NUCLEAR FUEL PELLETS AND MANUFACTURING METHOD THEREOF

Abstract

The present invention relates to nuclear fuel pellets and a manufacturing method thereof, and more particularly, to nuclear fuel pellets comprising a trapping material of fission gas and a manufacturing method thereof. A nuclear fuel pellet of the present invention comprises a nuclear fuel; and a trapping material of fission gas, wherein the trapping material of the fission gas includes an oxide containing at least one element selected from the group consisting of silicon (Si), aluminum (Al) and barium (Ba) to exhibit an excellent trapping ability selective and independent for fission gas.

Claims

1. A nuclear fuel pellet comprising: a nuclear fuel; and a trapping material of fission gas, wherein the trapping material of the fission gas comprises an oxide containing at least one element selected from the group consisting of silicon (Si), aluminum (Al), and barium (Ba).

2. The nuclear fuel pellet of claim 1, wherein the nuclear fuel is a uranium-based oxide.

3. The nuclear fuel pellet of claim 1, wherein the trapping material of the fission gas is included in a grain boundary of the nuclear fuel pellet.

4. The nuclear fuel pellet of claim 1, wherein the trapping material of the fission gas is included in an amount of 0.05 to 1 wt % based on the total weight of the nuclear fuel.

5. The nuclear fuel pellet of claim 1, wherein the fission gas includes at least one selected from the group consisting of cesium (Cs) and iodine (I).

6. The nuclear fuel pellet of claim 1, wherein the trapping material of the fission gas includes an oxide containing silicon, aluminum, and barium.

7. The nuclear fuel pellet of claim 1, wherein the oxide includes a compound represented by the following Formula 1.
Ba.sub.xAl.sub.ySi.sub.zO.sub.w (wherein, 0≤x≤2, 0≤y≤2, 0≤z≤3, and 0<w≤8, and 2x+3y+4z=2w.)  [Formula 1]

8. The nuclear fuel pellet of claim 1, wherein the trapping material has an average particle size (D.sub.50) of 0.1 to 100 μm.

9. A method for trapping fission gas using the nuclear fuel pellet according to claim 1.

10. The method for trapping the fission gas of claim 9, wherein the fission gas includes at least one selected from the group consisting of cesium (Cs) and iodine (I).

11. A manufacturing method of a nuclear fuel pellet comprising: mixing and then sintering a nuclear fuel raw material; and at least one oxide selected from the group consisting of a silicon oxide, an aluminum oxide, and a barium oxide.

12. (canceled)

13. (canceled)

14. The manufacturing method of the nuclear fuel pellet of claim 11, wherein a nuclear fuel raw material; and a silicon oxide, an aluminum oxide, and a barium oxide are mixed.

15. The manufacturing method of the nuclear fuel pellet of claim 11, wherein the nuclear fuel pellet comprises an oxide represented by the following Formula 1.
Ba.sub.xAl.sub.ySi.sub.zO.sub.w (wherein, 0≤x≤2, 0≤y≤2, 0≤z≤3, and 0<w≤8, and 2x+3y+4z=2w.)  [Formula 1]

16. The manufacturing method of the nuclear fuel pellet of claim 11, wherein the nuclear fuel pellet includes a trapping material of fission gas.

17. The manufacturing method of the nuclear fuel pellet of claim 11, wherein the sintering is performed at a temperature which is higher than an eutectic temperature of the nuclear fuel raw material and the oxide and lower than a melting temperature of each of the nuclear fuel and the oxide.

18. (canceled)

19. (canceled)

20. (canceled)

Description

DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings of this specification exemplify a preferred embodiment of the present invention, the spirit of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, and thus it will be understood that the present invention is not limited to only contents illustrated in the accompanying drawings.

[0016] FIG. 1 illustrates a photograph of UO.sub.2 nuclear fuel pellets according to an embodiment.

[0017] FIG. 2 illustrates a result of confirming a microstructure of a UO.sub.2 nuclear fuel pellet according to an embodiment through an optical microscope. In FIG. 2, a planar structure illustrates a pellet grain and a boundary structure between the surfaces illustrates a grain boundary. According to FIG. 2, a typical grain and a grain boundary structure of the nuclear fuel pellet are illustrated, and a separate secondary phase except for pores is not observed in the grain, and thus, it is estimated that a trapping material containing added oxides containing Ba, Al and Si is distributed in the grain boundary.

[0018] FIG. 3 illustrates an equilibrium diagram of a trapping material composition formed by using Al.sub.2O.sub.3, SiO.sub.2, and BaO at a mixed weight ratio of 1:1:1 as starting materials using a HSC chemistry according to an embodiment. The equilibrium diagram illustrates Al.sub.2O.sub.3*SiO.sub.2 (D), BaO*Al.sub.2O.sub.3, Ba.sub.2SiO.sub.4, BaSiO.sub.3, BaAl.sub.2Si.sub.2O.sub.8, Al.sub.2O.sub.3, BaSi.sub.2O.sub.5, SiO.sub.2, and Ba.sub.2Si.sub.3O.sub.8, respectively.

[0019] FIGS. 4A and 4B illustrate potentials (FIG. 4A: Cs potential, FIG. 4B: I potential) of trapping products formed by a trapping material composition according to an embodiment. At this time, in order to simulate a furnace environment, an O.sub.2 potential is applied as—450 kJ/mol. Equilibrium states representing each potential are as follows.

BaO*Al.sub.2O.sub.3+2Cs(g)+2SiO.sub.2+½O.sub.2(g)=2CsAlSiO.sub.4+BaO  A1:

BaAl.sub.2O.sub.4+2Cs(g)+2SiO.sub.2+½O.sub.2(g)=2CsAlSiO.sub.4+BaO  A2:

Al.sub.2O.sub.3*SiO.sub.2+2Cs(g)+SiO.sub.2+½O.sub.2(g)=2CsAlSiO.sub.4  A3:

BaSiO.sub.3+2Cs(g)+½O.sub.2(g)=Cs.sub.2O*SiO.sub.2+BaO  A4:

Al.sub.2O.sub.3+2Cs(g)+2SiO.sub.2+½O.sub.2(g)=2CsAlSiO.sub.4  A5:

BaO*Al.sub.2O.sub.3+I.sub.2(g)=BaI.sub.2+Al.sub.2O.sub.3+½O.sub.2(g)  B1:

BaAl.sub.2O.sub.4+2I.sub.2(g)=BaI.sub.2+Al.sub.2O.sub.3+½O.sub.2(g)  B2:

BaSiO.sub.3+I.sub.2(g)=BaI.sub.2+SiO.sub.2+½O.sub.2(g)  B3:

BaAl.sub.2SiO.sub.8+I.sub.2(g)=BaI.sub.2+Al.sub.2O.sub.3+2SiO.sub.2+½O.sub.2(g)  B4:

[0020] FIG. 5 illustrates an XRD analysis result after reaction heat-treatment of a Cs source of a trapping material including an oxide containing Ba, Al and Si according to an embodiment.

[0021] FIGS. 6A, 6B, 6C, and 6D illustrate SEM (6A) and EDS (6B, 6C, and 6D) analysis results after reaction heat-treatment of an I source of a trapping material including an oxide containing Ba, Al and Si according to an embodiment. A sample range for EDS analysis was marked by a square-dotted box on the SEM result of FIG. 6A.

BEST MODE

[0022] A nuclear fuel pellet of the present invention comprises a nuclear fuel; and a trapping material of fission gas, wherein the trapping material of the fission gas includes an oxide containing at least one element selected from the group consisting of silicon (Si), aluminum (Al) and barium (Ba).

[0023] The nuclear fuel may be an uranium-based oxide, and for example, may be uranium dioxide (UO.sub.2), triuranium octoxide (U.sub.3O.sub.8), or a mixture thereof. More specifically, the nuclear fuel may be UO.sub.2.

[0024] The trapping material of the fission gas comprises an oxide containing at least one element selected from the group consisting of silicon (Si), aluminum (Al), and barium (Ba). In an embodiment, the trapping material of the fission gas may be formed by sintering at least one oxide selected from the group consisting of a silicon oxide, an aluminum oxide, and a barium oxide, and specifically, the trapping material of the fission gas may include an oxide containing silicon, aluminum, and barium. The trapping material of the fission gas includes an oxide containing at least one element selected from the group consisting of silicon, aluminum, and barium, specifically an oxide containing silicon, aluminum, and barium to exhibit a trapping ability independently with respect to each of cesium and iodine.

[0025] The nuclear fuel pellet according to the present invention includes barium in the trapping material composition to exhibit an excellent trapping ability for I, as compared with a conventional trapping material of an oxide composition (Al—Si—O composition) containing silicon and aluminum having a trapping ability for Cs.

[0026] The trapping material of the fission gas is not limited thereto, but may be included in a grain boundary of the nuclear fuel pellet. As illustrated in FIG. 2, it was confirmed that the trapping material of the fission gas is distributed in a grain boundary through an optical micrograph of the nuclear fuel pellet of the present invention. Specifically, in the optical micrograph of the nuclear fuel pellet, it is observed that some pores are formed in a grain and in an area other than the grain boundary, traces of forming a secondary phase are not found.

[0027] In addition, in an aspect, the amount of the trapping material of the fission gas in the nuclear fuel pellet may be 0.05 to 1 wt %, for example, 0.1 to 0.5 wt % with respect to the weight of the nuclear fuel, but is not limited thereto.

[0028] In an aspect, it was confirmed that the amount of the trapping material of the fission gas estimated by measuring the amount of the fission gas trapped using the nuclear fuel pellet was 0.15 wt % with respect to the total weight of the nuclear fuel (ex. UO.sub.2).

[0029] The fission gas may include at least one selected from the group consisting of cesium (Cs) and iodine (I) and specifically, may include at least one selected from the group consisting of cesium and iodine. More specifically, the nuclear fuel pellet according to the present invention may exhibit an independent trapping ability for cesium and iodine while the trapping ability for each of cesium and iodine is selectively excellent. In addition, the trapping product is not limited thereto, but for example, CsAlSiO.sub.4, CsAlSi.sub.2O.sub.6 and BaI.sub.2 of the trapping products may exhibit a high trapping rate for Cs and I in a wider area in the nuclear fuel due to relatively high melting points.

[0030] As described above, the trapping material of the fission gas may comprise an oxide containing at least one element selected from the group consisting of Si, Al and Ba. In addition, in an aspect, the oxide includes a compound represented by the following Formula 1 to exhibit an excellent trapping ability independently for each of Cs and I.

Ba.sub.xAl.sub.ySi.sub.zO.sub.w (wherein, 0≤x≤2, 0≤y≤2, 0≤z≤3, and 0<w≤8, and 2x+3y+4z=2w.)  [Formula 1]

[0031] Preferably, the oxide includes a compound represented by the following Formula 2 to exhibit an excellent trapping ability independently for each of Cs and I.

Ba.sub.lAl.sub.mSi.sub.nO.sub.p (wherein, 0<l≤2, 0<m≤2, 0<n≤3 and 0<p≤8, and 2l+3m+4n=2p.)  [Formula 2]

[0032] In an aspect, the trapping material of the fission gas may include an oxide in the form of BaAl.sub.2Si.sub.2O.sub.8.

[0033] The nuclear fuel pellet includes the trapping material of the Ba—Al—Si—O composition as described above to exhibit an effect capable of independently trapping I as a trapping product in the form of BaI.sub.2. Accordingly, as compared with a conventional I trapping technique using a Cs trapping product in the form of CsAlSiO.sub.4, CsAlSi.sub.2O.sub.6, and the like, the nuclear fuel pellet may exhibit an excellent effect of preventing diffusion for Cs and I by stably trapping Cs without separating Cs from the trapping product.

[0034] The trapping material of the fission gas in the nuclear fuel pellet is not limited thereto, but for example, an average particle size (D.sub.50) may be 0.1 to 100 μm, specifically 10 to 50 μm.

[0035] A manufacturing method of the nuclear fuel pellet of the present invention includes mixing and then sintering a nuclear fuel raw material; and at least one oxide selected from the group consisting of a silicon oxide, an aluminum oxide, and a barium oxide.

[0036] The nuclear fuel raw material may be an uranium-based oxide, and specifically, may be a uranium oxide or a compound formed by mixing at least one selected from the group consisting of a plutonium oxide, a gadolinium oxide, and a thorium compound with the uranium oxide.

[0037] At least one oxide selected from the group consisting of a silicon oxide, an aluminum oxide, and a barium oxide which is mixed with the nuclear fuel raw material is a raw material for forming the trapping material of the fission gas. A mixed weight ratio of the nuclear fuel raw material and the oxide is not limited thereto, but for example, may be 0.05 to 1 wt % with respect to the total weight of the nuclear fuel raw material. As an example, the nuclear fuel pellet manufactured when the mixed weight ratio of the nuclear fuel raw material and the oxide is mixed in the range to manufacture the nuclear fuel pellet has an appropriate density range and may exhibit an excellent reactivity effect while exhibiting an excellent trapping ability for the fission gas. 0.05 wt % of a lower limit of the range is based on a minimum required amount calculated to trap Cs and I generated within the nuclear fuel, and 1 wt % of an upper limit is based on a maximum added amount (as the added amount is increased, the sintering density is reduced) capable of satisfying a sintering density (relative density of about 95%) reference as the nuclear fuel, respectively.

[0038] In the manufacturing method of the nuclear fuel pellet of the present invention, when the nuclear fuel raw material, the silicon oxide, the aluminum oxide, and the barium oxide all are mixed, the mixed weight ratio thereof is set so that there is a phase in which the generated trapping material spontaneously traps Cs and I, and is not particularly limited thereto.

[0039] In an embodiment, as a result of manufacturing the nuclear fuel pellet using a mixture of the silicon oxide, the aluminum oxide, and the barium oxide at a weight ratio of 1:1:1, it was confirmed that the nuclear fuel pellet exhibits an excellent trapping ability for Cs and I (FIGS. 4A and 4B and FIGS. 6A to 6D).

[0040] In the step of mixing and then sintering the nuclear fuel raw material and the oxide, the sintering may be performed under a reducing gas atmosphere, and the reducing gas atmosphere may include hydrogen and inert gas atmospheres. The sintering atmosphere may be generally performed under a sintering condition of the manufacturing method of the nuclear fuel pellet and an additional optimization process is not required and it is economic.

[0041] In addition, the sintering may also be performed at a temperature which is higher than an eutectic temperature of the nuclear fuel raw material and the oxide for forming the trapping material of the fission gas and lower than a melting temperature of each of the nuclear fuel raw material and the oxide. The sintering is performed at a temperature higher than the eutectic temperature of the nuclear fuel raw material and the oxide to form grains and grain boundaries, preferably a trapping material of the fission gas in the grain boundaries, thereby improving a problem of reducing the trapping ability of the nuclear fuel pellet due to the volatilization of the trapping material. Accordingly, it is possible to improve the durability of the nuclear fuel pellet.

[0042] Hereinafter, the present invention will be described in detail with reference to Examples for understanding. However, Examples according to the present invention may be modified in various forms, and it is not interpreted that the scope of the present invention is limited to the following Examples. Examples of the present invention will be provided for more completely explaining the present invention to those skilled in the art.

[0043] Manufacturing of Nuclear Fuel Pellets and Confirmation of Microstructure

[0044] The manufacturing of nuclear fuel pellets was performed in order of preparation of starting powder, powder weighing, mixing, molding, and sintering. Al.sub.2O.sub.3, SiO.sub.2 and BaO powders as the starting powder were weighed according to a weight ratio to be mixed with UO.sub.2 powder. The mixing was performed for 30 minutes to 2 hours using a turbula mixer to secure uniformity. The mixed powder was introduced to a metal mold for molding to be pressed in a pellet form by applying pressure of 1 to 5 ton/cm.sup.2 using a uniaxial molder. The molded pellets were introduced into a ceramic furnace and mounted in a heat-treatment furnace and then a reducing atmosphere was formed using hydrogen and carbon dioxide gases in a chamber and then isothermal sintering was performed for several times within about 1700° C. At this time, in order to minimize the heat shock of the sintering pellets, heating and cooling rates were within 5° C./min.

[0045] The microstructure of the nuclear fuel pellet may be checked by an optical microscope, an SEM, and the like. The pellet was cut in an axial direction or radial direction using a diamond saw to expose the inner surface of the sintered pellet and then mount the corresponding surface to be exposed using a resin. When the curing of the resin was completed, the cutout surface of the sintered pellet was polished using a polisher. The polishing was performed in order from rough polishing that held a side surface to fine polishing using a diamond paste, and the surface was observed with an optical microscope after the polishing was finished (FIG. 2).

[0046] Equilibrium Composition Analysis of Trapping Material

[0047] A result of predicting an equilibrium composition of a trapping material prepared by weighing Al.sub.2O.sub.3, SiO.sub.2 and BaO powders at a weight ratio of 1:1:1 and using the powders as starting powders using HSC chemistry was illustrated in FIG. 3.

[0048] FIG. 3 is an equilibrium prediction diagram in a thermodynamic aspect on whether starting powders (Al.sub.2O.sub.3, SiO.sub.2, BaO) of the trapping material are formed with any shape of compound (sintering phase) in sintering conditions (temperature and atmosphere). Since these powders directly act as the trapping material of trapping the fission product when used as the nuclear fuel in the future, in order to verify whether these powders may directly react with Cs and I, first, chemical forms of these powders need to be defined and sintered phases need to be predicted. In addition, since the sintered phase is determined by the type and composition of the starting material of the trapping material, the sintered phase needs to be thermodynamically predicted before an experiment. Through the result of FIG. 3, as an example, how much any form of compound by adding trapping material oxides at a ratio of 1:1:1 in a sintering condition (1700° C., 98H.sub.2-2CO.sub.2 atmosphere) of an example may be predetermined. The main phases obtained herein are set as a reactant, and hereinafter, reactivity with Cs and I was experimentally checked below and the result was illustrated (see FIG. 5 and FIGS. 6A to 6D).

Experimental Example 1. Confirmation of Trapping Performance for Cs and I

[0049] In order to verify spontaneous trapping performance for Cs and I reaction specimens using the trapping material prepared by weighing Al.sub.2O.sub.3, SiO.sub.2 and BaO powders prepared above at the weight ratio of 1:1:1, the potentials of a Cs trapping product and an I trapping product formed from the equilibrium phases were calculated and the result thereof was illustrated in FIGS. 4A and 4B (FIG. 4A: Cs trapping product, FIG. 4B: I trapping product). In order to simulate an environment in a nuclear reactor, an 02 potential is applied as—450 kJ/mol and a calculation result was illustrated. When a potential change (ΔG) value of each trapping reaction is negative (−), it is determined that the trapping reaction is spontaneous and it can be seen that as the temperature range is increased, an operation range of the trapping material is increased.

[0050] According to the results of FIGS. 4A and 4B, Cs is trapped by an Al—Si—O phase to form a trapping product with excellent high-temperature stability, Cs—Al—Si—O (CsAlSiO.sub.4 or CsAlSi.sub.2O.sub.6) or Cs—Si—O (Cs.sub.2O*SiO.sub.2). In a high-temperature area of 900 to 1000° C. or higher, some trapping reactions have changed unspontaneously, but some trapping reactions may be still expected as spontaneous trapping reactions to reach 1200° C. In addition, since the high temperature stability is excellent in CsAlSiO.sub.4 or CsAlSi.sub.2O.sub.6, it was confirmed that Cs trapping may be realized in the nuclear fuel (about 1200° C. or lower) of a nuclear reactor which is a normal environment. Meanwhile, from FIGS. 4A and 4B, it can be seen that a reaction of trapping I in the form of BaI.sub.2 by sintered phases formed by Ba—O until 1200° C. is spontaneous. In addition, in addition to the Cs and I compounds, since BaO, Al.sub.2O.sub.3, SiO.sub.2, or the like is formed as a product, when a trapping material is formed by a reaction therebetween, a reaction cycle may be formed by acting as the trapping material again.

Experimental Example 2. Confirmation of Trapping Performance for Cs and I—XRD, SEM/EDS

[0051] In order to verify trapping performance for Cs and I reaction specimens using the trapping material prepared by weighing Al.sub.2O.sub.3, SiO.sub.2 and BaO powders prepared above at the weight ratio of 1:1:1, as an example, a reaction heat-treatment experiment was performed at a reducing atmosphere at 650° C.

[0052] After reacting gas-phase Cs and I with the compound of the Ba—Al—Si—O composition through heat treatment, respectively, as a result analyzed by XRD and SEM/EDS, it was confirmed that in the Cs reaction specimen, CsAlSiO.sub.4 and CsAlSi.sub.2O.sub.6 were observed (FIG. 5), and in the I reaction specimen, I was detected (FIGS. 6B, 6C, and 6D) and the trapping reaction of Cs and I occurred. Through the presence of CsAlSiO.sub.4 and CsAlSi.sub.2O.sub.6 phases clearly distinguished with diffraction peaks on the XRD, it is verified that the trapping reaction between the trapping material of the Ba—Al—Si—O composition and the gas-phase Cs occurred. Meanwhile, as the SEM/EDS result in which I is detected, it is shown that I is present in a compound form which may be stable at a reaction heat treatment temperature of 650° C. In EDS mapping, it could be interpreted that since a distribution space of an element I tends to be matched with a spatial distribution of a lot of Ba, but is clearly different from an element distribution of a large amount of AI and Si, BaI.sub.2 is formed through the trapping reaction.