JPS5958389A - 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] TECHNICAL FIELD This invention relates to improvements in nuclear fuel elements.

Generally, a nuclear fuel element stores a plurality of nuclear fuel pellets 2 in a stack in a cladding tube 1 as shown in FIG.
The openings at both ends of the cladding tube 1 are j'! ! A plug 3a.

It's more dense than 3bK. Is the above fuel pellet 2 a fissile oxide fuel powder? , fi=lle can also be formed and sintered into cylindrical pellets with a length to diameter ratio of about 1. 4τ1 In the figure, 4 is a spring that has the function of forming a gas reservoir brenum 5 in the multi-tube 1 and the function of stably supporting the nuclear fuel pellets 2r.

By the way, in the nuclear fuel element configured as above-
The C1 cladding tube 1 has the function of preventing the coolant from coming into contact with the nuclear fuel pellets 2 and preventing chemical reactions from occurring, and the function of preventing radioactive fission products released from the fuel from mixing in the coolant. A function is required to prevent this from occurring. Therefore, in the event that the cladding tube ruptures (d), the radioactivity level in the cooling system plant increases and the nuclear reactor is shut down to ensure safety. There are seven conditions in which the operation of the vehicle must be stopped.

On the other hand, cladding tubes for nuclear fuel used in water-cooled nuclear reactors are generally made of zirconium and zirconium alloy materials. Zirconium and its alloys have a small neutron absorption cross section, are strong and ductile at temperatures below about 400 C, and have the characteristics of not reacting with heat or water vapor used as a coolant.

However, according to the operating experience up to the present day, even with cladding made of zirconium and its alloys, there is a risk of interaction such as a decrease in material strength due to neutron irradiation and corrosion due to chemical reactions with nuclear fission products. #Sexual cracking occurs due to action. It's not that hopeful.''
Elephants are born and grow old in the following way. 1. ,! In order to efficiently transfer the heat generated by the nuclear fuel pellet 2 to the outer surface of the cladding tube 1, the gear formed between the inner surface of the cladding tube 1 and the nuclear fuel pellet 2 should be set to a thickness of 100 microns or less. There is a need to. On the other hand, during operation, the nuclear fuel pellet 2 generates heat, which causes the pellet itself to crack due to thermal stress, resulting in discrepancies in the fracture surfaces, and furthermore, volumetric expansion caused by the accumulation of fission products within the nuclear fuel pellet as it burns. As shown in FIG.
Stress expanded by force? receive. Although the average value of the stress in the circumferential direction that the jacket tube 1 receives is not very large, strain is locally concentrated on the wall of the sieve near the crack 6 that occurs in the nuclear fuel pellet 2, and this strain reaches more than the yield stress. . Further, with nuclear fission, corrosive gases such as iodine and iodine compounds, cesium and cesium compounds are generated from the nuclear/fuel pellet 2, and this corrosive gas is absorbed into the free space within the cladding tube 1, that is, the crack 6. etc. In other words, corrosive gas tends to collect near the part of the skinned pipe 1 where strain is particularly concentrated.

In general, when stress (particularly greater than yield stress) is applied in an atmosphere of 11 mo corrosive gas, the ductility of the material decreases and a brittle fracture phenomenon called stress corrosion cracking occurs. Stress corrosion cracking is caused by temperature, stress, corrosive gas concentration, dissolved acid accumulation,
It also depends on the composition of the alloy, heat treatment, degree of processing, etc.
The mechanism by which it occurs is not unique.

In order to prevent these undesirable destructions, for example, as a conventional example, a method of inserting a lubricant between the fuel pellet 2 and the cladding tube 1 is shown in US Pat. No. 3,018,238, and As an example of providing a barrier between fuel pellet 1 and fuel pellet 2, 1) AS1238115 describes a titanium layer. Further, nuclear fuel rods surrounding a wire mesh 11% f fuel of Nb, Ta, Mo, and Zr are also known. Other barrier materials include stainless steel leeches, glassy substances, and At.

Be, Mg, Cu, etc. are disclosed in US Pat. No. 3,080,893 and US Pat.
No. 085059-, No. 3212788, No. 32917
No. 00 No. 3230150 specification and JP-A-1989-10
It is publicly known from the publication No. 9397.

Similarly, the concept of lining the cladding tube is well known, and is disclosed in U.S. Pat. No. 3,502,549, U.S. Pat.
In No. 69796 and No. 51-71497, Mo, W, Nb, C1, and NH are used as the inner lining.

Fe, Mg, Cu, pure 7r, ΔA, Ni-Cr alloy,
Aluminized coatings, silicided coatings, etc. are shown.

Furthermore, for the purpose of alleviating the stress concentration caused by the above-mentioned frictional force, the fuel pellet 2 is coated with a high lubricant such as Q':, graphite, molybdenum disulfide, etc., or with another gold maple material called a barrier. The 4!i' plan inserted between 4jt'E#1 is ¥T'3 US time.
018238 specification trace and JP-A-50-109396
It is stated in the No.

However, some of the barrier materials and lining materials mentioned in the above prior art have the disadvantage that the cross-sectional size of the medium 1't+suction tlt7 decreases the economical efficiency of the furnace. Additionally, in some of the above-cited levee proposals, the material used as a barrier and defense may be an ugly material that is compatible with nuclear fuel pellets, or a material that is difficult to be compatible with cladding. fυ, it cannot be said that a fundamental solution to the local chemical-mechanical interaction between the nuclear fuel and the cladding, which is a recent problem, has been reached.

The present invention has been developed in view of the above circumstances, and is designed to prevent stress corrosion cracking from occurring when stress is applied to the cladding tube due to interaction with fuel in corrosive gases, and to prevent stress from occurring on the inner surface of the tube when the cladding tube is damaged. Providing nuclear fuel elements that can prevent rapid external oxidation and improve reliability? This is the purpose.

In the nuclear fuel element of the present invention, fuel pellets are filled in a zirconium alloy-based cladding tube of the same size, and the openings at both ends of the cladding tube are end plugs. At least a portion of the inner surface of the cladding tube is lined with a liner made of a low-oxygen zirconium alloy material. The present inventors have discovered that among zirconium alloys that have excellent corrosion resistance when burned in water, zirconium alloys with low oxygen concentrations also have excellent resistance to stress corrosion cracking due to interaction with fuel. This low-oxygen zirconium alloy is lined on the inner surface of the system cladding tube.

In the following, explanation of one embodiment of the nuclear fuel element of the present invention having the same structure as the conventional one will be omitted. This will be explained using diagram f23. FIG. 3 is a graph showing the relationship between the oxygen concentration of Zircaloy-2 and the Vickers hardness, with the horizontal axis representing the degree of oxygen and the vertical axis representing the Vickers hardness. Hypoxic sponge zirconium contains 1% il, 59% iron, 0.16'3'o iron, respectively.
Chromium 0.11%, nickel 0.06%! ! -added;
A low-oxygen Zircaloy-2 ingot with an oxygen a degree of 5001 or less was obtained and processed into a thick circular shape. next,
oxygen? Approximately 13001-containing Zircaloy-2 thick-walled pipe with the above-mentioned low-oxygen Zircaloy-2 thick-walled tube on the inner surface of U? The tube was inserted into a tube having an outer diameter of 12.52 m and a wall thickness of 0.86 mm by a combination of cold working using a Pilger rolling mill and annealing. In the Zircaloy-2 sterilized tube processed in this way, a liner of phosphoric acid and Zircaloy-2 is formed on its inner surface. From the autoclave test results of a portion of the above-mentioned low-oxygen Zircaloy-2 liner, it was found that the corrosion resistance against high-temperature water was equivalent to that of the conventional Zircaloy-2 containing about 12001% oxygen.

Furthermore, the hardness of some of the hypoxic Zircaloy-2 liners is
As shown by curve A in the figure, it was found that the oxidation concentration decreased as the oxidation concentration decreased. Hardness is the ease with which the material can be deformed,
For example, it is well known that there is a proportional relationship with yield stress. Therefore, the low-oxygen Zircaloy-2 liner, which is part of the liner, has a lower yield stress than the cladding material, and is highly effective against stress corrosion cracking of the fuel cladding caused by the mechanical interaction between the cladding and fuel pellets. It can be said that it is a material.

In order to investigate the characteristics of a cladding tube lined with low-oxygen Zircaloy-2, we inserted the entire hollow nuclear fuel pellet into the cladding tube and filled the hollow part of the nuclear fuel pellet with a cylindrical pure aluminum rod. , iodine concentration 3+ng/l
cc, the aluminum rod was compressed in the longitudinal direction in an atmosphere with a cladding tube temperature of 350 C, and circumferential stress was applied to the cladding tube through the hollow nuclear fuel pellet. Then, the elongation at break that occurred in the cladding tube that was polished at this time was determined. As a result, the fourth
The characteristics shown in bar line B of the bar graph in the figure were obtained.

On the other hand, for comparison, a similar experiment 2 was conducted using a conventional cladding tube, and as a result, it was observed that the elongation at break had decreased as shown by bar C in FIG. 4.

As is clear from the results shown in Figure 4, in the nuclear fuel element of this example, when stress is applied to the cladding due to interaction with fuel in corrosive gas, stress corrosion cracking of the 'FfJ cladding occurs and That is, the low-oxygen Zircaloy-2 liner has a one-to-one movement in preventing stress corrosion cracking, and has strong resistance and allows full elongation.

Further, rapid oxidation of the inner surface of the pipe when water enters in the direction of the coating can be prevented, and reliability can be improved.

In the above embodiment, a cladding tube made of Zircaloy-2 and a liner material made of Zircaloy-2 were explained, but the same effect can be obtained in the case of a cladding tube made of other zirconium alloys such as Zircaloy-4. have an effect. Furthermore, since corrosion cracking is most likely to occur in the axially intermediate portion of the cladding tube, it is effective to arrange the liner only in this intermediate portion.

As described above, the nuclear fuel element of the present invention is resistant to stress corrosion cracking when stress is applied to the cladding tube due to interaction with fuel in corrosive gas, and rapid oxidation of the inner surface of the tube when the cladding tube breaks. Is there an effect that can prevent this and improve reliability? It is something that you have.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a normal nuclear fuel element, FIG. 2 is an explanatory diagram of problems that tend to occur in the nuclear fuel element of FIG. Explanatory diagram of the relationship between oxygen content, υ degree, and Vickers hardness, Part 4
The figure is an explanatory diagram for comparing the characteristics of the cladding tube of the embodiment of the nuclear fuel element of the present invention and the characteristics of a conventional cladding tube. 1... Cladding tube, 2... Nuclear fuel pellet, 3... End plug. 1st figure

Claims (1)

[Scope of Claims] 1. A cladding tube made of a zirconium alloy material is filled with fuel pellets, and both openings of the cladding tube are sealed via end plugs, wherein the inner surface of the cladding tube is sealed with end plugs. A nuclear fuel element characterized by having a liner layer formed from a low-oxygen zirconium alloy material in at least a portion of the element. 2. The above 'ti11 tube is formed from Zircaloy-2 or Zircaloy-4 ribs, and is made of Zircaloy-2 or Zircaloy-4 with an oxygen concentration of about 500 P or less.
4. The nuclear fuel element according to claim 1, wherein a single layer of liner formed from a material with a monthly rate of 4.5% is lined over the entire inner surface of said nuclear cladding tube.