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Decontamination & decommissioning

Decontamination prodedure

In radiation protection, the term "decontamination" refers to the removal of loose or fixed surface radioactivity, and this operation is always done whenever wipe tests reveal the presence of radioactive contamination.

Proper cleaning method

The selection of a cleanser, or decontaminant depends on the nature of the item to be decontaminated and the amount of dirt trapped in the contamination. Either a commercial detergent or soap may be used. The key to effective decontamination is to use plenty of cleanser, a good brush or scouring pad, lots of water rinses, and absorbent paper to dry the area. Inadequate rinsing and drying may yield falsely elevated counts due to chemiluminescence. A common problem associated with cleanser residue buildup during post cleaning wipe test operations. Decontamination should always be followed by wipe tests to confirm that radiation has been reduced to acceptable levels.

All used cleaning materials are radioactive waste, and they should not be discarded in the regular garbage. And, be sure to read the MSDS for the decontaminant being used. This way you will know the associated hazards and which protective clothing you should wear when working with the product.

Maximum allowed contamination

CNSC (Canadian Nuclear Safety Commission) regulations and McGill policy require that radioactive material be removed from accessible areas whenever wipe tests indicate contamination exceeding 0.5 Becquerels per centimeter squared (Bq/cm2).

Storage of contaminated equipement

Laboratory equipment that is contaminated after use should be stored in an isolated and safe location. If necessary, carefully dismantle any equipment prior to decontamination procedures. Contaminated equipment should not leave the laboratory for repair, or any other purpose, until the level of activity has been reduced to a safe limit. If the half-life is short, like P-32, P-33 or S-35, then it would be easier to store for decay, rather than to attempt decontamination.

For longer half-life, equipment pieces can be replaced; if not then proceed to decontamination. Metals may be decontaminated with dilute mineral acids (nitric), a 10% solution of sodium citrate, or with ammonium bifluoride. When all other procedures fail for stainless steel use hydrochloric acid - this process is effective but unfortunately it will etch the surface. As for brass, commercial polish cleansers will work very well. Plastics may be cleaned with ammonium citrate, dilute acids, or organic solvents.

Decommissioning

This limit also applies to areas to be decommissioned. In radiation protection, "decommissioning" refers to the termination of radioactive work. Therefore the 0.5 B Bq/cm2 limit is still applicable for beta, gamma or x-ray radioactive work. The exception is alpha where the limit now drops to 0.05 Bq/cm2.

Remember to keep all records of measurement in the Radiation Log Book for at least 3 years.