Patent Number: 052981967
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the extraction of tritium and preparation of radioactive waste for disposal. More particularly, the present invention relates to tritium recovery during the process of heating and melting irradiated nuclear targets. 2. Discussion of Background Tritium is a radioactive isotope of hydrogen. Because it is frequently found in the form of a diatomic gas or bound in a water molecule, it is relatively mobile and difficult to recover and contain once it has escaped into the environment. Preventing the escape of tritium is an important priority in the operation of facilities where tritium is created as a product or a by-product. Tritium is created when lithium metal in target bundles is irradiated in a nuclear reactor. Nuclear reactor targets, after use, are referred to as "spent". Spent target bundles must be stored in carefully constructed disposal cells for a few hundred to thousands of years depending on the length of time the radioisotopes remain significantly radioactive and on government requirements. The volume required for long-term storage of radioactive waste should be minimized because of cost and other considerations. Using heat to drive tritium from nuclear targets for recovery is well known. See for example both U.S. Pat. No. 4,532,102 and 3,100,184 issued to Cawley and Abrahams, respectively. A prior art furnace, crucible and target bundle is illustrated in FIG. 1. A target bundle after heating is illustrated in its crucible in FIG. 2. A target bundle 20 is placed in a crucible 22 that holds it and encloses the bundle laterally, the top of the crucible is covered with a perforated plug 24 and has an integral hook 26. Crucible 22 is lowered into a retort 34 by a lid 36 that seals against a top flange 38 of retort 34. Retort 34 is positioned in a furnace 50 having electric resistance heaters 52 inside its wall 54. An inlet 56 and an exit 58 allow the circulation of coolant to prevent the shell 60 of the furnace from overheating. An off-gas pipe 62 allows tritium to be drawn from the interior of retort 34. After heating, bundle 20 occupies a much smaller volume than before heating, as depicted in FIG. 2 where a melted bundle 70 is shown in a crucible 72. The melted bundle, slumped to the bottom of the crucible, is disposed of still in the crucible. Although gases from the furnace are drawn off during heating, opening the furnace causes some tritium to escape to the environment. The current furnace, therefore, is unsatisfactory because it allows too much tritium to escape. Furthermore, the melted bundle comprises only about one tenth of the volume of the crucible. Thus, a substantial fraction of the interior of the crucible, as sent to a waste repository, is empty space. SUMMARY OF THE INVENTION According to its major aspects and broadly stated, the present invention is a crucible and sleeve assembly for holding and laterally enclosing an object, such as a target bundle, during transport to and from an inverted furnace where the object is heated to extract recoverable gases. When the object is heated, it gives off gases and vapors of radioactive materials that are drawn from the furnace interior through a trap filled with metal shavings. The hot vapors of radioactive materials condense on the shavings. Gases passing through the trap are collected in tanks downstream as the product. After the object is heated, it occupies less volume than before it is heated. The crucible is sized to hold and laterally enclose the object after it is heated so that, when a lid is sealed to it, the object is fully enclosed and can be transferred to a disposal or storage facility, the sealed crucible have substantially no empty space within. The crucible and sleeve assembly, on the other hand, is dimensioned to hold and laterally enclose the object before it is heated. Several components of the invention individually and in cooperation act to prevent the loss of gases to the environment. The furnace is inverted so that gases lighter than air tend to remain in the furnace when it is opened to remove the object. The portion of the furnace that includes the opening is housed within a containment. Airlocks prevent contamination of the atmosphere of the containment when the object is introduced into or taken from the containment. Finally, strippers remove gases from the containment atmosphere that have escaped from the inverted furnace. The crucible and sleeve assembly is an important feature of the present invention. The two cooperate to securely hold and laterally enclose the object before it is heated. Although a single container can serve the same function, being able to separate the sleeve from the crucible after the object has been heated and its volume is reduced to the preselected dimension of the crucible alone results in a substantial reduction in the volume of the material actually disposed of and enables the reuse of the sleeve. The crucible and sleeve fit together easily and securely so that full lateral support without leakage is provided. Both have flanges that engage each other, and the sleeve has a skirt that depends from the flange into the crucible. The inverted furnace is another important feature of the present invention. By inverting the furnace, the prevention of the escape of the gases given off as a result of heating, such as tritium when a nuclear target bundle is heated, is substantially simplified. The trap of the present invention is another important feature. The trap contains metal shavings on which the radioactive vapors condense, thus simplifying recovery of radioactive vapors that should not be vented to the atmosphere. Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of a Preferred Embodiment presented below and accompanied by the drawings.