JPS60183584A - 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] [Field of application of the invention] The present invention relates to the improvement of nuclear fuel enrichment.

[Background of the invention]

FIG. 1 is a longitudinal cross-sectional view of a conventional nuclear fuel element.

- As shown in FIG.
Both end openings are sealed by end plugs 3a and 3b. Fuel pellet 2 is a fissile oxide Im only ↓ name to name 51
141 pieces of rice were sintered into columnar pellets of 61 pieces. Reference numeral 4 denotes a spring that has the function of forming a gas reservoir plenum 5 within the cladding tube 1 and the function of stably supporting the nuclear fuel pellets 2.

By the way, in the nuclear fuel element configured as described above, the cladding tube 1 has the function of completely preventing the occurrence of chemical reactions when the coolant is inserted between the nuclear fuel pellet 2 and the nuclear fuel pellet 2, and the function of completely preventing the occurrence of chemical reactions between the cladding tube 1 and the nuclear fuel pellet 2. The ability to prevent products from entering the coolant is required. Therefore, in the event that the cladding tube 1 that does not satisfy these functions, that is, the cladding tube 1 is damaged, the radioactivity level in the cooling system plant will increase, and the reactor operation will have to be stopped to ensure safety. This is a situation where it is necessary to do so.

On the other hand, cladding tube 1 of nuclear fuel element used in water-cooled nuclear reactor
are generally made of zirconium and its alloys. Zirconium and its alloys have a small neutron absorption cross section, are strong and ductile at temperatures below about 400C, and can be cooled. It has H1 Hachimaya Seijuku.

However, according to operational experience to date, even if the cladding tube 1 is made of zirconium and its alloys, interactions such as a decrease in material strength due to neutron irradiation and corrosion due to chemical reactions with fission products may occur. Brittle cracking has occurred due to Such an undesirable phenomenon is thought to occur as follows. That is, in order to efficiently transfer the heat generated by the nuclear fuel pellet 2 to the outer surface of the cladding tube 1, it is necessary to set the gap formed between the inner surface of the cladding tube 1 and the nuclear fuel pellet 2 to several tens of microns or less. There is. 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. 2, which is an explanatory diagram of problems that tend to occur in nuclear fuel elements, the cladding tube 1 is expanded by the nuclear fuel pellets 2 and is subjected to stress.

Although the average value of the strain to which the cladding tube 1 is subjected in the circumferential direction is not so large, strain is exerted locally on the wall near the crack 6 that has occurred in the nuclear fuel pellet 2, and this strain reaches more than the yield stress. In addition, nuclear fission becomes a cow and nuclear fuel pellets 2, etc.
Corrosive gases such as iodine and iodine compounds, cesium and cesium compounds are generated, and this corrosive gas is
It gathers in the free space inside, ie, in the crack 6, etc. That is,
Corrosive gas tends to collect particularly in the vicinity of parts of the cladding tube 1 where strain is concentrated.

Generally, when stress (% yield stress or higher) is applied in a corrosive gas atmosphere, the ductility of the fuel decreases and a brittle fracture phenomenon called stress corrosion cracking occurs. Stress corrosion cracking is influenced by temperature, stress, concentration of corrosive gas, composition of dissolved oxygen 2 alloy, heat treatment, degree of processing, etc., and the mechanism by which it occurs is not unique.

For the purpose of preventing these undesirable destructions, for example, as a conventional example, a method of inserting a lubricant between the fuel pellets 2 and the cladding tube 1 is shown in US Pat. No. 3,018,238, and , DA81238115 describes a titanium layer as an example of providing a barrier between the cladding tube 1 and the fuel pellets 2. Furthermore, Nb, Ta, Mo,
Nuclear fuel rods in which the fuel is surrounded by a Zr wire mesh membrane are also known. Other barrier materials include stainless steel, glassy substances, At, Mg, Cu, etc.
It is known from the specification of No. 1 and Japanese Patent Application Laid-Open No. 109397/1983.

Similarly, the concept of lining a cladding tube is well known, as in U.S. Pat.
JP-A No. 51-69792, same No. 51-69795, same <51-69796 and same No. 51-7149
In Publication No. 7, Mo, W, Nb, and C are used as lining materials.
r, Ni, F'e, Mg, Cut pure Zr, At, Ni-
Cr alloys, aluminized coatings, silicided coatings, etc. are shown.

However, the barrier material and lining material described in the above prior art have a neutron absorption capacity of 171).
Hez+2 (Mela/7')ne561g Surname? -IT-T
(, 5J-2, -AG77'l/FA.Also, in some of the proposals cited above, the materials used as barriers are materials that are incompatible with nuclear fuel pellets, but there are also materials that are incompatible with cladding tubes. However, it cannot be said that the above-cited proposals reach a fundamental solution to the local chemical-mechanical interaction between the nuclear fuel and the cladding, which has recently become a problem. do not have.

[Object of the Invention] The present invention has been made in view of the above-mentioned situation, and is aimed at reducing stress corrosion cracking when stress is applied to the cladding tube due to the interaction between the fuel and the cladding tube in corrosive gas. It's hard to get up,
The aim is to provide nuclear fuel elements that can improve reliability.

[Summary of the invention]

In the nuclear fuel element of the present invention, a cladding tube made of a zirconium alloy material is filled with nuclear fuel pellets, and both openings of the cladding tube are sealed through end plugs, and a liner is layered on the inner surface of the cladding tube. The liner layer is bonded to the inner surface of the cladding tube with a rough membrane that leaves a non-contact area over the entire inner surface of the cladding tube. That is, because the liner layer is loosely bonded to the cladding tube, the magnitude of stress concentration occurring in the cladding tube is reduced compared to when the liner layer is tightly bonded to the cladding tube over the entire surface. This can improve health.

[Another example]

Hereinafter, the nuclear fuel element of the present invention will be described as an example, and parts that are the same as those of the conventional one will be denoted by the same reference numerals, and explanations of the structures of the same parts will be omitted.
This will be explained with reference to FIG. FIG. 3 is a cross-sectional view of the nuclear fuel element, and FIG. 4 is an enlarged view of section A in FIG. 3. In the figure,
Reference numeral 8 denotes a single layer liner made of zirconium, which is fitted onto the inner periphery of the cladding tube 1. Cladding tube 1 with liner single layer 8
is obtained by the following steps. A groove-like notch is provided in the longitudinal direction of the inner surface of the cladding tube 1 of the Zircaloy-2 raw tube that satisfies ASTM standards, and a cylindrical zirconium is fitted into the reinstated tube 1 of the Zircaloy-2 raw tube, and the cylindrical zirconium is rolled as a complete pilger. Cold rolling is performed by the method. Vacuum annealing is performed for each stage of rolling to give a final outer diameter of 12V52a + inner diameter of 10.80 mm. After that, a recrystallization heat treatment is applied, and a dimensional inspection is performed.

As shown in FIG. 4, the cladding tube 1 with a single layer of liner 8 produced as described above has a groove-like cut formed in the longitudinal direction so that the single layer of liner 8 does not contact the entire inner circumferential surface of the cladding tube 1. It has a non-contact part 9 formed by a notch and is fitted with a contact part 10.

From the results of the characteristic evaluation test, it was found that it is preferable that the area of the non-contact portion 9 is larger than that of the contact portion 10, that is, the contact area is preferably within 1/2.

Further, the effect of this embodiment is obvious for the reasons described below. As shown in Fig. 5(a), the stress intensity factor (K) in fracture mechanics is used as a parameter representing the magnitude of stress concentration in the cladding tube when a crack 7 occurs in one liner layer 8 having a thickness of a. .

In this case, K at the tip of the crack 7 can be expressed by equation (1).

K = AσV1− ・・・・・・・・・(1) However,
A: constant determined from the shape, σ: stress in the circumferential direction acting on the cladding tube 1, that is, when a crack 7 occurs in the liner layer 8, stress concentration at the tip of the crack 7 on the inner surface of the cladding tube 1 It can be seen that the magnitude increases in proportion to the square root of the thickness a of the liner layer 8.

On the other hand, in the liner single layer cladding tube 1 of this embodiment, even if the liner layer 8 is cracked, the liner layer 8 and the cladding tube 1
Since it is not a continuum, K at the tip of the crack 7 and the ura is determined by equation (2), regardless of the thickness a of the liner layer 8, depending on the size of the four dimensions (b) of the cladding tube 1 shown in Figure 5 (b). It is determined as follows.

K=Aσf t (2) Therefore, in the liner single-layer cladding tube 1 of this embodiment, even if a crack 7 occurs in the liner single layer 8, the degree of stress concentration in the cladding tube 1 will be lower than the liner layer cladding tube 1. One layer 8
is constant without increasing according to the thickness a. This relationship will be explained more clearly in FIG. The dimension of the concave portion of the liner single-layer cladding tube 1 of this example is approximately 10 μm, and the stress intensity factor corresponding to this value is set to the reference value (i,
o), the liner thickness is plotted on the horizontal axis and the normalized stress intensity factor is plotted on the vertical axis, as shown in curve B of FIG. 6, which is 1.0 regardless of the thickness of the liner layer 8. Become.

On the other hand, in the conventional structure of the cladding tube 1 with single layer 8 of Lyra,
The stress intensity factor at the crack tip increases as shown in curve C;
There is a possibility that the standard value will be exceeded.

In this way, in the nuclear fuel element of this embodiment, the liner layer fitted into the inner surface of the cladding tube is bonded in a rough state with no contact over the entire inner surface of the cladding tube, leaving a non-contact area. As shown in the above analysis example, even if cracks occur in the liner reservoir due to interaction with fuel in corrosive gas, stress corrosion cracking is less likely to occur and reliability can be improved.

〔Effect of the invention〕

As described above, the nuclear fuel element of the present invention has the effect that stress corrosion cracking is unlikely to occur when stress is applied to the cladding tube due to the interaction between the fuel and the cladding tube in corrosive gas, thereby improving reliability. It is.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-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. 1, and FIG. 3 is a cross-sectional view of an embodiment of the nuclear fuel element of the present invention. Figure 4 is an enlarged view of part A in Figure 3, and Figure 5 (a) is an explanation of stress concentration on the inner surface of the cladding tube at the tip of a crack in the single layer of liner when the liner has matured without providing a non-contact part on the inner surface of the cladding tube. Figure, (b)
4 is an explanatory diagram of stress concentration on the inner surface of the cladding tube at the tip of a crack in a single layer of liner, and FIG. 6 is a diagram illustrating a comparison of characteristics of the cladding tube of FIG. 3 and the cladding tube of FIG. 5. 1... Cladding tube, 2... Fuel pellets, 3a, 3b.
... End plug, 8 ... Liner single layer, 9 ... Non-contact part, 1
0...Contact part. Agent: Patent Attorney Akio Takahashi Figure 5 (A) (O) Y4 liner - 4 near C〃Kaji)

Claims (1)

[Claims] 1. Nuclear fuel pellets are filled in the cladding made of zirconium-coated zirconium-based material, and the openings at both ends of the cladding tube are sealed via end plugs.
One layer of liner is inserted into the inner surface of α, and one layer of liner 6 has a non-contact part that does not touch the entire inner surface of the cladding tube. and nuclear fuel elements.