Fuel pellet

Abstract

A uranium oxide fuel pellet having an inner region and an outer rim region about the inner region, and that the fuel pellet is cylindrical and the inner region and outer rim region are coaxial cylindrical regions. The outer rim region has an excess of oxygen in comparison to the inner region, wherein high burnup structure (HBS) formation will be suppressed or delayed. Preferably, the excess oxygen is obtained by a chemical treatment by immersing the pellet in hydrogen peroxide (H.sub.2O.sub.2) or potassium permanganate (KMnO.sub.4) in solution.

Claims

1. A uranium oxide fuel pellet comprising an inner region and an outer rim region about the inner region, and that the fuel pellet is cylindrical and the inner region and outer rim region are coaxial cylindrical regions, wherein the outer rim region has an excess of oxygen in comparison to the inner region, wherein high burnup structure (HBS) formation will be suppressed or delayed, and wherein said excess oxygen in the outer rim region of the pellet is 5% extra O, by molar content.

2. The uranium oxide fuel pellet according to claim 1, wherein the excess of oxygen is obtained by adding oxygen only to the outer rim region of the pellet.

3. The uranium oxide fuel pellet according to claim 1, wherein the excess oxygen is obtained by a chemical treatment by immersing the pellet in hydrogen peroxide (H2O2) in solution.

4. The uranium oxide fuel pellet according to claim 1, wherein the excess oxygen is obtained by a chemical treatment by immersing the pellet in potassium permanganate (KMnO4) in solution.

5. The uranium oxide fuel pellet according to claim 1, wherein the outer rim region has a maximum thickness of 100 μm.

6. A fuel rod comprising a cladding tube in which a plurality of uranium fuel pellets according to claim 1 are packed in axial alignment.

7. The fuel rod according to claim 6, wherein the cladding tube is at its inner surface provided with an oxide coating, and wherein said oxide coating is obtained by a chemical treatment by immersing the cladding tube in hydrogen peroxide (H2O2) or potassium permanganate (KMnO4) in solution.

8. A fuel rod assembly comprising a plurality of fuel rods according to claim 6.

9. A method of preparing a uranium oxide fuel pellet comprising an inner region and an outer rim region about the inner region, and that the fuel pellet is cylindrical and the inner region and the outer rim region are coaxial cylindrical regions, wherein the method comprises providing an excess of oxygen in the outer rim region in comparison to the inner region, wherein high burnup structure (HBS) formation will be suppressed or delayed, and wherein said excess oxygen in the outer rim region of the pellet is 5% extra O, by molar content.

10. The method according to claim 9, comprising immersing the pellet in hydrogen peroxide (H2O2) in solution for obtaining the excess oxygen by a chemical treatment by.

11. The method according to claim 9, comprising immersing the pellet in potassium permanganate (KMnO4) in solution for obtaining the excess oxygen by a chemical treatment.

12. The method according to claim 9, comprising adding the excess oxygen after the pellet has been formed and ground.

13. A method in relation of a fuel rod comprising a plurality of uranium fuel pellets obtained by the method according to claim 9, and providing a cladding tube in which said fuel pellets are intended to be packed in axial alignment, and wherein the method comprises providing an oxide coating at the inner surface of the cladding tube.

14. The method according to claim 13, comprising providing the oxide coating by a chemical treatment by immersing the cladding tube in hydrogen peroxide (H2O2) or potassium permanganate (KMnO4) in solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a fuel pellet according to the present invention.

(2) FIG. 2 is a cross-sectional view along a longitudinal axis of the fuel pellet according to the present invention.

(3) FIG. 3 is a cross-sectional view perpendicular to the longitudinal axis of the fuel pellet according to the present invention.

(4) FIG. 4 is a schematic perspective view of a fuel rod according to the present invention.

(5) FIG. 5 is a schematic perspective view of a fuel rod assembly according to the present invention.

DETAILED DESCRIPTION

(6) The uranium oxide fuel pellet, the fuel rod, the fuel rod assembly and the method will now be described in detail with references to the appended figures. Throughout the figures the same, or similar, items have the same reference signs. Moreover, the items and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

(7) With references to FIGS. 1-3 showing schematic illustrations of various views of a uranium oxide fuel pellet 2 the present invention will now be described in detail.

(8) The fuel pellet 2 comprises an inner region 4 and an outer rim region 6 about the inner region 4. The fuel pellet 2 is cylindrical and the inner region 4 and the outer rim region 6 are coaxial cylindrical regions. The outer rim region 6 has an excess of oxygen in comparison to concentration of oxygen in the inner region 4, wherein high burnup structure (HBS) formation in the outer rim region will thereby be suppressed or delayed.

(9) Preferably, the outer rim region 6 has a maximum thickness of 100 μm.

(10) The excess of oxygen is obtained by adding oxygen only to the outer rim region 6 of the pellet, and the excess oxygen is approximately 5% extra O, by molar content.

(11) The addition of the oxygen is performed after the pellet has been formed, e.g. sintered, and ground.

(12) In one embodiment the excess oxygen is obtained by a chemical treatment by immersing the pellet in hydrogen peroxide (H.sub.2O.sub.2) in solution.

(13) In another embodiment the excess oxygen is obtained by a chemical treatment by immersing the pellet in potassium permanganate (KMnO.sub.4) in solution. KMnO.sub.4 is a strong oxidizing agent. After the chemical treatment the outer rim region 6 will not only contain UO.sub.2, but rather UO.sub.2+x.

(14) The concentration of hydrogen peroxide or potassium permanganate in solution is preferably higher than 25 millimolar (mM), i.e. 25 mM/litre water.

(15) The temperature of the solution is within the range of 15 to 80° C.

(16) Also other substances being strong oxidizing agents may be considered in order to achieve the excess of oxygen in the outer rim region. Examples include oxygen gas (possibly diluted in an inert gas), other permanganate solutions, and NO.sub.x gas.

(17) The time duration of the immersion of the pellets in the solution varies considerably in dependence of various conditions, but according to some non-limiting examples typical time durations may be in the range of 5-15 minutes, e.g. 10 minutes, in 10% hydrogen peroxide solution at 50 degrees Celsius. The times, temperature and solution molarity can be varied to provide the ideal excess oxygen cross section through the radius of the pellet.

(18) As an alternative, excess oxygen can be added through exposure to oxidizing gases such as O.sub.2 or NO.sub.x, releasing oxygen into the UO.sub.2 rim. Although possible, the low temperature solution methods provide a more controllable reaction compared to the potentially spontaneous gaseous methods.

(19) The present invention also relates to a fuel rod 8 (see the schematic illustration in FIG. 4) that comprises a plurality of uranium fuel pellets 2 of the kind that has been described above. Conventionally the fuel rod comprises a cladding tube in which the fuel pellets are packed in axial alignment.

(20) In one embodiment the cladding tube 10, in which the fuel pellets are packed in axial alignment, is at its inner surface provided with an oxide coating, and that the oxide coating is obtained by a chemical treatment by immersing the cladding tube in hydrogen peroxide (H.sub.2O.sub.2) or potassium permanganate (KMnO.sub.4) in solution.

(21) In another variation the oxide coating instead is achieved by heating the cladding tube in air or a controlled oxygen containing atmosphere with an oxygen torch. Depth and adhesion of the oxide layer can be tailored by altering the gaseous composition, speed to the torch and temperature of the torch.

(22) In FIG. 5 is illustrated a fuel rod assembly 12 that comprises a plurality of fuel rods 8 of the type that has been described above, i.e. fuel rods provided with fuel pellets having an outer rim region having an excess of oxygen in comparison to the oxygen concentration of an inner region. The fuel rod also comprises a cladding tube that preferably at its inner surface is provided with an oxide coating as described above. The illustrated fuel rod assembly 12 has an essentially square cross-section, but other cross-sectional shapes could naturally also be used, e.g. circular.

(23) The present invention also relates to a method of preparing a uranium oxide fuel pellet comprising an inner region and an outer rim region about the inner region, and that the fuel pellet is cylindrical, and the inner region and the outer rim region are coaxial cylindrical regions. The method comprises providing an excess of oxygen in the outer rim region in comparison to the inner region, wherein high burnup structure (HBS) formation will be suppressed or delayed. The step of obtaining the excess oxygen is achieved by adding oxygen only to the outer rim region of the pellet.

(24) In one embodiment the method comprises immersing the pellet in hydrogen peroxide (H.sub.2O.sub.2) in solution for obtaining the excess oxygen by a chemical treatment.

(25) In another embodiment the method comprises immersing the pellet in potassium permanganate (KMnO.sub.4) in solution for obtaining the excess oxygen by a chemical treatment.

(26) The concentration of hydrogen peroxide or potassium permanganate in solution is preferably higher than 25 millimolar (mM), i.e. 25 mM/litre water.

(27) The temperature of the solution is within the range of 15 to 80° C.

(28) According to a further embodiment a method is provided in relation to a fuel rod comprising a plurality of uranium fuel pellets as described above and intended to be packed in axial alignment in a cladding tube.

(29) In one embodiment the method comprises providing an oxide coating at the inner surface of the cladding tube, and preferably, the method comprises obtaining the oxide coating by a chemical treatment by immersing the cladding tube in hydrogen peroxide (H.sub.2O.sub.2) or potassium permanganate (KMnO.sub.4) in solution.

(30) In another embodiment the method comprises providing the oxide coating by heating the cladding tube in air with an oxygen torch.

(31) The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.