Aiming at the problem of radioactive waste disposal, the concept and mechanism of self-burial by deep rock melting are presented. The rationality and feasibility of self-burial by deep rock melting are analyzed by comparing with deep geological burial. The heat threshold during the process of contact melting around a spherical heat source is defined. The descent velocities and burial depths of spherical waste containers with varying radius are calculated. The calculated depth is much smaller than that obtained in the related literature. The scheme is compared with the deep geological burial that is currently carried out by the main nuclear countries. It is found that, at the end of melting, a radioactive waste container can reach deep strata that are isolated from groundwater.
Radioactive Wastes Are Temporarily Stored In A Spherical Container
According to the latest report from the OECD/NEA [1], nuclear energy will be the only choice to replace the carbon fuels and satisfy the global energy needs. However, for nuclear energy, the problem of radioactive waste disposal is inevitable. The problem has existed for more than half a century since the development of the first nuclear power plant. There are available deep geological disposal solutions in France, Sweden, Finland, or USA, but at the moment there is no operating facility available for radioactive waste from civil use of nuclear energy. In 2020 also first disposal operating facility is expected in Finland. At the present time, most of the waste has to be stored temporarily, and the long-term storage has been proven to be safe and efficient. A lot of countries are in process of long-term storage prolongation from 50 to 100 years [1].
From the above analysis of the process of self-burial melting by deep rock melting, some conclusions can be drawn as follows.(1)By the scheme of self-burial by deep rock melting, four protective zones or near-field barriers can be formed to isolate radioactive waste from groundwater at the end of the melting process.(2)A larger radius of the spherical container will remarkably prolong the melting time and increase the final depth of self-burial. In addition, the shorter the waste storage time is, the deeper the final depth of self-burial is.(3)A container filled with radioactive waste can reach great depth by contact melting, and thick and safe near-field barriers can be formed. Self-burial by deep rock melting will be a valuable scheme for radioactive waste disposal.(4)The descent velocity will depend upon the type of rock [9]. A high melting temperature is chosen in this study, which results in a minimum estimate of depth of self-burial. Although spent fuel is used in this analysis, in fact, any waste with decay heat bigger than the heat threshold can be used as the heat source for self-burial by deep rock melting.
The deposits of native (pure) copper in the world have proven that the copper used in the final disposal container can remain unchanged inside the bedrock for extremely long periods, if the geochemical conditions are appropriate (low levels of groundwater flow). The findings of ancient copper tools, many thousands of years old, also demonstrate the long-term corrosion resistance of copper, making it a credible container material for long-term radioactive waste storage.
The concept consists of drilling a borehole into basement rock to a depth of up to about 5000 metres, emplacing waste canisters containing used nuclear fuel or vitrified radioactive waste from reprocessing in the lower 2000 metres of the borehole, and sealing the upper 3000 metres of the borehole with materials such as bentonite, asphalt or concrete. The disposal zone of a single borehole could thus contain 400 steel canisters each 5 metres long and one-third to half a metre in diameter. The waste containers would be separated from each other by a layer of bentonite or cement.
The repository would exist 300 metres underground in an unsaturated layer of welded volcanic tuff rock. Waste would be stored in highly corrosion-resistant double-shelled metal containers, with the outer layer made of a highly corrosion-resistant metal alloy, and a structurally strong inner layer of stainless steel. Since the geological formation is essentially dry, it would not be backfilled but left open to some air circulation. Drip shields made of corrosion-resistant titanium would cover the waste containers to divert possible future water percolation and provide protection from possible falling rock or debris. Containment relies on the extremely low water table, which lies approximately 300 metres below the repository, and the long-term durability of the engineered barriers.
The UK's Nirex Phased Disposal Concept (or Phased Geological Disposal Concept) has been developed for relatively large volumes of ILW and LLW, usually cemented into stainless steel containersk. These containers would be emplaced into a repository in a host rock environment below the water table. The waste would be monitored and remain retrievable and the groundwater managed to prevent contact with the wastes, until such a time that the repository is sealed. When this happens, the waste will be surrounded (backfilled) by specially formulated cement and the repository allowed to resaturate. The cement would provide a long-lasting alkaline environment that contributes to containment of the waste by preventing many radionuclides from dissolving in the groundwater. Similar cement-based schemes for ILW disposal have been proposed in France, Japan, Sweden and Switzerland.
The HLW in liquid or solid form could be placed in an excavated cavity or a deep borehole. The heat generated by the wastes would then accumulate resulting in temperatures great enough to melt the surrounding rock and dissolve the radionuclides in a growing sphere of molten material. As the rock cools it would crystallize and incorporate the radionuclides in the rock matrix, thus dispersing the waste throughout a larger volume of rock. There are some variations of this option in which the heat-generating waste would be placed in containers and the rock around the container melted. Alternatively, if insufficient heat is generated the waste would be immobilized in the rock matrix by conventional or nuclear explosion.
Another early proposal was for the heat-generating wastes to be emplaced in weighted, heat-resistant containers such that they would melt the underlying rock, allowing them to move downwards to greater depths with the molten rock solidifying above. This proposed option resembles similar self-burial methods proposed for disposal of HLW in ice sheets (see section below on Disposal in ice sheets).
In the 1990s there was renewed interest in this option, particularly for the disposal of limited volumes of specialized HLW (particularly plutonium) in Russia and in the UK. A scheme was proposed in which the waste content of the container, the container composition, and the placement layout would be designed to preserve the container and prevent the wastes becoming incorporated in the molten rock. The host rock would be only partially melted and the container would not move to greater depths.
Disposal at sea involves radioactive waste being dropped into the sea in packaging designed to either: implode at depth, resulting in direct release and dispersion of radioactive material into the sea; or sink to the seabed intact. Over time the physical containment of containers would fail, and remaining radionuclides would be dispersed and diluted in the sea. Further dilution would occur as the radionuclides migrated from the disposal site, carried by currents. The amount of radionuclides remaining in the seawater would be further reduced both by natural radioactive decay, and by the removal of radionuclides to seabed sediments by the process of sorption.
For the sub-seabed disposal option, radioactive waste containers would be buried in a suitable geological setting beneath the deep ocean floor. This option has been suggested for LLW, ILW, and HLW. Variations of this option include:
The disposal of radioactive wastes in a repository constructed below the seabed has been considered by Sweden and the UK. In comparison to disposal in deep ocean sediments, if it were desirable the repository design concept could be developed so as to ensure that future retrieval of the waste remained possible. The monitoring of wastes in such a repository would also be less problematic than for other forms of sea disposal.
Burial of radioactive waste in deep ocean sediments could be achieved by two different techniques: penetrators or drilling placement. The burial depth of waste containers below the seabed can vary between the two methods. In the case of penetrators, waste containers could be placed about 50 metres into the sediments. Penetrators weighing a few tonnes would fall through the water, gaining enough momentum to embed themselves into the sediments. A key aspect of the disposal of waste to seabed sediments is that the waste is isolated from the seabed by a thickness of sediments. In 1986, some confidence in this process was obtained from experiments undertaken at a water depth of approximately 250 metres in the Mediterranean Sea. The experiments provided evidence that the entry paths created by penetrators were closed and filled with remoulded sediments of about the same density as the surrounding undisturbed sediments.
In the 1980s, the feasibility of the disposal of HLW in deep ocean sediments was investigated and reported by the Organization for Economic Co-operation and Development (OECD). For this concept, radioactive waste would be packaged in corrosion-resistant containers or glass, which would be placed beneath at least 4000 metres of water in a stable, deep seabed geology chosen both for its slow water flow and for its ability to retard the movement of radionuclides. Radionuclides that are transported through the geological media, to emerge at the bottom of the seawater volume, would then be subjected to the same processes of dilution, dispersion, diffusion, and sorption that affect radioactive waste disposed of at sea (see section above on Sea Disposal). This method of disposal therefore provides additional containment of radionuclides when compared with the disposal of wastes directly to the seabed. 2ff7e9595c
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