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discovered_02_2015 - Title: Corrosion has its advantages

WWW.HZDR.DE discovered 02.15 TITLE // The chemistry involved in repositories for highly radioactive material. "We just assume the worst-case scenario," explains Vinzenz Brendler of the HZDR Institute of Resource Ecology. Anyone researching the chemistry involved in running a future repository for highly radioactive material should really take this pessimistic approach to heart. After all, the safety of a repository such as this, which would presumably be constructed deep beneath the Earth's surface contained in its own geological formation somewhere in Germany, must be guaranteed for at least one million years. Chemists like Vinzenz Brendler are well aware, however, that even the most improbable reactions can occur if given enough time. And one million years is a very long time. It is for exactly this reason that the 30 employees of the Surface Processes Division take into consideration anything there that could possibly react. And there is quite a lot to keep track of, since many of the elements listed in the periodic table of elements tend to occur in repositories. "In our work we concentrate on reactions with radioactive elements. Our institute is especially well-prepared for this," Vinzenz Brendler reports. Chemistry not involving radioactive substances can just be researched in "normal" labs after all. This selection allows scientists to narrow down the enormous mountain of reactions considerably, but a great many of them still remain. Mountain for chemists The repository will mainly house the used up fuel rods from nuclear reactors. Originally they consist predominantly of uranium. Since only a small portion is burned up in the reactor, there is very little change in the rods even after they served their purpose. Contrary to popular belief, the overwhelming majority of material in the rods does not emit strong amounts of radiation. Uranium itself, however, is an extremely poisonous heavy metal and is therefore taken very seriously. A small portion of the uranium is, however, split in the reactor. The fragments produced by this process have varying weights and consequently represent a wide variety of different elements that are now also contained in the fuel rod. Some products of the split, certain chlorine and cesium isotopes for example, are highly radioactive. In any case, a portion of this will decay relatively quickly. So after few decades in intermediate storage the radiation level will already be considerably lower. Nevertheless, there will still be plenty of highly radioactive elements in the used up fuel rods. Enough time Nuclear reactions in the reactor produce quite a few neutrons, many of which act as catalysts for the chain reaction and with others remaining trapped in uranium atoms. These will often trigger further nuclear reactions that in turn produce heavier and correspondingly highly radioactive elements such as americium, neptunium, plutonium, and curium. The researchers working with Vinzenz Brendler are also engaged in carefully tracking these "activation products". But there are also a series of other compounds that play a role in the repository and that could be involved in chemical reactions. The uranium of the fuel rods is placed in a zirconium casing. This is a heavy metal with low toxicity that, unlike many other metals, corrodes only very slowly. All the same, there is still the possibility that it will react with the radioactive elements in its environment - it is sure to have enough time for that in the repository. Ingredients for corrosion As it currently stands, the plan is to place the fuel rods in special Pollux containers made of steel, which are placed in casings made of cast iron and graphite, which are then placed in the repository. Oxygen from the air could then oxidize the iron in these casings. And this gives Vinzenz Brendler and his team another substance to keep an eye on. Once the repository is closed, the oxygen will slowly be used up, thus seemingly removing the basis for corrosion. It won't be long, however, until a fresh supply is available: The surrounding stones are sure to contain oxygen and perhaps even water - regardless of where the underground repository is located. These are the two essential ingredients that cause many metals to corrode. Especially when, as is the case in the repository, there is enough time. In time, rust could eat holes in the steel casings of the Pollux containers - at least if you assume the worst as Vinzenz Brendler does. What would happen in the repository then? The uranium in the fuel rods is present in the form of uranium dioxide, which is relatively inert. In this form organisms rarely absorb it and at first the heavy metal will hardly have any poisonous effects at all. This changes abruptly, however, when it oxidizes to a uranyl compound. Unlike uranium dioxide and the compounds it forms, these uranyl particles dissolve well in water. Not only can they be transported over long distances, but they can also easily be absorbed by life forms, thus poisoning them. _TEXT . Roland Knauer CORROSION HAS ITS ADVANTAGES