Patent Number: 043200281
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A cable in accordance with the present invention is shown in FIG. 1 and is indicated generally by the reference numeral 11, said cable being composed of fibers of a vitreous material such as borosilicate glass, the composition of said vitreous material including radioactive waste. The fibers are sufficiently thin and the number of fibers in said cable is such that said cable can be wound on a support. The diameter of the fibers may vary widely, depending upon the number in said cable and the curvature of the support on which said cable is to be mounted. However, the average diameter of said fibers may approximate 100 to 200 micrometers. Cable 11 has a central radioactive section 12 that may be up to several kilometers long. The rearward end 13 and the forward end 14 of central section 12 merge into non-radioactive leaders 15 and 16 at the boundaries indicated, the fibers in said cable extending from one end of the cable to the other. Fibers 17, as shown in FIG. 2, may be braided, woven or otherwise bundled together to form said cable 11. The leader sections of the fibers are formed by drawing first a length of ordinary glass, then without break in continuity the long section consisting of a mix of glass and radioactive waste, and finally once more a length containing glass alone. When these fibers are cabled together, the resulting cable is thus provided with nonradioactive leaders at both ends. Cable 11 may be color-coded by incorporation of suitable pigments in the composition or by coating one or more of the fibers with a vitreous layer 18 incorporating such a pigment. Cable 11 may be of any convenient shape in cross-section but a generally flat configuration, so that the cable is belt-like, is preferred both for improvement of heat-transfer characteristics thereof and for flexibility needed for winding said cable on a support. The belt-like configuration is shown in FIG. 2. While the cable 11 need have no covering thereover, an envelope 19 of a flexible material such as polytetrafluoroethylene may be provided, preferably in mesh net form for ventilation, said envelope serving to retain fragments of fiber that may be produced during the processing and transport of the cable. In view of the great hazards involved in the manufacture, transport and storage of the cable, it is extremely important that means be provided for monitoring the condition of the cable, the continuity, freedom from cracks and temperature of the cable being particularly important features. In addition, in view of the well-recognized danger of theft of radioactive materials by terrorists, for instance, it is essential that the actual presence and location of the cable be monitored. Such monitoring means are exemplified by fiber optics 21, which are arranged to form a complete loop so that a light pulse can be sent into the loop for monitoring the return thereof. By suitable arrangement, such monitoring means can be adapted to provide indication of the location of a break in the cable should such a break occur. The presence of multiple microscopic cracks in the monitor fiber, symptomatic of a similar condition in the waste-carrying fibers, would be signaled by increased optical attenuation. Also, it is essential that the temperature of the cable not exceed specified limits. For this purpose, temperature-monitoring means such as a cell of thermosensitive dye or liquid crystal 22 may be provided as an adjunct to the cable 11. Such a cell would modulate the light pulses traversing the optical-fiber circuit with information on the temperature at the cell's location. Conveniently, both the temperature-monitoring and integrity-monitoring means 22 and 21 may be incorporated in the envelope 19 so that they constitute part of the cable 11; placement of said monitoring fibers at the extremities of the cable cross-section minimizes radiation exposure. A plant for the manufacture, transport and storage of the cable in accordance with the present invention is shown in FIG. 3. In general, such a plant will be located proximate a source of radioactive waste material, such as an operating nuclear power reactor, a reprocessing plant or an interim radioactive-waste storage in the manufacture site. The plant of the present invention, shown in schematic form in FIG. 3, as aforenoted, has associated therewith a source 23 of radioactive waste products from which the waste materials are taken to a blender 24 to be mixed with components for preparing a vitreous composition. Needless to say, all operations with radioactive substances carried out in the plant of the present invention are carried out by machine under remote control, so that exposure of personnel to radiation is avoided completely. However, because the initial connections are to the nonradioactive leader, these may be made manually and unnecessary complexity avoided. From the blender, the fiber constituents are transferred to melting tank 26 and the melt is then drawn into fibers in the fiber-fabrication stage indicated by the reference numeral 27. The fibers may be produced by extrusion or by drawing or by a combination thereof. A large number of fibers are drawn simultaneously. At first, valve 26a in FIG. 3 will be open, and fibers of ordinary glass will be drawn to the length desired for the initial leader section. Valve 26b is then opened and valve 26a is closed, causing the fiber output to become radioactive while the fiber remains continuous. When the radioactive section of each fiber has attained the desired length, valve 26a is again opened and 26b closed, and the final leader is drawn. The fibers are slowly passed through a water bath to provide for decay of most of the short-term radioactivity. The individual fibers are then bundled into a cable at cable fabricator stage 28. It may not always be possible to match the speed of duct transport to the output rate of the fabricator. Short-term buffer storage is therefore provided within the fabrication portion of the plant, said fabrication portion being indicated generally by the reference numeral 29. The short-term buffer storage is indicated by the reference numeral 31. The cable of the present invention is intended for long-term storage, particularly for underground storage. A suitable location for such underground storage may not be immediately adjacent the source of nuclear waste, thereby introducing the problem of transporting the cable between fabrication plant 29 and the underground storage facility indicated generally by the reference numeral 32. Transport of radioactive products, and particularly waste products, is beset with serious problems arising from the fact that the radioactivity produced and emanating from such products is extremely dangerous to the biosphere and, particularly, to humans. Recently a number of governmental entities have passed laws prohibiting or restricting the transport of such products through the corresponding geographical areas. The form of the cable of the present invention provides a particularly advantageous means of coping with this problem. A shielded conduit 33 is provided between the fabrication plant 29 and a wellhead 34 located over storage region 32. The retrievability of the waste in this system makes the geologic stability of the formation far less critical than for disposal by billets, which are irretrievable. For this reason it can be expected that a suitable site will be found within a few tens of kilometers of practically every source of radioactive waste. The prefered shielding is earth 36, conduit 33 being disposed deeply enough within the earth so that the amount of radiation penetrating the shielding is considered safe on the basis of traffic and land use thereabove, about 4-5 m, as indicated above, being sufficient. It is envisioned that conduit 33 will be located in general within government installations, on former railroad rights of way or within other regions wherein traffic above the conduit can be controlled so as to further ensure that exposure to the radiation is held within safe limits. Conduit 33, as shown in FIG. 2, is fitted with devices such as roller 37 to support cable 11 for low-friction transport of said cable through conduit 33. The cable is sufficiently flexible so that it can be transported around bends as shown in FIG. 3. Also, since the cable 11 will in general be evolving substantial quantities of heat, cooling air may be circulated through said conduit, a source of cooling air or other coolant being indicated as entering conduit 33 through pipe 38 and exiting through pipe 38a. In the arrangement shown in FIG. 3, the coolant introduced through pipe 38 also serves to cool cable 33 during buffer storage at stages 39 and 31. The storage chambers are designed to make maximal use of air convection currents driven by the temperature gradients present to transfer heat to the chamber walls. Cable 11 is lowered through vertical well 41 into storage chamber 32, where support devices 43, preferably in the form of conical reels, and automatic winder means 44 are provided for emplacing the cable 11 and the front cable leader 14 on the support 43. A single conduit and waste-receiving facility could serve a cluster of several repositories. As aforenoted, the length of the period for which the cable must be stored is so great that the integrity of chamber 32 may be threatened or even breached by unforeseen thermal, geological or hydrologic processes despite the initial apparent safety thereof. Accordingly, therefore, means for retrieval of cable 11 from storage chamber 32 must be provided; buffer storage stage 39 is provided for this purpose; rear cable leader 15 is secured at the well-head and constructed so that it can retrieve the contents of chamber 32. The well-head chamber and conduit provide sufficient space so that, should retrieval for purposes of safety become necessary, they can together accommodate the entire contents of chamber 32 for a period of time sufficient so that another storage place can be developed. Retrieval may be necessary or desirable for other purposes, such as harvesting of isotopes present in the nuclear waste. Such harvested radioisotopes can be useful in energy generation, medical treatment and diagnosis, and for other possible applications. An example of a large-scale application being investigated is the treatment of sewage by exposure to gamma rays from cesium 137. Means for such harvesting are indicated by stage 44 within the fabrication plant 29. For this purpose, it is necessary that conduit 33 be constructed for two-way transfer of the cable. The paths in the retrieval phase are indicated by dashed lines. After harvesting of the desired isotope, the remaining components may be recycled to the blender 24, for fabrication again into fiber cable. Another reason for retrieving the cable may be to take advantage of the fact that the radioactivity thereof has decreased to a level such that it is no longer economical to devote storage space to same. Under such circumstances, the cable can be recycled to blender 24 where it is recombined with additional radioactive waste from source 23 and then sent through the fabrication process once more. Alternatively, the inert content of the fiber can be decreased, preferably using harvesting stage 44 for this purpose. A major advantage of the cable, the process for fabricating same and the storage chamber comprising the plant is that it can be monitored closely, as indicated by monitoring station 46, to make certain of the integrity of the cable, the temperature thereof, and the temperature of the storage space 32. This station or console is the terminal for all monitor-fiber pairs included in a cable. There, light pulses from light-emitting diodes or lasers are transmitted into the cable and, if the integrity of the cable has not been breached, received back from the cable. The temperature-sensing fibers pass into and out of detector cells consisting of optically thermoactive materials, either dyes or liquid crystals, which modulate the light according to the temperature at their location. Cracking or the development of other imperfections in the glass, symptomatic of possible cable damage, can be observed from the resulting light attenuation. Furthermore, on the basis of information supplied by monitor 46 to remote control means 47, the operation of the fabrication plant 29, supply of coolant to said plant to well-head 34 and storage space 32 can be adjusted and decision as to possible retrieval of cable can be made on a fail-safe basis. As is evident from the above, the process, plant and cable of the present invention make it possible to incorporate radioactive waste in a form such that the waste is transportable without hazard either to the environment or the inhabitants thereof, and to store the waste under conditions such that anticipated and unanticipated changes can be coped with. The techniques for incorporating the waste into glass are wellknown and the knowledge and skills involved in their processing into fibers and cables are well understood. The process is adaptable so that valuable isotopes can be retrieved from the cable and, if desired, the heat evolved by the waste products during storage can be utilized. It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above process, in the described product, and in the constructions set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.