Patent Number: 048287601
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT The present invention provides a method of removing sodium from fuel assemblies which is uniquely suited for fuel assemblies removed from a breeder reactor. In a breeder reactor, a coolant, typically sodium, potassium or a mixture thereof, is circulated through a reactor core wherein it is heated and subsequently the heat is extracted from the coolant. The reactor core comprises an array of fuel assemblies provided with passageways for the circulation of the coolant therethrough. Each fuel assembly 14 is comprised of a plurality of elongated pressurized metallic fuel pins. The fuel pin typically is a stainless steel cylinder or tube which is sealed at each end and contains, throughout a substantialy portion of its length, fuel pellets. The outer metal portion of the pin generally is referred to as the cladding. Generally, the fuel pellet is formed from an oxide or carbide of uranium and/or plutonium, some of the uranium may be converted to plutonium during service as a result of exposure to fast neutrons. As a result of the exposure to neutrons a high pressure fission gas is generated in each fuel pin of the fuel assembly. Because of the high pressure fission gas, the heating and cooling parameters, explained in more detail herein below, must be carefully controlled to preclude rupturing of the fuel pins during the decontamination or cleaning process. During service, it is not uncommon for an individual fuel pin to crack or rupture such that the alkali metal coolant seeps within the metal tube. This, of cource, complicates cleaning. To reprocess the fuel from a breeder reactor, it is essential that all of the alkali metal be removed as it will have a detrimental effect on the subsequent chemical reprocessing. In addition, during service, the alkali metal becomes radioactive, which is not removed could complicate the handling and shipment of the fuel assemblies. Generally, breeder reactor facilities include an adjacent fuel handling building or facility (also called a "cell") which is maintained under an inert atmosphere, typically argon gas. New fuel assemblies for loading into the reactor was well as spent fuel assemblies removed from the reactor, are temporarily stored in such a facility. It is an advantage of the present invention that it is particularly suited for use in such an enviroment. FIG. 1 shows in part a fuel storage building housing a fuel handling system for a sodium cooled breeder reactor. The spent fuel assembly is removed from the reactor into a remotely operated, inert gas filled, fuel handling cell or facility. In the cell, the fuel assembly is moved to the spent fuel cleaning device wherein the adhering alkali metal is removed from the fuel assembly outer surface. The cleaned spent fuel assembly is then transferred, via a gas lock or under floor transporter, to another remotely operated fuel handling cell or facility. The second cell may have an inert atmosphere or be air-filled, The spent fuel assembly may be stored under water to cool the fuel assembly or may be loaded into a spent fuel shipping cask for transportation to a reprocessor. Referring now to FIG. 2, therein is depicted apparatus 10 for use in practicing the method of the present invention. The apparatus includes a sealed chamber 12 for containing a spent fuel assembly 14. Fuel assembly 14 rests on a baffle member 16 and extends therethrough. Baffle member 16 engages the outer periphery of fuel assembly 14 to insure that those gases entering an upper portion 18 of chamber 12 must flow through fuel assembly 14 and into a lower portion 20 of chamber 12. Lower portion 20 of chamber 12 is provided with a recirculation outlet conduit 24 which is in fluid communication with a blower 26 which discharges into a recirculation inlet conduit 28 through a three-way valve 55 which conduit 28 is located above baffle member 16. and communicates into chamber 12 as shown in FIG. 2. Advantageously, conduit 28 also is provided with an electrical heater 30. Blower 26 is driven by a motor 32 which is interconnected to blower 26 via a magnetic coupling 34. Typically there also will be provided some means for preventing the transfer of heat from blower 26 back to magnetic coupling 34. As depicted, this would be accomplished by a cooling jacket 36 provided with an inlet and outlet for the flow of a cooling fluid therethrough. Chamber 12 also includes a conduit 38 and valve 40 for the introduction of an inert gas into chamber 12 in upper portion 18. Any inert gas may be used, typically the inert gas will be argon, particularly when the method of the present invention is practiced within a fuel storage or handling cell is maintained under an inert atmosphere of argon. An upper end of chamber 12 is provided with a discharge conduit 42 for conducting gas exiting upper portion 18 of chamber 12 to a condenser 44. Condenser 44 includes means for passing a coolant through an internal cooling coil 46 to condense any sodium vapors contained in the gas passing therethrough. Generally the coolant will be an organic fluid which is inert with respect to the alkali metal to prevent any reaction in the event of a leak. Condenser 44 further includes a sump portion 48 for the collection of condensed alkali metal coolant. A conduit 50 provides fluid communication between condenser 48 and a cryogenic trap 52 and also a bypass conduit 54 which connects to the three-way valve 55. Downstream of cryogenic trap 52 are two vaccum pumps 56 and 58. Pumps 56 and 58 are in fluid communication with cryogenic trap 52 via conduits and valves 60,61 and 62. Pump 58 is also provided with a discharge conduit 64. In accordance with the practice of the method of the present invention, a fuel assembly 14 is placed within chamber 12 which is then sealed. An inert gas, typically argon, is introduced into chamber 12 through conduit 38 and valve 40. Generally, spent fuel assembly 14 will have an initial temperature of about 400.degree. F. (204.degree. C.). Power is supplied to motor 32 which drives blower 26 via magnetic coupling 34 to cause circulation of the argon from lower portion 20 of chamber 12 through conduit 24 and valve 55 and back to upper portion 18 of chamber 12 via conduit 28. Power also is supplied to electric heater 30 until the temperature of the fuel assembly is increased to about 800.degree. F. After the fuel assembly has been heated to a desired temperature, power to the electric heater is turned off, valve 61 is opened and vacuum pump 58 started. Typically, vacuum pump 58 will be a dry, reciprocating vacuum pump which is operated for a sufficient time to decrease the chamber pressure from atmospheric to approximately 10 mm of mercury, during which time blower 26 is maintained in operation. Thereafter, secondary vacuum pump 56 (typically an oil-sealed rotary pump) is started. Valves 60 and 62 are opened and 61 closed. The chamber pressure is then further decreased from 10 mm of mercury to at least 0.05 mm of mercury. Preferably the secondary vacuum pump 56 is operated until the pressure within chamber 12 is reduced to 0.005 mm of mercury of less. During this time, blower 26 is inoperative. During the vacuum drying time, the gas and entrained sodium vapor is withdrawn via conduit 42 and cooled in condenser 44. Any residual sodium vapor leaving through conduit 50 is removed in cryogenic trap 52. The condensed sodium may be recovered at a later point in time. Typically, this would be accomplished in condenser 44, for example, by increasing the coolant temperature to melt the sodium and then draining it from sump 48. During the vacuum drying time, the fuel assembly temperature will continue to increase as a result of the decay heat. When the temperature reaches the maximum safe temperature for the cladding of the individual fuel pins, generally about 1000.degree. F., the vacuum treatment is stopped. Valves 60, 61 and 62 are closed and pumps 56 and 58 turned off. Valve 40 is open and chamber 12 is filled with argon gas to one atmosphere via conduit 38. After chamber 12 is filled with gas, valve 40 is closed and valve 55 repositioned to close conduit 28 and open conduit 54. Power is supplied to motor 32 to drive blower 26 and the argon gas is circulated through valve 55 and conduit 54 in a reverse direction through condenser 44 and back to chamber 12 via conduit 42. Typically, heater 30 would not be required for the remainder of the cleaning cycle backpressure created by the presence of heater 30 in conduit 28. The gas is circulated through chamber 12 and fuel assembly 14 until the temperature of the fuel assembly is reduced back to a desired level, typically below 800.degree. F. The vacuum treatment and cooling are repeated as required to insure substantially complete removal of the radioactive alkali metal contaminant. The number of cycles required is readily determinable through experimentation. It will be appreciated that various other valves and instrumentation such as pressure sensors, temperature sensors, also would normally be incorporated as well as additional redundant gas cleaning techniques. However, those matters are well within the skill of those versed in the art. The foregoing description and example illustrate a specific embodiment of the invention and what is now considered to be the best mode of practicing it. Those skilled in the art, however, will understand that changes may be made in the form of the invention without departing from its generally broad scope. Accordingly, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as is specifically illustrated and described.