Decontamination is the process of transferring one or more radioactive substances present on a surface or in a material to a packagable environment (waste, flakes, films, towels) for storage. The effectiveness of decontamination decides the destiny of the decontaminated support. It is assessed according to two methods:
- Measurement of decontamination factor FD = Initial activity / Final activity,
- Measurement of the reduction factor of dose rate FRDD = Initial dose debit / Final dose debit.
There are two types of contamination:
- Labile (not fixed): Surface contamination transferable by suspending or training.
- Fixed : Contamination strongly linked to the support by physical or chemical bonds.
The main processes of decontamination are:
- Physical decontamination which consists in mechanically removing the contamination by friction or pulling the material. Examples: decontamination of concrete by nibbling, decontamination of metals by machining, melting decontamination, peelable film.
- Chemical decontamination that involves suspending the contamination in a solution or gel or trapping it in flakes. There may be erosion or not of the support.
- Joint decontamination = chemical + physical. Examples: towels impregnated with a decontaminating solution, ultrasonic baths with decontaminating solution.
To choose a decontamination process, several criteria must be taken into account:
- Decontamination objective : maintenance, decommissioning or dismantling,
- Characteristics of the contaminated material (nature, shape, surface),
- Nature of the contaminants,
- Assessment of waste and effluents generated by the process,
- Risks associated with the exposure of personnel,
- Cost of the operation.
Chemical decontamination can be used on labile as fixed contamination. It has the advantage of not generating volatile dust and allowing remote interventions with soft and light material. The products used can be liquid, foam or gel, at neutral, acid or alkaline pH, corrosive or oxidizing. The choice is according to the type of contamination, the decontamination objective, the nature of the material (steel, stainless steel, concrete, plastic, etc.), the form (soil, vertical wall, pipes, etc.), the surface state (presence of fat, crystalline boron, oxidation, etc.) and whether effluents can be generated.
A product is effective when properly applied. The choice of spray equipment is essential. This choice is made depending on the type of product to use, the time allotted to the operation, the sources of energy and the space available, the amount of product required and the intervention or not of a remote manipulator.