Year : 2022 | Volume
: 45 | Issue : 2 | Page : 63--64
Radiological assessment of natural radioactivity in building materials: The common approach
Editor, RPE, Ex. Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
D D Rao
Editor, RPE, Ex. Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
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Rao D D. Radiological assessment of natural radioactivity in building materials: The common approach.Radiat Prot Environ 2022;45:63-64
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Rao D D. Radiological assessment of natural radioactivity in building materials: The common approach. Radiat Prot Environ [serial online] 2022 [cited 2023 Mar 23 ];45:63-64
Available from: https://www.rpe.org.in/text.asp?2022/45/2/63/364560
The earth's crust and the materials on it like soil, rocks, and vegetation, etc., contain natural radionuclides of uranium, thorium, their daughter products, and Potassium-40. Building materials are generally produced from the rocks and soil and thus also contain natural radionuclides. The daughter products of U-238 (Pb-212, Bi-212, and Ra-226), Th-232 (Pb-214, Bi-214, Ac-228, and Tl-208), and K-40 emit gamma rays and cause external gamma radiation exposure. UNSCEAR gives dose conversion factors (DCFs) in nGy/h per Bq/kg for Th-232, U-238, and K-40 in an undisturbed soil to estimate the external gamma radiation dose to an exposed individual. The DCFs are 0.0417, 0.462, and 0.604 nGy/h per Bq/kg for K-40, U-238, and Th-232, respectively. The calculated outdoor terrestrial gamma dose rate is 54 nGy/h. The global average concentrations of soil radioactivity are K-40:400 ± 24 Bq/Kg, U-238:37 ± 4 Bq/Kg, and Th-232:33 ± 4 Bq/kg. Using 0.7 Sv/Gy conversion factor for absorbed dose to effective dose, 0.2 occupancy factor, the annual average external exposure to adults due to natural gamma radionuclides is 0.066 mSv/yr.
Inadvertently or unknowingly or in a hurried situation or due to the popularity of using UNSCEAR DCFs, many investigators apply the soil DCFs of UNSCEAR for building materials also, to estimate the external gamma dose rates and there from the annual effective dose (AED). UNSCEAR has not given DCFs for building materials, although their publications contain enough data regarding the natural radioactivity in building materials. The purpose of this editorial is to give a clear direction or message to apply appropriate conversion factors to arrive at the correct effective dose from the natural radioactivity in building materials.
European Commission's Radiation Protection report -112 gives principles for the assessment of natural radioactivity in building materials and relates an index to the effective dose. All building materials contain various amounts of natural radionulides. Materials derived from the rock and soil contain mainly radionuclides of U-238 series, Th-232 series and K-40. The most common building materials are as follows: concrete, clay bricks, sand lime bricks, building stones, gypsum, coal fly ash, and furnace slag. The radiation exposure due to building materials can be both external and internal exposures. For instance, an individual living in an apartment block made of concrete structure containing average concentrations of Ra-226 (often referred to U-238), Th-232 and K-40 of 40 Bq/kg, 30 Bk/kg, and 300 Bq/kg, respectively, receives AED of 0.25 mSv/yr-based on computer-simulated calculations. The report also gives AED for several other considerations of enhanced radioactivity concentrations. The internal exposure is caused by the inhalation of Rn-222 and Rn-220 and their short-lived decay products. As per the EU guidelines, building materials can be exempted from all restrictions, if the excess gamma radiation exposure dose from them increases the AED to a member of public by 0.3 mSv/a at the most. This is in excess of outdoor gamma dose received by the member of public.
As per EC report 112 and IAEA SSG-32, an investigation level concentration index (I) is recommended to relate the concentrations of U-238 (Ra-226), Th-232, and K-40 in building materials to AED.
CRa, CTh, and CK represent the concentration of Ra-226 (U-238), Th-232, and K-40 in Bq/kg, respectively.
The parameter values for calculating the absorbed dose for a room structure with floor, ceiling, and walls constructed with concrete material are 0.92 nGy/h per Bq/kg, 1.1 nGy/h per Bq/kg, and 0.08 nGy/h per Bq/kg, for U-238, Th-232, and K-40, respectively. The other model parameters considered are of 4 m × 5 m × 2.8 m room dimensions, 20 cm wall thickness, 7000 hours (0.8 factor of the day), 50 nGy/h of background and 0.7 Sv/Gy.
Average concentrations of U-238 (40 Bq/kg), Th-232 (30 Bq/kg), and K-40 (400 Bq/kg) in building materials will lead to an effective dose of 0.25 mSv/a in excess of background. Furthermore, for I<1 for bulk materials such as bricks and concrete, the AED from exposure to gamma radiation from radionuclides in building materials will be less than the reference level of 1 mSv. Such building materials should not be subject to restrictions on their use (IAEA SSG-32). The EC report also gives the parameter values for various other considerations of material and room structure.
Most researchers use the ready reckoner parameters of UNSCEAR or EU reports for the assessment of radiological status of building materials. There is a plenty of scope to work on other topics of modelling by considering several other model parameters and generate the conversion factors. Hope this editorial will be of some help to appropriately use the proper conversion factors for the assessment for each case of radioactivity in soil and building materials according to the situation.