Patent Number: 058825525
Section: claims

1. A method for manufacturing new fuel pellets by recycling defective fuel pellets of UO.sub.2 or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium, comprising the following steps (a) heating said defective fuel pellets at a temperature in the range of about 300.degree. C. to about 800.degree. C. in an oxidizing gas to oxidize UO.sub.2 to U.sub.3 O.sub.8 in order to make fuel particles of U.sub.3 O.sub.8 or U.sub.3 O.sub.8 containing an oxide of plutonium, gadolinium or erbium;  (b) mixing said fuel particles with fresh fuel powder of UO.sub.2 or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium to form a sintering powder comprising said fresh fuel powder and at least about 20% by weight of said fuel particles;  (c) mixing said sintering powder uniformly with a sintering aid, which sintering aid comprises an element selected from the group consisting of aluminum, magnesium, niobium, titanium, vanadium, chromium, lithium, silicon, tin and mixtures thereof, to make a product A;  (d) pressing said product A into green pellets of about 40% to about 65% theoretical density; and thereafter  (e) sintering said green pellets at a temperature in the range of about 1500.degree. C. to about 1800.degree. C. in a reducing atmosphere to produce new fuel pellets.  (a) heating said defective fuel pellets at a temperature of 400.degree. C. under flowing air for 4 hours to to oxidize UO.sub.2 to U.sub.3 O.sub.8 in order to make fuel particles of U.sub.3 O.sub.8 ;  (b) passing said fuel particles through a 200 mesh sieve to remove large agglomerates;  (c) mixing said fuel particles with fresh fuel powder of UO.sub.2 to form a sintering powder comprising said fresh fuel powder and at least 20% by weight of said fuel power;  (d) mixing said sintering powder uniformly with 0.3-0.5%, calculated as niobium, by weight of Nb.sub.2 O.sub.5, based on the weight of said sintering powder to make a product A;  (e) pressing said product A into green pellets; and thereafter  (f) sintering said green pellets at a temperature of 1680.degree. C. for 4 hours in hydrogen having a dew point of about 20.degree. C. to produce new fuel pellets.  (a) heating said defective fuel pellets at a temperature in the range of about 300.degree. C. to about 800.degree. C. in an oxidizing gas to oxidize UO.sub.2 to U.sub.3 O.sub.8 in order to make fuel particles of U.sub.3 O.sub.8 or U.sub.3 O.sub.8 containing an oxide of plutonium, gadolinium or erbium;  (b) mixing said fuel particles with fresh fuel powder of UO.sub.2 or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium to form a sintering powder;  (c) mixing said sintering powder uniformly with a sintering aid, which sintering aid comprises an element selected from the group consisting of aluminum, magnesium, niobium, titanium, vanadium, chromium, lithium, silicon, tin and mixtures thereof, to make a product A;  (d) pressing said product A into green pellets of about 40% to about 65% theoretical density; and thereafter  (e) sintering said green pellets at a temperature in the range of about 1500.degree. C. to about 1 800.degree. C. in a reducing atmosphere to produce new fuel pellets. 2. The method of claim 1, wherein said sintering step is carried out for about 1-20 hours. 3. The method of claim 1, wherein said fuel particles are reduced to a lower oxidation state to produce an oxide B before step (b). 4. The method of claim 3, further comprising oxidizing and reducing said oxide B at least once before step (b). 5. The method of claim 1, further comprising screening said fuel particles to remove agglomerates before step (b). 6. The method of claim 5, wherein said screening step is conducted by passing said fuel particles through a 200 mesh sieve. 7. The method of claim 1, wherein said fuel particles are milled before step (b). 8. The method of claim 1, wherein said sintering powder consists of at least 20% by weight of said fuel particles with the remainder being said fresh fuel powder and a grinding sludge. 9. The method of claim 1, wherein said sintering powder is mixed with an amount of said element of about 0.02% to about 2% by weight, based on the weight of said sintering powder. 10. The method of claim 9, wherein said sintering powder is mixed with niobium oxide in an amount, calculated as niobium, of about 0.1% to about 1% by weight, based on the weight of of said sintering powder. 11. The method of claim 9, wherein said sintering powder is mixed with a titanium oxide in an amount, calculated as titanium, of about 0.05% to about 1% by weight, based on the weight of said sintering powder. 12. The method of claim 1, wherein said reducing atmosphere is selected from the group consisting of hydrogen, a mixture of hydrogen and inert gases, a mixture of hydrogen and carbon dioxide, a mixture of hydrogen and carbon monoxide, and mixtures thereof. 13. The method of claim 12, wherein said sintering aid comprises an oxide of aluminum or magnesium and said reducing atmosphere consists of about 5% to about 40% carbon dioxide by volume and the remainder being hydrogen. 14. The method of claim 1, wherein water vapor is added to said reducing atmosphere to control the oxidation potential of said reducing atmosphere. 15. The method of claim 14, wherein water vapor is added to said reducing atmosphere in an amount yielding a reducing atmosphere with a dew point of about 20.degree. C. 16. The method of claim 1, wherein said step (a) comprises heating said defective fuel pellets at a temperature in the range of about 350.degree. C. to about 700.degree. C. in flowing air to make fuel particles of U.sub.3 O.sub.8 or U.sub.3 O.sub.8 containing an oxide of plutonium, gadolinium or erbium. 17. The method of claim 1, wherein the oxidizing gas is selected from the group consisting of air, oxygen, a mixture of air and an inert gas, and a mixture of oxygen and an inert gas. 18. A method for manufacturing new fuel pellets by recycling defective fuel pellets of UO.sub.2, comprising the following steps 19. A method for manufacturing new fuel pellets by recycling defective fuel pellets of UO.sub.2 or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium, consisting essentially of the following steps 20. The method of claim 19, wherein said sintering step is carried out for about 1-20 hours. 21. The method of claim 19, wherein said fuel particles are reduced to a lower oxidation state to produce an oxide B before step (b). 22. The method of claim 21, further consisting essentially of oxidizing and reducing said oxide B at least once before step (b). 23. The method of claim 19, further consisting essentially of screening said fuel particles to remove agglomerates before step (b). 24. The method of claim 19, wherein said fuel particles are milled before step (b). 25. The method of claim 24, wherein said screening step is conducted by passing said fuel particles through a 200 mesh sieve. 26. The method of claim 19, wherein said sintering powder consists of about 10% to about 100% by weight of said fuel particles with the remainder being said fresh fuel powder. 27. The method of claim 19, wherein said sintering powder consists of about 10% to about 100% by weight of said fuel particles with the remainder being said fresh fuel powder and a grinding sludge. 28. The method of claim 19, wherein said sintering powder is mixed with an amount of said element of about 0.02% to about 2% by weight, based on the weight of said sintering powder. 29. The method of claim 28, wherein said sintering powder is mixed with niobium oxide in an amount, calculated as niobium, of about 0.1% to about 1% by weight, based on the weight of of said sintering powder. 30. The method of claim 28, wherein said sintering powder is mixed with a titanium oxide in an amount, calculated as titanium, of about 0.05% to about 1% by weight, based on the weight of said sintering powder. 31. The method of claim 19, wherein said reducing atmosphere is selected from the group consisting of hydrogen, a mixture of hydrogen and inert gases, a mixture of hydrogen and carbon dioxide, a mixture of hydrogen and carbon monoxide, and mixtures thereof. 32. The method of claim 31, wherein said sintering aid comprises an oxide of aluminum or magnesium and said reducing atmosphere consists of about 5% to about 40% carbon dioxide by volume and the remainder being hydrogen. 33. The method of claim 19, wherein water vapor is added to said reducing atmosphere to control the oxidation potential of said reducing atmosphere. 34. The method of claim 33, wherein water vapor is added to said reducing atmosphere in an amount yielding a reducing atmosphere with a dew point of about 20.degree. C. 35. The method of claim 19, wherein said step (a) comprises heating said defective fuel pellets at a temperature in the range of about 350.degree. C. to about 700.degree. C. in flowing air to make fuel particles of U.sub.3 O.sub.8 or U.sub.3 O.sub.8 containing an oxide of plutonium, gadolinium or erbium. 36. The method of claim 19, wherein the oxidizing gas is selected from the group consisting of air, oxygen, a mixture of air and an inert gas, and a mixture of oxygen and an inert gas.