Dose assessment to workers in a dicalcium phosphate production plant

The production of dicalcium phosphate (DCP) uses phosphate rock (PR) as a raw material. Sedimentary phosphate rocks are enriched with relevant concentrations of natural radionuclides from the ²³⁸U decay chain (around 10³ Bq·kg^-1), leading to the need of controlling potential exposures to radiation of workers and members of the public in accordance with IAEA safety standards. Indeed, phosphate industries are classified as Naturally Occurring Radioactive Material (NORM) industries. Thus, the aim of this work is to assess the radiological risk of the workers in a DCP production plant located in the Iberian Peninsula (South-West Europe), which digests PR with hydrochloric acid. In the present study ²³⁸U, ²³⁰Th, ²²²Rn, ²¹⁰Pb and ²¹⁰Po concentrations in aerosols (indoor and outdoor areas) are reported. Aerosols showed concentrations between 0.42-92 mBq·m^-3 for ²³⁸U, 0.24-33 mBq·m^-3 for ²³⁰Th, 0.67-147 mBq·m^-3 for ²¹⁰Pb and 0.09-34 mBq·m^-3 for ²¹⁰Po. Long-term exposure (four months) of passive ²²²Rn detectors provided concentrations that ranged from detection limit (< DL) to 121 Bq·m^-3 in outdoor areas and from < DL to 211 Bq·m^-3 in indoor areas, similar to concentrations obtained from short-term measurements with active detectors from < DL to 117 Bq·m^-3 in outdoor areas and from < DL to 318 Bq·m^-3 in indoor places. ²²⁶Ra accumulation in ebonite and pipe scales were the most important contributions to the ambient dose equivalent H*(10), resulting in 0.07 (background)-27 μSv·h^-1 with a median value of 1.1 μSv·h^-1. Average ²²²Rn air concentrations were lower than the 300 Bq·m^-3 limit and therefore, according to European Directive 2013/59/EURATOM, ²²²Rn concentration is excluded from the worker operational annual effective dose. Thus, considering the inhalation of aerosols and the external dose sources, the total effective dose determined for plant operators was 0.37 mSv·y^-1.