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The proposed system in Sweden and Finland for the construction of a geological repository for long-term host high-level nuclear waste (high level waste) has been challenged by a group researchers who have made experiments on the corrosion of copper which will be covered waste containers. The copper may corrode faster than they in fact provide the designers and expose the waste, compromising the security deposit. But the SKB, the joint venture formed by the Swedish energy industries in charge of construction, rejected the allegations saying the quality of the project [1].
technology for storage waste in a deep geological repository is called KBS-3 and provides for the inclusion of nuclear waste in a container lined with copper. The deposit is dug to a depth of 500 m in a geological environment that must meet stringent requirements. In the depths of the site is then excavated cylindrical holes in the ground where the containers are stacked. The space between the container and the wall of the hole is filled with bentonite and, once filled, is deposited a plug of bentonite. At some distance is made a new hole next to the containers. disposal_tunnel
barriers that need to isolate the waste from the environment are mainly three:
the copper coating
2.
the layer of bentonite surrounding the containers
3.
the surrounding rocks
The main enemy is made up of underground water flows, which can corrode the containers and transporting radioactive materials off-site through the rocks. For this reason it is necessary that the system possesses geological flows and poor water containers are waterproof and corrosion proof. The waterproofing is done through the bentonite, copper and provides protection against corrosion.
everything seems relatively simple, if one considers the enormous amount of time at stake. The drums must resist corrosion for at least 100,000 years, the time needed to invalidate the radioactive material at a level comparable to that emitted from a reservoir of natural uranium. In this period of time are provided of the ice ages, during which Finland and Sweden will be covered with a layer of thick ice some miles and be able to create a high pressure on the rocks of the deposit [5]. You will be prompted to discover and model the current hydrological situation and the changes it will undergo in the next 100,000 years. According
SKB, the characteristics of the site after its closure (which occurs when the storage becomes full, after about 100 years) is such corrosion of the copper coating. In fact the environment of the site will be free of oxygen and therefore, according to SKB, that will not cause corrosion significantly. But a group of researchers at the Royal Institute of Technology in Stockholm has found that copper can corrode instead of pure water in this environment [4]. Mechanisms of corrosion
During the centuries, the bentonite is saturated by water present in the rocks. The bentonite swells and reacts to become an impermeable barrier. But with the passage of time is impossible to prevent water from entering, although in small quantities. At that depth the water is deprived of oxygen, although small amounts oxygen are present in the pores of the bentonite before saturation. The copper reacts with oxygen present in the bentonite to form copper oxide. When the oxygen is consumed, the copper reacts with water containing sulfide ions that are found naturally in the bentonite to form copper sulfide. Then come into play chloride ions to form copper hydroxide water. These chemical reactions are accepted without major debate.
the situation is the presence of another reaction that takes place in an environment free of oxygen [1]: the reaction between copper and water molecules that form other copper hydroxide and hydrogen gas. In addition, the hydrogen can be absorbed by the metal, a phenomenon which would cause an increase in the fragility of the coating of copper. The chemical reaction that would happen is as follows:
Cu + H2O -> Cu hydroxide + H + H2 (in copper)
and would represent a further stage of corrosion of the metal, which would continue until the equilibrium pressure hydrogen, at a value of about 1 mbar [1]. Hultqvist said that scientific evidence was found of the presence of a reaction product and that molecular hydrogen can be measured both in gaseous form within the metal [1]. According
Szakálos these results are not in conflict with the principles of thermodynamics and can be explained by the formation of an amorphous copper hydroxide. Also according to the researcher, the corrosion of copper in oxygen-free water is a well known phenomenon in industries that use copper in cooling systems and synchrotrons, since all these types of systems have a corrosion of copper at a rate of about one micrometer per year [1].
In November 2009 a special group of experts was convened to clarify the issue [3]. Experts have proposed several experiments to confirm the formation of hydrogen, the experimental conditions of study and Hultqvist Szakálos examining the reaction products, measured the resistance of the copper over time and quantify the thickness of the coating. Conclusions
Since two of the three barriers (bentonite and rock) are designed only to delay the release of radionuclides into the environment, the most important barrier is formed from the case. In fact, the bentonite is not a permanent barrier, but a means to slow the penetration of water around the container. Moreover, the same SKB [6] notes that "today you can not exclude the possibility that the bentonite be eroded if the water has a low salinity. Today, the site of Forsmark, this condition does not occur. However, during a future ice age, the salinity is sufficiently low in both sites [Forsmark and Laxemark ed], although it is less likely to Forsmark. Furthermore, since the hydrological fluxes are lower Forsmark, this phenomenon will involve fewer than Laxemar deposition sites. If the layer of bentonite disappear, the container may be damaged in a very long period of time from corrosion caused by sulfur [6]. Corrosion cited is not enough to damage the containers SKB second over the period, but the analysis does not consider the recent discoveries.
All this makes it even more important is the need to conduct comprehensive analysis and the new phenomenon of corrosion. It shows how the permanent storage of high-level radioactive waste in a geological repository is not a permanent solution is fully resolved. The solution is particularly difficult because of the enormous amount of time in which the three barriers must operate. To do a little comparison, Homo sapiens sapiens appeared 50,000 years ago. If the first modern human had made a similar filing, today we would be only half of the period of isolation of the waste.
Notes:
[1] Nuclear Engineering Institute The Copper Controversy ", November 6, 2010
[2] SKB," Long-term safety for KBS-3 repositories at Forsmark and Laxemar – a first evaluation " Technical Report TR-06-09, October 2006, http://www.skb.se/upload/publications/pdf/TR-06-09webb.pdf
[3] Swedish National Council for Nuclear Waste, “ Mechanisms of Copper Corrosion in Aqueous Environments ”, November 16, 2009, http://www.mkg.se/uploads/Swedish_National_Council_for_Nuclear_Waste_Report_2009-4e_Mechanisms_of_Copper_Corrosion_in_Aqueous_Environments.pdf
[4] P. Szakálos, G. Hultquist, G. Wikmark, " Corrosion of Copper by Water ", Electrochemical and Solid-State Letters, 10 (11) C63-C67 (2007) DOI: 10.1149/1.2772085
http://mkg.nu/pdf/Corrosion_Copper_Water_Hultquist_Szakalos_Wikmark_2007.pdf
[5] Finland's Nuclear Waste Solution , IEEE Spectrum, Dec 2009
[6] SKB - Final Repository for Spent Fuel in Forsmark - Reasons for decision and basis for site selection , June 2009, p. 4
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