How gel formulations can aid decontamination28 January 2000
Using gels for decontamination can help minimise exposure, improve safety, maximise decontamination factors and reduce costs. French groups have developed new gel formulations.
Many processes for chemical decontamination operate by immersing components in concentrated and aggressive solutions. In these applications the components that are being treated must be dismantled and immersed in decontaminating baths containing large amounts of liquid. Prohibitive volumes of liquid radioactive waste are generated and, to make the process more difficult, in some cases the baths need to be heated or recycled. This kind of technology requires huge infrastructures, high costs, a high hazard risk in dismantling and a high exposure of the workers.
Two other chemical decontamination methods are electrodecontamination and the use of foaming agents. Again, there are several drawbacks. The first technique is limited to simple surfaces and can be dangerous – there are both electrical and chemical risks, including gaseous release. The second is particularly useful for hollow and complex systems, but it generates a large amount of organic waste.
STMI (Société des Techniques en Milieu Ionisant) has been collaborating with a French CNRS laboratory since 1996 to develop processes using gels instead of solutions, foams or electrolytic processes. The aim is to avoid the well-known disadvantages of chemical methods, while retaining their high efficiency. Gels can be projected directly onto the plant components, and onto all the types of surfaces that need to be decontaminated, including stainless steels, alloys and painted surfaces. The subjects do not have to be dismantled and the process does not require large amounts of gel. After several hours of treatment, the gels are rinsed off with water. High decontamination factors exceeding 2000 can be obtained, while the exposure and the generated waste are minimised.
Development work on the decontaminating gels has been completed, and was reported in 1998. The process has been patented and licensed to STMI.
The aim of the decontamination process is to remove the radioactivity at the surface of plant components. On site, the use of gel for the decontamination makes maintenance safer and makes it possible to bring the plant back into operation immediately after maintenance is completed. During decommissioning, decontamination can allow components to be considered as non-radioactive materials for disposal or recycling. Even in the case of highly contaminated systems, it is possible to remove enough of the radioactivity so that the items can be classified in a lower category of radioactive waste.
The gel-based decontamination process has already been proved at several nuclear plants and decommissioning sites. In 1997 and 1998, the principal French plants and a few foreign sites were treated. This includes decontamination of refuelling pools or spent fuel pools at several EDF plants (Chinon, Cruas, Dampierre, Blayais, Cattenom, Saint-Laurent-des-Eaux etc), tanks and glove bags from Cogema at La Hague, different parts (ceiling, floor and walls) of a moderately contaminated room in a Japanese plant, and stainless steel rooms made of 304 L at the National Institute of Radioelements in Belgium.
Gel production now amounts to several tons per year.
SAFEty AND CONTROL
The decontamination process by gel is perfectly suited to on-site maintenance of operational plants and decommissioning of those at the end of their life. It consists of applying the gel by spraying it onto the items to be treated. The gel adheres to any surface (reverse, vertical or complex) and operates by dissolving the radioactive deposit, along with a thin layer of the support, so that the radioactivity trapped at the surface can be removed. No consequent damage to the materials treated has been observed. After a few hours of application (typically less than six hours), the gel is rinsed with water under high pressure and the waste generated is recuperated for treatment and filtration. The radioactive waste is then captured, in solid form or in solution, and is subjected to well-known and standard treatments.
The process is particularly efficient for the decontamination of all types of surfaces (vertical, horizontal, cylindrical) such as walls, ceilings, floors, pools, pipes, because of the thixotropic property of the gels (see panel). The gels are liquid under stress (when vigorously shaken), allowing easy projection. When they are stationary they regain their solid state, to give perfect adherence to the surfaces to be decontaminated. Another important aspect of the gels is that small quantities can be used (approximately 1 kg per square metre surface area treated). A few hours of treatment are sufficient for the decontamination, with a material loss of 1-2 µm per application. Since the plant components can be easily cleaned the waste generated can be simply limited.
The process is also suitable for the decontamination of complex systems such as valves, heat exchangers, and glove bags.
The dissolving agent is a corrosive solution constituted of a strong oxidising reagent stabilised in a concentrated acidic medium. The strong oxidant is a metallic salt of cerium (IV), silver (II) or cobalt (III) with a high redox potential and a high stability in aqueous solution; the acids used are typically strong inorganic acids (nitric or sulphuric).
To obtain a gel from this solution, we use a mineral support that yields the best viscosing properties for the gel, such as small mineral oxides (typically fumed silica). The weight percentage is less than 5%, which makes the gel highly soluble in water and thus it can be easily rinsed at low pressure. When it is used, an organic coviscosing compound is added to the gel (typically between 1 and 4 wt%). This may be a surfactant, hydrosoluble polymer or an association of both. The smaller the amount of silica, the higher the concentration of organic polymer. The organic compound has to be soluble in saline solution and chemically resistant to strongly acidic and oxidising media.
Other types of gels can be formulated in the same way. Basic or reducing gels, for example, are suitable for degreasing of plant components. Instead of metallic salts and an acidic medium, these gels use concentrated basic media and strong reducing agents. Sometimes different decontamination processes by gels are combined or used as pre-treatment to improve decontamination results.
In the manufacturing procedure, most of the gel constituents are mixed at the same time or successively. In this way, several formulations can be prepared, either for the gels to be used quickly or for them to be stocked for several months before application.
The gels are also easy to handle: thanks to the thixotropic effect, they can be projected easily and adhere to any type of surface without leaking. To apply the gels efficiently and safely, a self-contained piece of equipment has been developed. It is equipped with an agitator, a pump, a control panel and a blast pipe. The blast pipe can be handled manually or automatically, and by remote control. Rinsing can be operated with the same equipment. Exposures are minimised, because the machine works at distance.
The corrosion efficiency is designed so that most of the time one gel application is sufficient for decontamination. For highly contaminated items or for higher decontamination factors on moderately contaminated items, a few more applications may be required.
Another benefit of the process is corrosion control and materials innocuousness. In the gels the diffusion rate of the corrosion species are high, as are those of products from the metallic surface, and the corrosion control is governed by the concentrations of the oxidative reagent and the acid. These concentrations can be chosen to consume, in a given contact time, the overall quantities of the corrosion species. One can evaluate the surface layer removed or the gel amount required for any type of contamination and any type of material. With such a controlled loss rate, the pieces treated show no localised corrosion damage. The plant components can thus be re-used or decommissioned.
After treatment, the gels are rinsed with equipment similar to that described above. Using high water pressures allows excellent cleaning (with no residual gel marks at the surface) and ensures that volumes remain low. Generated waste amounts to a maximum of 10 litres per square metre of the surface area treated. The reccovered mixture can be subjected to additional waste processing (such as neutralisation, precipitation and organic matter decomposition) and finally filtered to release a purified solution; alternatively it can be evaporated directly.
Another interesting aspect is the lifetime of the gels, which do have a limited action in time. They lose their thixotropic property (so that they cannot adhere any more to a vertical surface) after a period between a few days and several months, depending on the formulation. It is the coviscosing organic compound that governs the lifetime of the gels. Due to the strong oxidising medium, surfactants or polymers deteriorate more or less rapidly over time, and the good viscosity of the gels is no longer assured. Because of the organic matter oxidation, the oxidising agent tends to be reduced, allowing corrosion to be controlled and the metallic salt to be treated more easily.
As part of decontamination for maintenance and to allow the use of the gels without any damage of the overall surfaces to be treated, qualification tests have been performed on most of the metallic alloys used in EDF power plants. These include 304 L, 308 L, 316 L stainless steels, 600 and 690 Inconel, Z3 CN 20 09 M, Z8 CND 17 04, and Z15 CN 16 02. Some items made of bronze or of UA10N cannot be treated by these gels, because the surface corrodes too much (by up to 25-30 µm).
Decontamination results from several plant items made of 316 L stainless steel show that decontamination factors (DF) vary from 2 to more than 1900, depending on the initial radiation rate and the complexity of the components to be treated.
The best results are obtained from simple systems with an initial contamination of more than 1500 Bq/m2 which, after three applications of six hours, decreased to 0.8 Bq/m2, leading to a global DF>1900. Some items have a DF>40 with only one treatment. The components that have low DF (<10) are those with very complex shapes (tubular systems, etc) and relatively high initial contamination rates.
|The gel-based decontamination process does not suffer from the drawbacks of other chemical processes, thanks to the thixotropic property of the gels. Defined as a decrease in apparent viscosity with time under stress, followed by gradual recovery at rest, thixotropy is a rheological phenomenon that involves the simultaneous breakdown and build-up capabilities of the microscopic structure of the fluid. When the gel is under stress (eg vigorously shaken), it is liquid and allows easy projection; when the gel is stationary, it is solid and adheres well to any surface. In applying gels for decontamination, typical viscosities are less than 0.5 Pa.s under stress and more than 3 Pa.s at rest. After being sprayed, the gels adhere to a vertical surface without leaking for a minimum of two hours, sufficient time in general for decontamination. The gels retain their viscosity, allowing diffusion of corrosion species toward the metal surface. Thixotropy is also characterised by a rebuild time when the gel structure evolves from breakdown to recovery states. The rebuild time is essentially governed by the amount of mineral support and the coviscosing organic compound in the gel. It can vary from a few tenths of seconds up to one second, giving an efficient decontaminating gel.|