Uranium dioxide nuclear fuel pellet having ceramic microcells

Abstract

A uranium dioxide nuclear fuel pellet has about 50 to about 400 m (with respect to a 3-dimentional size) microcells formed of a ceramic material having a chemical attraction with fission products generated in the nuclear fuel pellet to absorb and trap the fission products, such that the extraction of the fission product may be retrained in a normal operation condition and that the performance of the nuclear fuel may be enhanced by mitigating PCI. In addition, highly radioactive fission products including Cs and I having a large generation amount or a long half-life enough to affect the environments can be trapped in the pellet in an accident condition, without being released outside.

Claims

1. A uranium dioxide nuclear fuel pellet comprising: uranium dioxide grains; and micro-partitions partitioning and contacting the uranium dioxide grains, wherein the micro-partitions consist essentially of a ceramic material that melts in a temperature range of 1200-1800 C., wherein the ceramic material is a mixture comprising two or more selected from the group consisting of Si-compounds, Ti-compounds, Al-compounds, Mg-compounds, and Mn-compounds, and wherein the mixture further comprises the Ti-compounds in an amount of at least 37.08 wt % or the Mn-compounds in an amount of 14.0 wt %.

2. The uranium dioxide nuclear fuel pellet of claim 1, further comprising: metallic particles dispersed in the uranium dioxide grains and configured to react with oxygen more easily than uranium dioxide does.

3. The uranium dioxide nuclear fuel pellet according to claim 2, wherein the metallic particles comprise Cr or Mo.

4. The uranium dioxide nuclear fuel pellet according to claim 2, wherein an average size of the metallic particles is about 0.3 m to about 10 m.

5. The uranium dioxide nuclear fuel pellet according to claim 1, wherein the micro-partitions define a plurality of microcells, wherein each of at least some of the microcells contains a single grain of uranium oxide.

6. The uranium dioxide nuclear fuel pellet according to claim 1, wherein one of the micro-partitions is located between two immediately neighboring grains of the uranium dioxide grains and contacts the two immediately neighboring grains.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual diagram illustrating microcells arranged in an uranium dioxide fission nuclear pellet according to the present invention;

(2) FIG. 2 is a conceptual drawing of trapping a fission product by arranging the ceramic microcells in the uranium dioxide nuclear fuel pellet according to the present invention;

(3) FIG. 3 is a conceptual drawing of trapping excess oxygen and the fission product by additionally arranging metallic particles in the ceramic microcells arranged in the uranium dioxide nuclear fuel pellet according to the present invention;

(4) FIG. 4 is a drawing illustrating a phase diagram of SiO.sub.2TiO.sub.2Al.sub.2O.sub.3 according to a first embodiment of the present invention;

(5) FIG. 5 is an optical micrograph image of a ceramic microcell structure of an uranium dioxide nuclear fuel pellet fabricated according to the first embodiment of the present invention;

(6) FIG. 6 is an optical micrograph image of a ceramic microcell structure of an uranium dioxide nuclear fuel pellet fabricated according to a second embodiment of the present invention; and

(7) FIG. 7 is an optical micrograph image of a ceramic microcell structure of a uranium dioxide nuclear fuel pellet fabricated to a third embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(8) Various embodiments of a fission product trapping pellet with ceramic microcells arranged therein and a fabricating method thereof will be described as follows, referring to the accompanying drawings. Reference will now be made in detail to the specific embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

(9) In uranium oxide pellets without any barriers or walls between grains, the fission product is produced within grains and diffused to grain boundary, and exists as bubbles. When the fission product reaches a predetermined amount, a bubble tunnel is formed along the grain boundary, and the fission product is released from the pellet through the bubble tunnel.

(10) As the grain size of the pellet increases, the diffusion distance of fission product to the grain boundary becomes longer. Therefore, the fission product remains within the pellet for a longer time, thus reducing a released amount of the fission product. Thus, high burnup nuclear fuel pellet may have an increased grain size.

(11) UO.sub.2 nuclear fuel pellet is inserted in a zirconium alloy unclear fuel cladding which is deformed inwardly during the burn-up and the nuclear fuel pellet is swollen outwardly by neutron irradiation, such that the nuclear fuel pellet and the cladding may contact with each other to generate stress. Especially, it is more likely to operate a nuclear fuel for an ultrahigh burnup level in an extreme situation such as a high power or a transition operation. When the output power is increased for a relatively short time, the temperature of the nuclear fuel pellet is increased and a predetermined pressure is applied to the fuel cladding by heat expansion. When high stress is applied to the fuel cladding at a high burnup level for a relatively short time, there might be damage on the fuel cladding. Accordingly, to reduce the pressure applied to the fuel cladding generated by the thermal expansion of the nuclear fuel pellet, a new pellet having an increased amount of initial deformation and an increased rate of creep deformation is developed and Pellet-Clade interaction (hereinafter, PCI) characteristics are enhanced.

(12) When the grain size of the pellet is increased, the movement distance of the fission product is increased, and may slow down the extraction of the fission products. Further, an additive may be provided to heighten the creep deformation rate of the fuel pellet such that the stress applied to a fuel cladding can be reduced effectively.

(13) FIG. 1 is a conceptual diagram illustrating microcells arranged in a uranium dioxide fission nuclear pellet according to the present invention.

(14) There may be provided a method of trapping a fission product in a nuclear pellet (UO.sub.2 pellet) having microcells arranged therein.

(15) FIG. 2 is a conceptual drawing of trapping a fission product by arranging the ceramic microcells in the uranium dioxide nuclear fuel pellet according to the present invention. Ceramic microcells having a chemical attraction with fission products are arranged in the uranium dioxide nuclear fuel pellet, such that extraction of fission products in a normal operational condition may be restrained and that PCI is mitigated, so as to enhance the performance of the nuclear fuel and to enable the nuclear fuel pellet to effectively trap radioactive fission products including Cs and I having a long half-life and a large amount enough to affect the environments out of fission products in an accident condition to prevent release of the radioactive fission products outside.

(16) The ceramic material used in forming the microcells arranged in the uranium nuclear fuel pellet may be a material having a chemical attraction with the fission products. Especially, such the ceramic material may be one or more chemical elements selected from a group configured of a Si-compound, a Ti-compound, an Al-compound, an Mg-compound, a Mn-compound, a Na-compound, a Ca-compound and a Ba-compound.

(17) The ceramic material has a chemical attraction with Cs and I having a large generation amount and a long half-life out of the fission products enough to affect the environments if they are extracted outside in an accident. Accordingly, the fission products including Cs and I generated during the irradiation reaches a wall of the cell, the material composing the cell wall absorbs and traps the fission products, to restrain the release of the fission products.

(18) In embodiments, the size of the microcell is within about 50 to about 400 m. In embodiments, a suitable number of microcells can be formed with a small amount of an additive powder and when the average size of the microcell is in the range of about 50 to about 400 m.

(19) The ceramic material content of the microcell is in a range of about 0.1% to about 8.0% with respect to the weight of the uranium dioxide. The content of the ceramic material is in the range of about 0.1% to about 8.0% that can form the appropriate microcells in the nuclear fuel pellet and maintain the appropriate amount of the uranium per unit volume of the nuclear fuel pellet.

(20) The ceramic material of the microcell is partially or entirely changed into liquid at about 1200 to about 1800 C. In embodiments, the ceramic material is changed into liquid below about 1800 C. that is the upper limit of the sintering temperature, to form the microcells during the sintering effectively and to set the nuclear fuel pellet not changed to liquid below about 1200 C. that is the upper limit temperature of the normal operation condition.

(21) As mentioned above, the ceramic material starting to be changed into liquid ceramic in the range of about 1200 to about 1800 C. is used and then the cell wall is softened, such that the initial deformation amount and the creep deformation rate may be increased enough to enhance PCI characteristics of the nuclear fuel pellet.

(22) The ceramic material compound may be one or more ones selected from a group configured of metal, oxide, sulfide, fluoride, chloride, stearate, carbonate, nitrate and phosphate.

(23) In addition, the ceramic microcells are formed per grain unit.

(24) FIG. 3 is a conceptual drawing of trapping excess oxygen and the fission product by additionally arranging metallic particles in the ceramic microcells arranged in the uranium dioxide nuclear fuel pellet according to the present invention.

(25) Referring to FIG. 3, the ceramic microcells are arranged and metallic particles capable of forming an oxide stably by reacting with the excess oxygen, prior to UO.sub.2, are additionally arranged in the uranium dioxide nuclear fuel pellet, only to absorb and trap the fission products. Together with that, the movement speed of the fission products is slowed down and the nuclear fuel pellet can have an enhanced nuclear fuel performance and stability.

(26) A metallic particle arranged in the microcell of the uranium diode nuclear fuel pellet reacts with the excess oxygen generated at a burnup level of about 70,000 MWD/MTU, prior to UO.sub.2, and the metallic particle absorbs and isolates the excess oxygen not to react with UO.sub.2. It is preferable that the metallic particle is Cr or Mo.

(27) An average size of the metallic particle may be about 0.3 to about 10 m. The size of the metallic particle may be in the range of about 0.3 to about 10 m that is the range capable of arranging an appropriate number of the particles per unit weight.

(28) Next, a fabricating method of the uranium dioxide nuclear fuel pellet having the ceramic microcells arranged therein according to the present invention will be described as follows.

(29) The fabricating method includes steps of fabricating powder mixture by mixing the uranium dioxide powder with the additive powder that consists of one or more elements selected from the group of the Si-compound, Ti-compound, Al-compound, Mg-compound, Mn-compound, Na-compound, Ca-compound and Ba-compound; fabricating a green pellet by compressing the powder mixture; and sintering the green pellet at about 1600 to about 1800 C. under the reducing gas atmosphere.

(30) According to one embodiment of the present invention, the compounds provided in the additive powder added in the powder mixture fabricating step may be at least one selected from the group of metal, oxide, nitride, sulfide, fluoride, chloride, stearate, carbonate, nitrate and phosphate.

(31) According to one embodiment of the present invention, the content of the additive is about 0.1 to about 8.0% of the overall weight in the powder mixture fabricating step. The limited content of the ceramic material is the range of about 0.1 to about 8.0% that can form the appropriate microcells in the nuclear fuel pellet and maintain the appropriate amount of the uranium per unit volume of the nuclear fuel pellet.

(32) According to one embodiment of the present invention, the selected additive may be partially or entirely changed into liquid below about 1800 C. that is the upper limit temperature of the typical sintering temperature and it may not start to be changed into liquid below about 1200 C. that is the upper limit of the normal operation condition. The liquid formed in the sintering may make the grain grow rapidly and the liquid is disposed along a boundary of the growing grain. Accordingly, the grain unit ceramic microcell having an appropriate size can be formed effectively and the nuclear fuel pellet may be maintained during the irradiation, with no liquid grain boundary.

(33) The size of the microcell is limited within about 50 to about 400 m. In embodiments, a suitable number of microcells can be formed with a small amount of an additive powder and when the average size of the microcell is in the range of about 50 to about 400 m.

(34) According to one embodiment of the present invention, the reducing gas atmosphere in the sintering step may be hydrogen-containing gas atmosphere. Especially, the hydrogen-containing gas may be a hydrogen-containing gas mixture formed of a hydrogen gas mixed with at least one selected from a group of carbon dioxide, vapor and inert gas or a hydrogen.

(35) Next, embodiments of the present invention will be described in detail as follows. Here, the embodiments which will be described as follows are examples of the present invention and the scope of the present invention is not limited by the embodiments.

(36) First Embodiment

(37) 1.0% of SiO.sub.2, TiO.sub.2 and Al.sub.2O.sub.3 powder with respect to UO.sub.2 powder is added to uranium dioxide (UO.sub.2) powder and they are mixed with each other by a mixer for 2 hours, to prepare the powder mixture. At this time, the weight ratio of the SiO.sub.2 powder, TiO.sub.2 powder and Al.sub.2O.sub.3 powder added at this time may be 35.86%, 37.08% and 27.06%.

(38) A green pellet is fabricated by compressing the powder mixture with 3 ton/cm.sup.2.

(39) The green pellet is heated at a heating rate of 300 C. per hour under hydrogen gas atmosphere, to make the temperature of the pellet reach 1720 C. The heated pellet is maintained for 4 hours and cooled to be a normal temperature at a rate of 300 C. per hour under the same atmosphere, such that the uranium dioxide unclear fuel pellet may be fabricated.

(40) The density of the fabricated nuclear fuel pellet is measured based on Archimedes' principle and a cross section of the nuclear fuel pellet having the density measured is polished like a mirror. After that, the size and structure of the microcell is observed.

(41) FIG. 4 is a drawing illustrating a phase diagram of SiO.sub.2TiO.sub.2Al.sub.2O.sub.3 according to a first embodiment of the present invention and shows a composition ratio selected according to the embodiment. The temperature of a liquidus line possessed by the selected additive is about 1650 C.

(42) It is measured that the density of the nuclear fuel pellet fabricated in the process mentioned above is 96.9% of TD and that an average size of the microcell is 87 m.

(43) FIG. 5 is an optical micrograph image of a ceramic microcell structure of a uranium dioxide nuclear fuel pellet fabricated according to the first embodiment of the present invention. As shown in FIG. 5, the microcells are formed along the grain boundary.

(44) Second Embodiment

(45) 1.0% of SiO.sub.2, Al.sub.2O.sub.3 and MnO powder with respect to UO.sub.2 powder is added to uranium dioxide (UO.sub.2) powder and they are mixed with each other by a mixer for 2 hours, to prepare the powder mixture. At this time, the weight ratio of the SiO.sub.2 powder, Al.sub.2O.sub.3 powder and MnO powder added at this time may be 40.5%, 45.5% and 14.0%.

(46) The powder mixture is compressed and sintered in the same process mentioned in the first embodiment and the UO.sub.2 pellet is fabricated.

(47) It is measured that the density of the nuclear fuel pellet fabricated in the process mentioned above is 97.3% of TD and that an average size of the microcell is 96 m.

(48) FIG. 6 is an optical micrograph image of a ceramic microcell structure of a uranium dioxide nuclear fuel pellet fabricated according to a second embodiment of the present invention. As shown in FIG. 6, the microcells are formed along the grain boundary.

(49) Third Embodiment

(50) 0.8% of TiO.sub.2 and MgO powder with respect to UO.sub.2 powder is added to uranium dioxide (UO.sub.2) powder and they are mixed with each other by a mixer for 2 hours, to prepare the powder mixture. At this time, the weight ratio of the TiO.sub.2 powder and MgO powder added at this time may be 70% and 30%.

(51) The powder mixture is compressed and sintered in the same process mentioned in the first embodiment and the UO.sub.2 pellet is fabricated.

(52) It is measured that the density of the nuclear fuel pellet fabricated in the process mentioned above is 97.2% of TD and that an average size of the microcell is 102 m.

(53) FIG. 7 is an optical micrograph image of a ceramic microcell structure of a uranium dioxide nuclear fuel pellet fabricated to a third embodiment of the present invention. As shown in FIG. 6, the microcells are formed along the grain boundary.

(54) It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.