JPH0442094A - Structural body of nuclear reactor core - Google Patents

Abstract

PURPOSE:To obtain small cross-section for neutron absorption and high durability by providing a main body made of zyrcalloy, and a coating layer consisting of an Fe or Ni alloy which is provided at a whole surface or an outer surface thereof. CONSTITUTION:Inside a reactor core 1 is of cell structure and is constituted of a number of fuel assemblies 11 and control rods being arranged among the assemblies. The assemblies 11 have a spacer 15 which is inserted between fuel rods 14, a handle 16, a finger spring 17 and a tie-plate 18. The fuel rod 14 consists of fuel rods 14a made of low enriched uranium dioxide, and a fuel cladding tube 14b. Main bodies and the like of the fuel cladding tube 14b, a channel 13 and the spacer 15 as a structural bodies of a reactor core, are constituted of a zyrcalloy, and a coating layer constituted of an Fe or Ni alloy is formed on a whole surface or an outer surface of the main body. Thickness of the coating layer is preferably not exceed 30% of total thickness of each reactor core structural body and thickness of less than 100mum is much preferable.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) This invention relates to a fuel cladding tube,
It relates to nuclear reactor core structures such as fuel channels and spacers.

(Prior Art) Zirconium alloys, which have a small neutron absorption cross section, are used as materials for nuclear reactor core structures from the viewpoint of fuel economy. Zirconium alone has poor corrosion resistance in high-temperature, high-pressure water, which is the atmosphere of a nuclear reactor, so zirconium is combined with Sn and Fe.

An alloy called "Zircaloy" containing Ni, Cr, etc. in a total amount of 2% or less is used. Zircaloy has good corrosion resistance in the above atmosphere and can be used safely for four years, which is the lifetime of the core structure.

(Problem to be solved by the invention) As the proportion of nuclear power generation in total electricity increases,
There is a desire to increase reactor availability, and it has therefore been considered that the reactor core structure can be used for longer periods of time than currently available. When Zircaloy, which is currently in use, is used for a long time, white spot-like corrosion products called nodular corrosion can occur on its surface, and zirconium alloys with high nodular corrosion resistance have begun to be actively developed.

However, until now, tin as an additive element,
There is no definitively superior material for iron, nickel, chromium, niobium, molybdenum, etc., and we are still at the stage of searching. That is, in order to use fuel pellets efficiently, there is a need for a core material that can be used in a nuclear reactor for a long period of time.

This invention was made in consideration of these points, and the purpose is to provide a nuclear reactor core structure that maintains a low neutron absorption cross section and has high durability under a nuclear reactor atmosphere. do.

[Structure of the Invention] (Means for Solving the Problem) A nuclear reactor core structure according to the present invention includes a main body made of Zircaloy, and a Fe-based alloy or Ni-based material provided on the entire surface or outer surface of the main body. It is characterized by having a coating layer made of an alloy.

Conventionally, Fe-based alloys or N
I-based alloys such as ferritic steel, austenitic steel, and Inconel are used.

The present inventors have found that these members remain unchanged and sound even during long-term use of the nuclear reactor. In addition, the present inventors made a nodular susceptibility testing device using Inconel at 500° C. and 105 atm steam, which simulates the reactor water conditions of a nuclear reactor, and conducted tests using a test specimen holder made of 5US316 stainless steel. As a result, Inconel also has 5US3
No. 16 stainless steel also had extremely good corrosion resistance against repeated use.

In fact, when a nodular corrosion test is performed on a zirconium alloy using this test device and holder, the zirconium alloy corrodes in 24 hours, and even a material with good corrosion resistance corrodes by 50 m.
The corrosion weight increase is about g / d m 2, and it can reach up to 2000 mg / d m 2 for materials with poor corrosion resistance, whereas the above-mentioned Inconel shows almost no corrosion increase and only shows an interference color, and 5US316 stainless steel Steel will turn black, but the corrosion increase will be 3 mg/d.
m 2 or less.

As described above, the inventors of the present application focused on the fact that Fe-based alloys and Ni-based alloys have excellent corrosion resistance in the special atmosphere inside a nuclear reactor, and developed them into core structures such as fuel cladding tubes, fuel channels, and spacers. The idea was to apply an Fe-based alloy or a Ni-based alloy.

On the other hand, the reason why zirconium alloy is used as a constituent material of a nuclear reactor is that, as mentioned above, it has a small neutron absorption cross section. For example, the neutron absorption cross section of zirconium is 0.18 barns, whereas iron is 2
．． 52 burns, and 4.6 burns for nickel, which is 14 times and 25 times that of zirconium.

Therefore, if the core structure is made of an Fe-based alloy or a Ni-based alloy, neutron economy will be reduced. Since it is desirable that neutrons contribute to the fission of uranium without being absorbed by the reactor core structure, it is not preferable to use Fe-based alloy or Ni-based alloy for the entire reactor internal structure.

As a result of considering the above, the present invention having the above structure has been completed. In other words, only the portion of the core structure that comes into contact with reactor water, that is, the surface portion, is made of Fe, which has good corrosion resistance.
By constructing the base alloy or Ni-base alloy and constructing the main body of Zircaloy, it is possible to maintain a low neutron absorption cross section and improve corrosion resistance.

This invention will be explained in detail below.

FIG. 1 is a sectional view showing a schematic structure of a nuclear reactor core, and FIG. 2 is a perspective view showing a schematic structure of a fuel assembly. The inside of the reactor core 1 has a cell structure and is composed of a large number of fuel assemblies 11 and control rods arranged between them.

The fuel assembly 11 includes a bundle of fuel rods 14, a channel 13 covering the outside thereof, a spacer 15 interposed between the fuel rods 14, a handle 16, and a finger spring 17.
and a tie plate 18. The fuel rod 14 is
Fuel 14a made of low enriched uranium dioxide and fuel cladding tube 1
4b. Such a fuel assembly 11 is fixed to the core body by an upper grid plate and a core support plate (not shown).

Among these core structures, the fuel cladding tube 14, channel 13, spacer 15, in particular, come into contact with reactor water and are exposed to a corrosive environment.

In this invention, the fuel cladding as a core structure! ,
The channel, spacer body, etc. are made of Zircaloy,
A coating layer made of an Fe-based alloy or a Ni-based alloy is formed on the entire surface or outer surface of the main body.

Examples of Fe-based alloys applied to the coating layer include Fe-Ni alloy, Fe-Cr alloy, and Fe-Ni-Cr alloy.
Alloys are preferred.

In addition, the preferable range of the composition of the Fe-based alloy and the Ni-based alloy here is as follows.

Fe-based alloy (wt%) C=≦0.08 Si:≦1.00 Mn:≦2.00 P:≦0.040 S:≦0.03O Ni:8. OO~15.00 Cr: 16.00~20.00 M0=≦3.0O Nb:≦1.00 Ta:≦1600 Fe: Balance Ni Mn Cr Ni Fe ≦0.20 ≦0.75 ≦1.50 ≦ 0. 040≦0. 030 23, 00-30. 00 ≦ 0.50 0, 10-0.25 Balance ■ Ni: 33-50 Fe: Balance Ni-based alloy (wt%) C: 0.03-0.15 Fe: 20 or less Co: 30 or less Cr; 6- 50 Mo: 18 or less W: 5 or less Nb: 7 or less Ti: 0.1 to 5 AN: 0.1 to 5 Ni: balance 5U as an Fe-based alloy belonging to the above composition range
There are austenitic stainless steels such as S304, 316, and 347, ferritic steels such as Al5I446, etc.
Examples of i-based alloys include Inconel and Hastelloy.

In addition, the coating layer composed of Fe-based alloy and Ni-based alloy was
It is desirable that the corrosion weight increase when kept in water vapor at 105 atm at 105°C for 24 hours is 20 mg/dm2 or less.

Note that if the parts of the core structure that come in contact with reactor water are made of the same material (for example, only Fe-based material or only Ni-based material), there will be no problem of local batteries in the contact parts, and it will be resistant to contact corrosion and crevice corrosion. Furthermore, continuous corrosion of channels near the upper grid plate and control rod hanging handles does not occur.

The thickness of the coating layer made of Fe-based alloy or Ni-based alloy is preferably as thin as possible because the neutron cross section of Fe or Ni is large, and the thickness should not exceed 30% of the total thickness of each core structure. desirable. moreover,
It is more desirable that the thickness is 100 μm or less.

This range is particularly important in fuel cladding.

Further, it is desirable that the coating layer is metallurgically bonded to the main body, and a diffusion layer of several atoms or more is formed at the interface thereof.

The fuel cladding in the core structure has the role of transmitting the heat generated when uranium as a fuel undergoes nuclear fission to the cooling water flowing outside the cladding, so the thermal conductivity of the cladding material is The higher the better. The thermal conductivity of Zircaloy is 0. 0
4cal/cs-sec ・"C, and the Fe-based alloy and N1-based alloy of the coating layer constituent materials in the present invention are equivalent to or higher than Zircaloy. In particular, ferritic stainless steel of Fe-based alloy has a high thermal conductivity. The thermal efficiency is 1.5 times or more that of Zircaloy, and the thermal efficiency is better than that of Zircaloy alone, so it is preferable.

When the core structure has a layered structure as in the present invention, it is necessary that the adhesion between the main body and the coating layer be good. In order to maintain this adhesion, the coefficient of thermal expansion of these layers is an important factor, and the coefficient of thermal expansion of the coating layer is preferably 0.25 to 4 times the coefficient of thermal expansion of Zircaloy. The coefficient of thermal expansion of Zircaloy is 4.9X10-6
/''C, whereas in Fe-based alloys and Ni-based alloys it is usually 10 to 17 x 10-6/'C, which is about two to three times that of Zircaloy.

By the way, since the inside of the fuel cladding tube is in contact with the high-temperature uranium pellets and the outside is in contact with the low-temperature cooling water, the overall balance is good if the coefficient of thermal expansion is low on the inside and high on the outside. According to the structure of the present invention, the outer side is made of Fe having a high coefficient of thermal expansion.
Since it is made of base alloy or Ni-based alloy, and the inner side is Zircaloy with a low coefficient of thermal expansion, there are extremely few adverse effects due to differences in thermal expansion during use.

Fuel cladding tubes have conventionally been Zircaloy tubes lined with pure zirconium for the purpose of high burnup and high performance. Therefore, if the present invention is applied to such a cladding tube, the cladding tube will have a three-layer structure in which the outer layer is an Fe-based alloy or Ni-based alloy coating layer and the inner layer is a pure zirconium lining.

A cladding tube lined with pure zirconium is called a liner tube, and the thickness of the litchi part is approximately 90 μm.
Its thickness can be measured non-destructively with good precision over the entire length using the eddy current method. The thickness of the coating layer in core structures such as cladding tubes, channels, and spacers having a coating layer of the Fe-based alloy or Ni-based alloy of the present invention can also be easily measured by the eddy current method.
Acceptance inspection can be carried out quickly. Therefore, the thickness can be guaranteed, and its lifespan and safety can be fully guaranteed.

Note that in the core structure, the spacer is a thin plate, so
The entire structure may be made of Fe-based alloy or Ni-based alloy without forming a coating layer of Fe-based alloy or Ni-based alloy.

Next, an example of a method for manufacturing a core structure according to the present invention will be described.

In the case of fuel cladding, it is manufactured as follows. First, a cylindrical billet of pure zirconium that forms the innermost circumference,
A cylindrical billet of Zircaloy-2 forming the center portion and a cylindrical billet of Fe-based alloy or Ni-based alloy forming the outermost periphery are prepared, and these three types of billets are fitted after cleaning. Next, EB welding is performed along the boundaries of two concentric circles at both ends of this fitting billet to form a combined billet. Since this combined billet is EB welded, the interface between the three cylinders is in a vacuum.

Therefore, by hot rolling thereafter, diffusion bonding is easily performed and an integral billet can be obtained. This composite integral billet is cold rolled and annealed in a conventional manner to form a fuel cladding tube. In the case of a cladding tube that does not have an innermost tube made of pure zirconium, the outermost billet and the zircaloy billet may be integrated to form the cladding tube.

In the case of channels and spacers, they are manufactured as follows. First, a Zircaloy plate is made of Fe-based alloy or N
The structure is sandwiched between i-based alloy plates, and two boundary lines on both end faces perpendicular to the rolling direction are EB-welded. Thereafter, hot rolling is performed to diffusion bond the interface between the Fe-based alloy or Ni-based alloy and the Zircaloy. The composite is repeatedly cold rolled and annealed to form channels and spacers.

(Example) Examples of the present invention will be described below.

Example 1 A fuel cladding tube with a three-layer structure in which a cylindrical billet made of Inconel with a thickness of 90 μm is used as the outer periphery, a cylindrical billet made of pure zirconium with a thickness of 90 μm is used as the inner periphery, and a Zircaloy tube is sandwiched between these. was manufactured. This cladding tube was cut to a length of 5011 pieces, an Inconel end plug was welded,
500℃.

A corrosion test in 105 atm water vapor, a so-called nodular corrosion test, was conducted. As a result, the corrosion increase after 24 hours was 0.
．． 5 mg/dm2, and the outer surface only showed a pale yellow interference color.

As a comparative example, a liner tube made of pure zirconium with a thickness of 90 μm on the inner periphery was prepared, cut into lengths of 50 layers, welded with Zircaloy end plugs, and subjected to the same nodular corrosion test. I did it. As a result, the corrosion increase was 270 m
g/dm2, and white speckled nodules were generated.

Note that end plug welding is performed in the nodular corrosion test because when the pure zirconium on the inner periphery is exposed to a corrosive atmosphere, it turns into a white product of zirconium oxide during the nodular test.
Due to the significant increase in weight and the ability to distinguish the external characteristics,
This is to prevent the pure zirconium on the inner periphery from being exposed to a corrosive atmosphere.

Next, core structures such as channels, spacers, upper lattice plates, tie plates, and control rods were fabricated by providing an Inconel coating layer on the Zircaloy surface. Also,
These were also made using only Inconel. As a result of conducting a test simulating the state of the reactor core using these, no change was shown even after 3000 hours had passed.

Example 2 A fuel channel having a structure in which a coating layer of ferritic steel (AISI446) was formed on a Zircaloy plate was fabricated.

This channel was subjected to a nodular corrosion test in the same manner as in Example 1. The corrosion weight increase after 24 hours is 2 mg/
It exhibited a purple interference color at d m 2.

Example 3 Austenitic steel (AIS 131
A spacer having a structure in which the coating layer of 6) was formed was produced. This spacer was subjected to a nodular corrosion test. Corrosion increase after 24 hours is 3mg/dm2
showed a purple interference color.

Example 4 A spacer was manufactured from Fe-4ONi alloy and subjected to a nodular corrosion test. As a result, the corrosion weight increase was 0.5 mg/d.
m'' showed a yellowish interference color.

[Effect of the invention] According to this invention, the core structure is formed by F on the surface of Zircaloy.
Since the structure is formed with a coating layer of an e-based alloy or a Ni-based alloy, durability in a nuclear reactor can be improved while maintaining the low neutron absorption cross section of Zircaloy.

In addition, the manufacturing method is simple and the thickness of the coating layer can be measured by non-destructive testing, so it is highly reliable.

Furthermore, Fe-based alloy used as a coating layer,
Since there are a wide variety of materials for Ni-based alloys, there is a wide range of materials to choose from.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a schematic structure of a nuclear reactor core, and FIG. 2 is a perspective view showing a schematic structure of a fuel assembly forming a part of the reactor core. 1; Core, 11; Fuel assembly, 12; Control rod, 13; Channel, 14; Fuel rod, 14a; Fuel, 14b; Fuel cladding tube, 15; Spacer, 16; Handle, 17; Spring, 18; Tie plate number 1 Figure Applicant's agent Patent attorney Takehiko Suzue Figure 2

Claims (2)

[Claims] (1) A nuclear reactor core structure comprising a main body made of Zircaloy and a coating layer provided on the surface of the main body and made of an Fe-based alloy or a Ni-based alloy. (2) The nuclear reactor core structure according to claim 1, wherein the coating layer is metallurgically bonded to the main body, and a diffusion layer of several atoms or more is formed at the interface thereof.