KR100828414B1 - Method and Furnace for Sintering of U-Zr Powder Compacts in a Low-vacuum Environment - Google Patents

Abstract

The present invention relates to a low vacuum sintering method and a sintering furnace of a uranium-zirconium powder compact. In particular, the present invention relates to a sintering method and a sintering furnace of a uranium-zirconium mixed powder compact in a low vacuum atmosphere after mixing and molding uranium and zirconium powder.

According to the present invention, in the process of sintering the uranium-zirconium powder compact, the sintering furnace is sintered by directly charging an argon gas which is an inert gas into the sintering furnace chamber in a high vacuum state or by attaching a chamber capable of maintaining an argon gas atmosphere outside the high sintering furnace. By preventing the inflow of air into the furnace and controlling the degree of vacuum, it is possible to simultaneously suppress Zr (O) phase formation and uranium evaporation in the uranium-zirconium sintered alloy to provide a useful effect of producing a homogeneous uranium-zirconium sintered alloy. have.

Alloy Sintered Body, Uranium-Zirconium Powder Compact, Sintered, Argon Chamber, Low Vacuum, Vacuum Degree Control

Description

Method for Furnace and Sintering of U-Zr Powder Compacts in a Low-vacuum Environment

1 is a flow chart showing a manufacturing process of the U-Zr sintered alloy according to the present invention;

2 is a scanning electron microscope (SEM) photograph showing the microstructure of the U-Zr sintered alloy according to the present invention;

3 is a graph showing SEM / EDS component distribution of U-Zr sintered alloy according to various vacuum degrees according to the present invention (A: 1.0 × 10 ⁻⁴ torr, B: 1.8 × 10 ⁻⁴ torr, C: 3.0 × 10 ^{− 4} torr, D: 6.0 × 10 ⁻⁴ torr, E: 1.0 × 10 ⁻³ torr); And

Figure 4 is a schematic diagram of a sintering furnace easy to control the low vacuum atmosphere according to the present invention.

The present invention relates to a low vacuum sintering method and a sintering furnace of a uranium-zirconium powder compact. In particular, in the production of the uranium-zirconium sintered body, since it is sintered in a low vacuum atmosphere, a homogeneous uranium-zirconium sintered alloy is suppressed by simultaneously suppressing the formation of the Zr (O) phase and the uranium evaporation phenomenon generated in the conventional process of sintering in a high vacuum atmosphere. It relates to a low vacuum sintering method and a sintering furnace of a uranium-zirconium powder compact which can be produced.

Nuclear fuel, also known as atomic fuel, is a substance that can be charged into a nuclear reactor and get available energy by cascading nuclear fission. In nuclear power generation, dozens or hundreds of sintered bodies made of nuclear fuel materials are put in a zirconium alloy cladding tube, and both ends are sealed and welded to manufacture fuel rods, and bundles of tens to hundreds of fuel rods are manufactured. These bundles are loaded and used in hard water and heavy water reactors, and the heat generated from the sintered body is transferred to the cooling water flowing around the fuel rod through the cladding pipe through the sintered body.

In addition, currently used nuclear fuel for nuclear reactors may be divided into ceramic (UO ₂ ) fuel and metal (U) fuel. Among them, metal uranium fuel is used in various types of nuclear reactors due to its excellent thermal conductivity compared to ceramic nuclear fuel. The process for producing the uranium-zirconium alloy, which is one of the metal nuclear fuels, is used in the form of a sintered body by mixing, molding and sintering metal uranium and metal zirconium powder with each other.

The step of producing the nuclear fuel sintered body can be largely divided into three stages of mixing, molding, and sintering. The first mixing step uses a mixer such as Double Cone or V-Cone to blend the main raw materials and additives for the production of the product or to add lubricant, adjusting the appropriate rotation speed and mixing time. It is a very important process for the uniformity of quality during molding. The second molding step is the first process in which the mixed powder is produced in the form of a semifinished product, and the powder particles are sufficiently bonded to give a stable mechanical strength until the next process. That is, even in a molded article formed by applying high pressure, the bonding force between the powder particles is very low as a collection of individual powder particles only in a state before sintering, and can be easily broken even with a small stress. The final sintering step is that each particle in the powder is bonded by the adhesion between atoms by heating, thus increasing the strength of the whole powder and causing an increase in density and recrystallization by mass transfer. It is a step of combining only one component of the system with heating at a temperature lower than the melting point. That is, as a kind of heat treatment performed at a temperature below the melting point of the base metal, diffusion between the powder particles occurs and chemical bonding results in the required mechanical properties. This heat treatment is sintering. At this time, the sintering density is determined according to the sintering temperature and the sintering time.

On the other hand, the nuclear fuel sintered body has a sintered body density and grain size defined in the technical specifications, specifically, the density of the sintered body is 95 ± 1% TD and the grain size is 5㎛ or more. In order to satisfy these conditions and to manufacture fuel sinters economically, much research is being conducted.

In the manufacturing process of uranium-zirconium sintered alloy, which is a mixed fuel sintered body, the sintering density of the uranium-zirconium sintered body is determined according to the sintering temperature and the sintering time. In order to do so, the molded body is sintered in a high vacuum atmosphere. However, when sintering at high vacuum, air is leaked into the sintering furnace, and the air introduced into the sintering furnace oxidizes the uranium-zirconium powder compact, thereby forming a Zr (O) phase in the sintered alloy. In addition, since the sintering at a high vacuum, the phenomenon of evaporation of uranium on the surface of the uranium-zirconium powder compact occurs.

Accordingly, the present invention provides a low vacuum sintering method and a sintering furnace of a uranium-zirconium mixed powder compact for forming a homogeneous sintered metal by preventing the evaporation of uranium and the Zr (O) phase formed during the sintering process due to the inflow of air according to the high vacuum. The purpose is to provide.

In order to achieve the above technical problem, the present invention is a uranium-zirconium powder compact comprising the step of sintering a uranium-zirconium powder compact containing 10 to 90% by weight of zirconium and the balance of uranium in a low vacuum inert powder It provides a low vacuum sintering method of.

In addition, the present invention is a housing for receiving a heater for supplying heat for sintering the uranium-zirconium powder compact, the crucible for producing a uranium-zirconium powder compact and the outer periphery of the crucible, the A uranium, which is in communication with an opening at one side of the bottom of the housing, includes a vacuum system for adjusting the degree of vacuum of the housing, and an inert chamber accommodating the housing, the inlet and outlet having an inlet and outlet of an inert gas at predetermined positions on the upper and lower sides thereof. Provided is a sintering furnace for sintering zirconium powder compacts.

Hereinafter, the present invention will be described in detail.

The present invention includes a low vacuum sintering method of a uranium-zirconium powder compact.

Specifically, the present invention includes the step of sintering a uranium-zirconium powder compact containing 10 to 90% by weight of zirconium and the balance of uranium in a low vacuum inert atmosphere.

In the production of a mixed fuel sintered body, the present invention manufactures the sintered body in low vacuum for the purpose of controlling the density and microstructure of the sintered body. In the low vacuum reduction sintering process of the present invention, the powder compact is charged into a sintering furnace, and the oxygen composition of the sintered compact can be controlled by sintering in a low vacuum / reducing gas atmosphere. More specifically, the low-vacuum reduction sintering process of the mixed fuel sintered compact is performed by sintering the powder compact in a low vacuum / inert gas atmosphere for a predetermined time to produce a sintered compact and then lowering the temperature.

The present invention first precedes the process of mixing and compacting uranium powder and zirconium powder to produce a uranium-zirconium powder compact. The manufacturing process of the molding agent may be prepared by a conventional method in the field of the present invention, the present invention is not limited thereto. The mixing process is a very important process for determining the uniformity of the sintered body. In the present invention, the mixing process is a process for preparing metal powder from metal raw materials. The molten metal may be dropped onto the disk rotating at high speed to produce fine powder using centrifugal force.

Zirconium powder may be prepared by a hydrogenation-dehydride process of a zirconium sponge, for example, by a precipitation reaction in which a hydrogen catalyst is added to reduce hydrogen in a solvent. These obtained uranium and zirconium powders can be passed through a sieve of 125 mesh or less, preferably 50 to 125 mesh, to obtain a mixed powder. In the mixing process, the uranium-zirconium powder compact is preferably mixed so as to contain 10 to 90% by weight of Zr and the balance of uranium, which is to prepare low enriched uranium having a concentration of 20% or less of uranium. In the example, 38% of uranium was mixed, which corresponds to a low concentration of uranium of 19.75%.

The compacting process serves to allow the powder particles to sufficiently bind in the desired shape to impart minimal stability for subsequent processing.

Next, the present invention includes a step of sintering the molded body in a low vacuum inert gas atmosphere. Sintering is the most important step in the production of sintered alloys, and the process conditions of this step can produce homogeneous uranium-zirconium powder compacts, and the sintered density of the desired sintered compact is determined.

As suitable process conditions for sintering, the sintering temperature is preferably 1300 ~ 1500 ℃ and the holding time is 1 ~ 5 hours, in particular, the temperature is more preferably 1400 ~ 1500 ℃. When sintered under the process conditions in this range, it shows high theoretical density of 95% TD or more, and satisfies the range (95 ± 1% TD) specified in the technical specification.

The inert gas forms a low vacuum atmosphere of the crucible through the inflow and outflow thereof, and creates an environment for preventing oxidation of the sintered body, and argon gas or nitrogen gas can be used. The low vacuum state of the crucible is preferably maintained in the range of 1.0 × 10 ⁻⁴ to 1.0 × 10 ⁻¹ torr. In this range, the rate of air leakage is reduced to form Zr (O) and evaporate uranium. The phenomenon can be suppressed at the same time.

In the present invention, the low vacuum atmosphere is a method of continuously charging an inert gas containing argon gas directly into the crucible, or by attaching a chamber capable of maintaining an argon gas atmosphere to the outside of the high vacuum crucible to inert gas into the crucible It is possible to maintain a low vacuum state by using a continuous charging method. When manufacturing a sintered compact in a low vacuum atmosphere, the formation of the Zr (O) phase and the evaporation of uranium which occur in the conventional high sintering system can be suppressed simultaneously.

To this end, the present invention includes a sintering furnace for sintering the uranium-zirconium powder compacts that can easily control the low vacuum / inert gas atmosphere that can maintain an inert atmosphere outside the vacuum sintering furnace.

Specifically, the present invention is a housing for accommodating a crucible for producing a uranium-zirconium powder compact and a heater for supplying heat for sintering the uranium-zirconium powder compact to the crucible, Uranium is in communication with the bottom side opening of the housing and comprises a vacuum system for adjusting the degree of vacuum of the housing, and an inert chamber for accommodating the housing, the inlet and outlet of the inert gas in a predetermined position on the upper and lower sides A sintering furnace for sintering zirconium powder compacts.

Preferably, the housing is maintained by the vacuum system in a low vacuum atmosphere in the range of 1.0 × 10 ⁻⁴ to 1.0 × 10 ⁻¹ torr, particularly preferably at 3.0 × 10 ⁻⁴ torr. .

The inert chamber is a device for further maintaining the low vacuum of the housing due to the inflow and outflow of argon, and serves as a buffer space to prevent outside air from entering. The inert gas containing nitrogen or argon gas is continuously charged in the housing to maintain a low vacuum state, or an inert gas chamber is attached to the outside of the high vacuum housing to prevent the inflow of air and to control the degree of vacuum. Therefore, formation of the Zr (O) phase and uranium evaporation which occur when the uranium-zirconium powder compact is sintered in a high vacuum atmosphere can be suppressed at the same time.

The heater is a device for supplying heat to the crucible for the sintering treatment of the uranium-zirconium powder compact, it is preferably heated to a heat of 1300 ~ 1500 ℃, preferably 1400 ~ 1500 ℃ range. When sintered under the process conditions in this range, it shows a high theoretical density of 95% or more TD.

In the present invention, the operation of the sintering furnace for the production of uranium-zirconium sintered alloy is as follows.

The manufactured uranium-zirconium powder compact is charged into a crucible of a sintering furnace and a heater is operated to heat the temperature of the crucible to 1300-1500 ° C., and the housing is connected to a bottom side opening by using a vacuum system (for example, a vacuum pump). Sintering is carried out in a vacuum in the range of 10 × 10 ⁻⁴ to 1.0 × 10 ⁻¹ torr. At this time, the inert gas containing nitrogen or argon gas is continuously charged into the inert gas chamber attached to the outside of the housing to prevent the inflow of external air into the housing, thereby suppressing the formation of Zr (O) phase in the sintered body and the evaporation of uranium. can do.

Hereinafter, the present invention will be described in more detail with reference to Examples and the accompanying drawings. However, the following examples and drawings are provided to aid the understanding of the present invention, and the contents of the present invention are not limited or limited by the embodiments of the following drawings.

Preparation Example Manufacture of U-Zr Mixed Powder Compact

Metal uranium powder, which is used as a raw material of the uranium-zirconium metal fuel sintered body, may be manufactured from a uranium lump through a centrifugal atomization process. Zirconium powder was prepared by the zirconium sponge (hydride-dehydride process). Uranium and zirconium powders produced in this process were passed through a sieve, and the powder passed through the sieve was weighed at a constant weight ratio (38 wt% uranium and 62 wt% zirconium). The weighted powder was mixed homogeneously in a mixer, which was charged to a V-type mixing vessel and then carried out for 2 hours at a rotational speed of 75 revolutions per minute (rpm) in an argon atmosphere. Uranium and zirconium mixed powder was charged into a molding mold apparatus (mold and punch) in a molding machine, and then a green pellet was prepared at a pressure of about 428 MPa.

Example 1 Preparation of U-Zr Sintered Alloy in High Vacuum Atmosphere

The uranium-zirconium mixed powder compact prepared according to the preparation example was sintered for 2 to 100 hours at a temperature range of 1100 ° C. to 1500 ° C. under a low vacuum atmosphere of various vacuum ranges in an environment in which low vacuum was maintained by inflow of argon gas. A uranium-zirconium sintered alloy was produced. As a result of sintering the uranium-zirconium powder compact according to Example 1, the sintering density according to the sintering temperature and the sintering time in a high vacuum of 1.0 × 10 ^-4 torr is shown in Table 1 .

Example 2 Preparation of U-Zr Sintered Alloy in Low Vacuum Atmosphere

The uranium-zirconium mixed powder compact prepared according to the preparation example was sintered at a temperature of 1450 ° C. for 3 hours under a low vacuum atmosphere ranging from 1.8 × 10 ⁻⁴ to 1.0 × 10 ⁻³ torr to prepare a uranium-zirconium sintered alloy.

Experimental Example Analysis Using SEM and SEM / EDS

In order to determine the microstructure and the content of the sintered body prepared according to an embodiment of the present invention, the following assay was performed. After the longitudinal section of the prepared sintered body was taken, the surface microstructure and component distribution of uranium and zirconium were evaluated for a constant area (300 × 300 μm) in the inner direction from the outside of the specimen. Analysis was performed using a scanning electron microscope (SEM) and SEM / EDS (SEM / energy dispersive spectroscopy) equipment.

Sintering Temperature (℃) Sintering time (h) Sintered Density (g / cm3) Theoretical density (% TD) 1100 100 8.04 92.7 1300 30 8.18 94.3 1350 30 8.44 97.3 1400 10 8.47 97.7 1450 3 8.50 98.0 1500 2 8.56 98.7

As shown in Table 1 , the sintered density after sintering for 100 hours at 1100 ℃ sintered 8.04 g / cm 3 (92.7% TD-theoretical density), 95% TD or less even after 30 hours at 1300 ℃ sintered very low Seemed. However, when sintered at a temperature of more than 1350 ℃ exhibited a high theoretical density of more than 95% TD.

1 is a flow chart showing the manufacturing process of the uranium-zirconium sintered alloy according to the present invention. As shown in FIG. 1 , the manufacturing process of the uranium-zirconium sintered alloy consists of first mixing, pressing and molding uranium and zirconium powder, followed by sintering under a reducing gas atmosphere.

Figure 2 is an electron micrograph showing the microstructure of the uranium-zirconium sintered alloy according to the present invention. As shown in FIG. 2 , microscopic images of the uranium-zirconium sintered bodies prepared by sintering at 1450 ° C. for 3 hours in a high vacuum ( FIG. 2A ) or low vacuum ( FIG. 2B ) atmosphere were observed. The sintered compact exhibited a microstructure in which a gray α-Zr phase was deposited on δ-UZr ₂ , which is a light matrix. In addition, fine pores are uniformly distributed. In the case of sintering in a high vacuum atmosphere, a dark gray mass of Zr (O) phases was observed outside the sintered alloy. On the other hand, almost no Zr (O) phases were observed in the sintered metal produced by maintaining a low vacuum. The Zr (O) phase is formed by combining Zr with oxygen introduced into the chamber by the inflow of air, and is composed of 10 to 20 wt% of oxygen (O) as an impurity. Therefore, the Zr (O) phase was not observed in the low vacuum sintering process because the inflow rate of air was decreased as the vacuum degree of the sintering furnace was lowered, thereby suppressing the oxidation of zirconium.

3 is a graph showing the distribution of components of the uranium-zirconium sintered alloy according to the degree of vacuum according to the present invention. As shown in FIG. 3 , the sintering behavior of the uranium-zirconium powder compact was observed according to the degree of vacuum. Sintering was performed at 1450 ° C. for 3 hours, and in this process, the degree of vacuum was controlled by continuously charging argon gas into the sintering chamber.

In the process of sintering the uranium-zirconium powder compact, the component distribution of the uranium-zirconium sintered body obtained under various vacuum conditions (A, B, C, D, and E) can be seen. In the sintered body manufactured by sintering at high vacuum, uranium evaporated on the surface, so the uranium concentration was very low and the content of zirconium was relatively high. On the other hand, as a result of lowering the degree of vacuum by charging argon gas during the sintering process, the concentration of uranium on the surface of the sintered body was increased by suppressing evaporation of uranium.

Figure 4 is a schematic diagram of a sintering furnace easy to control the low vacuum / argon atmosphere according to the present invention. As shown in Figure 4 , it can be seen a view of the sintering furnace easy to control the low vacuum atmosphere that can maintain the argon atmosphere inside the vacuum sintering furnace. Sintering the uranium-zirconium powder compact in a sintering furnace that can easily control a low vacuum atmosphere suppresses the inflow of air during the sintering process, thereby suppressing the formation of the Zr (O) phase in the uranium-zirconium sintered alloy. In addition, since the sintering in low vacuum, it is possible to suppress the evaporation of uranium during the sintering process.

As described above, according to the present invention, in the process of sintering the uranium-zirconium powder compact, the inert gas argon gas may be directly charged into the high vacuum sintering chamber or the argon gas atmosphere may be maintained outside the high vacuum sintering furnace. By attaching a chamber to prevent the inflow of air into the sintering furnace and controlling the degree of vacuum, thereby suppressing Zr (O) phase formation and evaporation of uranium in the uranium-zirconium sintered alloy, thereby producing a homogeneous uranium-zirconium sintered alloy. It can provide a useful effect.

Claims (5)

Uranium-zirconium powder comprising the step of sintering a uranium-zirconium powder compact containing 10 to 90% by weight of zirconium and the balance of uranium in a low vacuum inert atmosphere in the range of 1.0 × 10 ⁻⁴ to 1.0 × 10 ⁻¹ torr Low vacuum sintering method of molded body. The low vacuum sintering method of uranium-zirconium powder compact according to claim 1, wherein the low vacuum is maintained in a range of 1.0 × 10 ⁻³ torr. The method of claim 1, wherein the sintering temperature is 1300 ~ 1500 ℃, the holding time is low vacuum sintering method of the uranium-zirconium powder compact, characterized in that it further comprises 1 to 5 hours. A housing for preparing a uranium-containing powder compact and a heater disposed at an outer periphery of the crucible for receiving a heater for supplying heat to the crucible for sintering the uranium-zirconium powder compact; A vacuum system in communication with the bottom side opening of the housing and adapted to adjust the degree of vacuum of the housing; And An inert chamber accommodating the housing and provided with an inlet and an outlet of an inert gas on upper and lower sides Sintering furnace for sintering the uranium-zirconium powder compacts comprising a in a low vacuum inert atmosphere in the range of 1.0 × 10 ^-4 ~ 1.0 × 10 ^-1 torr. The method of claim 4, wherein the housing is maintained in a low vacuum atmosphere in the range of 1.0 × 10 ⁻⁴ to 1.0 × 10 ⁻¹ torr by the vacuum system, and heat in the range of 1300 ° C. to 1500 ° C. for sintering the powder compact. A sintering furnace for sintering a uranium-zirconium powder compact, characterized in that supplied by the crucible in a low vacuum inert atmosphere.

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