JPH01304391A - Production of oxide nuclear fuel body - Google Patents

Abstract

PURPOSE:To allow molding and processing even in the case of raw material powder with which crystal grains relatively hardly grow by executing a 3-stage treatment to assure a prescribed oxygen partial pressure by incorporating and mixing air into and with gaseous CO2 or gaseous N2. CONSTITUTION:Oxide nuclear fuel pellets having fine double structures in which UO2+x powder having <=3m<2>/g specific surface area value (BET) is used as the raw material are first presintered for <=1 hours in a presintering region 2 in a gaseous H2 or H2+N2 atmosphere at 1,300-1,600 deg.C treatment temp. The treatment is then progressed to the prescribed sintering stage by the combination of <=20 hours treatment time, 1,300-1,600 deg.C treatment temp. and the oxygen partial pressure at the treatment temp. of the prescribed range in the sintering region 2 of the gaseous mixture atmosphere composed of the air and CO2. Further, the 3-stage continuous treatment to execute the reduction treatment for <=1 hours in the reduction region 6 of the gaseous H2 or H2+N2 atmosphere is executed. The molding and processing are enabled in this way even in case of the raw material powder with which the grown grains of the crystal grains relatively hardly grow.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing an oxide nuclear fuel assembly used in nuclear reactors for power generation and the like.

[Prior art] In conventional UO□ pellets, generally UO2+
The powder of X is pressed to form a compact called a clean pellet, which is heated and sintered in a slightly humidified hydrogen gas stream at a processing temperature of 1,700°C or higher for 8 hours or more, and its density is 95% TD (theoretical density).

Theoretical DensiL), the average crystal grain size is about several gm, the grain size distribution in the pellet radial direction is almost -like, and the oxygen to uranium metal atomic ratio is 0/U.
The ratio has a stoichiometry of 2.00.

It is also known that the thermal dimensional stability during long-term reheating at 1700° C. causes a gradual increase in density.

Patent application No. 62-1799 filed by the same applicant as this application
UO2 which is relatively easy to sinter as disclosed in No. 85
+8 powder is pressed under relatively high pressure to form clean pellets, pre-sintered in an H2 gas atmosphere at a processing temperature of 1400 to 1600°C, further sintered at the same temperature in a CO2 gas atmosphere, and then When the reduction treatment is again carried out at the same temperature in an H2 gas atmosphere, an oxide nuclear fuel pellet is obtained which has a double microstructure of a large particle size region in the center and a small particle size region in the outer periphery.

The large crystal grain size in the center reduces the release of fission product gas (hereinafter referred to as FP gas) from the pellet, and the small crystal grain size in the outer periphery reduces plastic deformation on the pellet side. As a result, the degree of contact between the pellet and the cladding tube is improved, and the mechanical interaction between the pellet and the cladding tube (hereinafter referred to as PCM I) is improved.
CIad Mechanical Interaction
n) is relaxed.

However, in the case disclosed in Japanese Patent Application No. 179986/1986, the double microstructure was noticeable only when UO2+U powder, which is relatively easy to sinter, was used.

Here, the raw material powder that is easy to sinter is the specific surface area value (BET)
or 3 tn'/g.

[Problems to be Solved by the Invention] In the conventional method for producing an oxide nuclear fuel body, pellets having a pronounced double microstructure can be obtained only when UO2+8 powder, which is relatively easy to sinter, is used.

However, from the perspective of irradiation behavior, nuclear reactor operation is more flexible than the conventional UO2 pellets, which have an average crystal grain size in the radial direction of approximately several 1, if there is a margin in the nuclear fuel design. In the case of sintering improved large-grain size U02 pellets having a large-grain double microstructure, it is difficult to find a method that enables molding and processing in the case of raw material powder in which crystal grains are relatively difficult to grow. There wasn't.

In addition, here, pellets having a large particle size double microstructure are:
The pellet has a small crystal grain size region of about 10 gm on the outer periphery, which is the same as conventional pellets, and a large crystal grain size region of 10 gm or more in the center.

In addition, the raw material powder that is difficult to sinter is defined by the specific surface area value (
BET) refers to a relatively inert powder of 3rn'/g or less.

This invention was made in order to solve the above-mentioned problems, and is an improved large-grain UO having a large-grain double microstructure that allows for greater margin in nuclear fuel design and flexible reactor operation.
An object of the present invention is to provide a method for manufacturing an oxide nuclear fuel body that enables molding and processing even in the case of raw material powder in which crystal grains are relatively difficult to grow by sintering two pellets.

[Means for Solving the Problems] The method of the present invention involves rough molding using a raw material, such as UO2 powder, which is relatively difficult to sinter and has a specific surface area value (BET) of 3 ml/g or less. A slurry is prepared, granulated and a suitable lubricant such as zinc stearate or polyhyneal alcohol is added to form a granulated powder that is easily molded.

In addition, in order to set the density of the sintered body to a predetermined density from the viewpoint of design and use, in addition to a small amount of the above-mentioned lubricant as a pore-forming agent, pretreatment is applied to the raw material powder for the purpose of forming pores of a predetermined size. By adding crushed or sized ammonium oxalate, ammonium tartar, starch, sucrose, etc.
Mixing deviation is good.

The above granulated powder is processed under relatively high molding pressure, e.g.
´ or more to make a clean pellet with high enrichment.

Next, the above clean pellets are treated at a treatment temperature of 1300~
At 1600℃, N2 gas or H2+N, which is called a safety gas,
Pre-sintering for less than 1 hour in the mixed gas atmosphere of 2.
C0 to ensure a specified oxygen potential at lower temperatures.
Mix air with the two gases and hold for 20 hours or less in a sufficiently mixed atmosphere or an inert gas atmosphere, such as the cheapest N2 gas atmosphere, with a mixing ratio of air and CO2 or N2 gas of [1,00] to 10. to proceed with sintering, then use N2 gas or safety gas (H2 and N2) again at the same temperature.
(mixed gas) for less than 1 hour.

The three-zone sintering furnace used in the three-stage wood treatment method is an external heating type, and the external heating furnace inside the shell can raise the temperature to about +5[deg.]C.

Here, the three-stage treatment method refers to a method for sintering oxide nuclear fuel pellets, and includes three types of sintering atmospheres, each with three zones provided in a sintering furnace, and continuous treatment. This refers to a method in which hatch processing is performed using a sintering furnace with a similar atmosphere.

[Function] One of the conventional sintering methods for refractory nuclear fuel Beret 1~
Unlike the one-stage processing method in which the sintering atmosphere is set up in one region with humidified hydrogen and continuous processing is performed, the present invention uses CO2
Since it is a three-stage processing method that secures the oxygen potential of the sintering atmosphere by mixing and mixing air with gas or N2 gas, the resulting UO
Regarding thermal dimensional stability during long-term resintering of the two pellets at 1700°C, gradual densification occurs as in the case of conventional sintering in a hydrogen gas atmosphere.

In addition, in the case of the three-stage processing method of the present invention, uranium oxide powder with a relatively poor sintering property of 3 m'/g or less, that is, uranium oxide powder that is difficult to increase density and grain size during sintering, is used as a raw material. It is also possible to produce oxide nuclear fuel pellets with improved large particle size and double microstructure.

By the way, UO2 was obtained by heating and sintering clean pellets as raw material powder at 1750°C for 4 hours in a humidified N2 gas atmosphere using normal rough molding, granulation, and molding pressure of 4 tons.
The density of the pellets is 95% TD, and the average grain size is approximately 6
It is gm.

[Example] Hereinafter, the present invention will be explained in detail based on FIG.

Figure 1 shows a sintering furnace with a processing temperature of 1,300 to 1,600°C used in an embodiment of the present invention, a three-stage processing method in which air is mixed with CO2 gas to ensure a predetermined oxygen partial pressure. It is a schematic explanatory diagram.

In Figure 1, first, arrow 4 points to the pellet loading chamber 1 at room temperature.
Clean pellets, which are the processing material, are loaded in the direction shown, and the processing temperature is 1300 ~] 6 [10°C, 9 processing times, 1 hour or less, and the formation of closed pores is completed in the H2 or safe gas atmosphere preliminary sintering area 5. The process is then pre-sintered.

Next, the low-density pellets with a stoichiometric composition in which closed pores have been formed pass through a weir 7 formed by a N2 gas curtain to create a mixed gas atmosphere of air and CO2 that maintains a predetermined oxygen partial pressure. In the sintering zone 2, a predetermined sintering process is carried out under a relatively high oxygen potential for a processing time of 20 hours or less.

Here, the predetermined sintering process means that the density is, for example, 95
The density is controlled by adding and mixing the above-mentioned pretreated pore-forming material so as to satisfy the usage density of %TD.

Next, the processing temperature is such that an oxide nuclear fuel beam 1 having a double microstructure which is preferable from the point of view of irradiation behavior is obtained;
A predetermined sintering process is achieved by a combination of treatment time and oxygen partial pressure.

For example, in order to obtain an oxide nuclear fuel pellet having a preferable double microstructure in which the grain size at the center is 23 gn+ and the grain size at the outer periphery is 1 gm, the treatment temperature is 1400°C, the treatment time is 6 hours, and the oxygen partial pressure is 0. A combination of 0.01 atm is fine.

Further, the pellets having a superstoichiometric composition that have undergone a predetermined sintering process pass again through a weir 7 formed by a N2 gas curtain, and are returned to a stoichiometric composition in an H2 or safe gas atmosphere reduction area 6. The O/U ratio, which is the specification of the thing, is 1.
．． Reduction processing is performed in a processing time of 1 hour or less so as to satisfy the ratio of 99 to 2.02.

Then, the predetermined oxide nuclear fuel pellets are cooled in the cooling/pellet removal chamber 3, and then taken out and sent to the next process.

The means for mixing air into CO2 gas to ensure a predetermined oxygen partial pressure is as follows.

C02 gas is supplied to a CO2 gas cylinder from 13, controlled to a predetermined supply pressure by a pressure reducing valve 12, and supplied at a predetermined flow rate by a float type flow meter 11.

On the other hand, in order to prevent dust from entering the air,
Air cleaning filters 15 are installed upstream and downstream of the air compressor 916 to supply air at a predetermined pressure, and the air mass flow controller 1-roller 14 controls the air flow rate to a predetermined flow rate.

Both the above CO2 gas and air have a constant supply pressure in the gas mixing tank 9, and are sufficiently mixed in the gas mixer 8 using a static mixer or a stirrer.
The air and CO2 mixed gas atmosphere of the sintering furnace is supplied to the sintering zone 2.

Furthermore, when N2 gas is used instead of the above-mentioned CO2 gas, sintering is carried out in the same manner as in the case of CO2 gas, except that the gas cylinder 13 is replaced with one containing N2 gas.

A specific example carried out using the sintering furnace shown in FIG. 1 will be described below.

The UO2 pellets obtained in the examples of the present invention were made from green pellet powder with a normal rough molding, granulation, and molding pressure of 4 tons, 8ffi', and were provided with clean pellets that were subjected to equivalent molding. be.

Figure 2 shows the relationship between the porosity of U02 pellets obtained by sintering in an atmosphere containing CO2 gas mixed with air and the flow rate ratio of air and CO2 gas, and Figure 3 shows the relationship between the porosity of U02 pellets obtained by sintering in an atmosphere in which air is mixed with CO2 gas, and Figure 3 FIG. 3 is a diagram showing the relationship between the crystal grain size in the center and the flow rate ratio of air and CO2 gas.

Incidentally, the UO2 pellets obtained in the examples of the present invention are obtained by using the above powder as a raw material and subjecting it to the same molding to provide green pellets, which are sintered in an atmosphere in which air is mixed with N2 gas. The relationship between the porosity of the obtained UO2 pellets and the flow rate ratio of air and N2 gas is the same as in Figure 2,
Furthermore, it was experimentally confirmed that the relationship between the crystal grain size at the center of the pellet and the flow rate ratio of air and N2 gas is the same as that shown in FIG.

As a first example, pre-sintering is performed for 1 hour in an H2 gas atmosphere at a processing temperature of 1400°C, and then at the same temperature and under a mixed gas of air and CO2, with a flow rate ratio of 0.05 and an oxygen partial pressure of 0. ．． The density of the pellets was 97% TD, O/
The U ratio is 2.00, the crystal grain size at the center is 23 gm, and the crystal grain size at the outer periphery is 11 gm.

By the way, the density of the UO2 pellets obtained under the same heating conditions as above when only CO2 gas is used without mixing air is 95% TD, the O/U ratio is 2.00, and the center grain size is 101. , the outer peripheral crystal grain size is 8 gm.

Next, as a second example, pre-sintering is performed for 1 hour in an H2 gas atmosphere at a treatment temperature of 1500°C, and then the oxygen content is further sintered at the same temperature under a mixed gas of air and CO2 at a flow rate ratio of 0.45. The pressure was set to be equivalent to 0.07 atm, and sintering was allowed to proceed for 6 hours under this oxygen potential. Afterwards, the pellets were reduced again under an H2 gas atmosphere, and the density of the pellets was 97%.
TD, O/U ratio is 2.00, and center grain size is 3.
3 Im, and the outer peripheral crystal grain size is 14 River m.

By the way, the density of the UO2 pellets obtained under the same heating conditions as in Section 1 using only CO2 gas without mixing air was 96% TD, the O/U ratio was 2.00, and the central crystal grain The diameter is 16 μm, and the outer peripheral crystal grain size is 12ILIIl.

As is clear from each of the above, C02 gas or N2 gas
Even if the raw material is a powder that is relatively difficult to sinter by mixing air with gas to increase the oxygen partial pressure, U O2 with high density has a double microstructure with large grain size and excellent irradiation behavior. It can be seen that manufacturing to Beretsu 1 is possible.

In addition, in the case of a light water reactor, the amount of Gd2O3 in the gal-linear doped beret is about 10% by weight, which is below the solid solubility limit of about 30% by weight, and its sintering behavior includes a solid solution reaction. Since this is equivalent to the case of UO2, the three-stage sintering method in which air is mixed with CO2 or N2 gas according to the embodiment of the present invention can be applied.

Furthermore, since (pH, U) O2 is a solid solution entirely, its sintering behavior is not significantly different from that of UO2.

In addition, in the case of the three-stage processing method of the present invention, C02° with a specific surface area of 3IT1'/g or less of BET, which has relatively poor sinterability, that is, increases in density during sintering makes it difficult for crystal grain size to grow. Even when X powder is used as a raw material, an improved large particle size double-layered microstructured oxide nuclear fuel can be obtained. The mechanism will be described below.

First, in the preliminary sintering process in an N2 gas atmosphere, the clean bellets 1 to 1 are subjected to reduction treatment and sintering progresses to about 90% TD, and the formation of closed pores is almost completed.

Since the inside of the closed pores is dominated by N2 gas, gradual densification is performed without an increase in the pressure inside the closed pores during the long-term resintering at 1700° C. as described above.

Next, CO2 gas is mixed with air to produce CO. When sintering in an atmosphere with a higher oxygen partial pressure than in the case of only gas, the initial stage is a low-density sintered body with a stoichiometric composition, or the sintered body is sintered in an atmosphere with a high oxygen partial pressure. The supply to the sintered body is relatively slow; at the outer periphery of the sintered body, oxygen enters between the crystal lattices and immediately moves to the center. They tend to exist as interstitial atoms, and the superstoichiometric composition on the central side promotes sintering.

Here, in Figure 4, M, H, RAND and O, KlIBAS.
t': The Thermoch written by HEWSKI
chemical properties of Urani
um Compounds'', 01iver and
Boyd, Edinburgh.

G of U3O8 and C409 shown in 1963
It shows the relationship between the equilibrium oxygen partial pressure found from the standard production energy of i b b s and temperature, and also shows the upper limit level and range of the oxygen partial pressure of the mixed gas of air and CO2 that can be sintered in the embodiment of the present invention. The predetermined range is indicated by diagonal lines.

In addition, since the temperature range of the standard energy of formation shown by M, Il, RAND et al. is up to 1122°C, the high temperature side is extrapolated and shown by the broken line, or the oxygen of the mixed gas of air and 002 of the present invention is The upper limit level of partial pressure is the equilibrium oxygen partial pressure of U3O8.
It was confirmed through this experiment that the sintered body did not lose its shape even though it was rotated twice.

This is 1300~], 50 in sintering on the wood grain side.
[In the oxygen partial pressure range shown in Figure 4 under heating at 1'C, the sintered body is in an equilibrium state of U, O, and the resistance reaction does not proceed if the treatment time is 20 hours or less. It is thought that.

In the next reduction treatment process under an N2 gas atmosphere, the reduction action is delayed in the release of excess oxygen toward the center of the pellet because the pellet is a high-density sintered body.

On the other hand, excess oxygen in the outer periphery is liberated relatively quickly.

'' - Corresponding to the distribution of excess oxygen in the t-radial direction of the pellet, crystal grain growth progresses in the center of the pellet compared to the outer side.

As a result, even when a powder that is relatively difficult to sinter is used as a raw material, oxide nuclear fuel pellets having a double microstructure with a large grain size region in the center and a small grain size region in the outer periphery can be obtained.

In addition, by mixing air with N2 gas and controlling the oxygen partial pressure, the oxygen potential of the sintering atmosphere is secured, promoting the progress of sintering, and producing an oxide with a double microstructure with large grain size. The mechanism by which nuclear fuel pellets are obtained is also the same as in the case where air is mixed with CO2 gas.

In this way, the large crystal grain size at the center reduces the release of FP gas from the pellet, and the small crystal grain size at the outer periphery improves the contact situation between the pellet outer periphery and the cladding tube, resulting in PCMI or be relieved.

[Effects of the invention] Long-term reburning at 1700°C of an oxide nuclear fuel assembly obtained by the three-stage treatment method in which air is mixed with 002 gas or N2 gas of the present invention to ensure a predetermined oxygen partial pressure. Thermal dimensional stability at 1,700°C or higher
As in the case of the oxide nuclear fuel pellets obtained by sintering in a two-gas atmosphere, gradual densification is produced and swelling, which is the volume increase during irradiation of the oxide nuclear fuel pellets 1 to 1, is alleviated.

Regarding the stoichiometric composition of the oxide nuclear fuel pellets obtained by the present invention, the O/U ratio, which is the specification, is 1.
．． 99 to 2.02, and the variation as a product is small. Furthermore, regarding the fine structure of the retardant nuclear fuel pellets obtained by the present invention, even when the raw material is UO2° powder with a specific surface area of 3 m'/g or less of BET, which has poor sinterability, it has a large crystal grain size. It has a double fine structure.

This double microstructure consists of a large grain size region at the center of the oxide nuclear fuel beam and a small grain size region at the outer periphery.

The excellent characteristics of this double microstructure are due to the fact that the crystal grain size is larger in the central part where the irradiation temperature of the pellet is relatively high, from the viewpoint of reducing FP gas release from oxide nuclear fuel pellets.
Because the diffusion and movement of P gas is delayed, the F of the pellet is
P gas holding capacity will improve. This suppresses the increase in the internal pressure of the fuel rod due to the release of FP gas, thereby preventing deformation of the cladding tube, and also preventing a decrease in thermal conductivity in the gap between the pellet and the cladding tube.

In addition, the excellent characteristics of the double microstructure are also due to the fact that from the viewpoint of PCMI, the grain size of the outer periphery of oxide nuclear fuel pellets is small, so the pellet side is easily plastically deformed, so it is necessary to cover the outer periphery of the oxide nuclear fuel pellet during fluctuations in reactor output. The stress exerted on the tube is reduced and the amount of deformation of the cladding tube is also reduced, thereby preventing damage to the fuel rods. As mentioned above, the excellent properties of oxide nuclear fuel pellets with a double microstructure and large grain size result in favorable irradiation behavior, which can be used to increase burnup and load following operation for the purpose of improving economic efficiency in nuclear power generation, etc. This makes it possible to design fuels and operate nuclear reactors with margins in order to ease power distribution regulations.

Furthermore, since the sintering temperature of the three-stage treatment method of the present invention is lower than the conventional hydrogen sintering temperature, equipment costs and operating costs are reduced.

Furthermore, the air used for atmosphere control in the three-stage treatment method of the present invention is taken from the atmosphere, and N2 gas is cheaper than Co2 gas, making it possible to reduce operating costs.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a sintering furnace with a processing temperature of 1,300 to 1,600°C, which is used in an embodiment of the present invention, a three-stage processing method in which air is mixed with CO2 gas to ensure a predetermined oxygen partial pressure. Explanatory diagram, Figure 2 is a diagram showing the relationship between the porosity of UO2 pellets obtained by sintering in an atmosphere in which air is mixed with CO2 gas and the flow rate ratio of air and 002 gas, and Figure 3 is a diagram showing the relationship between the flow rate ratio of air and 002 gas. Figure 4 shows the relationship between the crystal grain size in the center and the flow rate ratio of air and 002 gas.
FIG. 2 is a diagram showing the relationship between the equilibrium oxygen partial pressure and temperature, which can be found from the Gibbs standard energy of formation of 4O9. In the figure. ■ Pellet loading chamber 2 Air and CO2 mixed gas or air and N2 mixed gas atmosphere Sintering area 3 Cooling and pellet removal chamber 5 N2 gas or N2 and N2 mixed gas atmosphere Pre-sintering area 6 N2 gas or N2 and N2 mixed gas atmosphere Reduction area 7 Bottle 8 Gas mixer 9 Gas mixing tank 10: Valve 11 Float flow rate λ1 ] 2 Pressure reducing valve 13: Gas phone 14: Mass flow controller for air 15: Filter for used air n1 16: Compression for air Machine Agent Patent Attorney 1) Haru Kitatake

Claims (5)

[Claims] (1) UO_ with a specific surface area value (BET) of 3 m^2/g or less
Oxide nuclear fuel pellets with a double microstructure made from 2_+_X powder are pre-sintered at a processing temperature of 1300 to 1600°C in an H_2 or H_2+N_2 gas atmosphere for 1 hour or less, and then sintered in an air and CO_2 mixed gas atmosphere. Processing time of 20 hours or less, 1300-1600°C
The sintering process is progressed to a predetermined sintering process using a combination of processing temperature and oxygen partial pressure at a predetermined range of processing temperatures, and then,
A method for producing an oxide nuclear fuel assembly, comprising a three-stage continuous process of performing a reduction treatment for one hour or less in an H_2 or H_2+N_2 gas atmosphere. (2) UO_ with a specific surface area value (BET) of 3 m^2/g or less
Oxide nuclear fuel pellets with a double microstructure made from 2_+_X powder are pre-sintered at a processing temperature of 1300 to 1600°C in an H_2 or H_2+N_2 gas atmosphere for 1 hour or less, and then sintered in an air and N_2 mixed gas atmosphere. The sintering process is progressed to a predetermined sintering process using a combination of a treatment time of 20 hours or less, a treatment temperature of 1300 to 1600°C, and an oxygen partial pressure at a treatment temperature in a predetermined range.
A method for producing an oxide nuclear fuel assembly, comprising three consecutive stages of reduction treatment in an atmosphere of _2 or H_2+N_2 gas for one hour or less. (3) UO_ with a specific surface area value (BET) of 3 m^2/g or less
Oxide nuclear fuel pellets with a dual microstructure made from 2_+_X powder are made of ground or granulated ammonium oxalate, ammonium tartrate, starch, sucrose or naphthalene, and zinc stearate or zinc stearate, which also acts as a lubricant. The method for producing an oxide nuclear fuel assembly according to claim (1) or claim (2), characterized in that polyvinyl alcohol is used as a pore-forming agent. (4) According to any one of claims (1) to (3), wherein UO_2 pellets, (Pu, U)O_2 or (Gd, U)O_2 pellets are used as the oxide nuclear fuel pellets. A method for producing an oxide nuclear fuel body. (5) To ensure an oxygen partial pressure within a predetermined range as a sintering atmosphere, air is mixed with CO_2 gas or N_2 gas at a mixing ratio of 0.001 to 10, or CO
The method for producing an oxide nuclear fuel assembly according to any one of claims (1) to (4), characterized in that an inert gas is used instead of the _2 gas or the N_2 gas.