Effect of organic carbon on the mobility of uranium and neptunium in a geological disposal facility

What is it about?

It is likely that radioactive wastes will be placed in an underground geological disposal facility. Such a facility will be dry during its operational phase but will likely become saturated with groundwater after closure. Extensive use of concrete will form part of the engineering of the facility. Once re-saturated with groundwater this will create a highly alkali (high pH) environment. Dissolved organic carbon is the brown discolouration you see in streams draining peat moorland and is the product of the decay of once living plants and organisms (the tannic acid that makes your black tea is another example). This organic carbon is present in all groundwaters and is an important chemical in the mobility of dissolved metals - including radioactive elements. This study reports experimental results showing the interactions of oxidised neptunium and uranium forms with organic matter.

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Why is it important?

Once closed a geological disposal facility will be saturated with groundwater and the interaction with concrete will create a high pH environment. Calculations for three simulated cement leach waters (representing different phases expected as the concrete degrades over decades/centuries) suggest uranium and neptunium are not likely to be significantly bound to organic matter under these conditions and therefore will not contribute significantly to transport of these radioactive metals from a repository.

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