The unintentional radon chamber: ventilation of NORM stores

Full text: During a routine audit of a store for unsealed radioactive material, elevated surface contamination levels (0.3 – 2.2 Bq.cm^-2) were detected on the clothing and bodies of the two consultants conducting the audit. All controls for safe handling of the material in the store had been adhered to, and by exclusion it was inferred that the contamination levels were due to elevated levels of radon and thoron decay products from a quantity of NORM material in the store. This was supported by the rapid decrease in contamination on all surfaces once the consultants left the store. A potential source of contamination was identified (container of NORM). Wipe tests taken from inner surfaces of the container were assessed by gamma spectroscopy. Elevated levels of Pb210 and evidence of radon decay products were confirmed. Further investigation and monitoring were conducted over the following weeks showing radon at levels exceeding the ARPANSA reference levels for radon in workplaces. Grab samples indicated elevated radon decay products at levels that would result in exceedance of the member of public radiation dose limit within five standard working weeks. While regulatory frameworks in Australia require that radioactive material is stored securely and provide high level guidance on measures to ensure safe storage that will maintain radiation doses to within constraint levels, the specific controls are left to the owner of the material to determine and implement. Where source inventories include NORM materials, the owners should be cognisant of the potential for significant build up of radon and decay products that can result from a relatively small quantity of material under conditions where air exchange is not adequate. (author)