In the U.S. alone over 70,000 metric tons of spent nuclear fuel at 70 sites in 35 states is awaiting disposal. Over 12,000 metric tons is generated anew each year. This yearly amount is approximately the size of a two story building placed on a basketball court. It is not really a large amount, but it adds up, and it is a major constraint to the expansion of the nuclear power industry. This is all or nearly all high level waste which most likely will have to be buried deeply, sealed away for about 10,000 years or more when it may no longer represent a hazard. One has to be confident that the container seals will last and that the burial area will remain geologically inactive for that period as well.
Low level and intermediate level radioactive waste is generally less of a problem. It can be buried in a sealed-off, designated location encapsulated in containers or in the ground where it will lose most or nearly all of its radioactivity in the next 100 to 200 years. Low level radioactive materials are represented by Uranium contaminated wastes from mining operations, hospital or research lab radioactive waste, and diluted waste from nuclear power plant decommissioning sites or nuclear fuel reprocessing sites. All these forms of low to intermediate level waste are simply a much easier problem than those requiring really long term storage.
The most major concern for long term storage of high level waste is that at some point it will escape its containers and seep into the ground water. Tunnels mined from shale formations represent an option, but it seems difficult to set up a 10,000 year plan to watch over them. Thus, deep boreholes are also under consideration In one plan we would drill holes about three miles deep with a width of about 20 inches across. We would then store sealed canisters containing high level waste into these holes. Given the amount of waste we have on hand and the rate at which it is growing, we would need about 700 such holes.
Several problems emerge in this approach. First, we have never machined such holes out of the Earth before, but that’s a problem we could likely solve. Secondly, we have some exposure risk associated with workers who will have to fit highly radioactive materials into the designated canisters. We also need a site into which these holes can be carved while the site itself remains confidently, seismically inactive for the next ten millennia. In the U.S. so far no state seems ready to designate a site for placement of this or any other similar deep storage facility. Several have been discussed, but so far no decisions appear to be made.
Additional possibilities remain. First, following the robotic clean up beginning now at the Fukushima Nuclear facility, we could create a legion of robots to tend to properly sealed containment vessels. Such robots would check containers for leaks on a regular basis, fix leaks when they occur before they become more major. They would be prepared for clean up if necessary, and regularly report to a command and control center. Thus, high level radioactive waste would be kept in many centers and thus not concentrated in one location, but the robotic staff monitoring the sites would report directly to a centralized command location from which a response could be mounted should the problems encountered by the robotic staff at any individual location become more than they can handle.
Initially robotic monitoring may not be as streamlined as one makes it sound above. Indeed, an initial period of 50 to 100 years or more may be required to develop and refine the system so that safe monitoring and overall control could be effected, but after that new waste facilities could be more confidently developed and monitored as safety is assured. Further, as radioactive decay occurs over the centuries, robots could also be taught to reprocess and concentrate the stored wastes so as to reduce inventories. Given the fact that much of the radioactivity may not have decayed for several centuries or more, there is clearly no reason the have to think in terms of burying it and forgeting about it. Even though humans do not have to be involved in monitoring and upgrading storage faciilities, there’s no reason it cannot be done by robots.
Further, as the centuries move on it is possible that new uses may be found for the stored radioactivity. At that point, the stored radioactive materials can be processed and purified by the robots and safely provided for the intended use. Even now some have suggested that highly radioactive waste might be used to eradicate pathogenic organisms in human sewage. That might make the pathogen-inactivated sewage available to prepare safe fertilizers. One can envision miles of pipes containing an outer double layer stuffed with highly radioactive waste sealed off from a flowing mass of human sewage flowing through the inner pipe. After flowing for a mile or more the pathogens present in the sewage would be inactivated. The pathogen-free sewagevwould then be collected and processed. Admittedly this is a crude application, yet others might be developed in the future when stored radioactive waste might be brought out of storage and used in other applications we are not capable to envisioning just yet.