JPH0470594A - Fabrication of nuclear fuel pellet of oxide with niobia additive - Google Patents

Abstract

PURPOSE:To reduce cost of formation and fabrication of nuclear fuel pellets of oxide with niobia additine by keeping sintering environment at a determined level and making such large grain size as to reduce gas release at relatively low temperature. CONSTITUTION:The sintering promoter, niobium oxide is processed in the gas flow of hydrogen or mixture of hydrogen and nitrogen at about 800 deg.C for a proper time to reduce pentavalent Nb3O5 into tetravalent Nbo2 powder. The NbO2 powder is added with 0.2-1.0 weight % to uranium oxide or mixture of uranium oxide and plutonium oxide stuff powder, mixed and formed to green pellets. These pellets are heat processed for reduction in the gas flow of hydrogen or mixture of hydrogen and nitrogen at about 800- 1,000 deg.C fro a proper time. Then, it is sintered in the sintering temperature range of 1,000-2,000 deg.C which corresponds to the inverse absolute temperature lines between -0.25--3.2 and in the gas flow range of the logarithmic flow ratio of CO2/CO lines between 2.4-0.62.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing oxide nuclear fuel pellets used in nuclear power reactors, particularly niobia-added oxide nuclear fuel pellets.

[Conventional technology]

When a small amount of niobium oxide is added to uranium dioxide (102) powder, the sintered pellets have a large crystal grain size, which increases the diffusion distance of fission product (hereinafter referred to as FP) gas, which reduces FP gas release. , the pellets tend to creep more easily, resulting in increased resistance to P
It is said that this method is effective in improving tClad Interaction).

According to Japanese Patent Publication No. 1-20399, Niobia (N
12 by adding b20s) to uranium dioxide
Pellets with a large particle size of ~80 μm are obtained, and the sintering conditions are a sintering temperature of 1550°C or higher, and hydrogen containing a trace amount of water vapor or carbon dioxide between 1000 volume ppm and 20,000 volume ppm. It is said that by controlling the atmosphere, it is possible to produce specified pellets with few impurities.

[Problem to be solved by the invention]

Oxide nuclear fuel pellets used in nuclear reactors for power generation are
There are impurity regulations for carbon, fluorine, nitrogen, etc., and impurities are kept as low as possible, and the chemical composition is oxygen and gold.
It is necessary that the ratio of ig (IJ, Pu) be maintained at a stoichiometric composition of about 2.0.

Therefore, in the above publication, the sintering temperature is 1550°C or higher,
The firing atmosphere was 1000 ppm by volume and 20. There is a restriction that hydrogen contains a trace amount of water vapor or carbon dioxide of between 1,000 ppm by volume, and there is room for consideration in reducing manufacturing costs from the viewpoint of the life of the sintering furnace and heater, the required power, etc.

In view of the above-mentioned state of the art, the present invention has been developed to achieve economical requirements under a wider range of firing conditions, that is, with large grain size and fewer impurities.
It is an object of the present invention to provide a method for producing niobium oxide-added oxide nuclear fuel pellets having a stoichiometric composition.

[Means to solve the problem]

The present invention provides (1) a method for producing oxide nuclear fuel pellets in which uranium oxide or a mixture of uranium oxide and plutonium oxide is molded and sintered as a raw material powder, in which niobium oxide, which is a sintering accelerator; The product is subjected to reduction heat treatment at 800° C. for an appropriate time in a stream of hydrogen or a mixed gas of hydrogen and nitrogen to convert pentavalent Nb20s to tetravalent Nb02 powder, and convert the Nb02 powder 0. 2 to 1.0% by weight is added to the above raw material powder, mixed and molded to form green pellets, and the green pellets are heated at 800 to 1000°C for an appropriate time in a stream of hydrogen or a mixed gas of hydrogen and nitrogen. A reduction heat treatment of
The upper and lower limits of the logarithm of the flow rate ratio of carbon dioxide and carbon monoxide are respectively set to the sintering atmosphere corresponding to
℃ 2,4 to -0.62, and sintering temperature 200
In the case of 0°C, carbon dioxide and carbon monoxide are approximately linearly enveloped in the relationship between the logarithm of the flow rate ratio of carbon dioxide and carbon monoxide and the reciprocal of the absolute temperature of the sintering temperature, as -0.25 to -3.2 at 0°C. A method for producing oxide nuclear fuel pellets with high density, large particle size, stoichiometric composition, and low impurities, characterized by sintering in an air flow with a flow rate ratio of .

(2) For the sintering atmosphere corresponding to the sintering temperature range of 1000 to 2000°C, the upper and lower limits of the logarithm of the flow rate ratio of carbon dioxide and hydrogen are 2.3 when the sintering temperature is 1000°C.
~-0.50, and 0.0 at a sintering temperature of 2000°C.
15 to -2.5, sintering in an air flow with a flow rate ratio of carbon dioxide and hydrogen that is approximately enveloped by a straight line in the relationship between the logarithm of the flow rate ratio of carbon dioxide and hydrogen and the reciprocal of the absolute temperature of the sintering temperature. The method for producing oxide nuclear fuel pellets as described in item (1) above.

(3) The upper and lower limits of the logarithm of the flow rate ratio of steam and hydrogen are 2.6 to -0.
42, and 0.45 to 0.45 at a sintering temperature of 2000°C
-2.5 is characterized by sintering in an air flow within a range of flow rate ratios of water vapor and hydrogen that are almost linearly enveloped in the relationship between the logarithm of the flow rate ratio of water vapor and hydrogen and the reciprocal of the absolute temperature of the sintering temperature. The method for producing oxide nuclear fuel pellets according to item (1) above.

FIG. 1 shows the manufacturing process of the niobia-added oxide nuclear fuel pellets of the present invention.

The oxide nuclear fuel powder used as the raw material is U024X powder, which has been used in the past and has different powder characteristics, that is, different formability and sinterability.

Niobium oxide, which is a trace additive that acts as a sintering accelerator for one of the raw materials, is generally the highest-order oxide5.
Pentavalent Nb2O5 is used as a raw material because it is easy to handle, but in the present invention, pentavalent Nb2O5 powder is heated at 800°C in a flow of hydrogen gas or a mixed gas of hydrogen and nitrogen (hereinafter referred to as safety gas). It is heated for an appropriate period of time to perform a reduction heat treatment to reduce evaporative impurities, and is used as a sintering accelerator as a tetravalent NbO2 powder.

Next, 0.2 to 1.0% by weight of NbO2 powder, which is an amount sufficient to promote sintering, is added to the above UO2 powder and mixed mechanically using a ball mill or the like.

The above-mentioned mixed powder is roughly compacted, granulated, lubricated, and molded using a conventional cold compression method under the following molding conditions to produce desired green pellets.

The rough molding conditions are such that approximately 30±10 g of the above mixed powder is pressed under relatively low pressure to form a slug-like rough molded body. Next, these rough compacts are processed into granulated powder having a size of, for example, 0.1 to 0.01 mm. A small amount of low boiling point substances such as zinc stearate and polyvinyl alcohol are added and mixed as a lubricant to these granulated powders to facilitate molding. A fixed amount is put into the die of the mold and pressed with upper and lower punches under relatively high pressure, generally about 10a+m diameter, 10! A cylindrical green pellet of DID height is prepared.

In addition, in order to make the sintered body density a predetermined density from the viewpoint of design specifications, ammonium oxalate, ammonium tartrate,
Just add and mix starch, sucrose, etc. When using a pore-forming agent, if green pellets are manufactured without using a pore-forming agent, the density of a general oxide nuclear fuel sintered body is assumed to be 95% TD (theoretical density); Also, pore-forming agents are used when it becomes necessary to artificially suppress the density from the viewpoint of fuel design. Pretreatment of the pore-forming agent refers to drying and passing through a sieve to prevent agglomeration during addition and mixing, and pulverization to ensure the size of pores in the sintered body.

Next, the green pellets of the above mixed powder are heated at 800°C in a hydrogen gas or safety gas stream for an appropriate time to perform a reduction heat treatment to form treated pellets containing a trace amount of NbO2 and evaporate. Reduces impurities.

After the above treatment, as shown in FIGS. 2, 3 and 4, treated pellets of IJO2 containing a trace amount of NbO2 were sintered with Co,
/CD, CO2/H2, and H,0/H, in a sintering atmosphere having a flow rate ratio of 1,0/H, and sintering by heating for an appropriate time.

The niobia-doped oxide nuclear fuel pellet obtained by the production method of the present invention has a predetermined high density, a stoichiometric composition, a sufficiently low content of impurities, and good thermal dimensional stability. There is. Further, in order to obtain a desired crystal grain size, it is possible to control the sintering temperature by a combination of the flow rate ratio of the above-mentioned mixed gas that ensures the oxygen partial pressure of the sintering atmosphere with an appropriate sintering time.

[Effect]

FIG. 1 shows the process for producing niobia-added oxide nuclear fuel pellets of the present invention, and the effects of the individual steps of the present invention will be explained below in accordance with the order of the steps.

■ Tetravalent Nb is obtained through reduction heat treatment of pentavalent Nb and 05 powder.
By using O2 powder, the following effects can be achieved.

(a) Since the niobium oxide in the sintering temperature range and oxygen partial pressure range of the sintering atmosphere of the present invention is tetravalent NbO2, excess oxygen can be liberated in advance.

Cb) NbJs and 10. , , cracks occurred in the treated pellets due to the dimensional changes accompanying the reduction of both, but in the case of Nb
In the case of O2 and uO□□, the dimensional change due to reduction is 102
+X only, and it is possible to prevent cracks from occurring.

(C) Impurities contained in niobium oxide powder can be reduced.

In addition, 800℃ in hydrogen gas or safety gas flow
It has been confirmed from the weight change before and after the heat treatment and the X-ray diffraction test results that the pentavalent NbJs powder turns into the tetravalent NbO7 powder by the reduction heat treatment for an appropriate time.

(2) Green consisting of N b 02 and En2+X
By subjecting the pellets to reduction heat treatment to obtain treated pellets consisting of NbO2 and [102], the following effects can be achieved.

(a) Since the 0/U ratio of the niobia-added oxide nuclear fuel pellets in the sintering temperature range of the present invention and the oxygen partial pressure range of the sintering atmosphere maintains a stoichiometric composition of 2.00, excess oxygen is removed in advance. Can be released.

(b) Reduce impurities contained in green pellets of niobia-doped oxide nuclear fuel. This is due to the temperature rise process and sintering process until the sintering temperature is reached when the flow rate ratio of the mixed gas that provides a relatively high oxygen partial pressure in the sintering temperature range and oxygen partial pressure range of the sintering atmosphere of the present invention is selected. Since the formation of closed pores is rapid in the early stage of the sintering process, there is a possibility that the release of impurities from the treated pellets will be inhibited, so the impurities contained in the green pellets are released in advance at a temperature below the temperature at which sintering does not start. be able to.

Therefore, the level of impurities contained in the pellets is below the permissible value, and thermal dimensional stability is also ensured.

The closed pores of a ceramic sintered body are voids that are formed when sintering enters the final stage and are isolated and confined inside the sintered body. The reason why the formation of closed pores is accelerated is that the oxygen partial pressure in the sintering atmosphere of the first stage sintering in the method for producing niobia-added oxide nuclear fuel pellets of the present invention can be made relatively high. Sintering continues even during the heating process, and as a result, closed pores begin to form in the sintered body of relatively low density.

In addition, 800℃ in hydrogen gas or safety gas flow
NbO2 and UO3,
It has been confirmed from the weight change before and after the reduction heat treatment and from the X-ray diffraction test results that the green pellets made of .

Flow rate of each mixed gas to bring the treated pellets consisting of 0.2 to 1.0% by weight of NbO2 and UL into the sintering temperature range of the present invention and the oxygen partial pressure of the sintering atmosphere corresponding to these sintering temperatures. One-stage sintering in the ratio range allows for a thermally dimensionally stable product with a predetermined high density, a desired grain size, a stoichiometric composition with an O/11 ratio of 2.00, and low impurities. A good niobia-added oxide nuclear fuel pellet is achieved by the following actions.

(a) For uranium oxide, the temperature-dependent equation for the equilibrium oxygen partial pressure at which the O/U ratio is 2.00 is given by S, Aronson and J,
Be1le (J, Chem, Phys, 29 [1] 1
51-58 (1958)) is given by the following formula.

LogPo2-1.881-14332/Tk where,
Po2 (atm): Oxygen partial pressure Tk (K): Absolute temperature The upper limit of the oxygen partial pressure in the sintering atmosphere of the present invention is 002. . . The sintering promotion effect was obtained by setting the equilibrium oxygen partial pressure to .

On the other hand, regarding the lower limit of the oxygen partial pressure in the sintering atmosphere of the present invention, the lower limit of the oxygen partial pressure in the sintering atmosphere is 02. . . 1 of the equilibrium oxygen partial pressure of
It was confirmed that even at a level lowered by 1/06, there is a sufficient effect of promoting sintering.

(b) Oxygen in the sintering atmosphere such that the upper limit is the equilibrium oxygen partial pressure at which the 0/[1 ratio of uranium oxide is 2.00, and the lower limit is a level that is 1/106th of the oxygen partial pressure. The following three types of mixed gases that provide partial pressure can be used to form a sintering atmosphere that promotes sintering.

(i) The temperature-dependent equation for the equilibrium oxygen partial pressure in the case of a mixed gas of [l'02/CD is J, P, (:oughlin
(Ll, S.

Mines Bull, No. 542 (1954))
According to the following formula % formula % Here, CO2/Co: Flow rate ratio of both gases (ii
) The equilibrium oxygen partial pressure in the case of a mixed gas of Co2/N2 is
The equilibrium of CO2, CO, N20, and N2.02 was calculated using the standard free energy of formation.

The above flow rate ratios of Co2/CO and Co2/H2 can be determined by filling supply cylinders in advance at predetermined composition ratios, or by controlling the flow rates from each individual gas supply cylinder using a float flowmeter or mass flow controller. However, the pellets may be supplied to the sintering furnace from the downstream or upstream side of the pellet flow through a gas mixer.

(iii) The temperature-dependent equation for the equilibrium oxygen partial pressure in the case of a mixed gas of N20/l12 is T, L, Markin
et al (J.

+norg, nucl, chem,, 30 (1
968) 807 to 817) is given by the following formula.

LogPo2=-25026/Tk+1.96Log
Tk-0,966+2Log (N20/H2) Here, N20/H2: Flow rate ratio of both.In the case of sintering of this mixed gas of 8.0/H2, the nuclear fuel as oral tradition in JP-A No. 62-180292 By using a humidified hydrogen atmosphere control device in the pellet sintering furnace, it is possible to accurately ensure the oxygen partial pressure in the sintering atmosphere, and similarly to the above, the mixed gas in the sintering atmosphere is controlled from the downstream or upstream side of the pellet processing flow. supply

Further, Ar, He, or N2 gas may be used as a carrier gas in consideration of the oxygen partial pressure in the above-mentioned mixed gas.

FIG. 2, FIG. 3, and FIG. 4 show the relationship between the flow rate ratio range of three types of mixed gases and the sintering temperature, respectively, according to the present invention.

Table 1 shows the upper and lower limits of the oxygen partial pressure in the sintering atmosphere corresponding to each temperature in the sintering temperature range of 1000 to 2000°C of the present invention, and three types of mixtures that provide these oxygen partial pressures in the sintering atmosphere. Indicates the gas flow rate ratio.

(C) Figure 5 shows that the O/U ratio of uranium oxide is 2.00.
Together with the relationship between equilibrium oxygen partial pressure and temperature, we show the equilibrium oxygen partial pressure when the three types of mixed gases are CD2/CD = 1, CO2/H2-1, and )120/H2 = 1. In the flow ratio range of the present invention at the sintering temperature, the D of the treated pellet is
/U ratio is 2. It can be seen that the sintered pellets are kept as OO, and the sintered pellets after being taken out after sintering has progressed sufficiently have a stoichiometric composition, and during this sintering process, the 1/U ratio is 2. It can be seen that there is an effect that is ensured by 00.

(d) Furthermore, in the process before sintering, the treated pellets made of NbO, powder, and NbO2 and mouth 02 were made by reducing heat treatment, so the content of impurities and excess oxygen in the treated pellets was small, and the sintering atmosphere was It can be seen that these actions enable the formation of a sintering atmosphere and the production of a low impurity sintered body.

(e) Niobia-added oxide nuclear fuel pellets produced in a sintering atmosphere with an oxygen partial pressure range corresponding to the sintering temperature of the present invention have a predetermined O/ll ratio of, for example, 1.99 to 2,02
The present inventors have found through experiments that the growth of crystal grains is promoted by increasing the oxygen partial pressure of the sintering atmosphere to the upper limit of the range of the present invention.

Therefore, by controlling the oxygen partial pressure of the sintering atmosphere, it is possible to produce niobia-added oxide nuclear fuel pellets having a desired crystal grain size at a relatively low sintering temperature.

(f) For oxide nuclear fuel pellets containing about 30% by weight PuO2, PLI02 and UO3 are entirely solid solution, so their sintering behavior involves a solid solution reaction, but 1. I
These effects of the present invention will be confirmed by others in the art since there is no significant difference compared to the case of 02 alone.

[Example 1] UO2,,2 raw material powder was made into green pellets by normal rough molding, granulation, addition of lubricant, and molding pressure of 4 (Ton/cm2), and hydrogen containing about 1% volume water was used. Single oxide nuclear fuel pellets of 102 sintered by heating at 1750°C for 5 hours in a gas stream have a density of 96% TD and a crystal grain size of 8μ.
It is m.

The uranium oxide powder used in this example was commercially available and was commercially available.
uranate) method.

The rough molding pressure here is about 1 ton/cm2, and the granulation process produces granulated powder between -13 and +100 mesh, and 0.2% by weight of zinc stearate is added and mixed to this. Approximately 7 g of granulated powder containing zinc stearate was filled into a mold and cold compressed to produce cylindrical green pellets with a diameter of approximately 10 mm.

The following niobia-added oxide nuclear fuel pellet obtained according to the present invention is the same powder as the above-mentioned UO□, +2 powder.

High-purity Nb2O5 powder used for boat fishing was subjected to reduction heat treatment at 800°C for 5 hours in a safety gas flow to produce NbO2 powder. The safety gas used in this
It is a reducing gas with 1 part hydrogen gas and 2 parts nitrogen gas, and there is no danger of explosion.

Both the above 1Qb02 powder and [102,,2 powder were weighed and mechanically mixed using a ball mill so that the amount of NbD2 added was approximately 0.3% by weight.

Green pellets were made from the mixed powder obtained above using the same general rough molding, granulation, addition of lubricant, and molding pressure of 4 (tons/cm2) as described above.

The green pellets obtained above were subjected to reduction heat treatment at 800°C for 5 hours in a safety gas flow to produce NbO2.
and En. A processed beret consisting of was made.

The sintered body data is shown when the above niobia-added pellets are sintered in the following sintering temperature range of the present invention with a Co2/Co mixed gas within the oxygen partial pressure range corresponding to these temperatures.

The heating conditions for sintering were 1400° C. for 5 hours.

[Example 2] Niobia-added pellets prepared in the same manner as in Example 1 above were treated with 002 and H2 within the oxygen partial pressure range of the present invention as described below.
The data of the sintered body when sintered in a mixed gas with different flow rate ratios is shown.

The heating conditions for sintering were 1100 tons for 5 hours.

[Example 3] Niobia-added pellets made in the same manner as in Example 1 above were heated to a Co2/Co flow rate ratio within the oxygen partial pressure range corresponding to these temperatures in the following sintering temperature range of the present invention. −
The sintered body data when sintered in No. 1 is shown.

[Example 4] Niobia-added pellets prepared in the same manner as in Example 1 above were heated to a temperature of H, 0/H2 within the oxygen partial pressure range corresponding to these temperatures in the following sintering temperature range of the present invention. The data of the sintered body when sintered with mixed gas is shown.

The heating conditions for sintering were 1750° C. for 5 hours.

As can be seen from the above examples, even when using uranium oxide powder, which is relatively difficult to sinter, as a raw material, the comparison can be made by combining the addition of a small amount of niobium oxide and the oxygen partial pressure of the sintering atmosphere. The relatively low sintering temperature makes it possible to produce oxide nuclear fuel pellets with a grain size large enough to sufficiently reduce the release of FP gas.

〔Effect of the invention〕

(a) By maintaining the sintering temperature in the production of the niobia-added oxide nuclear fuel pellets of the present invention at a desired level with the oxygen partial pressure in the sintering atmosphere, sufficient FP can be achieved even at relatively low temperatures.
It is possible to have a grain size that reduces outgassing.

This lowering of the sintering temperature reduces the equipment cost, operating cost, and maintenance cost of the sintering furnace, and thus contributes to reducing the cost of forming and processing oxide nuclear fuel pellets.

ω) Niobium oxide, which is a sintering accelerator in the production of niobia-added oxide nuclear fuel pellets of the present invention, is replaced with pentavalent NbJ.
The reduction heat treatment from s to tetravalent NbO□ reduces the contamination of impurities into the pellets due to the addition of a small amount of niobia, and also reduces the occurrence of cracks in the treated pellets during the downstream process of reduction heat treatment of niobia-added green pellets. can be avoided.

(C) NbO2 and 102. By applying reductive heat treatment to niobia-added green pellets consisting of Since impurity gases generated by thermal decomposition of lubricants and the like are promptly released at the target pellet stage, it is possible to produce oxide nuclear fuel pellets with good thermal dimensional stability and low impurity content.

(cl) The oxygen partial pressure in the sintering atmosphere in the production of the niobia-added oxide nuclear fuel pellets of the present invention has an upper limit of the equilibrium oxygen partial pressure at which the 0/U ratio is 2.00, corresponding to the sintering temperature, and CO□ and CD, which provide an oxygen partial pressure of
In an air flow of a mixed gas of D, H2 or H2O and H2, the O/II ratio will not exceed 2,00 even in the process of increasing the temperature to the sintering temperature and in the process of decreasing the temperature from the sintering temperature. It is possible to produce oxide nuclear fuel pellets with a predetermined 0/[1 ratio of 2.0Ω to 2.01.

(e) The niobia-added oxide nuclear fuel pellets of the present invention reduce the release of FP gas from the pellets, making it easier to use high-burnup nuclear fuel including load following operation of nuclear reactors, thereby improving economic efficiency. Enables improvement.

[Brief explanation of drawings]

FIG. 1 is a process diagram of the method for producing niobia-added oxide nuclear fuel pellets of the present invention. FIG. 2 is a correlation diagram between the logarithmic range of the flow rate ratio of carbon dioxide and carbon monoxide that provides a sintering atmosphere for the niobia-added oxide nuclear fuel pellets of the present invention and the reciprocal of the absolute temperature of the sintering temperature. FIG. 3 is a correlation diagram of the logarithmic range of the flow rate ratio of carbon dioxide, carbon monoxide, and hydrogen that provides a sintering atmosphere for the niobia-added oxide nuclear fuel pellets of the present invention and the reciprocal of the absolute temperature of the sintering temperature. FIG. 4 is a correlation diagram between the logarithmic range of the flow rate ratio of water vapor and hydrogen that provides a sintering atmosphere for the niobia-added oxide nuclear fuel pellets of the present invention and the reciprocal of the absolute temperature of the sintering temperature. FIG. 5 is a correlation diagram between the sintering temperature of the niobia-added oxide nuclear fuel pellet of the present invention and the flow rate ratio of various mixed gases corresponding to the oxygen partial pressure of the sintering atmosphere.

Claims (3)

[Claims] (1) In a method for producing oxide nuclear fuel pellets in which uranium oxide or a mixture of uranium oxide and plutonium oxide is molded and sintered as a raw material powder, niobium oxide, which is a sintering accelerator, is Alternatively, by subjecting pentavalent Nb_2O_5 to powder of tetravalent NbO_2 by subjecting it to reduction heat treatment at 800°C for an appropriate time in a mixed gas flow of hydrogen and nitrogen, 0.2 to 1.0% by weight of the NbO_2 powder is The above raw material powder is added, mixed and molded to form green pellets, and the green pellets are subjected to reduction heat treatment at 800 to 1000°C for an appropriate time in a stream of hydrogen or a mixed gas of hydrogen and nitrogen. and a treated pellet with an atomic ratio of about 2.0,
Furthermore, for the sintering atmosphere corresponding to the sintering temperature range of 1000 to 2000°C, the upper and lower limits of the logarithm of the flow rate ratio of carbon dioxide and carbon monoxide are 2.4 when the sintering temperature is 1000°C.
~-0.62, and -0 at a sintering temperature of 2000°C
．． 25 to -3.2, the air flow in the range of the flow rate ratio of carbon dioxide and carbon monoxide that is almost linearly enveloped in the relationship between the logarithm of the flow rate ratio of carbon dioxide and carbon monoxide and the reciprocal of the absolute temperature of the sintering temperature. A method for producing oxide nuclear fuel pellets having high density, large particle size, stoichiometric composition, and low impurities, characterized by sintering in a medium. (2) For the sintering atmosphere corresponding to the sintering temperature range of 1000 to 2000°C, the upper and lower limits of the logarithm of the flow rate ratio of carbon dioxide and hydrogen are 2.3 to - in the case of a sintering temperature of 1000°C.
0.50, and 0.15 at a sintering temperature of 2000°C
-2.5, sintering in an air flow with a flow rate ratio of carbon dioxide and hydrogen that is almost linearly enveloped in the relationship between the logarithm of the flow rate ratio of carbon dioxide and hydrogen and the reciprocal of the absolute temperature of the sintering temperature. The method for producing oxide nuclear fuel pellets according to claim (1) above. (3) The upper and lower limits of the logarithm of the flow rate ratio of steam and hydrogen are 2.6 to -0.
42, and 0.45 to - at a sintering temperature of 2000°C
2.5 is characterized in that sintering is carried out in an air flow within a range of flow rate ratios of water vapor and hydrogen that are almost linearly enveloped in the relationship between the logarithm of the flow rate ratio of water vapor and hydrogen and the reciprocal of the absolute temperature of the sintering temperature. The method for producing oxide nuclear fuel pellets according to claim (1).