Patent Number: 047568526
Section: summary

BACKGROUND OF THE INVENTION This invention relates to systems for storing nuclear waste material and, more particularly, to apparatus for venting nuclear waste storage containers in a manner that allows gases generated by the stored waste material to escape, while simultaneously minimizing the intrusion of water. One of the pressing problems currently facing society is the storage and disposal of nuclear waste. Given the magnitude and prolonged duration of the dangers inherent in storing nuclear waste, storage systems must satisfy exacting criteria over long periods of time. Thus, nuclear waste is generally stored in an impervious system specially designed for the application. A typical constraint on the design of such a system is that the waste must be contained, without leakage, for a period of 300 years. Development of a suitable storage system is further complicated by the variety of potential storage locations employed. For example, is frequently stored at the generation site initially. During this time, the storage container is accessible to personnel working at the site, making it susceptible to tampering or accidental damage. The container eventually may be buried at an underground site selected for its geological stability. Burial storage minimizes the likelihood of human interference with the stored waste. In most cases, clay, sand, rock, or salt burial sites are selected to provide a relatively dry storage environment for the container and to minimize the possibility of groundwater contamination. From the preceding discussion, it can be seen that successful storage of nuclear waste requires the system to be resistant to the effects of radiation, erosion, vibration, biodegradation, thermal cycling, burial loading forces, impact forces sustained by the container, and chemical action of the waste and environment on the container. As noted, the specific problem of nuclear waste storage addressed by this invention is the venting of gas generated within the container. Should these gases cause the internal pressure of the container to become too great, the container structure could become overpressurized, allowing the stored waste to contaminate the environment. Three sources of gas generation within the container must be considered in order to realize a satisfactory venting system. First, the container material itself may generate gas when exposed to the radiation of its contents. Second, ion-exchange resins, which are used to reduce the radioactivity of fluids in nuclear power systems, may undergo radiolytic gas generation when stored in the container. Third, gas may be generated by the biodegradation of organic waste stored in the container (e.g., contaminated grease, solvents, oils, or organic materials attached to the ion-exchange resins). The rate at which gas is generated depends, among other things, on the total radiation dose exposure of the container and contents, the container and ion-exchange resin materials, the amount of organic waste present in the stored material, and the amount of oxygen within the container. From the preceding discussion, it is clear that a precise determination of the amount of gas will be generated within the container would be difficult at best. Thus, given the need to ensure the structural integrity of the storage container under any set of conditions, a means for venting the interior of the container to the environment must be provided. In that manner, pressure differences between the interior of the container and the environment will be minimized, preventing the container from becoming overpressurized. It is extremely doubtful that conventional venting devices can meet the design constraints for venting nuclear waste storage containers. For example, the natural venting characteristics of high-density polyethylene, as a container material, are generally incapable of producing the degree of venting required. Small check valves have good water restriction characteristics, but uncertainty exists as to their operation and ability to reseal over the 300-year design life of the container. Filters made of a porous metallic material would appear to have a number of drawbacks. First, their water restriction characteristics appear to be insufficient for nuclear waste storage container applications. Second, the material has a tendency to become wetted and trap water, greatly increasing the pressure required to pass gases through the material. Finally, the use of a metallic material can establish a galvanic couple between the container and the filter and lead to corrosive failure. Activated charcoal filters, while noncorrosive, resistant to gamma radiation, and readily available, generally have a low resistance to the ingress of water. SUMMARY OF THE INVENTION In accordance with the invention, there is provided a passive vent having as its primary component a reversibly porous, air-diffusible, water-restrictive, polymer plug secured in a port provided in the wall of the container. The air-diffusible nature of the material allows gases to flow through the plug in both directions. Thus, variations in pressure between the inside of the container and the container environment may be relieved. In the presence of water, water flow through the plug is restricted by a swelling of the plug material, minimizing the possibility of groundwater contamination from the waste stored in the container. The degree of waterflow restriction exhibited by the plug is directly proportional to the amount of water retained by the plug. Airflow through the plug is also inhibited in direct proportion to the amount of water retained in the plug. Even with the plug material saturated with water, however, some venting takes place. In addition, the reversible porosity of the material allows the reduction in air-diffusibility of the material attendant liquid saturation to be reversed by allowing the material to dry. The characteristics of the plug material selected also include a high resistance to the effects of radiation, chemicals, corrosion, biodegradation, and thermal cycling. A means for securing the plug in the wall of the housing is provided that will ensure plug retention in the wall over the life of the system even when subject to environmental effects, such as vibration. Finally, in the currently preferred embodiments, a sealing means, impervious to the flow of both gas and liquid, is placed between the wall of the port, and the plug. In these currently preferred embodiments the plug material is a low-density, linear porous polyethylene having an average pore diameter of less than 5 microns. Threads provided on the sides of the plug for engagement with similar threads provided in the container wall port constitute the means for securing the plug in the container wall. A thread sealant is applied to the threads. In the preferred embodiments, the plug has a cross-sectional area of less than 0.5 square inch (approximately 3.2 square centimeters), limiting the size of the port required to be made in the wall of the housing. Thus, even if the vent should fail, the constrictive effect of the relatively small port cross section will minimize the ingress and egress of liquids, protecting the environment. The plug also has an outer portion that includes means (such as a screwdriver slot) for receiving a tool capable of driving the plug into the port until the plug is properly seated. This outer portion of the plug constitutes an excess region that protrudes from the outer wall of the container when the plug is inserted. After insertion, this excess region is removed, minimizing the possibility that the plug will be tampered with or subject to forces incurred by the container wall from the environment. Additionally, with the portion of the plug containing the means for receiving the insertion tool removed, there are no depressions on the plug surface to collect water. A slight protrusion of material may also be left to minimize the collection of groundwater around the vent. According to the invention, a process for installing the plug in the wall of the housing is revealed. The process consists of applying a sealant to the threads of the plug, inserting the plug a predetermined distance in a port provided in the wall of the housing, and removing the excess portion of the plug protruding from the wall of the housing.