Patent Number: 048287601
Section: claims

1. A method of cleaning a fuel assembly including surfaces thereof prior to decladding, each assembly surface contaminated with a radioactive alkali metal and comprising a plurality of pressurized metallic fuel pins containing a spent fissible material, the method comprising the sequential steps of: (a) placing the fuel assembly in a sealed chamber;  (b) passing a heated, inert gas through the chamber to heat the fuel assembly to a temperature sufficient to cause volatilization of the alkali metal but insufficient to rupture the pressurized metal pins;  (c) evacuating the chamber to a pressure of less than 0.5 mm of Hg to further enhance volatilization and removal of the alkali metal and maintaining the chamber at that pressure until the decay heat of the fissile materials causes the temperature of the fuel assembly to increase to a level which would be detrimental to the integrity of the metal pins;  (d) cooling the fuel assembly by passing a cool, inert gas through the chamber to reduce the temperature of the fuel assembly to a desired level;  (e) repeating the evacuation and cooling steps as required to insure removal of substantially all of the radioactive alkali metal from the assembly surface; and  (f) recovering the cleaned fuel assembly from the chamber.  (a) placing the fuel assembly in a sealed chamber,  (b) passing a heated inert argon gas through the chamber to heat the fuel assembly to a temperature of about 800.degree. F.,  (c) evacuating the chamber to a pressure of less than about 0.05 mm of mercury and maintaining the fuel assembly and chamber at that pressure until the temperature of the fuel assembly is about 1000.degree. F.,  (d) passing a cool inert argon gas through the chamber and the fuel assembles to reduce the temperature of the fuel assembly to about 800.degree. F.,  (e) repeating steps c and d as required to ensure removal of substantially all of the radioactive alkali metal, and  (f) recovering the decontaminated fuel assembly from the chamber.  (a) placing the fuel assembly in a sealed chamber;  (b) passing a heated inert argon gas through the chamber to heat the fuel assembly to a temperature of about 800.degree. F.;  (c) evacuating the chamber to a pressure of less than about 0.05 mm of mercury and maintaining the fuel assembly and chamber at that pressure until the temperature of the fuel assembly is about 1000.degree. F.;  (d) passing a cool inert argon gas through the chamber and the interior of the fuel assembly to reduce the temperature of the fuel assembly to about 800.degree. F.;  (e) repeating steps c) and d) as required to insure removal of substantially all of the radioactive alkali metal from the assembly surface; and  (f) recovering the cleaned fuel assembly from the chamber. 2. A method of claim 1 wherein said alkali metal is sodium. 3. A method of claim 1 wherein said alkali metal is a mixture of sodium and potassium. 4. The method of claim 1 wherein in step c) the chamber is evacuated to a pressure of less than about 0.005 mm of mercury. 5. The method of claim 1 wherein the fuel assembly free of contaminants is processed for the recovery of fissile material therefrom. 6. The method of claim 1 in which gas exiting the sealed chamber from steps c) and d) is passed in indirect heat exchange relationship with a coolant for the condensation and removal of any vaporized alkali metal therefrom. 7. The method of claim 6 wherein the gas from step c) is further conducted through a cryogenic trap to insure substantially complete removal of any remaining radioactive alkali metal from the gas. 8. A method of decontaminating a fuel assembly contaminated with a radioactive alkali metal, each fuel assembly comprising a plurality of metal pins containing a spent fissile material selected from a group consisting of carbides and oxides the method comprises the sequential steps of: 9. A method of cleaning a fuel assembly including surfaces thereof prior to decladding, each assembly surface contaminated with a radioactive alkali metal and comprising a plurality of pressurized metallic fuel pins containing a spent fissible material selected from the group consisting of carbides and oxides, the method comprising the sequential steps of: 10. The method of claim 9 in which gas exiting from the sealed chamber in steps c) and d) is passed in indirect heat exchange relatipnship with a coolant for the condensation and removal of vaporized alkali metal therefrom. 11. The method of claim 10 wherein the gas from step c) is further conducted through a cryogenic trap to insure substantially complete removal of any remaining radioactive alkali metal from the gas. 12. The method of claim 11 wherein said alkali metal is sodium. 13. The method of claim 11 wherein said alkali metal is a mixture of sodium and potassium. 14. The method of claim 11 wherein in step c) the chamber is evacuated to a pressure of less than 0.005 mm of mercury. 15. The method of claim 14 wherein the fuel assembly which is free of contaminants is processed for the recovery of fissile material therefrom.