GB2320800A

GB2320800A - Nuclear fuel pellets

Info

Publication number: GB2320800A
Application number: GB9727259A
Authority: GB
Inventors: Yoshiharu Takahashi; Haruo Tsuchiya
Original assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan; Mitsubishi Materials Corp
Current assignee: Doryokuro Kakunenryo Kaihatsu Jigyodan; Mitsubishi Materials Corp
Priority date: 1996-12-27
Filing date: 1997-12-23
Publication date: 1998-07-01
Anticipated expiration: 2017-12-23
Also published as: GB2320800B; FR2757993A1; GB9727259D0; JPH10186075A; JP3170468B2; FR2757993B1

Abstract

Nuclear fuel pellets are made by pulverising powder, rolling the powder into agglomerates, and disposing the agglomerates in a mould to form pellets. In an embodiment, an aqueous solution of Pu and U nitrates having a desired Pu enrichment is prepared, the solution is subjected to MH denitrification at ambient temperature and normal pressure in the atmosphere, and oxidized to UO 3 and PuO 2 , the oxides are roasted for 2 hours at 750{C in the atmosphere to obtain U 3 O 8 and PuO 2 , reduced for 4 hours at 750{C in an atmosphere of N 2 and H 2 to give UO 2 and PuO 2 , the oxides are pulverized in a ball mill, an agglomerating agent added e.g. stearic or cinnamic acid, and the product rolled into agglomerates. The pellet raw material powder has improved fluidity and good processing characteristics, and does not easily scatter.

Description

METHOD OF MANUFACTURING NUCLEAR FUEL PELLETS FIELD OF THE INVENTION: This invention relates to a method of manufacturing nuclear fuel pellets, and more specifically to a method of manufacturing nuclear fuel pellets wherein a nuclear fuel substance is rolled into agglomerates which do not easily scatter when formed into pellets.

BACKGROUND OF THE INVENTION: Fig. 8 shows a flowchart of a conventional method of manufacturing reprocessed nuclear fuel pellets. In the following description, an example is taken of a powdered nuclear fuel substance recovered from spent nuclear fuel which is used as a raw material for making pellets.

As shown in Fig. 8, the method for making reprocessed nuclear fuel pellets may broadly be divided into a rolling process and a pellet-making process.

The rolling process comprises: (1) a solution preparing step wherein a nuclear fuel substance recovered by the reprocessing of spent nuclear fuel, is used to make a raw material solution having a predetermined nuclear fuel enrichment, (2) a denitrifying step wherein nitrate radicals are removed from the raw material solution, (3) a coarse crushing step wherein the denitrified raw material is coarse crushed, (4) a roasting step wherein the coarse crushed powder is roasted so as to obtain an oxide of the coarse crushed powder, (5) a reducing step wherein the coarse crushed oxide powder is reduced so as to obtain an agglomerate-forming powder, (6) a pulverizing step wherein the agglomerate-forming powder is pulverized in a ball mill, and (7) a lot blending step wherein the product is lot blended in a blade-type mixer so as to obtain a predetermined nuclear fuel enrichment.

After lot blending, the material is stored until pelletmaking.

The aforesaid denitrifying step (2) may generally be either a direct denitrification (MH method, fluid bed method) or a precipitation method (oxalic acid precipitation, coprecipitation).

Fig. 8 shows a rolling process using MH denitrification, which is a type of direct denitrification. MH denitrification refers to microwave heating direct denitrification, and since the operating procedure for th-s method is relatively simple, it has come into widespread use in recent years.

The pellet-making process comprises: (8) a cross-blending step wherein the stored powder is cross-blended in a delta mixer, (9) a homogenization blending step wherein the cross-blended powder is weighed with a desired nuclear fuel substance or recycled powder in order to achieve a desired fuel specification, and the resulting powder is homogenized in a ball mill, (10) a PF blending step wherein abinder is added, and PF (performer) blending is performed in a VI mixer, (11) a press particle-making step wherein the PF-blended binder and powder are pressed into tablets in a rotary press, (12) a crushing and grading step wherein the agglomerates are crushed and graded in a rotary crusher, (13) a lubricant addition step wherein a lubricant is added to the crushed, graded nuclear fuel and blended in a VI mixer, (14) a pellet-fcrming step wherein the pellet-forming powder obtained in the step for adding a lubricant (e.g., zinc stearate), is pressed into a dice and formed into pellets, (15) a degreasing step wherein the pellets so formed are degreased, (16) a roasting step wherein the degreased pellets are roasted, (17) a polishing step wherein the roasted pellets are polished, and (18) an inspect on step wherein the pellets are inspected.

Describing the rolling process in further detail, as shown in Fig. 8, an aious solution of plutonium and uranium nitrates comprising Pu/U = 1/1 is first prepared (step (1)), this aqueous solution is denitrified by the MH method at ambient temperature and normal pressure (step (2)), and the product is coarse crushed to a size of 5mm or less. A powder comprising UO3, PuO2 is thereby obtained. Next, this powder comprising UO3, PuO2 is sintered in the atmosphere at 750C for approximately 2 hours (step (4)) . A powder comprising U3O8, PuC2 is thereby obtained. Reduction is performed for approximately 4 hours in an atmosphere of nitrogen gas (N2) and hydrogen gas (H2) at 750t for approximately 4 hours, so as to obtain a powder comprising UO2, PuO2 which is the agglomerate-forming powder (step (5)). The product is then pulverized to 2Cun or less in a vibrating mill (step (6)). Lot blending is performed in a blade-type mixer (step (7)), and a nuclear fuel raw material (referred to hereafter as "MOX powder") wherein Pu/U = 1/1 is thereby obtained. This MOX powder is stored until pellet making.

In an effort to improve homogenizing and blending properties of Pu, U and pellet-making properties, fine powders (less than 2Sm) pulverized in a pulverizer such as a ball mill were conventionally used as particle-making powders [(Pu, U)O] for pellet fuel processing as describedhereabove. This is because if a fine powder is not used, agglomerated Pu is formed (i.e., Pu spot) resulting in the production of fuel pellets which do not conform to the fuel specification.

However even when for example a powder converted by the microwave heating direct denitrification method (MH denitrification) is pulverized to a fine powder as described hereabove, the surfaces of the powder particles are rough and the fluidity of the powder is poor. Further, since it is a fine powder, it tends to scatter.

When the fluidity of the agglomerate-forming powder is poor, powder tends to be retained in the equipment in the pellet-forming material preparation process from the aforesaid step (4), roasting, to (14), the pellet-forming step, and this causes a decline of product yield. Moreover, as the powder tends to scatter in the pellet-forming process, more scrap is generated, and workers who carry out manual equipment maintenance are liable to suffer increased radiation exposure due to increase of the spatial dosage rate. In this context, the term "scrap" means that the scattered fine powder becomes mixed with the product, altering the fuel specification (e.g., ratio of Pu, U) of the product so that it can no longer be used as fuel. In this case, the whole process must be repeated starting from the initial adjustment of the fuel specification, and this leads to an increase of fuel manufacturing cost.

SUMMARY OF THE INVENTION: It is therefore an object of this invention, which was conceived in view of the aforesaid problems, to obtain a pellet-forming raw material powder having improved fluidity which is easy to process into fuel, and which does not easily scatter.

It is a further object of this invention to provide a method of manufacturing reprocessed fuel pellets which simplifies a basic manufacturing process.

To resolve the aforesaid problems, the method of manufacturing reprocessed fuel pellets according to the present invention comprises the following features.

(1) After pulverizing raw material powder derived from a nuclear fuel substance, a rolling process for rolling the powder into agglomerates, and a pellet-forming process for disposing these agglomerates in a mold so as to form pellets, are provided.

Due to rolling after pulverizing the raw material powder derived from the nuclear fuel substance, substantially spherical agglomerates which have fluidity and which do not easily scatter are obtained. This prevents retention of powder in the equipment during handling and improves product yield. Also there is no scattering of powder in the steps up to pellet-forming, so scrap is not generated and exposure of workers to radiation hazard is substantially reduced. Further in the rolling process, agglomerates having a particle size suitable for forming the pellets are generated. therefore as shown in Fig. 8, the steps of (7) lot blending, (8) cross-blending, (9) homogenization blending, (10) PF blending, (11) press agglomerating and (12) crushing/grading are rendered unnecessary so that the pellet manufacturing process is shortened.

(2) In the aforesaid nuclear fuel pellet manufacturing method (1), an agglomerating agent may first be added to the raw material powder in the aforesaid rolling process before rolling into agglomerates.

Due to the addition of this agglomerating agent, agglomeration of the fine powder is enhanced.

(3) In the aforesaid nuclear fuel pellet manufacturing method (2), in the rolling process, the agglomerating agent and raw material powder are heated as they are rolled to form the agglomerates.

When heat is applied in the rolling process, the agglomerating agent melts and its adhesion to the powder increases, so substantially spherical agglomerates form. Further, agglomerates of high density are formed, and therefore pellet density after pellet-forming and sintering is also high.

(4) In the aforesaid nuclear fuel pellet manufacturing methods (2) and (3), the aforesaid agglomerating agent may be at least one type of organic compound having either a low melting point or a high melting point.

When the agglomerating agent has a low melting point, it easily vaporizes during heating in the rolling process, or during sintering of the pellets. On the other hand when the agglomerating agent has a high melting point, it gradually melts during heating in the rolling process, so agglomeration is enhanced and pellets of high sintered density are obtained.

(5) In any of the aforesaid nuclear fuel pellet manufacturing methods (1) to (4), the raw material powder may be subjected to microwave heating direct denitrification, sintering and reduction steps.

In this case, since the agglomerates are obtained by rolling a raw material powder which has been subjected to the reduction step of the conventional nuclear fuel pellet manufacturing method, substantially spherical agglomerates of high fluidity and which do not easily scatter can be obtained without making extensive modifications to manufacturing equipment. Hence retention of powder in the equipment during handling is prevented, and product yield is improved. Also, since the powder does not scatter in the steps up to pellet-forming, generation of scrap is prevented and the radiation exposure hazard to workers is largely reduced.

(6) In any of the aforesaid nuclear fuel pellet manufacturing methods (1) to (4), the raw material powder may be subjected to only microwave heating direct der.itrification and slntering steps. The agglomerates are then reduced in a reduction step provided after the rolling step, the reduced agglomerates then being disposed in a mold to form pellets.

Generally, when the agglomerates are rolled after sintering and reduction, PuO2/UO2 is reoxidized during storage so that part is converted to PuO2/U303, and from the viewpoint of fuel efficiency, it was necessary to reduce it again when it was used as a raw material for nuclear fuel pellets.

However when rolling is performed after the sintering step and reduction is postponed until later, the agglomerates comprising the stable oxides PuO2/U3Oswhich are produced in the sintering step may be stored. Reduction can be performed at any time when the agglomerates are used as a raw material for nuclear fuel pellets, thereby rendering further reduction unnecessary and cutting down manufacturing costs.

As in the case of the aforesaid (1), in the rolling process, spherical powder particles which have fluidity and which do not easily scatter are obtained. This prevents retention of powder in the equipment during handling and improves product yield. Also as there is no scattering of powder in the steps up to pellet-forming, scrap is not generated and exposure of workers to radiation hazard is substantially reduced. Further, in the rolling process, agglomerates having a particle size suitable for pellet-forming are produced, so the pellet manufacturing process is shortened in comparison to the process of the prior art.

(7) In the nuclear fuel pellet manufacturing method described in (1), a step for adding an agglomerating agent to the surface of the agglomerates is provided after the rolling step, and since the agglomerates are formed in the rolling step without adding an agglomerating agent, agglomerates of high density can be obtained.

Also, as the agglomerating agent adheres to the surface of the agglomerates after rolling, the slip properties of the agglomerates are improved by adding a smaller amount of agglomerating agent than if agglomerating agent were added in the rolling step.

(8) In the nuclear fuel pellet manufacturing method described in (7), heat is applied during addition of the agglomerating agent.

By applying heat during the agglomerating agent addition step, the agglomerating agent melts and adheres to the surface of the agglomerates, so substantially spherical powder particles are formed during rolling. Also, high density agglomerates are formed, so the density of the pellets after sintering is high.

(9) In any of the aforesaid nuclear fuel pellet manufacturing methods (2) to (8), the aforesaid agglomerating agent is stearic acid and/or cinnamic acid.

Stearicacid (meltingpoint o.5t) andcinnamicacid (melting point 68t) are low melting point organic compounds which are solids at ambient temperature. Therefore when heat is applied in the rolling process, the agglomerating agent melts and adhesion to the powder increases, substantially spherical agglomerates are formed during rolling, and the agglomerates are formed without modifying the oxidation number of the nuclear fuel substance. As a result, the agglomerates have an oxidation number suited for nuclear fuel pellets and have a high density.

BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a production flowchart of a nuclear fuel pellet manufacturing method according to a first embodiment of this invention.

Fig. 2 is a production flowchart of a nuclear fuel pellet manufacturing method according to a second embodiment of this invention.

Fig. 3 is a flowchart of a method of adding an agglomerating agent in a nuclear fuel pellet manufacturing method according to the first, second and a fourth embodiment of this invention.

Fig. 4 is a flowchart showing a nuclear fuel pellet manufacturing method according to a sixth embodiment of this invention.

Fig. 5 is a flowchart showing a nuclear fuel pellet manufacturing method according to the sixth embodiment of this invention.

Fig. 6 is a flowchart showing a nuclear fuel pellet manufacturing method according to the sixth embodiment of this invention.

Fig. 7 is a flowchart showing a nuclear fuel pellet manufacturing method according to the sixth embodiment of this invention.

Fig. 8 is a flowchart showing a conventional nuclear fuel pellet manufacturing method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS: The nuclear fuel pellet manufacturing methods described below are examples using a powdered substance recovered from spent nuclear fuel which is used as a raw material for making nuclear fuel pellets.

Embodiment 1 A method of manufacturing nuclear fuel pellets according to a first embodiment will now be described. Features which are the same as those of the prior art will be omitted.

The method of manufacturing nuclear fuel pellets according to this embodiment comprises a preparing step for preparing a raw material solution of a recycled nuclear fuel substance obtained by reprocessing spent nuclear fuel, this solution having a desired enrichment, a denitrifying step for removing nitrate radicals from the raw material solution so as to produce a denitrified powder, a roasting step for roasting this denitrified powder so as to obtain an oxide, a reducing step for reducing the oxide of the denitrified powder so as to produce an agglomerate-forming powder (the raw material powder of the first embodiment), a rolling process wherein the raw material powder is pulverized and then rolled so as to cause it to form agglomerates, and a pellet-forming process wherein the agglomerates are disposed in a mold to form pellets.

One example of the pellet manufacturing method of this first embodiment will be described referring to Fig. 1. First, in the conversion process, a raw material solution of Pu, U nitrates wherein the plutonium (Pu) enrichment has been adjusted, is subjected to MH denitrification at ambient temperature and normal pressure in the atmosphere, and oxidized to U03, PuO2 (step (1)).

Next, the UO;, PuO2 are roasted for approximately 2 hours at 7500C in the atmosphere (step (2)) so as to obtain U3Os, PuO2. Next, reduction is performed for approximately 4 hours at 750cm in an atmosphere of nitrogen gas (N2) and hydrogen gas (H2) so as to obtain UO2, PuO,(which are other oxide) (step (3)) . Subsequently, the raw material UQ, PuO- is pulverized in a ball mill, an agglomerating agent is added to the finely crushed, pulverized powder, and the product is rolled to give agglomerates (step (4)).

Next, in the pellet-making process, a lubricant is added to the agglomerates, and blending of a PF (performer, i.e., a density-lowering agent such as avicyl (melting point 2500C)) is carried out in a VI mixer (step (5)) . Pellets are then manufactured from the agglomerates obtained via (14) a pellet-forming step as shown in Fig. 8 wherein the pellet-forming powder is packed into dice, (15) a degreasing step for degreasing the pellets so formed, (16) a roasting step for roasting the degreased pellets, (17) a polishing step for polishing the pellets after roasting, and (18) an inspection step for inspecting the pellets. Since the pulverized powder is handled only from the pulverization of the step (4) to the rolling process, there is little chance of the powder scattering, so scrap is not generated and exposure of workers to radiation hazard is substantially reduced. Further, in the rolling process, the nuclear fuel raw material powder is formed into agglomerates so that spherical powder particles having fluidity and which do not easily scatter are obtained. This prevents retention of powder in the equipment during handling and improves product yield. Also as there is no scattering of powder in the steps up to pellet-forming, scrap is not generated and exposure of workers to radiation hazard is substantially reduced. Further, in the rolling process, agglomerates having a particle size suitable for forming pellets are produced. Therefore as shown in Fig. 8, the steps of (7) lot blending, (8) cross-blending, (9) homogenization blending, (10) PF blending, (11) press agglomerating and (12) crushing/grading are rendered unnecessary so that the pellet manufacturing process is shortened.

When it is desired to adjust the enrichment of the nuclear fuel pellets, a raw material solution having the desired enrichment may be prepared prior to denitrification as described hereabove, or a powder of a nuclear fuel substance may be added so as to obtain a desired enrichment of the nuclear fuel after the reduction step.

When the adjustment is performed prior to denitrification, the enrichment is already adjusted depending on the atomic reactor for which the fuel is intended, so there is no need to add nuclear fuel powder to adjust the enrichment prior to the rolling process.

There are therefore fewer steps wherein the powder is handled, and the process of pellet production is still further shortened.

Embodiment 2 Fig. 2 shows a method of manufacturing nuclear fuel pellets according to a second embodiment of this invention. Features which are the same as those of the prior art will be omitted.

The nuclear fuel pellet production method according to this embodiment comprises a preparing step for preparing a raw material solution of a recycled nuclear fuel substance obtained by reprocessing spent nuclear fuel, this solution having a desired enrichment, a denitrification step for denitrifying the raw material solution so as to remove nitrate radicals and produce a denitrified powder, a roasting step for roasting this denitrified powder so as to obtain an oxide, a rolling process wherein the raw material powder is pulverized and then rolled so as to cause it to form agglomerates, a reducing step for reducing the agglomerates, and a pellet-forming process wherein the agglomerates are disposed in a mold to form pellets.

An example of the pellet-forming process will now be given with reference to Fig. 2.

First, in the conversion process, a raw material solution of Pu, U nitrates wherein the plutonium (Pu) enrichment has been adjusted, is subjected to MH denitrification at ambient temperature and pressure in the atmosphere (step (1)) . Next, the raw material solution whereof the enrichment has been adjusted, is subjected to MH denitrification at ambient temperature and normal pressure in the atmosphere so as to oxidize the product to U03, PUO2. The U03, PuO2 are then roasted for approximately 2 hours at 750C in the atmosphere (step (2)) so as to obtain U3Oe, PuO2. Next, the UO2, PuO2 which is the raw material powder is pulverized in a ball mill, an agglomerating agent is added to the finely crushed, pulverized powder, and the product is rolled to give agglomerates (step (3)).

This Pu enrichment-adjusted MOX powder is stored.

Next, in the pellet-making process, the stored MOX powder is reduced for approxinately 4 hours at 7509C in an atmosphere of nitrogen gas (N) and hydrogen gas (H-) so as to obtain oxidized UO:, PuO2 (step (4)).

If the agglomerates have not disintegrated in the aforesaid reducing step, a lubricant is added to the agglomerates, and blending of a PF (performer) is carried out in a VI mixer (step (5)). The pellet-forming agglomerates obtained are then formed into pellets via the pellet-forming step (14) shown in Fig. 8 and intermediate steps up to the inspection step (18).

When the agglomerates have disintegrated in the reduction step, the disintegrated agglomerates are pulverized in a ball mill, an agglomerating agent is again added to the finely crushed, pulverized powder, the resulting product is rolled into agglomerates (step (5)), and blending with a PF (performer) is performed in a VI mixer (step (6)). After undergoing the pellet-forming step (14) of Fig. 8 to the inspection step (18), the desired nuclear fuel pellets may be obtained.

Generally, in agglomerates which have been rolled after sintering and reduction, PuO2/UO2 is reoxidized during storage so that part is converted to PuO2/U3Oa, and from the viewpoint of fuel efficiency, it was necessary to reduce it again when it was used as a raw material for nuclear fuel pellets.

However when rolling is performed after the sintering step and reduction is performed later as described hereabove, the agglomerates comprising the stable oxides PuO/U;Oaproduced in the sintering step may be stored. Reduction can be performed at any time when the agglomerates are used as a raw material for nuclear fuel pellets, thereby rendering further reduction unnecessary and cutting down manufacturing costs. As in the case of the aforesaid first embodiment, in the rolling process, spherical powder particles which have fluidity and which do not easily scatter are obtained. This prevents retention of powder in the equipment during handling and improves product yield. Also as there is no scattering of powder in the steps up to pellet-forming, scrap is not generated and exposure of workers to radiation hazard is substantially reduced. Further, in the rolling process, agglomerates having a particle size suitable for pellet-forming are produced, so the pellet manufacturing process is shortened in comparison to the process of the prior art.

There are therefore even fewer steps wherein the powder is handled, and the process of pellet production is still further shortened.

Embodiment 3 A method of manufacturing nuclear fuel pellets according to a third embodiment will now be described. Features which are the same as those of the prior art will be omitted.

According to the method of this embodiment for manufacturing nuclear fuel pellets, the denitrification and roasting of the aforesaid first and second embodiments are performed simultaneously.

A raw material solution of the nuclear fuel substance is for example placed in a SiC container (e.g., a dish). When microwave heating direct denitrification is performed (MH denitrification), the SiC emits heat, nitrate radicals in the nuclear fuel substance are eliminated, and the denitrifiedpowder is roasted. In this way, a denitrified powder that has oxidized to PuO2, U3Oe is obtained, and the pellet production process can be still further shortened.

Embodiment 4 A method of manufacturing nuclear fuel pellets according to a fourth embodiment will now be described. Features which are the same as those of the prior art will be omitted.

According to the pellet producing method of this embodiment, a step is provided for adding the agglomerating agent to the surface of the agglomerates after the rolling process in the methods of the first and second embodiments.

In the pellet-forming method of the aforesaid first and second embodiments, the agglomerating agent (e.g., 0.5-3.0% relative to the amount of raw material powder) is added to the raw material powder (e.g., 20g), and after crush blending, the mixture is rolled into agglomerates (e.g., rotated for an agglomerating time of 10 min by inclining a flat bottom beaker of internal dia. 90mmf at 60 and rotating at 30rpm) with heating (e.g., to the melting point of the agglomerating agent), as shown in Fig. 3 (a) However in the pellet manufacturing method of this embodiment, the raw material powder (e.g., 20g) which has passed through the roasting or reducing steps and been pulverized, is rolled into agglomerates (e.g., rotated for an agglomerating time of 10 min by inclining a flat bottom beaker of internal dia. 90mix at 600 and rotating at 30rpm), the agglomerating agent is then added to the agglomerates (or agglomerated powder) obtained, the product is rolled again (e.g., rotated for a further 5 min under the aforesaid conditions), and then heated (e.g., to the melting point of the agglomerating agent) as shown in Fig. 3(b). According to the method of Fig. 3(b), "core shell" agglomerates are obtained.

Since the agglomerates are initially formed during rolling without adding the agglomerating agent, agglomerates of high density are obtained. Moreover, as the agglomerating agent is added to the surface of the agglomerates once they are already formed after rolling, there slip properties can be improved by adding a smaller amount of agglomerating agent than when the agglomerating agent is added during rolling.

Further, when pellets are manufactured using these core shell agglomerates, an equivalent sintered density (e.g., 95-98% TD) is obtained to that achieved using the agglomerates produced by the method of 3(a).

Embodiment 5 A method of manufacturing nuclear fuel pellets according to a fifth embodiment will now be described. Features which are the same as those of the prior art will be omitted.

According to the pellet manufacturing method of this embodiment, no agglomerating agent is added during rolling of the fine powder in the aforesaid rolling process.

When it is possible to cause a fine powder having a suitable resilience to agglomerate without the addition of an agglomerating agent, agglomerating agent does not vaporize from within the pellets during sintering after the pellet-forming step, hence pellets of high sintered density (approximately 95-98E TD, and 97.6! TD by the MH same as those of the prior art will be omitted.

According to the pellet manufacturing method of this embodiment, rolling of the mixture of pulverized powder and agglomerating agent in the rolling process is performed either with or without the application of heat.

As shown in Fig. 4, after pulverizing an MH denitrifiedpowder comprising UO2or U02-30%Pu in a ball mill, 2% stearic acid (melting point 69.6"C) was added to the denitrified powder and mixed in a sample mill, and the mixture rolled into agglomerates with heating (60"C-80"C) or without heating (referred to also as "no heating").

These agglomerates were formed into pellets (3 ton/cm2), and the product was sintered for 5 hours at 1750 C in a N2-H2 atmosphere.

Stearic acid vaporizes at approximately 90C.

When heat was applied during rolling, pellets having a high sintered density of 93.38 TD were obtained. When the amount of stearic acid added was 0.2E relative to the amount of denitrified powder as shown in Fig. 5, pellets of a high sintered density of 95.5t TD or more were obtained. This sintered density is effectively equivalent to that of the pellets obtained by the conventional manufacturing method. The same effect was obtained using cinnamic acid of melting point 68"C instead of stearic acid.

When on the other hand heat was not applied, pellets having a low sintered density of approximately 85= TD were obtained. When 3% stearic acid was added to denitrified UO2 powder obtained by MH denitrification after drying at ambient temperature for 12 hours and rolling the product into agglomerates without heating, pellets having a sintered density of 85.1% TD were obtained. Pellets of 85% TD are suitable for use in the fast breeder reactor "Monju".

So far, only pellets of denitrified UO2 powder produced by MH denitrification have been described, but pellets of a high sintered density of 95.1% TD were obtained also when denitrified U02 powder produced by precipitation was mixed with denitrified UO2 powder produced by the MH method, and 1% stearic acid was added to the mixed denitrified powder as shown in Fig. 6.

It was therefore confirmed that pellets having the desired density could be obtained using denitrified powder produced by either denitrification method.

The aforementioned description has focused on the manufacture of pellets comprising only UO2 denitrified powder, however a sintered density equivalent to that of UO2 powder is obtained also from an MH denitrified powder comprising U02-30% Pu. In this case however, as Pu heats spontaneously unlike U, the melting point of the agglomerating agent must be controlled.

Embodiment 7 A method of manufacturing nuclear fuel pellets according to a seventh embodiment will now be described. Features which are the same as those of the prior art will be omitted.

According to the pellet manufacturing method of the aforesaid sixth embodiment, the agglomerating agent was a low melting point organic compound, however the agglomerating agent used in the pellet manufacturing method of this embodiment is a high melting point organic compound, e.g., cinnamic acid (melting point temperature 1360C) . When cinnamic acid is used, the same sintered density as when stearic acid is used was obtained by heating to a temperature in the vicinity of the melting point. It may be noted that as the oxidation number of the nuclear fuel substance can vary slightly, reduction may be performed after agglomeration if necessary.

Example 8 A method of manufacturing nuclear fuel pellets according to an eighth embodiment will now be described. Features which are the same as those of the prior art will be omitted.

According to the pellet manufacturing method of this embodiment, when the nuclear fuel substance recovered by reprocessing spent nuclear fuel is only uranium, the agglomerating agent may be either water or an alcoholic solvent, or both. The aforesaid alcoholic solvent may for example be polyvinyl alcohol (PVA).

In the case of plutonium, water or alcoholic solvents react and therefore cannot be used, but when the fuel comprises only uranium, water or an alcoholic solvent may be used as agglomerating agent. As water or alcoholic solvents are generally liquids, the nuclear fuel substance and the agglomerating agent may be blended homogeneously together in the rolling process, so the sintered density of the pellets formed is improved. Further, as the agglomerating agent adheres uniformly to the surface of the product after agglomeration, the slip properties of the agglomerates are improved.

Hence according to the nuclear fuel pellet manufacturing method of this invention, in the rolling step after the reduction step, a denitrified powder of the nuclear fuel substance is caused to agglomerate, and substantially spherical agglomerates are obtained which have fluidity and which do not easily scatter. This prevents retention of powder in the equipment during handling and improves product yield. Also as there is no scattering of powder in the steps up to pellet-forming, scrap is not generated and exposure of workers to radiation hazard is substantially reduced.

Further, in the rolling process, agglomerates having a particle size suitable for pellet-forming are produced, so the pellet manufacturing process is shortened in comparison to the process of the prior art.

When rolling is performed after the roasting step, the agglomerates stored comprising PuO2/U O which is a stable oxide formed in the roasting step can be stored, and as reduction may be performed at any time when the product is used as a raw material for nuclear fuel pellets, an additional reduction step is rendered unnecessary so that manufacturing costs are cut.

Claims

WHAT IS CLAIMED:

1. A nuclear fuel pellet manufacturing method characterized in comprising: a rolling process wherein a raw material powder derived from a nuclear fuel substance is pulverized and rolled into agglomerates, a pellet-forming process wherein said agglomerates are disposed in a mold and formed into pellets.

2. A nuclear fuel pellet manufacturing method as defined in Claim 1, wherein an agglomerating agent is further added to said raw material powder and said powder is then rolled into agglomerates in said rolling process.

3. A nuclear fuel pellet manufacturing method as defined in Claim 2, wherein said agglomerating agent and raw material powder are heated when they are rolled into said agglomerates in said rolling process.

4. A nuclear fuel pellet manufacturing method as defined in Claim 2 or Claim 3, wherein said agglomerating agent is at least one type of low melting point or high melting point organic compound.

5. A nuclear fuel pellet manufacturing method as defined in any of Claims 1 to 4, wherein said raw material powder has been subjected to a microwave heating direct denitrification step, roasting step and reduction step.

6. A nuclear fuel pellet manufacturing method as defined in any of Claims 1 to 4, wherein said raw material powder has been subjected to a microwave heating direct denitrification step and roasting step, further comprising a reduction step for reducing said agglomerates after said rolling process, and said reduced agglomerates are disposed in a mold so as to form pellets.

7. A nuclear fuel pellet manufacturing method as defined in Claim 1, comprising a step for adding an agglomerating agent to the surface of said agglomerates after said rolling process.

8. A nuclear fuel pellet manufacturing method as defined in Claim 7, wherein heat is applied during said agglomerating agent addition step.

9. A nuclear fuel pellet manufacturing method as defined in any of Claims 2 to 8, wherein said agglomerating agent is stearic acid and/or cinnamic acid.

10. A nuclear fuel pellet manufacturing method as claimed in claim 1 substantially as hereinbefore described with reference to any one of the Embodiments and Examples.