Method of making a nuclear fuel pellet for a nuclear power reactor

Abstract

A method of making a nuclear fuel pellet for a nuclear power reactor. The method includes: providing a nuclear fuel material in powder form, pressing the powder such that a green pellet is obtained; providing a liquid that comprises an additive which is to be added to the green pellet; contacting the green pellet with the liquid so the liquid, with the additive, penetrates into the pellet; and sintering the treated green pellet. The additive is such that larger grains in the nuclear fuel material are obtained with the additive.

Claims

1. A method of making a nuclear fuel pellet for a nuclear power reactor, the method comprising the following steps: providing a nuclear fuel material in powder form, wherein the nuclear fuel material is based on UO.sub.2; pressing the powder thereby obtaining a green pellet is obtained; providing a liquid that comprises an additive which is to be added to the green pellet, the additive comprises B and/or Cr; contacting the green pellet with the liquid wherein the liquid, with the additive, penetrates into the pellet; and sintering the so treated green pellet, wherein with said additive larger grains in the nuclear fuel material are present in the pellet after the sintering step as compared with the grain size obtained if the additive had not been added but otherwise produced in the same manner.

2. A method according to claim 1, wherein said additive is in the form of particles dispersed in said liquid.

3. A method according to claim 1, comprising a step of controlling the penetration depth of the liquid, and thereby of the additive, into the green pellet.

4. A method according to claim 3, wherein said step of controlling the penetration depth is done by selecting one or both of the following: the viscosity of the liquid with included additive, the amount of the liquid, with the additive, which is added to the green pellet when contacting the green pellet with the liquid, with the additive.

5. A method according to claim 3, wherein the penetration depth of the liquid, with the additive, into the green pellet is controlled to obtain an outer portion of the green pellet containing substantially more liquid, and thereby more additive, than an inner portion of the green pellet, wherein the sintered pellet has a larger grain size in the outer portion than in the inner portion.

6. A method according to claim 1, wherein said liquid with additive is selected and said method is performed wherein the liquid with additive will penetrate into the pores which exist between the grains in the green pellet.

7. A method according to claim 6, wherein said liquid with additive is selected and said method is performed wherein the liquid with additive will not penetrate into the pores which exist in the grains in the green pellet.

8. A method according to claim 6, wherein said liquid with additive is selected and said method is performed wherein the liquid with additive will penetrate also into the pores which exist in the grains in the green pellet.

9. A method according to claim 1, wherein said liquid is selected and said method is performed wherein at least 99% of the liquid will leave the pellet before or during the sintering step.

10. A method according to claim 1, wherein said additive constitutes or includes a substance which causes said larger grains in the sintered pellet, wherein said substance is selected and the method is performed wherein at least 90% of the substance leaves at least an outer portion of the pellet before and/or during the sintering step.

11. A method according to claim 10, wherein at least 95% of the substance leaves at least an outer portion of the pellet before and/or during the sintering step.

12. A method according to claim 11, wherein at least 99% of the substance leaves at least the outer portion of the pellet before and/or during the sintering step.

13. A method according to claim 12, wherein the substance completely leaves at least the outer portion of the pellet before and/or during the sintering step.

14. A method according to claim 1, wherein said additive comprises B and wherein at least 90% of said B is .sup.11B.

15. A method according to claim 1, wherein said liquid is selected so the additive does not dissolve in the liquid, and wherein the nuclear fuel material in the green pellet is not dissolved by the liquid.

16. A method according to claim 1, wherein said liquid is an oil, preferably a mineral oil.

17. A method of making and using nuclear fuel, comprising: making a plurality of nuclear fuel pellets according to the method of claim 1; arranging the nuclear fuel pellets in cladding tubes; arranging the cladding tubes, with the nuclear fuel pellets, in the core of a nuclear power reactor in a nuclear power plant, wherein at least 20% of the nuclear fuel material in said core are made of pellets made in accordance with the aforementioned method of making the plurality of nuclear fuel pellets; and operating the nuclear reactor to produce energy.

18. A method of making and using nuclear fuel according to claim 17, wherein at least 50% of the nuclear fuel material in said core are made of pellets made in accordance with the aforementioned method of making the plurality of nuclear fuel pellets.

19. A method of making and using nuclear fuel according to claim 18, wherein 100% of the nuclear fuel material in said core are made of pellets made in accordance with the aforementioned method of making the plurality of nuclear fuel pellets.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematically a flow chart of a manner of carrying out the method according to the invention.

(2) FIG. 2 shows schematically an example of how the grain size in a pellet produced in accordance with the present invention may vary in the radial direction of the pellet.

DETAILED DESCRIPTION

(3) Since a person skilled in the art knows how to produce nuclear fuel pellets from a powder, all the details of such a method will not be described herein. However, the main steps which are relevant to the present invention are described.

(4) FIG. 1 shows schematically the main steps of a manner of carrying out the method according to the present invention.

(5) A nuclear fuel material in powder form is provided. The nuclear fuel material may be based on UO.sub.2, which is enriched concerning .sup.235U. The powder may also comprise other materials, for example binder materials, U.sub.3O.sub.8, burnable neutron absorbers, pore formers and sintering aid. The powder may contain at least 60%, preferably at least 80%, UO.sub.2.

(6) The powder is pressed such that a green pellet is formed. The green pellet will be porous. For example 50% of the pressed pellet may consist of pores.

(7) An additive is provided. The additive is such that it will increase the grain size of the sintered pellet. The additive may for example comprise B (which will cause larger grains), for example in the form of UB.sub.4. According to one embodiment, the B is in the form of .sup.11B.

(8) According to another alternative, the additive may be Cr.sub.2O.sub.3.

(9) The additive is preferably in the form of particles, i.e. a powder. The size of the particles should be small enough so that the particles can penetrate into the pores in the green pellet, into which it is intended that the particles should penetrate. The particle size may for example be about 1 m.

(10) A liquid is provided. The liquid may be a mineral oil. The mineral oil may be selected to have a desired viscosity, for example a kinematic viscosity of 320 centistokes.

(11) The additive is mixed with the liquid. Preferably, the additive particles are dispersed in the liquid, i.e. the liquid is selected such that the additive particles do not dissolve in the liquid, and also such that the nuclear fuel material in the green pellet is not dissolved by the liquid.

(12) The green pellet is brought into contact with the liquid with the additive. The green pellet may for example be dipped into the liquid with additive or the liquid with additive may be sprayed onto the green pellet.

(13) The penetration depth of the liquid, and thereby of the additive, into the green pellet is controlled. This can be done by selecting a suitable viscosity of the liquid or by controlling the amount of liquid, with the additive, which is added to the green pellet. This can be done for example by spraying a certain amount of the liquid onto the pellet or by dipping the green pellet in the liquid, with additive, during a predetermined time. It is also possible to control the penetration depth by the particle size or the particle size distribution.

(14) According to one manner of carrying out the method according to the present invention, the penetration depth is controlled such that the additive is added only to an outer portion of the green pellet.

(15) By controlling for example the viscosity of the liquid, with the additive, or the size of the additive particles, it is also possible to control into which pores in the green pellet that the additive will enter. For example, it may be controlled that the additive will substantially only enter into the pores which exist between the grains in the green pellet. Alternatively, it may be controlled that the additive will enter also into the pores which exist in the grains in the green pellet.

(16) The so treated green pellet is then sintered. This can be done by a normal sintering process, for example in a furnace which contains different zones where the pellet is heated up to a final temperature of about 1800 C.

(17) The liquid is preferably selected such that it will evaporate during the heating process. There may be a separate heating step before the actual sintering in order to evaporate the liquid. However, no such separate heating step may be necessary, since the liquid will evaporate during the sintering process.

(18) According to one manner of carrying out the method according to the invention, also the substance that causes the larger grains in the sintered pellet will leave the pellet (evaporate) during a heating process, for example during the sintering step.

(19) When it is desired to increase the grain size in the whole nuclear fuel pellet, the additive, and the viscosity of the liquid, may be selected such that the whole pellet is infiltrated with the liquid with the additive. However, as explained above, it is possible to control the penetration depth of the liquid with the additive. According to a preferred manner of carrying out the present invention, the penetration depth is controlled such that the additive will substantially enter only into an outer peripheral portion of the green pellet. When the green pellet is then sintered, larger grains will be obtained mainly in an outer portion of the pellet.

(20) FIG. 2 illustrates schematically how the grain size may vary in a pellet treated in this manner. The x-axis shows the radius of the sintered pellet. The radius r 1.0 is thus the outer periphery of the pellet. The radius of the pellet may for example be about 4.6 mm. The y-axis in FIG. 2 shows the average 2D grain size. The curve in FIG. 2 thus shows how the average 2D grain size varies with the radius. FIG. 2 thus illustrates that according to this embodiment of the invention, a substantially larger grain size is obtained in the outer portion of the sintered pellet. This has in particular the advantage that the occurrence of the above described HBS can be prevented or delayed.

(21) A plurality of nuclear fuel pellets are produced according to the method of the present invention.

(22) The produced pellets are arranged in cladding tubes.

(23) The cladding tubes are arranged in the core of a nuclear power reactor, such that the core includes several thousand cladding tubes with pellets produced in accordance with the present invention.

(24) The nuclear reactor is operated in order to produce energy.

(25) The present invention is not limited to the examples described herein, but can be varied and modified within the scope of the following claims.