JPH01119650A - Manufacture of channel box for nuclear reactor fuel assembly - Google Patents

Abstract

PURPOSE:To manufacture a channel box in which corrosion resistance and creep deformation characteristics are optimized by using a Zr alloy containing specific amounts of Nb, Sn, and Mo and also specifying hot rolling temp., temp. of annealing between cold working stages, hardening temp., and heat-treatment conditions after assembly by welding. CONSTITUTION:A refined ingot of an alloy having a composition consisting of, by weight, 0.5-2.5% Nb, 0.5-2.0% Sn, 0.1-1.0% Mo, and the balance Zn is forged at a temp. in a beta-phase temp. region. The resulting forged stock is heated up to a temp. in the beta-phase temp. region again and cooled rapidly to undergo solution heat treatment. Subsequently, the above stock is hot-rolled at about 650-750 deg.C and subjected to alternate repetition of cold rolling and annealing at about 550-650 deg.C, which is then heated up to about 790-870<= in the (alpha+beta)-phase temp. range, cooled rapidly, and subjected to final cold rolling and bending. Successively, a square-shaped cylinder is formed by means of welding, by which a channel box for nuclear reactor fuel assembly is prepared. After assembly by welding, stress is applied and heat treatment is then applied to the above channel box at about 400-650 deg.C in the alpha-phase temp. range. By this method, the channel box having high strength and excellent in corrosion resistance can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a nuclear reactor fuel assembly, and particularly to a method for manufacturing a channel box suitable for long-term use in a nuclear reactor.

[Conventional technology]

As shown in FIG. 2, a BWR fuel assembly includes a large number of fuel rods 1 and spacers 2 that hold them at predetermined intervals.
．． Furthermore, there is a rectangular channel box 3 to store them.
, an upper tie plate 4 and a lower tie plate 5 that hold both ends of the fuel rod 1 containing fuel pellets in the fuel cladding tube.

It consists of a handle for transporting the whole thing.

Furthermore, these fuel assemblies are manufactured through a complicated manufacturing process, and each structure is assembled by welding.

The fuel channel box houses fuel rods assembled with fuel spacers therein, and is used with the fuel rods fixed between an upper tie plate 4 and a lower tie plate 5. FIG. 3 shows an enlarged view of the fuel channel box, which has a rectangular cylindrical shape made by welding 10 together two divided plates. This member works to forcibly guide high-temperature water and steam generated in the fuel rods during plant operation to the top, and the square tube is used for a long time under constant stress that spreads outward. .

During use, the fuel assembly channel box is exposed to high-temperature, high-pressure reactor water and neutron irradiation. Also, since the pressure inside the rectangular tube is higher than that outside, it is subject to internal pressure. As a result, corrosion resistance under high temperature and high pressure environments and high creep deformation resistance under neutron irradiation are required.

Zirconium-based alloys generally have high corrosion resistance.

Neutron absorption cross section is small. These characteristics make it suitable as a material for fuel assemblies for nuclear reactors, and it is used for fuel cladding tubes, channel boxes, spacers, etc. that constitute fuel assemblies. Zirconium-based alloys used in these applications include Zircaloy-2 (Sn1.2-1.7 wt
%, Fe0.07-0.2wt%, Cr0.05-0.
15 wt%, NiO, 03-0.08 wt%, balance Z
r), Zircaloy-4 (Sn1.2-1.7 wt%
, Fe0.18~0.24wt%, Cr'0.05~0
．． 15 wt%, remaining Zr), Zr-1wt%Nb-
Alloy, Zr-3,5wt%5n-0,8wt%Nb-0
, 8wt% Mo alloy (Exce1 alloy), Zr-1wt
%Sn-1wt%Nb-0,5wt%Fe alloy Zr-N
b(0.5-5.0wt%)-8n(0-3.0wt%
)-Fe, Ni, Cr, Tat pci, Mo.

There is an alloy of any one type of W (up to 2 wt%).

Zr-5n-Fe-Cr-(Ni
) alloys undergo local oxidation (
Nodular corrosion) occurs. The occurrence of nodular corrosion reduces the wall thickness of the healthy part of the component, and the hydrogen generated during the corrosion reaction is absorbed into the component, forming brittle hydrides in the alloy component, which causes a decrease in strength. . Since the corrosion phenomenon progresses over time, it is generally believed that under high burnup operating conditions where these members are used for a long period of time, corrosion of the members becomes a factor that determines the life of the fuel assembly.

Zirconium-niobium alloys containing Nb have high strength and
It is known to have excellent creep properties and low hydrogen absorption rate. Localized corrosion called nodular corrosion also does not occur.

These have favorable properties as materials for fuel assembly members, but there is a problem in that corrosion of welded parts and heat-affected zones is accelerated and thick white oxides that are peelable are formed.

In addition, as a niobium-zirconium based multi-component alloy, 0,5-2,O
wt% Nb, up to 1.5 wt% Sn, and Fe.

A zirconium alloy containing up to 0.25 wt% of a tertiary alloying element selected from the group consisting of Cr, Mo, V, Cu, Ni and W is presented, with a special microstructure imparting corrosion resistance in high temperature steam environments. It has been disclosed that it has

However, the alloy actually exemplified is Zr-Nb-8n-
Only the Fe alloy is used, and the white corrosion of the welded part is not disclosed.

U.S. Pat. No. 3,121,034 discloses a Zr-based binary alloy (Zr-0.5~5wt%Nb), a ternary alloy (Zr-based
-0,5~5wt%Nb -0~3wt%Sn)
Or quaternary alloy (Zr-0,5~5wt%Nb-o~3
wt%5n-Fe, Ni, Cr, Ta.

550~2 wt% of any one of Pd, Mo, and W after cold rolling (working degree: 50~60%)
It is disclosed that high corrosion resistance is increased by annealing at 600° C. for 1 to 240 hours and cooling in air.

However, in welded structural members, it is difficult to subject the welded portion to severe processing of several tens of percent after welding.

The technology disclosed herein cannot be applied to nuclear reactor fuel assemblies that have a welded structure.

[Problem that the invention seeks to solve]

A channel box applied to a highly economical high burnup core must have the following characteristics.

First, it has high corrosion resistance, and even if the operating period is extended, there is little decrease in wall thickness of healthy parts due to corrosion.

There is little material deterioration due to hydrogen absorption.

Second, even though the wall thickness is thinner than that of conventional channel boxes, it has sufficient strength and has high resistance to creep deformation. This is because a thick channel box reduces neutron economy.

Third, it is easy to manufacture, and can be manufactured using conventional manufacturing equipment without requiring special manufacturing equipment.

There are various problems when trying to apply the conventional technology to spacers for high burnup cores. Zircaloy-2 and Zircaloy-4 undergo nodular corrosion, and the strength of 1 is lower than that of Zr-N.
lower than that of b-based alloys.

Zr--Nb alloys have high strength and high resistance to creep deformation, but peeling white corrosion occurs in welds and heat-affected zones, making them unsuitable for use. Furthermore, since Zr-Nb alloys have high strength, their workability is low, and when various heat treatments are applied to improve workability, corrosion resistance or strength characteristics deteriorate. Ta.

It is an object of the present invention to solve this problem and provide a method for manufacturing a spacer having the above-mentioned first to third characteristics.

[Means for solving problems]

The present invention solves the above problems by limiting the alloy composition and the heat treatment temperature and processing temperature in the manufacturing process. That is, Nb: 0.5-2.
5 wt%, Sn: 0.5-2, 0 wt%, Mo:
Using a Zr alloy containing 0.1 to 1.0 wt%, in the manufacturing process shown in Figure 1, the hot rolling temperature was
50℃~750'C1 Annealing temperature between cold working and cold working: 550℃~650℃, quenching temperature before final cold working: α+β phase temperature range, heat treatment temperature after welding assembly 2500℃ By setting the temperature to 650°C, the corrosion resistance, tensile properties, creep deformation properties, and manufacturability of the spacer were optimized.

[Effect]

The reason why white corrosion occurs in the welded part of the spacer made of Zr--Nb alloy and its heat-affected zone is because a non-equilibrium phase appears in the metal structure.

The equilibrium phase in the equilibrium phase diagram of the zirconium niobium alloy is a hexagonal α phase Zr with approximately 1 significant % of Nb dissolved in it.
and β-phase Nb in which 15% by weight (hereinafter also referred to as wt%) or less of Zr is solidly dissolved.

By increasing or decreasing the amount of niobium, these equilibrium phases can exist alone or in a mixture.

When a metal structure consisting of these equilibrium phases (α phase Zr and β-Nb phase) is welded, a non-equilibrium phase that does not appear in the equilibrium phase diagram is generated during the cooling process. This non-equilibrium phase is formed in the shape of a needle, and is a martensitic phase (α'
phase), a β-Zr phase containing a large amount of Nb as a solid solution, and an α phase. In order to prevent the occurrence of white corrosion, it is effective to reduce the amount of non-equilibrium phase or to reduce the amount of Nb solid solution. This is because the amount of production can be significantly reduced. The reason for adding Mo is that Nb dissolved in solid solution in the non-equilibrium phase is precipitated as an intermetallic compound with the fine intermetallic compound phase containing Mo as the nucleus, reducing the amount of Nb dissolved in solid solution, and Decomposition of equilibrium phase (α phase Zr and Zr-Nb
This is because it has the effect of promoting decomposition into -Mo precipitated phase).

At the same time, these fine precipitates are extremely effective in increasing the static tensile strength and creep deformation resistance of the alloy.

For these reasons, ZrZr-Nb-8n- alloy is used as a channel box material, and Nb: 0.5~
2.5 wt%, Sn: 0.5-2. o wt%, M
o: A Zr alloy containing 0.1 to 1.0 wt% was selected.

Hot plastic working means working at a temperature equal to or higher than the recrystallization temperature (530°C), and in the present invention, means hot rolling of plate materials and hot extrusion of pipe materials. Hot working allows for a significant reduction in plate thickness, and is effective in reducing the thickness of forged materials. Figure 4 shows ZrZr-Nb used in the present invention.
-Deformation resistance and Zircaloy of 8n-alloy at various temperatures
A comparison of the deformation resistance of the four materials is shown. Hot rolling or hot extrusion of conventional Zircaloy-4 material is 600
It has been carried out at around ℃. The deformation resistance of the present invention material at 600°C is approximately 100 MP compared to Zircaloy-4 material.
a The deformation resistance is high, which means that it is necessary to increase the capacity of the hot rolling mill in order to process at this temperature, which increases manufacturing costs.

In ZrZr-Nb-8n-alloy, Zircaloy-4
The deformation resistance equivalent to that of ZrZ is obtained at a temperature of 650"C or higher.
When performing hot plastic working of r-Nb-5n-alloy, a rolling mill with the same capacity as the conventional one can be used.

Figure 5 shows the effect on the limit cold rolling degree after hot working.
The influence of hot plastic working temperature is shown. As the temperature increases, the critical cold rolling workability also increases.

By selecting a hot working temperature of 650℃ or higher,
It can be seen that cold rolling of 0% or more is possible. If the hot rolling temperature is 750° C. or higher, the metal structure will become coarser, which is not preferable. Therefore, the hot plastic working temperature is 6
A temperature of 50 to 750°C is suitable. By selecting this temperature range, excellent cold workability and hot workability can be obtained.

Figure 6 shows the effect of annealing temperature on the elongation of 50% cold rolled Zircaloy-4 material and ZrNbSn-Mo alloy. The elongation is restored by annealing at 550°C or higher.

Therefore, it can be seen that excellent cold workability can be obtained by setting the intermediate arm annealing temperature to 550° C. or higher. 6
Even if the temperature is raised to 50° C. or higher, the elongation will not increase and the crystal grains will become coarser, which is not preferable. Therefore, the appropriate intermediate arm annealing temperature is in the range of 550 to 650°C.

Hot plastic working, cold working, and intermediate annealing cause the precipitates in the metal structure to become coarser and the strength to decrease.

The quenching treatment downstream of the manufacturing process is a heat treatment aimed at refining coarse precipitates and increasing the strength again.

FIG. 7 shows the influence of quenching temperature and post-quenching heat treatment temperature on strength and elongation. Although the strength of the material quenched above the β-phase temperature increases, the ductility decreases significantly and the fracture formability becomes unstable, which is not preferable. The material quenched from the α+β temperature has high ductility and relatively high strength. This excellent property is caused by the coexistence of areas with fine precipitation and α-Zr phase crystal grains with high ductility and almost no fine precipitation. Figure 8 shows the relationship between corrosion weight increase and quenching temperature.

Regarding corrosion of this material, white corrosion does not occur at any quenching temperature, but corrosion weight tends to increase at temperatures above 880°C. In addition, the quenching temperature also affects the cold workability of the next process,
When the temperature exceeds around 50°C, workability decreases significantly (Figure 9).

Therefore, the quenching temperature is within the α+β phase temperature range (790~870
°C) is suitable, and 830-870 °C is preferred.

In addition, cold rolling after quenching is difficult for conventional Zr-Nb alloys.
Approximately 10% (range where recrystallization does not occur during subsequent aging treatment)
The usual method is to do so. This is intended to be strengthened by cold rolling. In this manufacturing process, since the steel is strengthened by adding Mo, it is not necessarily necessary to incorporate cold rolling.

The final heat treatment is performed in order to improve the strength and corrosion resistance by finely precipitating Nb dissolved in solid solution by α+β quenching.

The final heat treatment after welding 1 assembly process is 400 to 65
Perform at 0°C. As shown in Figure 10, the tensile strength is 40
Strengthening occurs due to the aging effect at 0 to 500°C, and maximum strength is obtained. The elongation is rather low at around 20%, and to achieve high ductility it is best to carry out the process at 600 to 650'C.

Regarding the strength of this alloy, the strength of the conventional material is Zr-2.5wt%.
Compared to Nb, sufficient high strength can be obtained even when annealing at a high temperature, and it has high ductility. This is due to the addition of Mo to Zr(
This is because the intermetallic compound Mo) precipitates finely. Final heat treatment also has a large effect on corrosion resistance. The treatment consists of non-equilibrium phase structures (β-Zr and ω
phase) and non-equilibrium phase structure (β-Zr phase) of the weld metal and weld heat-affected zone generated during the thermal cycle during welding.

(including ω phase) and improves corrosion resistance. FIG. 11 shows the influence of the final heat treatment temperature on the corrosion resistance of the base metal and weld zone. Corrosion resistance reduced by α+β quenching or welding is recovered by final heat treatment at a temperature of 400°C or higher, and high corrosion resistance can be obtained again.

The effect of improving corrosion resistance can be achieved at 400-650°C for 0.5-30 hours. At temperatures below 400°C, Nb does not precipitate and a non-equilibrium phase remains, making it impossible to obtain high corrosion resistance. Temperatures exceeding 650° C. are not preferred because the non-equilibrium phase disappears and although corrosion resistance is improved, the precipitates aggregate and become coarse, resulting in a decrease in strength.

Final annealing temperature and time also affect dislocation density. The dislocations introduced by cold working disappear with increasing annealing temperature and annealing time, reducing its density. Creep deformation varies depending on the initial dislocation density, and the lower the dislocation density, the smaller the amount of creep deformation. The preferred dislocation density is 3 x 10"ke/d or less. To achieve such a dislocation density, if the temperature is 500°C or higher, it is necessary to hold the temperature for at least one hour. Hold for about 24 hours. By doing so, a channel box with high creep deformation resistance can be obtained.

[Example] Hereinafter, the present invention will be explained in more detail with reference to Examples.

Table 1 shows the chemical composition of the alloy. Briquettes with adjusted alloy components and composition were vacuum arc melted to produce a primary ingot. This ingot was used as an electrode for the chemical composition of the alloy shown in Table 1 and was subjected to secondary vacuum arc melting to produce an ingot. This ingot was forged at 1020℃ to 75mm.
The cross section of Hot rolled twice at 700℃ to a thickness of 4nI+
It was made into a plate material. That is, the thickness was set to 40nvn in the first hot rolling, and the thickness was set to 4mm in the second hot rolling.

Thereafter, it was cold-rolled and intermediately annealed at 600°C for two hours to produce a plate with a thickness of 2.1 m. This plate material is heated to 870℃
After heating and holding for about an hour, it was cooled to room temperature at an average cooling rate of 50" C/S. This plate material was bent into a U-shape,
After butt-welding the two bent plates, the welded portion was rolled with a roll to flatten the bead after welding. Insert a stainless steel mandrel inside this and heat to 550'C.
It was held for ten hours. Since the coefficient of linear expansion of stainless steel is higher than that of Zr alloy, the channel box was expanded and shaped to predetermined dimensions, and at the same time, Nb and MO dissolved in solid solution were precipitated by α+β quenching, and the non-equilibrium phase disappeared. The tensile strength of the channel box manufactured by the above method was about 80 kg/mm" or more, and the elongation was about 22%. The dislocation density was reduced to 1X10"/d.

Cut out a test piece from this channel box weld,
It was immersed in high-temperature water at 288°C and a pressure of 86 kg/cd for 300 hours to examine its corrosion resistance. As a result, a black uniform oxide film similar to that of the base metal was formed on the welded part, and no abnormal white corrosion was observed.

Table 2 summarizes these results, and it can be seen that the hydrogen absorption rate is about 1/4 to 115 of that of the conventional Zircaloy-4 material, and there is no fear of hydrogen embrittlement.

〔Effect of the invention〕

According to the present invention, a channel box with high strength and excellent corrosion resistance can be manufactured, so that it can withstand long-term use inside a nuclear reactor.

[Brief explanation of the drawing]

Fig. 1 is a fuel channel box manufacturing process according to an embodiment of the present invention, Fig. 2 is a cross-sectional view of a fuel assembly, Fig. 3 is a perspective view of the channel box, and Fig. 4 is a diagram of the relationship between deformation resistance and processing temperature. , Figure 5 is a diagram showing the relationship between limit cold rolling workability and hot rolling temperature, Figure 6 is a diagram showing the relationship between elongation and annealing temperature, and Figure 7 is a diagram showing the relationship between tensile strength, elongation, and heat treatment after quenching. Figure 8 is a diagram showing the relationship between corrosion weight gain and quenching temperature, Figure 9 is a diagram showing the relationship between limit cold rolling workability and quenching temperature, Figure 10 is a diagram showing the relationship between tensile strength and final heat treatment temperature, FIG. 11 is a diagram showing the relationship between corrosion weight increase and final heat treatment temperature. DESCRIPTION OF SYMBOLS 1... Fuel rod, 2... Spacer, 3... Channel box, 4... Upper tie plate, 5... Lower tie plate, 1o... Welding part. 2nd mouth, 3rd mouth, 4th mouth, temperature ("c", 5 mouth jif4-Zr -/・Swt
t%S? 1-o5vt%M. @ Temperature of the 6th mouth RFl-: h -2wt%Nb-1wf, 5Sn-0
,5wt4rr'l. Quenching @ processing temperature (°C) 8th grain material + 4) lr-o fortune ~ 20wtirJb-10wtX
s9-o2~oSwt%M. Quenching temperature (°9) Material: Zy -0!545wt Nb -7.0wt
1 S? I -0-2~θ5vvt%M11) Quenching temperature (°C) Early 10 final light treatment 51 degrees (°C) Early/l Mouth #ripening temperature (°C)

Claims (1)

[Scope of Claims] 1. A fuel comprising a fuel rod made of fuel pellets built into a fuel cladding tube, a channel box housing a plurality of the fuel rods, and a spacer partitioning between the fuel rods in the channel box. Among the aggregates, Nb: 0.5~
2.5wt%, Sn: 0.5-2.0wt%, Mo: 0
．． In the manufacturing method of a channel box, which is a welded structural member made of an alloy of 1 to 1.0 wt% Zr and the balance, (1) the process of forging a molten ingot in the β-phase temperature range; (2) the process of forging the forged material again in the β-phase temperature range; (3) A step of hot rolling (4) A step of alternately repeating cold rolling and annealing (5) A step of rapidly cooling after heating to the α+β phase temperature range ( 6) The process of cold rolling (7) The process of bending (8) The process of forming a rectangular tube by welding (9) The process of applying stress and heat treatment in the α phase temperature range is performed in this order. A method for manufacturing a channel box for a nuclear reactor fuel assembly, characterized by: 2. A method for manufacturing a channel box for a nuclear reactor fuel assembly according to claim 1, characterized in that the step (6) is omitted. 3. Manufacturing a channel box for a nuclear reactor fuel assembly according to claim 1 or 2, characterized in that the temperature of the hot rolling process in step (3) is 600°C to 750°C. Method. 4. A method for manufacturing a channel box for a nuclear reactor fuel assembly according to claim 1 or 2, characterized in that the annealing temperature in step (4) is 550°C to 650°C. 5. In claim 1 or 2, the α+β phase heating temperature range in step (5) is 790°C to 870°C.
A method for manufacturing a channel box for a nuclear reactor fuel assembly, characterized in that the temperature is ℃. 6. A method for manufacturing a channel box for a nuclear reactor fuel assembly according to claim 1 or 2, characterized in that the phase heating temperature in step (9) is 400 to 650°C. 7. In claim 1, 2, or 5, the dislocation density in the alloy member in step (9) is 3×10^1
A method for manufacturing a channel box for a nuclear reactor fuel assembly, characterized by controlling heating time in the α-phase temperature range so that the temperature is below ^0ke/cm^3.