Abstract:
This invention collects and analyzes condensable gases using a cryogenic cooling device connected to a charcoal cold trap assembly. More particularly, it uses a removable cryogenic cooling device instead of liquid nitrogen to trap condensable gases. The invention has four major components: (1) a removable cryogenic cooling device (2) a vacuum canister, (3) a cold trap canister filled with activated charcoal, and (4) a cover assembly with welded inlet/outlet tubes and an o-ring seal between the cover assembly and the vacuum canister.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention collects and analyzes condensable gases using a cryogenic cooling device connected with a charcoal cold trap assembly. More particularly, it uses a removable cryogenic cooling device instead of liquid nitrogen to trap condensable gases. 
     2. Background and Description of Related Art 
     When irradiated fuel samples are annealed at high temperatures, it is necessary to obtain a quantitative evaluation of the radioactive fission gas that is released. A cold trap can be used to condense, collect, and analyze this gas, in which the hot gas is transported from the high temperature area by a carrier or purge gas to a cold trap assembly filled with a high-surface-area substance, preferably activated charcoal. In the prior art, the cold trap is cooled with liquid nitrogen. The cooled charcoal condenses and “traps” the hot, radioactive gas. A radiation detector, in close proximity to the trap monitors when and how much radioactive condensable gas has been trapped. A thermocouple may be used to monitor the temperature of the hot gases. A radiation source may be used to calibrate the radiation detector. A vacuum canister insulates the cold trap. After analyses are complete, the charcoal then must be heated to drive off condensed products from the charcoal. The trap may be heated by flowing warm gas, such as air, through the air inlet/outlet tube. In a particularly preferred embodiment, a thin, flexible, electric heating element is inserted in the air inlet/outlet tube to heat the cold trap canister. 
     Cooling with liquid nitrogen can be problematic. The lines are susceptible to freezing, thus limiting the flow of the liquid. This can cause the cold trap to heat up and evaporate or not condense and trap the gas, leading to lost data. The venting of nitrogen in confined, inhabited spaces is also a concern. Additionally, the liquid nitrogen cooled traps usually use a great amount of liquid nitrogen, making maintaining an adequate supply a problem. 
     Most cold traps are a single welded assembly. Therefore, the entire cold trap assembly must be treated as radioactive material for disposal, which can be very costly. The design of the present invention reduces the components exposed to radioactivity, thus minimizing the amount of material that must be treated as radioactive waste. 
     It is an object of the present invention to cool the cold trap with a removable cryogenic cooling device that does not become radioactively contaminated under normal operating circumstances. 
     It is another object of the present invention to use a cryogenic cooling device, rather than liquid nitrogen, to cool the cold trap. 
     It is yet another object of the present invention to limit the exposure to radiation to the cold trap, so that the vacuum canister, and removable cryogenic cooling device may be reused. 
     SUMMARY OF THE INVENTION 
     The present invention uses a cold trap that is cooled by an off-the-shelf, self-contained cryogenic cooling device, instead of liquid nitrogen. The trap has four major components: (1) a removable cryogenic cooling device (2) a vacuum canister, (3) a cold trap canister filled with activated charcoal, and (4) a cover assembly with welded inlet/outlet tubes and an o-ring seal between the cover assembly and the vacuum canister. 
     The removable cryogenic cooling device is a closed cooling system capable of keeping the cold trap canister cool enough to condense the hot gas, near liquid nitrogen temperatures, around −150° C. In a particularly preferred embodiment, an integral portable compressor is connected to the cryogenic cooling device with a flexible metal hose. The cryogenic cooling device is removable and is secured to the bellows with standard vacuum fittings. Because the cold trap canister is a sealed assembly, the cryogenic cooling device does not become radioactively contaminated under normal operating conditions, and can be removed and reused for other analyses. 
     The cover assembly has a flexible vacuum bellows welded to it, to which the cryogenic cooling device is attached using standard vacuum connections. Stainless steel tubes are attached to the cover assembly for purge gas inlet and outlets, a heated air inlet/outlet, a thermocouple, and a vacuum port. 
     The cold trap canister portion of the invention comprises a cylindrical canister with gas diverters radiating from its inner surface. The canister can be made from any conductive metal that does not react with the process gases and does not affect the seal integrity of the welded tubes. In a particularly preferred embodiment, the canister is made of copper and is filled with activated charcoal. The canister also has inlet and outlet tubes for purge gas flow. 
     The vacuum canister can be made from any durable material that can withstand a vacuum. In a particularly preferred embodiment, the vacuum canister is a modified stainless steel beaker. A cover assembly is bolted to the vacuum canister with an o-ring seal between the cover assembly and vacuum canister. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of the vacuum canister, cold trap canister, and cover assembly, showing the relationship of all the components. 
     FIG. 2 is an assembly drawing of the vacuum canister, cold trap canister, and cover assembly. 
     FIG. 3 is a drawing of the internals of the cold trap canister. 
     FIG. 4 is a system drawing showing the cryogenic cooling device, radiation detector, compressor, vacuum pump, and thermocouple. 
     FIG. 5 is a drawing of the internals of the heated air inlet/outlet tube. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention, a cold trap apparatus, collects condensable hot gas products for analysis. The preferred embodiment is adapted to condense, collect, and analyze hot radioactive gases that evolve from annealing fuel at high temperatures, however, the invention could be used to condense, collect, and analyze any hot, condensable gases. A purge gas carries the fission gas products from the furnace  80  and flows through a purge gas inlet tube  7  to a cold trap canister  5  filled with activated charcoal  17 . See FIG.  4 . The cold trap canister  5  is preferably made of copper and has a plurality of copper cooling gas diverters  3  that are soldered or welded to either the top or bottom cover of the cold trap canister  5  such that they radiate from the inner surface of the cold trap canister  5 . See FIGS. 1 and 3. In a particularly preferred embodiment, three cooling gas diverters  3  are used. The cold trap canister  5  and cooling gas diverters  3  provide optimum heat transfer between the charcoal  17  within the cold trap canister  5  and the environment within the vacuum canister  1 . The cold trap canister  5  also contains screens  18  at the purge gas inlet  7  and outlet  8  tubes to keep the charcoal  17  from escaping from the cold trap canister  5 . See FIG.  3 . 
     Stainless steel tubes, used for purge gas inlet  7  and outlet  8 , are welded to opposite sides of the cold trap canister  5 . See FIG.  3 . The purge gas inlet tube  7  is kept as low as possible to allow for ease of detection by the radiation detector  40  of any fission gas condensation on the cold portion of the purge gas inlet tube  7 . In a particularly preferred embodiment, the radiation detector  40  is a sodium iodide radiation detector and is located in close proximity to the bottom of the cold trap canister  5 . The cold plate  52  of the cryogenic cooling device  50  makes good operable contact with the malleable metal conductor  30  of the cold trap canister  5  due to the force of the bellows  15  caused by the vacuum in the vacuum canister  1 . In a particularly preferred embodiment, the malleable metal conductor  30  is made from indium. During operation, the vacuum inside the vacuum canister  1  provides a force to collapse the bellows  15  and force the cold plate  52  of the cryogenic cooling device  50  into contact with the malleable metal conductor  30  of the cold trap canister  5 . This arrangement allows for maximum heat transfer between the cryogenic cooling device  50  and the cold trap canister  5 . 
     A thermocouple tube  9 , sealed at the bottom  16 , is welded to the cover assembly  20  and is placed near the cold trap canister  5  in the vacuum canister  1 . See FIG.  4 . The thermocouple tube  9  is open at the top for insertion of a thermocouple  70 , which monitors the temperature within the cold trap canister  5 . Alternatively, a radioactive source may be inserted into the thermocouple tube  9  to calibrate the radiation detector  40 . The heated air inlet/outlet tube  10  is positioned such that a portion of the tube  10  is in contact with the outer surface of the cold trap canister  5 , and is welded to the cold trap canister cover  6 . See FIG.  1 . In a particularly preferred embodiment, the heated air inlet/outlet tube  10  partially encircles the outer surface of the cold trap canister  5 . This heated air inlet/outlet tube  10  contains an electric heating element  24  and passes heated air to drive off the condensed gas when the analyses are completed, or to vary the cool down, heat up, or soak temperature by varying the flow of a liquid or gas through the line. See FIG.  5 . 
     The cover assembly  20  contains heated air inlet/outlet tubes  10 , purge gas inlet  7  and outlet  8  tubes, a thermocouple tube  9 , and the vacuum port  19  all of which are seal welded to the cover  13 . The bellows  15  is also welded to the cover  13 . The cryogenic cooling device  50  will vacuum seal against the bellows  15  during operation when a vacuum is applied to the vacuum canister  1  using vacuum pump  90 . A contact between the cold plate  52  of the cryogenic cooling device  50  and the malleable metal conductor  30  of the cold trap canister  5  is further magnified when vacuum is applied, pulling the cryogenic cooling device downward by collapsing the bellows  15 . 
     In a preferred embodiment, the cryogenic cooling device  50  is a Cryotiger Tigertail available from APD Cryogenics, Inc. The cryogenic cooling device  50  is a self-contained refrigeration system that cools the charcoal cold trap to about −150° C. See FIG.  4 . The cryogenic cooling device  50  is attached to a compressor  60 . In a particularly preferred embodiment, the cryogenic cooling device  50  is attached to the compressor  60  by flexible steel lines  21 . The cryogenic cooling device  50  can be removed from the apparatus by removing a standard vacuum clamp  23 . See FIG.  4 . Because the radioactive portions of the cold trap apparatus (i.e., the inlet  7  and outlet  8  purge gas tubes and the cold trap canister  5 ) constitute a sealed system, the cryogenic cooling device  50  should not become contaminated during normal operating conditions, and can be easily removed, reused, or replaced. 
     A vacuum canister  1  is sealed to the cover assembly  20  by an o-ring  12  and hold down bolts  14 . See FIGS. 1 and 2. The vacuum canister maintains a vacuum and provides insulation around the cold trap canister  5 . The vacuum canister  1  is preferably a standard 1200 cc stainless steel beaker. An o-ring  12  and hold down bolt flange  11  are seal welded to the top of the vacuum canister  1 , which is bolted to the cover  13  of the cover assembly  20 . See FIGS. 1 and 2. 
     While this invention has been described as having preferred ranges, materials, and designs, it is understood that it is capable of further modifications, uses and adaptations of the invention following in general the principle of the invention, and including such departures from the present disclosure, as those come within the known or customary practice in the art to which the invention pertains and as may be applied to the central features set forth herein, and fall within the scope of the invention and of the appended claims. It is further understood that the present invention is not limited to the claims appended hereto.