exceeding its yield point (point B) and the outer tube is below the yield point, the pressure is removed. When removed, the tensile stress of the outer tube returns almost to the point (A point), while the inner tube reaches point G, where the compressive stress acts, from point E, where the tensile stress acts. When the internal pressure is below the yield point of the inner cladding tube 1811, the inner cladding tube does not undergo plastic deformation, so no compressive stress is applied. If the internal pressure is higher than the yield point (point C) of the outer cladding tube, the entire cladding tube undergoes plastic deformation and the shape of the cladding tube changes, which is unwise. Therefore, the magnitude of the internal pressure must be within the range of above the yield point of the inner cladding tube and below the yield point of the outer cladding tube.

For example, one end of the cladding tube is sealed, and an internal pressure p+ kg/cnf is applied from the other end.

1) The thickness of I must be Jl, and as mentioned above, the yield point of the inner 11j cladding tube must be σγ11 (g/piece 2 or more, and the yield point of the outer cladding tube σY must be within the range of Q kg/mm 2 or less) .

Now, measure the inner diameter of the entire lowered arm (Trrm), and the outer diameter (r). m
m, and assuming that the thickness of the inner handling tube is sufficiently smaller than the thickness of the outer cladding tube, approximately p.

range? , fi Ii is given by the following equation (1).

If the internal pressure is removed after plastic deformation has occurred in the inner cladding tube in this way, compressive stress will remain in the inner cladding tube, and the mechanical interaction between the fuel pellet and the cladding tube will be affected. Even if tensile stress is applied to the inner surface of the cladding tube, this residual compressive stress is canceled out and stress corrosion cracking of the IE tube can be prevented. It is effective to make the internal pressure p1 as large as possible within the range of the IJ formula.

Incidentally, since zirconium and zirconium alloys recover at high temperatures, in order to generate Fl- and shrinkage residual stress by applying internal pressure, it is necessary to apply internal pressure within the υ soaking degree range where this recovery does not occur. In general, zirconium strength is more likely to cause recovery than zirconium alloys. Pure zirconium has a relatively slow recovery rate in a temperature range of about 300 C'tMFB- or less, but in a temperature range of about 350'C or more, the rate of recovery rapidly increases as the temperature rises. Therefore,
The starting temperature is 35 (I' or below, preferably 300
The temperature range below t:' is preferable.

FIG. 3 shows a T'rMJ diagram of the upper end of a nuclear fuel element using the cladding tube shown in FIG. 1, where 5 is a fuel pellet and the inside is covered with a zirconium tube. The composition of the Zircaloy-2 outer tube is Zr
:98.4%, Sn: 1.50%, Fe: 0.112%
+N+'0-001'A+Cr: 0.
03%, and the zirconium inner tube also has high strength. 6 is an end plug, and 7 is a tin ring. In this two-M tube structure, before assembling the fuel elements into a bundle, one end of the tube is sealed by welding a zirconium alloy end plug 6, and high-pressure inert gas or oil is introduced from the other end. Then apply internal pressure. In this case, the applied internal pressure is
It varies depending on the yield stress of Zircaloy-2 and Zircaloy-2, temperature, and the dimensions of the cladding tube. In the case of the silconide liner tube with an outer diameter of 125 mm and an inner diameter of 10.7 mm, which was adopted in this example, the temperature is plotted on the horizontal axis. Yield stress f on the vertical axis
: From the relationship between temperature and yield stress in Figure 4, which shows the yield point-temperature relationship curve in Figure 4, the relationship between applied internal pressure and temperature is as follows: Temperature is on the horizontal axis and internal pressure is on the vertical axis. This is the hunting range AC/) shown in FIG.

In FIG. 4, 10 is a curve showing the relationship between the yield stress and temperature of zirconium, and 11- is a curve showing the relationship between the yield stress and temperature of Zircaloy-2.

Therefore, if the temperature of the cladding tube is checked before pressure treatment is applied to the cladding tube, and pressure is applied within the pressure range shown in Figure 5, compressive stress will be applied to the zirconium inner tube l inside the cladding tube. be able to. The internal pressure may be applied for 1 minute, and the temperature may be from room temperature to about 300C.

In this way, the nuclear fuel cladding tube of this example is formed in such a state that a compressive force is applied to one of the dual zones, so that stress on the cladding tube due to nuclear fission products, etc. It prevents corrosion cracking and maintains the same strength as the conventional one.

As described above, the Ishi 1 combustion absorption cladding tube of the present invention is
This has the effect of preventing stress corrosion cracking caused by fission products without reducing the strength by 1 bar.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a double tube consisting of a zirconium inner tube and a zirconium alloy outer tube in a framed embodiment of the nuclear fuel element of the present invention, and FIG. 2 shows an internal pressure applied to the cladding tube of FIG. 1. Figure 3 is a cross-sectional view of the upper end of the cladding tube in Figure 1. Figure 4 is the relationship between the yield point and temperature of Zircaloy-2 and zirconium. A simplified diagram, FIG. 5, is an explanatory diagram of the relationship between the pressure range of the cladding tube of the moon stone in FIG. 3 and the temperature. l...Zirconium inner tube, 2...Zircaloy-2
Outside V1 plan 21 audience 73 fig.

Claims (1)

[Scope of Claims] 1. In a cladding tube in which nuclear fuel material is loaded inside the cladding having a multilayer structure, the cladding 'C1 has a structure in which the innermost tube has a lower strength than the outer tube. the innermost tube undergoes plastic deformation exceeding its yield point in the state before the nuclear fuel elements are assembled into a bundle, and the outermost tube has a size below the yield point. A cladding tube for a nuclear fuel element, characterized in that the innermost tube of the cladding tube is formed in such a state that a compressive force is applied to the innermost tube.29. A cladding tube for a nuclear fuel element according to claim 1, which is formed from a zirconium alloy. 3. The cladding tube has a square tube structure in which the innermost tube is made of pure zirconium and the outer tube is made of a zircaloy alloy. A cladding agent for a nuclear fuel element according to claim 1 formed therein. 4. The cladding tube is subjected to an internal pressure in a vorticity range of 300 C or higher from vorticity to a compressive force acting on the innermost tube. A cladding tube for a nuclear fuel element according to claim 1, wherein the cladding tube is formed by: