Decontamination method comparison testing using simulated contamination

• Main Author: Demmer, Rick
• Published: Lancaster University 2016
• Subjects: 621.48
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.689215

Radiological decontamination of surfaces is challenging. Cleanup tasks are typically visual or even chemically measured, but radioactive contamination is invisible and measured at extremely low levels. Different contamination events have lead to the development of hundreds of decontamination processes. Their selection balances criteria such as cost effectiveness and waste minimization. While testing on the actual system where the contaminations arises (as "field" radioactive specimens) is appropriate, doing so is expensive and time consuming. Simulating contamination with substitute contaminants requires a unique understanding of the system. This provides a less expensive, more controlled and often more informative selection method. However, not all simulation methods are reliable, reproducible, have a useful range nor are relevant to the target activity. To address these needs, a range of simulated contamination systems have been developed: ·SIMCON 1 – to simulate loose contamination on stainless steel; ·SIMCON 2 – to simulate fixed contamination on oxide coated stainless steel; ·A radiological dispersal device (RDD) or so-called “dirty bomb” contaminant to simulate fixed contamination on urban material surfaces. Over 400 individual decontamination tests have been performed with these simulants, using a range of contaminants (Cs, Zr, Am, Sr, Co) and substrates (steel, concrete, marble, limestone, granite). Review and data mining of the results of these tests has allowed for the development of an understanding of the fundamental mechanisms of interaction between contamination and surface and their role in determining decontamination efficiency. Detailed analysis of the SIMCON results demonstrates that the availability of a deliberately designed contamination simulant system allows for the comparative assessment of a wide range of decontamination methods in a common frame of reference on a “level playing field”. Though somewhat intuitively obvious, the SIMCON methods evaluation unambiguously demonstrated that chemically different contaminants may behave quite differently during a decontamination action. The exact nature of this behavior depends upon the nature of applications (loose or fixed) and the decontamination method itself. If a wide range of decontamination methods are deployed on a well-designed contamination simulant, comparison of the results of the decontamination tests may provide insight into the mechanism of contamination e.g. how it is held (loose, fixed etc.), where it is held (on the surface, in a protective oxide layer etc.) and so forth. These conclusions are reinforced by detailed consideration of the urban RDD simulant decontamination data. With respect to the contaminants themselves, it is found that cations that exhibit a pH dependent speciation, such as americium or cobalt, can demonstrate radionuclide / material substrate specific chemistry that results in contaminant precipitation at the substrate surface, especially if the material has an intrinsically alkaline surface pH. For example, in the case of Am and Co, this results in the precipitation of Am and Co oxyhydroxide species at the outer surface of materials such as concrete with consequently high percentage removal efficiencies. For cationic radionuclides such as Cs+ that do not exhibit any nuclide/material surface specific chemistry the principal material property controlling the tenacity of that nuclide during decontamination from urban material substrates is the permeability of that substrate. For substrates of similar permeability, net surface negative charge on the material substrate plays a major role in determining the tenacity of non-reactive cations such as cesium; the larger the net negative charge on the substrate surface, the more tenacious the contamination. Finally, with respect to the decontamination agents themselves, harsh, high concentration chemical agents that utilize multiple decontamination processes (acids, bases, chelants) typically have an effectiveness advantage over more dilute, one component solutions. Strongly acidic solutions achieve the highest overall % removal decontamination results. This is partly because they tend to dissolve small amounts of the substrate surface and liberate imbibed contaminants. Finally, some strippable coating based methods are found to be surprisingly effective; likely because of their chelant/absorbtive character.