discovered 02_2012

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 36 37 WORKPLACE: Sketch of the proposed 2015 ESRF beamline; the two HZDR measuring stations are located at the beamline 20 (BM 20). Image credit: ESRF security of future nuclear waste repositories. Currently, the researchers are focusing on experiments with iron oxides. These minerals occur naturally in granite and clay rocks - together with salt the most important geological formations for storing high-level nuclear waste - but also on the surface of steel in the form of corrosion products. Which is reason enough for the scientists to focus on the interaction of plutonium, a very long-lived and toxic element, with these iron oxides because - quite evidently - it is of utmost importance to know what happens when the steel barrels containing waste from nuclear reactors begin to rust. Andreas Scheinost does not want to make a big secret out of his research findings. “The rusting containers still have a comparatively high capacity to retain their toxic content, as we were able to demonstrate for plutonium.“ Rust is inevitable Even far below the Earth‘s surface, metals are susceptible to corrosion. The oxygen required to fuel this process is not provided by the atmosphere, as there is no air in those depths, but comes from the ground water. And it is quite easy to conceive of a scenario involving water when one considers, say, the fact that the host rock granite actually contains fissures for conducting fluids. “We have to assume that, with time, all waste containers will inevitably begin to rust, even when made from stainless steel. What we‘re talking about here are processes that will take several thousands of years to happen,“ concedes Scheinost - a timeframe that, for obvious reasons, is impossible to examine in an experimental lab setting. Instead, researchers are able to look at only short reaction times ranging between several months and a few years, and they then have to find ways to extrapolate their findings in order to make accurate, long-term predictions. Thus far, only those samples with the shortest reaction times of two months have been analyzed, but long-term samples of up to three years will follow. It has been a long way to carry out those experiments - mostly due to the fact that they had to be conducted under experimental conditions mimicking those in real repositories, i.e. in the absence of atmospheric oxygen. That, of course, makes the results particularly valuable. In fact, the observed reactions deviate from previous work done under normal atmospheric conditions, i.e. in the presence of 20 percent oxygen. While the researchers had expected to find a much higher mobility under the - more realistic - anoxic conditions, this did not turn out to be true: plutonium was found to be firmly bound to the surface of the iron minerals. In order to perform the actual measurements at the ROBL beamline, which took “only” twelve 24-hour-shifts last year, radiochemistry experts from all over Europe participated BM 20

discovered 02_2012

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