Naturally occurring radioactive material (NORM) from a former phosphoric acid processing plant

Area of Science:

• Environmental Science
• Radiological Science
• Industrial Chemistry

Background:

• Non-nuclear industries processing ores and minerals can lead to radioactive contamination from naturally occurring radioactive material (NORM).
• Phosphoric acid production is a significant industrial process with potential for NORM accumulation in plant materials and waste.

Purpose of the Study:

• To investigate the nature and concentration of NORM in scale samples from a decommissioned phosphoric acid plant.
• To identify the specific plant areas and material compositions where NORM is retained.
• To assess the potential for radioactivity release from these materials.

Main Methods:

• Scale samples were collected from four key areas of a decommissioned phosphoric acid plant: Green Acid Plant, storage tanks, Purified White Acid plant, and solid waste landfill.
• Chemical composition of scale samples was analyzed, identifying fluorides, calcium sulphate, and mixed fluorides/phosphates.
• Radioactive inventory was determined, focusing on uranium-238 (238U) and its decay products, including radium-226 (226Ra).
• Water extraction tests were performed to evaluate the solubility and release potential of radioactivity.

Main Results:

• Scale samples predominantly consisted of fluorides, calcium sulphate, and mixed fluorides/phosphates.
• The radioactive inventory was dominated by 238U and its decay chain, with significant fractionation observed.
• Elevated concentrations of 238U (up to 8.8 Bq/g) were found in fluoride- and phosphate-rich areas.
• Enriched levels of 226Ra (up to 11 Bq/g) were associated with calcium sulphate precipitates.
• Water extraction tests revealed that many scale and waste materials readily release radioactivity into solution.

Conclusions:

• NORM, particularly 238U and 226Ra, concentrates in specific process scales and waste materials within phosphoric acid plants.
• The chemical composition of these materials influences NORM retention and fractionation.
• The solubility of these materials indicates a potential for environmental radiological risk through the release of radioactivity into aqueous solutions.