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, providing an additive, forming a so-called green pellet, wherein said additive is added either to said nuclear fuel material in powder form or to the green pellet, sintering the green pellet, wherein said additive is such that it causes larger grains in the nuclear fuel pellet, and wherein said additive is made of or includes a substance which causes the larger grains and which substantially leaves at least an outer portion of the pellet before and/or during the sintering step.

Claims

1-13. (canceled)

14. 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, providing an additive, pressing the powder such that a green pellet is obtained, wherein said additive is added either to said nuclear fuel material in powder form or to the green pellet, sintering the so obtained green pellet, wherein said additive is such that larger grains in the nuclear fuel material are present in the pellet after the sintering step as compared with the grain size obtained if a pellet, which is produced according to the above manner but without the addition of the additive, is sintered in the same manner, wherein said additive is made of or includes a substance which causes said larger grains in the sintered pellet, wherein said substance is selected and the method is performed such that at least to 90% of the substance, leaves at least an outer portion of the pellet before and/or during the sintering step.

15. A method according to claim 14, wherein the produced nuclear fuel pellet has a substantially cylindrical shape with a radius r and wherein said outer portion is the part of the nuclear fuel pellet that is located between 0.8 r and r, or between 0.9 r and r, or between 0.95 r and r.

16. A method according to claim 14, wherein said substance is made of, or comprises, B and/or Cr.

17. A method according to claim 16, wherein said additive to at least 60% is selected from the group consisting of B, UB.sub.4, B.sub.4C, ZrB.sub.2, Cr, CrO, CrO.sub.2 and Cr.sub.2O.sub.3 or combinations thereof.

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

19. A method according to claim 14, comprising arranging said additive such that an outer portion of the green pellet contains substantially more additive than an inner portion of the green pellet, such that the sintered pellet has a larger grain size in the outer portion than in the inner portion.

20. A method according to claim 14, wherein said additive is provided in the form of particles.

21. A method according to claim 20, comprising providing a liquid and arranging the additive in the liquid such that said additive is in the form of particles dispersed in said liquid, wherein the liquid with the additive is added either to said nuclear fuel material in powder form or to the green pellet.

22. A method according to claim 21, comprising adding said liquid with the additive to the green pellet by contacting the green pellet with the liquid such that the liquid, with the additive, penetrates into the green pellet and controlling the penetration depth of the liquid, and thereby of the additive, into the green pellet.

23. A method according to claim 22, 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.

24. A method according to claim 21, wherein said liquid is selected and said method is performed such that the liquid will at least to 99% leave the pellet before or during the sintering step.

25. A method according to claim 21, wherein said liquid is an oil.

26. A method according to claim 25, wherein said oil is a mineral oil.

27. A method according to claim 14, wherein at least 95% of the substance leaves at least an outer portion of the pellet before and/or during the sintering step.

28. A method according to claim 27, wherein at least 99% of the substance leaves at least an outer portion of the pellet before and/or during the sintering step.

29. A method according to claim 17, wherein said additive to at least 80% is selected from the group consisting of B, UB.sub.4, B.sub.4C, ZrB.sub.2, Cr, CrO, CrO.sub.2 and Cr.sub.2O.sub.3 or combinations thereof.

30. A method according to claim 29, wherein 100% of said additive is selected from the group consisting of B, UB.sub.4, B.sub.4C, ZrB.sub.2, Cr, CrO, CrO.sub.2 and Cr.sub.2O.sub.3 or combinations thereof.

31. A method of making and using nuclear fuel, comprising: making a plurality of nuclear fuel pellets as defined in the method of claim 14; 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, such that at least 20% of the nuclear fuel material in said core are made of pellets in accordance with the aforementioned method of making the plurality of nuclear fuel pellets; and operating the nuclear reactor to produce energy.

32. A method according to claim 31, wherein at least 50% of the nuclear fuel material in said core are made of pellets made in accordance with a method of making a nuclear fuel pellet for a nuclear power reactor comprising the following steps: providing a nuclear fuel material in powder form, providing an additive, pressing the powder such that a green pellet is obtained, wherein said additive is added either to said nuclear fuel material in powder form or to the green pellet, sintering the so obtained green pellet, wherein said additive is such that larger grains in the nuclear fuel material are present in the pellet after the sintering step as compared with the grain size obtained if a pellet, which is produced according to the above manner but without the addition of the additive, is sintered in the same manner, wherein said additive is made of or includes a substance which causes said larger grains in the sintered pellet, wherein said substance is selected and the method is performed such that at least to 90% of the substance, leaves at least an outer portion of the pellet before and/or during the sintering step.

33. A method according to claim 31, wherein 100% of the nuclear fuel material in said core are made of pellets made in accordance with a method of making a nuclear fuel pellet for a nuclear power reactor comprising the following steps: providing a nuclear fuel material in powder form, providing an additive, pressing the powder such that a green pellet is obtained, wherein said additive is added either to said nuclear fuel material in powder form or to the green pellet, sintering the so obtained green pellet, wherein said additive is such that larger grains in the nuclear fuel material are present in the pellet after the sintering step as compared with the grain size obtained if a pellet, which is produced according to the above manner but without the addition of the additive, is sintered in the same manner, wherein said additive is made of or includes a substance which causes said larger grains in the sintered pellet, wherein said substance is selected and the method is performed such that at least to 90% of the substance, leaves at least an outer portion of the pellet before and/or during the sintering step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] FIG. 1 shows schematically a flow chart of a manner of carrying out a method according to the invention.

[0057] FIG. 2 shows schematically a flow chart of another manner of carrying out a method according to the invention.

[0058] FIG. 3 shows schematically an example of how the grain size in a pellet produced in accordance one manner of carrying out a method according to the invention may vary in the radial direction of the pellet.

DETAILED DESCRIPTION

[0059] 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.

[0060] FIG. 1 shows schematically the main steps of a manner of carrying out a method according to the present invention.

[0061] 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 and U.sub.3O.sub.8.

[0062] An additive is provided. The additive constitutes or includes a substance that will increase the grain size of the sintered pellet. Furthermore, the substance is such that it will leave at least an outer portion of the pellet before and/or during a sintering step.

[0063] The additive may for example comprise B, for example in the form of UB.sub.4. According to one embodiment, the B is in the form of .sup.11B. B has the property of increasing the grain size when the green pellet is sintered. However, B will also to a large extent leave the pellet when it is heated during the sintering process, or before the sintering if a heating step is performed before the actual sintering.

[0064] According to another alternative, the additive may be for example Cr.sub.2O.sub.3. Also Cr has the property of increasing the grain size. Furthermore, also Cr will to a substantial degree leave at least an outer portion of the pellet if sufficient temperature and time are used for heating the pellet, before or during the sintering step.

[0065] The additive is preferably in the form of particles, i.e. a powder.

[0066] The additive powder is mixed with the nuclear fuel material in powder form.

[0067] The mixed powder is pressed such that a green pellet is formed.

[0068] The additive may be mixed with the whole nuclear fuel material in powder form. Alternatively, it is possible to mix the additive with only a part of the nuclear fuel material in powder form. According to the first mentioned alternative, the additive may thus be distributed in the whole green pellet. According to the second alternative, it is possible to arrange the nuclear fuel material in powder form without the additive in an inner portion and to add the mixture of the additive and the nuclear fuel material in powder form to an outer portion before the green pellet is pressed. According to the second alternative, the additive will thus be present only in an outer portion of the green pellet.

[0069] The green pellet is then sintered. This can be done for example in a furnace which contains different zones where the pellet is heated up to a final temperature of about 1 800° C. The temperature and the time is selected such that the substance in the additive which causes the larger grains in the sintered pellet will substantially leave (evaporate) at least an outer portion of the pellet during the sintering step. Alternatively, a separate heating step may be performed before the actual sintering in order to achieve this.

[0070] The sintered pellet is ground in order to obtain the correct diameter and surface finish.

[0071] The nuclear fuel pellet has a substantially cylindrical shape with a radius r. The outer portion of the pellet may for example be the part of the produced nuclear fuel pellet that is located between 0.9 r and r.

[0072] When a plurality of nuclear fuel pellets have been made in accordance with the present invention, the nuclear fuel pellets are arranged in cladding tubes. The cladding tubes are then positioned in nuclear fuel assemblies which are arranged in a nuclear reactor. The nuclear reactor is then operated in order to produce energy.

[0073] FIG. 2 shows a flow chart of another manner of carrying out a method according to the invention. The main difference in the method according to FIG. 2, as compared with the method according to FIG. 1, is that the additive is added after the green pellet has been formed.

[0074] According to FIG. 2, a nuclear fuel material in powder form is provided. The same nuclear fuel material as mentioned in connection with FIG. 1 may be used.

[0075] 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.

[0076] An additive is provided. The additive is made of or includes a substance which will increase the grain size of the sintered pellet. Furthermore, the substance, which causes the larger grains, is such that it will leave at least an outer portion of the pellet before and/or during a following sintering step. The additive may for example comprise B, for example in the form of UB.sub.4. According to one embodiment, the B is in the form of .sup.11B.

[0077] According to another example, the additive may be Cr.sub.2O.sub.3.

[0078] 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 μm.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] The penetration depth can be controlled such that the additive is added only to an outer portion of the green pellet.

[0084] 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.

[0085] The so treated green pellet is then sintered. This can be done for example by a sintering process in a furnace which contains different zones where the pellet is heated up to a final temperature of about 1 800° C.

[0086] 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.

[0087] The time and temperature for the heating/sintering process are selected such that also (at least) the substance which causes the larger grains will substantially leave at least an outer portion of the pellet before and/or during the sintering step. The outer portion of the pellet may, as explained above, for example be the part of the produced nuclear fuel pellet that is located between 0.9 r and r.

[0088] 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.

[0089] FIG. 3 illustrates schematically how the grain size may vary in a pellet produced in this manner or in the manner described in connection with FIG. 1 (if the additive is added in an outer portion of the pellet). 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. 3 shows the average 2D grain size. The curve in FIG. 3 thus shows how the average 2D grain size varies with the radius. FIG. 3 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 formation of the above described HBS can be prevented or delayed.

[0090] In the same manner as described in connection with FIG. 1, a plurality of nuclear fuel pellets are produced according to the method of the present invention.

[0091] The produced pellets are arranged in cladding tubes.

[0092] 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.

[0093] The nuclear reactor is operated in order to produce energy.

[0094] The present invention is not limited to the examples described herein, but can be varied and modified within the scope of the following claims.