Radiological risks and excess lifetime cancer risk of the topsoil around the coal-fired Plomin thermal power plant (Istria, Croatia) and long-term effects after ceasing use of the coal with elevated radionuclides activities

Abstract

Radiological and health risks arising from ²²⁶Ra, ²³²Th, and ⁴⁰K in topsoil due to coal combustion in Plomin thermal power plant were assessed: outdoor absorbed dose rate in air (D), annual outdoor effective dose rate (D_ef), external hazard index (H_ex), internal hazard index (H_in), and excess lifetime cancer risk outdoors (ELCR_out). Spatial distribution of risks around the plant was studied and relative contributions of ²²⁶Ra, ²³²Th, and ⁴⁰K to D (applies to D_ef and ELCR_out as well), H_ex, and H_in were determined. The risks were studied at two soil depths (A: 0–10 cm, B: 10–25 cm), radially around the plant at 1 km, 5 km, and 10 km distances from the plant, and in a downwind (SW) profile at 0.1–1 km distance from the plant. Elevated D, D_ef, H_in, and ELCR_out were determined, while H_ex was not elevated. Almost all D, D_ef, and ELCR_out values were above the world average for soils (58 nGy/h, 0.07 mSv/y, and 0.29 × 10⁻³, respectively). D, D_ef, and ELCR_out were: 32–338 nGy/h (mean value: 116 nGy/h), 0.039–0.414 mSv/y (mean value: 0.142 mSv/y), and 0.17 × 10⁻³–1.79 × 10⁻³ (mean value: 0.61 × 10⁻³), respectively. H_ex was in the 0.18–1.98 range (mean value: 0.69), with only two extreme values above the recommended limit of 1. H_in was in the 0.22–3.67 range (mean value: 1.02), with most of the values above the recommended limit of 1 in the downwind profile and at one station with extremes (1 km from the plant). A “hot spot” was determined for all risks at 1 km distance from the plant in the wind direction (SW from the plant). The next highest, elevated, risks were observed in the downwind profile stations. The most important parameters influencing spatial distribution of risks are ²²⁶Ra activities in soil, wind direction, and distance from the plant. ²²⁶Ra is generally the most important contributor to risks in soils, while ⁴⁰K is the least important. ²²⁶Ra and ²³²Th were found to be the most significant and comparable contributors to D, D_ef, H_ex, and ELCR_out. Only ²²⁶Ra was found as the most significant contributor to H_in in the studied area. Elevated risks are partially from the natural source (carbonate bedrock) and partially from the power plant (coal combustion and handling, ash deposition on soil).