Method for fabrication of oxide fuel pellets and the oxide fuel pellets thereby

Abstract

Disclosed herein is a method for manufacturing oxide fuel pellets. The method for manufacturing the oxide fuel pellets includes (step 1) preparing nuclear fuel powder containing uranium dioxide (UO2+x, x=0 to 0.20), (step 2) compacting the nuclear fuel powder prepared in step 1 to manufacture green pellets, sintering the green pellets manufactured in step 2 at a temperature of about 1,200° C. to about 1,400° C. by using an atmosphere gas, and reducing the green pellets sintered in step 3 at a temperature of about 800° C. to about 1,000° C. by using a reducing atmosphere gas. The method for manufacturing the oxide fuel pellets according to the present invention performs the sintering at a low temperature of about 1,200° C. to 1,400° C. to manufacture economical and safe oxide fuel pellets that are adequate for the nuclear fuel specification.

Claims

1. A method for manufacturing oxide fuel pellets, the method comprising: (step 1) preparing nuclear fuel powder containing uranium dioxide (UO.sub.2+x, x=0 to 0.20); (step 2) compacting the nuclear fuel powder prepared in step 1 to manufacture green pellets; (step 3) sintering the green pellets manufactured in step 2 at a temperature of about 1,200° C. to about 1,400° C. by using an atmosphere gas; and (step 4) reducing the green pellets sintered in step 3 at a temperature of about 800° C. to about 1,000° C. by using a reducing atmosphere gas.

2. The method as set forth in claim 1, wherein the nuclear fuel powder in step 1 further contains gadolinia (Gd.sub.2O.sub.3) or plutonium oxide (PuO.sub.2).

3. The method as set forth in claim 1, wherein the manufacturing of the green pellets in step 2 is performed under a compaction pressure of about 100 MPa to about 500 MPa.

4. The method as set forth in claim 1, wherein the manufacturing of the green pellets in step 2 is performed under a compaction pressure of about 150 MPa to about 450 MPa.

5. The method as set forth in claim 1, wherein the atmosphere gas in step 3 comprises at least one kind of compound selected from the group consisting of carbon dioxide, nitrogen, and argon.

6. The method as set forth in claim 1, wherein the sintering in step 3 is performed for about 2 hours to about 8 hours.

7. The method as set forth in claim 1, wherein the sintering in step 3 is performed for about 2 hours to about 5 hours.

8. The method as set forth in claim 1, wherein the atmosphere gas in step 4 comprises a hydrogen gas.

9. The method as set forth in claim 1, wherein the reducing in step 4 is performed for about 1 hour to about 5 hours.

10. The method as set forth in claim 1, wherein the sintering and reducing in steps 3 and 4 are continuously performed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a graph of a time-dependent temperature variation in a manufacturing method according to the present invention;

(3) FIGS. 2 and 3 are graphs illustrating compaction pressure-dependent sintering densities of oxide fuel pellets manufactured according to Embodiments 1 to 18 of the prevent invention and Comparative Examples 1 to 12;

(4) FIG. 4 is a graph illustrating sintering temperature-dependent grain sizes of the oxide fuel pellets manufactured according to Embodiments 1 to 18 of the prevent invention and Comparative Examples 1 to 12;

(5) FIGS. 5 and 6 are graphs illustrating compaction pressure-dependent sintering densities of oxide fuel pellets manufactured according to Embodiments 19 to 36 of the prevent invention and Comparative Examples 13 to 24;

(6) FIG. 7 is a graph illustrating sintering temperature-dependent grain sizes of the oxide fuel pellets manufactured according to Embodiments 19 to 36 of the prevent invention and Comparative Examples 13 to 24;

(7) FIGS. 8 and 9 are graphs illustrating compaction pressure-dependent sintering densities of oxide fuel pellets manufactured according to Embodiments 37 to 54 of the prevent invention and Comparative Examples 25 to 36; and

(8) FIG. 10 is a graph illustrating sintering temperature-dependent grain sizes of the oxide fuel pellets manufactured according to Embodiments 37 to 54 of the prevent invention and Comparative Examples 25 to 36.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) The present invention provides a method for manufacturing oxide fuel pellets. The method for manufacturing the oxide fuel pellets includes a process (step 1) of preparing nuclear fuel powder containing uranium dioxide (UO.sub.2+x, x=0 to 0.20), a process (step 2) of compacting the nuclear fuel powder prepared in step 1 to manufacture green pellets, a process (step 3) of sintering the green pellets manufactured in step 2 at a temperature of about 1,200° C. to about 1,400° C. by using an atmosphere gas, and a process (step 4) of reducing the green pellets sintered in step 3 at a temperature of about 800° C. to about 1,000° C. by using a reducing atmosphere gas.

(10) The present invention provides a low-temperature sintering reduction method for adjusting the oxide fuel pellets, which are used as a nuclear fuel and fed into a nuclear power plant, to be adequate for nuclear fuel specification. The oxide fuel pellets that are adequate for the nuclear fuel specification may have a sintering density ranging from about 94.0% to about 96.5% of a theoretical density (T.D) and a grain size of about 5 μm to about 25 μm.

(11) Hereinafter, the method for manufacturing the oxide fuel pellets in each of the processes according to the present invention will be described in detail.

(12) First, in the method for manufacturing the oxide fuel pellets according to the present invention, step 1 may be a process of preparing nuclear fuel powder containing uranium dioxide (UO.sub.2+x, x=0 to 0.20).

(13) In step 1, nuclear fuel powder containing uranium dioxide manufactured in a wet or dry manner is prepared as a raw material for manufacturing the oxide fuel pellets.

(14) Particularly, in step 1, the nuclear fuel powder may be prepared by performing a pretreatment process. In one example of the pretreatment process of the nuclear fuel powder, to improve fluidity of the uranium dioxide (UO.sub.2+x, x=0 to 0.20) manufactured in a wet or dry manner, a preliminary compaction process is performed at a predetermined pressure (about 300 MPa or less) to form preliminary slug, and then, the slug passes through a granulator having a sieve with a diameter of about 1 mm or less to form granules having a particle size of about 1 mm or less. Since the nuclear fuel powder that is formed with the granules through the above-described pretreatment process has good fluidity, the granules having a predetermined particle size may be manufactured. Also, an amount of lubricant may be mixed with the granules to reduce frication between granules and friction between the granules and a die wall, thereby prevent cracks from occurring.

(15) Here, the nuclear fuel powder manufactured in step 1 may further contain gadolinia (Gd.sub.2O.sub.3) or plutonium oxide (PuO.sub.2). Although the nuclear fuel powder contains only the uranium dioxide to manufacture the uranium dioxide fuel pellets, if the nuclear fuel powder containing burnable poison such as gadolinia (Gd.sub.2O.sub.3) is used as described above, mixed oxide fuel pellets such as UO.sub.2—Gd.sub.2O.sub.3 may be manufactured. Also, when the nuclear fuel powder contains the nuclear fission material such as plutonium oxide (PuO.sub.2), mixed oxide fuel pellets such as UO.sub.2—PuO.sub.2 may be manufactured.

(16) Next, in the method for manufacturing the oxide fuel pellets according to the present invention, step 2 may be a process for compacting the nuclear fuel powder prepared in step 1 to manufacture the green pellets.

(17) Particularly, the process of manufacturing the green pellets in step 2 may be performed under a compaction pressure of about 100 MPa to about 500 MPa. If the process of manufacturing the green pellets in step 2 is performed under a compaction pressure of about 100 MPa or less, it may be difficult to deal with the green pellets due to their weak properties and obtain a sintering density that is used for the oxide fuel pellets. On the other hand, if the process of manufacturing the green pellets is performed under a compaction pressure of about 500 MPa or more, it may be difficult to manufacture the green pellets, involve the possibility of an occurrence of cracks, and increase a sintering density corresponding to the increase in the compaction pressure.

(18) The compaction in step 2 may be performed through the conventional method. The green pellets manufactured by the compaction process may have an annular, solid, or rectangular shape that is adequate for the following process.

(19) Next, in the method for manufacturing the oxide fuel pellets according to the present invention, step 3 may be a process for sintering the green pellets manufactured in step 2 at a temperature of about 1,200° C. to about 1,400° C. by using an atmosphere gas.

(20) To manufacture the oxide fuel pellets adequate from the nuclear fuel specification in the related art, the green pellets are sintered at a high temperature (about 1,600° C. or more) for two hours or more under a hydrogen atmosphere. However, when the green pellets are sintered at a high temperature under the hydrogen atmosphere as described above, the risk of explosion of hydrogen may be involved. Also, electricity may be excessively consumed due to the sintering at the high temperature, and thus, the above-described manufacturing process may be uneconomical. Also, in the low-temperature sintering method according to the related art, the sintering process is performed at a low temperature (about 1,400° C. or less) under an atmosphere in which a ratio of carbon dioxide/carbon monoxide is adjusted. However, in the low-temperature sintering method, it may be difficult to adjust the ratio of carbon dioxide/carbon monoxide, and thus it may be difficult to manufacture oxide fuel pellets that are adequate for the nuclear fuel specification.

(21) For this, in step 3, the green pellets are sintered at a temperature of about 1,200° C. to 1,400° C. by using the atmosphere gas. In step 4 that is the next process, the sintered green pellets are reduced by using the reducing atmosphere gas to manufacture the oxide fuel pellets that are adequate for the nuclear fuel specification.

(22) Particularly, the sintering in step 3 may be performed under a gas atmosphere of carbon dioxide, nitrogen, or argon. When the sintering in step 3 is performed under the oxidative gas atmosphere such as carbon dioxide or the inert gas atmosphere such as nitrogen or argon, since an O/U ratio of the uranium dioxide power manufactured in the wet or dry manner is about 2.15, excessive oxygen may exist. Thus, the excessive oxygen may promote the sintering to increase the sintering density and grain size. On the other hand, when the sintering in step 3 is performed under the hydrogen gas atmosphere, the existing excessive oxygen may be reduced and thus do not exist. Thus, to obtain the desired sintering density and grain size, the sintering may be performed at a high temperature of about 1600° C. or more. When the sintering is performed under the oxidative gas atmosphere such as the carbon dioxide (CO.sub.2) than the inert gas atmosphere such as nitrogen or argon, the gas atmosphere may be oxidized to further increase the sintering density and grain size. However, since the oxidative gas atmosphere such as the carbon dioxide causes global warming, when the sintering is performed by using nitrogen or argon, the sintering process may be environmentally friendly.

(23) Also, the sintering in step 3 may be performed for about 2 hours or more, more preferably, about 2 hours to about 8 hours. If when the sintering in step 3 is performed for about 2 hours or less, the manufactured oxide fuel pellets may not be suitable in grain size for the nuclear fuel specification.

(24) Next, in the method for manufacturing the oxide fuel pellets according to the present invention, step 4 may be a process for reducing the green pellets sintered in step 3 at a temperature of about 800° C. to about 1,000° C. by using a reducing atmosphere gas.

(25) Since the green pellets sintered in step 3 do not have an O/U ratio of about 2.00 that is required for the nuclear fuel specification, the green pellets may be reduced at a temperature of about 800° C. to about 1,000° C. by using the reducing gas atmosphere while being cooled after the sintering to reduce the green pellets into UO.sub.2. Thus, the UO.sub.2 oxide fuel pellets may be manufactured from UO.sub.2+x through the reduction process.

(26) As the reduction in step 4 is performed, high-quality UO.sub.2 oxide fuel pellets without having cracks may be manufactured. Here, since the manufactured UO.sub.2 oxide fuel pellets have an O/U ratio of about 2.0, the UO.sub.2 oxide fuel pellets may be superior in quality.

(27) Although the reduction in step 4 is performed at the temperature of about 800° C. to about 1,000° C. for about 1 hour to about 5 hours, the present invention is not limited thereto.

(28) The sintering and reduction in steps 3 and 4 may be continuously performed. That is, after the sintering in step 3 is performed, a hydrogen gas may be injected to convert the gas atmosphere into the reducing atmosphere. Thus, the sintering and reduction may be continuously performed without intermission.

(29) Here, when the sintering is performed under the atmosphere gas such as carbon dioxide, nitrogen, or argon, the hydrogen gas may be immediately injected to create the reducing atmosphere.

(30) In an example of the sintering and reduction processes according to the present invention, as schematically illustrated in FIG. 1, the method for manufacturing the oxide fuel pellets according to the present invention performs the low-temperature sintering process at a temperature of about 1,200° C. to 1,400° C. under the activated gas atmosphere or inert gas atmosphere to convert the gas atmosphere into the reducing gas atmosphere while performing the cooling process, thereby performing the reduction process at a temperature of about 800° C. to about 1,000° C. As a result, the oxide fuel pellets that are adequate for the nuclear fuel specification may be manufactured.

(31) Also, the present invention may provide the oxide fuel pellets manufactured by the above-described method.

(32) The oxide fuel pellets manufactured according to the present invention may be suitable for the oxide fuel pellets that are adequate for the nuclear fuel specification. In the related art, to manufacture the oxide fuel pellets that are adequate for the nuclear fuel specification, the oxide fuel pellets may be sintered at a high temperature (about 1,600° C. or more) for about 2 hours or more under the hydrogen atmosphere or may be sintered at a low temperature (about 1,400° C. or less) under an atmosphere in which a ratio of carbon dioxide/carbon monoxide is adjusted. However, the manufacturing methods may have low economic feasibility and stability, and it may be difficult to adjust the ratio of the atmosphere gas.

(33) On the other hand, the oxide fuel pellets manufactured by the above-described method according to the present invention may be sintered at a temperature of about 1,200° C. to about 1,400° C. to solve the above-described low economic feasibility and stability. Also, since the oxide fuel pellets are manufactured under the single gas atmosphere, the manufacturing process may be easy.

(34) Here, the oxide fuel pellets may have a density ranging from about 94.0% to about 96.5% of a theoretical density (T.D) and a grain size of about 5 μm to about 25 μm. Thus, the oxide fuel pellets may be suitable for the oxide fuel pellets that are adequate for the nuclear fuel specification.

(35) Hereinafter, following embodiments and experimental examples according to the present invention will be described in detail.

(36) However, the following embodiments and experimental examples may be exemplified merely as illustrative purpose, and thus the technical scope of the present invention is not limited thereto.

Manufacture 1 of Oxide Fuel Pellet

(37) Step 1: Natural uranium dioxide powder (ex-ADU UO.sub.2, specific surface area: 5.72 m.sup.2/g, O/U ratio: 2.13) manufactured in a wet manner was prepared.

(38) Step 2: The uranium dioxide prepared in step 1 may be pressed at a pressure of about 150 MPa by using a hydraulic press to manufacture green pellets. Here, each of the green pellets has a diameter of about 10.0 mm, a length of about 10 mm, and a weight of about 3.6 g to about 4.5 g. Also, the green pellet has a density of about 42% to about 43%.

(39) Step 3: The green pellets manufactured in step 2 were sintered at a temperature of about 1,200° C. with a heating rate of about 4° C./minute for about 2 hours under a carbon dioxide (CO.sub.2) atmosphere.

(40) Step 4: After step 3 is performed, the green pellets were cooled up to a temperature of about 1,000° C. with a cooling rate of about 4° C./minute, and then the carbon dioxide was replaced with a hydrogen gas to perform a reduction process for 2 hours to manufacture oxide fuel pellets.

Manufacture 2 of Oxide Fuel Pellet

(41) The same process as Embodiment 1 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 1 was performed to manufacture oxide fuel pellets. When the green pellets are manufactured at the pressure of about 300 MPa, a green density was about 48% to about 49% of the theoretical density.

Manufacture 3 of Oxide Fuel Pellet

(42) The same process as Embodiment 1 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 1 was performed to manufacture oxide fuel pellets. When the green pellets are manufactured at the pressure of about 450 MPa, a green density was about 52% to about 53% of the theoretical density.

Manufacture 4 of Oxide Fuel Pellet

(43) The same process as Embodiment 1 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Manufacture 5 of Oxide Fuel Pellet

(44) The same process as Embodiment 4 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 4 was performed to manufacture oxide fuel pellets.

Manufacture 6 of Oxide Fuel Pellet

(45) The same process as Embodiment 4 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 4 was performed to manufacture oxide fuel pellets.

Manufacture 7 of Oxide Fuel Pellet

(46) The same process as Embodiment 1 except that the green pellets are sintered at a temperature of about 1,300° C. in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Manufacture 8 of Oxide Fuel Pellet

(47) The same process as Embodiment 7 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 7 was performed to manufacture oxide fuel pellets.

Manufacture 9 of Oxide Fuel Pellet

(48) The same process as Embodiment 7 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 7 was performed to manufacture oxide fuel pellets.

Manufacture 10 of Oxide Fuel Pellet

(49) The same process as Embodiment 7 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 7 was performed to manufacture oxide fuel pellets.

Manufacture 11 of Oxide Fuel Pellet

(50) The same process as Embodiment 10 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 10 was performed to manufacture oxide fuel pellets.

Manufacture 12 of Oxide Fuel Pellet

(51) The same process as Embodiment 10 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 10 was performed to manufacture oxide fuel pellets.

Manufacture 13 of Oxide Fuel Pellet

(52) The same process as Embodiment 1 except that the green pellets are sintered at a temperature of about 1,400° C. in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Manufacture 14 of Oxide Fuel Pellet

(53) The same process as Embodiment 13 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 13 was performed to manufacture oxide fuel pellets.

Manufacture 15 of Oxide Fuel Pellet

(54) The same process as Embodiment 13 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 13 was performed to manufacture oxide fuel pellets.

Manufacture 16 of Oxide Fuel Pellet

(55) The same process as Embodiment 13 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 13 was performed to manufacture oxide fuel pellets.

Manufacture 17 of Oxide Fuel Pellet

(56) The same process as Embodiment 16 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 16 was performed to manufacture oxide fuel pellets.

Manufacture 18 of Oxide Fuel Pellet

(57) The same process as Embodiment 16 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 16 was performed to manufacture oxide fuel pellets.

Manufacture 19 of Oxide Fuel Pellet

(58) The same process as Embodiment 1 except that the green pellets are not sintered under carbon dioxide (CO.sub.2) atmosphere, but sintered under an argon (Ar) atmosphere in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Manufacture 20 of Oxide Fuel Pellet

(59) The same process as Embodiment 19 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Manufacture 21 of Oxide Fuel Pellet

(60) The same process as Embodiment 19 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Manufacture 22 of Oxide Fuel Pellet

(61) The same process as Embodiment 19 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Manufacture 23 of Oxide Fuel Pellet

(62) The same process as Embodiment 22 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 22 was performed to manufacture oxide fuel pellets.

Manufacture 24 of Oxide Fuel Pellet

(63) The same process as Embodiment 22 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 22 was performed to manufacture oxide fuel pellets.

Manufacture 25 of Oxide Fuel Pellet

(64) The same process as Embodiment 19 except that the green pellets are sintered at a temperature of about 1,300° C. in step 3 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Manufacture 26 of Oxide Nuclear Pellet

(65) The same process as Embodiment 25 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 25 was performed to manufacture oxide fuel pellets.

Manufacture 27 of Oxide Fuel Pellet

(66) The same process as Embodiment 25 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 25 was performed to manufacture oxide fuel pellets.

Manufacture 28 of Oxide Fuel Pellet

(67) The same process as Embodiment 25 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 25 was performed to manufacture oxide fuel pellets.

Manufacture 29 of Oxide Fuel Pellet

(68) The same process as Embodiment 28 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 28 was performed to manufacture oxide fuel pellets.

Manufacture 30 of Oxide Fuel Pellet

(69) The same process as Embodiment 28 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 28 was performed to manufacture oxide fuel pellets.

Manufacture 31 of Oxide Fuel Pellet

(70) The same process as Embodiment 19 except that the green pellets are sintered at a temperature of about 1,400° C. in step 3 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Manufacture 32 of Oxide Fuel Pellet

(71) The same process as Embodiment 31 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 31 was performed to manufacture oxide fuel pellets.

Manufacture 33 of Oxide Fuel Pellet

(72) The same process as Embodiment 31 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 31 was performed to manufacture oxide fuel pellets.

Manufacture 34 of Oxide Fuel Pellet

(73) The same process as Embodiment 31 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 31 was performed to manufacture oxide fuel pellets.

Manufacture 35 of Oxide Fuel Pellet

(74) The same process as Embodiment 34 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 34 was performed to manufacture oxide fuel pellets.

Manufacture 36 of Oxide Fuel Pellet

(75) The same process as Embodiment 34 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 34 was performed to manufacture oxide fuel pellets.

Manufacture 37 of Oxide Fuel Pellet

(76) The same process as Embodiment 1 except that the green pellets are not sintered under carbon dioxide (CO.sub.2) atmosphere, but sintered under a nitrogen (N2) atmosphere in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Manufacture 38 of Oxide Fuel Pellet

(77) The same process as Embodiment 37 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Manufacture 39 of Oxide Fuel Pellet

(78) The same process as Embodiment 37 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Manufacture 40 of Oxide Fuel Pellet

(79) The same process as Embodiment 37 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Manufacture 41 of Oxide Fuel Pellet

(80) The same process as Embodiment 40 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 40 was performed to manufacture oxide fuel pellets.

Manufacture 42 of Oxide Fuel Pellet

(81) The same process as Embodiment 40 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 40 was performed to manufacture oxide fuel pellets.

Manufacture 43 of Oxide Fuel Pellet

(82) The same process as Embodiment 37 except that the green pellets are sintered at a temperature of about 1,300° C. in step 3 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Manufacture 44 of Oxide Nuclear Fuel Pellet

(83) The same process as Embodiment 43 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 43 was performed to manufacture oxide fuel pellets.

Manufacture 45 of Oxide Fuel Pellet

(84) The same process as Embodiment 43 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 43 was performed to manufacture oxide fuel pellets.

Manufacture 46 of Oxide Fuel Pellet

(85) The same process as Embodiment 43 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 43 was performed to manufacture oxide fuel pellets.

Manufacture 47 of Oxide Fuel Pellet

(86) The same process as Embodiment 46 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 46 was performed to manufacture oxide fuel pellets.

Manufacture 48 of Oxide Fuel Pellet

(87) The same process as Embodiment 46 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 46 was performed to manufacture oxide fuel pellets.

Manufacture 49 of Oxide Fuel Pellet

(88) The same process as Embodiment 37 except that the green pellets are sintered at a temperature of about 1,400° C. in step 3 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Manufacture 50 of Oxide Fuel Pellet

(89) The same process as Embodiment 49 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 49 was performed to manufacture oxide fuel pellets.

Manufacture 51 of Oxide Fuel Pellet

(90) The same process as Embodiment 49 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 49 was performed to manufacture oxide fuel pellets.

Manufacture 52 of Oxide Fuel Pellet

(91) The same process as Embodiment 49 except that the green pellets are sintered for about 5 hours in step 3 of Embodiment 49 was performed to manufacture oxide fuel pellets.

Manufacture 53 of Oxide Fuel Pellet

(92) The same process as Embodiment 52 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Embodiment 52 was performed to manufacture oxide fuel pellets.

Manufacture 54 of Oxide Fuel Pellet

(93) The same process as Embodiment 52 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Embodiment 52 was performed to manufacture oxide fuel pellets.

Comparative Example 1

(94) The same process as Embodiment 1 except that the green pellets are sintered at a temperature of about 1,000° C. in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Comparative Example 2

(95) The same process as Comparative Example 1 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 1 was performed to manufacture oxide fuel pellets.

Comparative Example 3

(96) The same process as Comparative Example 1 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 1 was performed to manufacture oxide fuel pellets.

Comparative Example 4

(97) The same process as Comparative Example 1 except that the green pellets are sintered for about 5 hours in step 3 of Comparative Example 1 was performed to manufacture oxide fuel pellets.

Comparative Example 5

(98) The same process as Comparative Example 4 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 4 was performed to manufacture oxide fuel pellets.

Comparative Example 6

(99) The same process as Comparative Example 4 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 4 was performed to manufacture oxide fuel pellets.

Comparative Example 7

(100) The same process as Embodiment 1 except that the green pellets are sintered at a temperature of about 1,100° C. in step 3 of Embodiment 1 was performed to manufacture oxide fuel pellets.

Comparative Example 8

(101) The same process as Comparative Example 7 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 7 was performed to manufacture oxide fuel pellets.

Comparative Example 9

(102) The same process as Comparative Example 7 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 7 was performed to manufacture oxide fuel pellets.

Comparative Example 10

(103) The same process as Comparative Example 7 except that the green pellets are sintered for about 5 hours in step 3 of Comparative Example 7 was performed to manufacture oxide fuel pellets.

Comparative Example 11

(104) The same process as Comparative Example 10 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 10 was performed to manufacture oxide fuel pellets.

Comparative Example 12

(105) The same process as Comparative Example 10 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 10 was performed to manufacture oxide fuel pellets.

Comparative Example 13

(106) The same process as Embodiment 19 except that the green pellets are sintered at a temperature of about 1,000° C. in step 3 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Comparative Example 14

(107) The same process as Comparative Example 13 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 13 was performed to manufacture oxide fuel pellets.

Comparative Example 15

(108) The same process as Comparative Example 13 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 13 was performed to manufacture oxide fuel pellets.

Comparative Example 16

(109) The same process as Comparative Example 13 except that the green pellets are sintered for about 5 hours in step 3 of Comparative Example 13 was performed to manufacture oxide fuel pellets.

Comparative Example 17

(110) The same process as Comparative Example 16 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 16 was performed to manufacture oxide fuel pellets.

Comparative Example 18

(111) The same process as Comparative Example 16 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 16 was performed to manufacture oxide fuel pellets.

Comparative Example 19

(112) The same process as Embodiment 19 except that the green pellets are sintered at a temperature of about 1,100° C. in step 3 of Embodiment 19 was performed to manufacture oxide fuel pellets.

Comparative Example 20

(113) The same process as Comparative Example 19 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 19 was performed to manufacture oxide fuel pellets.

Comparative Example 21

(114) The same process as Comparative Example 19 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 19 was performed to manufacture oxide fuel pellets.

Comparative Example 22

(115) The same process as Comparative Example 19 except that the green pellets are sintered for about 5 hours in step 3 of Comparative Example 19 was performed to manufacture oxide fuel pellets.

Comparative Example 23

(116) The same process as Comparative Example 22 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 22 was performed to manufacture oxide fuel pellets.

Comparative Example 24

(117) The same process as Comparative Example 22 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 22 was performed to manufacture oxide fuel pellets.

Comparative Example 25

(118) The same process as Embodiment 37 except that the green pellets are sintered at a temperature of about 1,000° C. in step 3 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Comparative Example 26

(119) The same process as Comparative Example 25 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 25 was performed to manufacture oxide fuel pellets.

Comparative Example 27

(120) The same process as Comparative Example 25 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 25 was performed to manufacture oxide fuel pellets.

Comparative Example 28

(121) The same process as Comparative Example 25 except that the green pellets are sintered for about 5 hours in step 3 of Comparative Example 25 was performed to manufacture oxide fuel pellets.

Comparative Example 29

(122) The same process as Comparative Example 28 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 28 was performed to manufacture oxide fuel pellets.

Comparative Example 30

(123) The same process as Comparative Example 28 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 28 was performed to manufacture oxide fuel pellets.

Comparative Example 31

(124) The same process as Embodiment 37 except that the green pellets are sintered at a temperature of about 1,100° C. in step 3 of Embodiment 37 was performed to manufacture oxide fuel pellets.

Comparative Example 32

(125) The same process as Comparative Example 31 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 31 was performed to manufacture oxide fuel pellets.

Comparative Example 33

(126) The same process as Comparative Example 31 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 31 was performed to manufacture oxide fuel pellets.

Comparative Example 34

(127) The same process as Comparative Example 31 except that the green pellets are sintered for about 5 hours in step 3 of Comparative Example 31 was performed to manufacture oxide fuel pellets.

Comparative Example 35

(128) The same process as Comparative Example 34 except that the uranium dioxide powder is pressed at a pressure of about 300 MPa by using the hydraulic press in step 2 of Comparative Example 34 was performed to manufacture oxide fuel pellets.

Comparative Example 36

(129) The same process as Comparative Example 34 except that the uranium dioxide powder is pressed at a pressure of about 450 MPa by using the hydraulic press in step 2 of Comparative Example 34 was performed to manufacture oxide fuel pellets.

(130) TABLE-US-00001 TABLE 1 Atmosphere Temperature Pressure Classification gas (° C.) (MPa) Time Embodiment 1 Carbon 1200 150 2 Embodiment 2 dioxide 300 Embodiment 3 (CO.sub.2) 450 Embodiment 4 150 5 Embodiment 5 300 Embodiment 6 450 Embodiment 7 1300 150 2 Embodiment 8 300 Embodiment 9 450 Embodiment 10 150 5 Embodiment 11 300 Embodiment 12 450 Embodiment 13 1400 150 2 Embodiment 14 300 Embodiment 15 450 Embodiment 16 150 5 Embodiment 17 300 Embodiment 18 450

(131) TABLE-US-00002 TABLE 2 Atmosphere Temperature Pressure Classification gas (° C.) (MPa) Time Embodiment 19 Argon 1200 150 2 Embodiment 20 (Ar) 300 Embodiment 21 450 Embodiment 22 150 5 Embodiment 23 300 Embodiment 24 450 Embodiment 25 1300 150 2 Embodiment 26 300 Embodiment 27 450 Embodiment 28 150 5 Embodiment 29 300 Embodiment 30 450 Embodiment 31 1400 150 2 Embodiment 32 300 Embodiment 33 450 Embodiment 34 150 5 Embodiment 35 300 Embodiment 36 450

(132) TABLE-US-00003 TABLE 3 Atmosphere Temperature Pressure Classification gas (° C.) (MPa) Time Embodiment 37 Nitrogen 1200 150 2 Embodiment 38 (N2) 300 Embodiment 39 450 Embodiment 40 150 5 Embodiment 41 300 Embodiment 42 450 Embodiment 43 1300 150 2 Embodiment 44 300 Embodiment 45 450 Embodiment 46 150 5 Embodiment 47 300 Embodiment 48 450 Embodiment 49 1400 150 2 Embodiment 50 300 Embodiment 51 450 Embodiment 52 150 5 Embodiment 53 300 Embodiment 54 450

(133) TABLE-US-00004 TABLE 4 Atmosphere Temperature Pressure Classification gas (° C.) (MPa) Time Comparative Carbon 1000 150 2 Example 1 dioxide Comparative (CO.sub.2) 300 Example 2 Comparative 450 Example 3 Comparative 150 5 Example 4 Comparative 300 Example 5 Comparative 450 Example 6 Comparative 1100 150 2 Example 7 Comparative 300 Example 8 Comparative 450 Example 9 Comparative 150 5 Example 10 Comparative 300 Example 11 Comparative 450 Example 12 Comparative Argon 1000 150 2 Example 13 (Ar) Comparative 300 Example 14 Comparative 450 Example 15 Comparative 150 5 Example 16 Comparative 300 Example 17 Comparative 450 Example 18 Comparative 1100 150 2 Example 19 Comparative 300 Example 20 Comparative 450 Example 21 Comparative 150 5 Example 22 Comparative 300 Example 23 Comparative 450 Example 24 Comparative Nitrogen 1000 150 2 Example 25 (N.sub.2) Comparative 300 Example 26 Comparative 450 Example 27 Comparative 150 5 Example 28 Comparative 300 Example 29 Comparative 450 Example 30 Comparative 1100 150 2 Example 31 Comparative 300 Example 32 Comparative 450 Example 33 Comparative 150 5 Example 34 Comparative 300 Example 35 Comparative 450 Example 36

Analysis of Sintering Density and Grain Size of Oxide Fuel Pellet

(134) (1) Analysis of Sintering Density and Grain Size of Oxide Fuel Pellet Depending on Sintering Under Carbon Dioxide Atmosphere

(135) To confirm the sintering density and grain size of the oxide fuel pellets manufactured by the manufacturing method according to the present invention, sintering densities of the oxide fuel pellets manufactured in Embodiments 1 to 18 and Comparative Examples 1 to 12 were measured in an immersion method, grain sizes were measured in the Heyn's lineal intercept (ASTM E112) method, and their results were illustrated in FIGS. 2 to 4.

(136) As illustrated in FIGS. 2 and 3, the oxide fuel pellets that are adequate for the nuclear fuel specification has a sintering density ranging from about 94.0% to about 96.5% of a theoretical density (T.D), and manufacturing conditions of the oxide fuel pellets having a density within the above-described sintering density range are as follows.

(137) When a sintering time is about 2 hours, a compaction pressure is about 400 MPa or more in case of a sintering temperature of about 1,000° C., a compaction pressure is about 280 MPa or more in case of a sintering temperature of about 1,100° C., a compaction pressure is about 210 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 420 MPa or less in case of a sintering temperature of about 1,400° C.

(138) Also, when a sintering time is about 5 hours, a compaction pressure is about 300 MPa or more in case of a sintering temperature of about 1,000° C., a compaction pressure is about 210 MPa or more in case of a sintering temperature of about 1,100° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 250 MPa or less in case of a sintering temperature of about 1,400° C.

(139) Further, as illustrated in FIG. 4, it was confirmed that the oxide fuel pellets that are adequate for the nuclear fuel specification has a grain size of about 5 μm or more, and manufacturing conditions of the oxide fuel pellets having a grain size within the above-described grain size range are as follows: a temperature of about 1,200° C. or more and a sintering time of about 2 hours or more. Here, it was confirmed that the compaction pressure has a slight influence on the grain size.

(140) As described above, conditions adequate for manufacturing the oxide nuclear fuel pellets, that are adequate for the nuclear fuel specification, having the conditions in which the theoretical density (T.D) of about 94.0% to about 96.5% and the grain size of about 5 μm or more are as follows.

(141) When a sintering time is about 2 hours, a compaction pressure is about 210 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 420 MPa or less in case of a sintering temperature of about 1,400° C. When a sintering time is about 5 hours, a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 250 MPa or less in case of a sintering temperature of about 1,400° C.

(142) (2) Analysis of Sintering Density and Grain Size of Oxide Fuel Pellet Depending on Sintering Under Argon Atmosphere

(143) To confirm the sintering density and grain size of the oxide fuel pellets manufactured by the manufacturing method according to the present invention, sintering densities of the oxide fuel pellets manufactured in Embodiments 19 to 36 and Comparative Examples 13 to 24 were measured in an immersion method, grain sizes were measured in the Heyn's lineal intercept (ASTM E112) method, and their results were illustrated in FIGS. 5 to 7.

(144) As illustrated in FIGS. 5 and 6, the oxide fuel pellets that are adequate for the nuclear fuel specification has a sintering density ranging from about 94.0% to about 96.5% of a theoretical density (T.D), and manufacturing conditions of the oxide fuel pellets having a density within the sintering density range are as follows.

(145) A sintering time is about 2 hours, a compaction pressure is about 370 MPa or more in case of a sintering temperature of about 1,000° C., a compaction pressure is about 220 MPa or more in case of a sintering temperature of about 1,100° C., a compaction pressure is about 220 MPa or more in case of a sintering temperature of about 1,200° C., and a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C.

(146) Also, when a sintering time is about 5 hours, a compaction pressure is about 270 MPa or more in case of a sintering temperature of about 1,000° C., a compaction pressure is about 170 MPa or more in case of a sintering temperature of about 1,100° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 300 MPa or less in case of a sintering temperature of about 1,400° C.

(147) Further, as illustrated in FIG. 7, it was confirmed that the oxide fuel pellets that are adequate for the nuclear fuel specification has a grain size of about 5 μm or more, and manufacturing conditions of the oxide fuel pellets having a grain size within the above-described grain size range are as follows: a temperature of about 1,200° C. or more and a sintering time of about 2 hours or more. Here, it was confirmed that the compaction pressure has a slight influence on the grain size.

(148) As described above, conditions adequate for manufacturing the oxide nuclear fuel pellets, that are adequate for the nuclear fuel specification, having the conditions in which the theoretical density (T.D) of about 94.0% to about 96.5% and the grain size of about 5 μm or more are as follows.

(149) When a sintering time is about 2 hours, a compaction pressure is about 200 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,400° C. When a sintering time is about 5 hours, a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 300 MPa or less in case of a sintering temperature of about 1,400° C.

(150) (3) Analysis of Sintering Density and Grain Size of Oxide Fuel Pellet Depending on Sintering Under Nitrogen Atmosphere

(151) To confirm the sintering density and grain size of the oxide fuel pellets manufactured by the manufacturing method according to the present invention, sintering densities of the oxide fuel pellets manufactured in Embodiments 37 to 54 and Comparative Examples 25 to 36 were measured in an immersion method, grain sizes were measured in the Heyn's lineal intercept (ASTM E112) method, and their results were illustrated in FIGS. 8 to 10.

(152) As illustrated in FIGS. 8 and 9, the oxide fuel pellets that are adequate for the nuclear fuel specification has a sintering density ranging from about 94.0% to about 96.5% of a theoretical density (T.D), and manufacturing conditions of the oxide fuel pellets having a density within the sintering density range are as follows.

(153) A sintering time is about 2 hours, a compaction pressure is about 350 MPa or more in case of a sintering temperature of about 1,000° C., a compaction pressure is about 200 MPa or more in case of a sintering temperature of about 1,100° C., a compaction pressure is about 190 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,400° C.

(154) Also, when a sintering time is about 5 hours, a compaction pressure is about 270 MPa or more in case of a sintering temperature of about 1,000° C., a compaction pressure is about 190 MPa or more in case of a sintering temperature of about 1,100° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 190 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 320 MPa or less in case of a sintering temperature of about 1,400° C.

(155) Further, as illustrated in FIG. 10, it was confirmed that the oxide fuel pellets that are adequate for the nuclear fuel specification has a grain size of about 5 μm or more, and manufacturing conditions of the oxide fuel pellets having a grain size within the above-described grain size range are as follows: a temperature of about 1,200° C. or more and a sintering time of about 2 hours or more. Here, it was confirmed that the compaction pressure has a slight influence on the grain size.

(156) As described above, conditions adequate for manufacturing the oxide nuclear fuel pellets, that are adequate for the nuclear fuel specification, having the conditions in which the theoretical density (T.D) of about 94.0% to about 96.5% and the grain size of about 5 μm or more are as follows.

(157) When a sintering time is about 2 hours, a compaction pressure is about 190 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,400° C. When a sintering time is about 5 hours, a compaction pressure is about 190 MPa or more in case of a sintering temperature of about 1,200° C., a compaction pressure is about 150 MPa or more in case of a sintering temperature of about 1,300° C., and a compaction pressure is about 320 MPa or less in case of a sintering temperature of about 1,400° C.

(158) In the method for manufacturing the oxide fuel pellets according to the present invention, the sintering may be performed at a low temperature of about 1,200° C. to about 1,400° C. to manufacture the economical and safe oxide fuel pellets that are adequate for the nuclear fuel specification.

(159) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.