Population Exposure to Natural Radiation Sources in Romania

Transcription

1 Population Exposure to Natural Radiation Sources in Romania O. Iacob, E. Botezatu Radiation Hygiene Laboratory, Institute of Public Health, 14,V. Babes Str, Ro Iasi, Romania. Abstract. The present paper updates figures for the radiation doses received by the Romanian population from annual exposure due to the natural radiation background. The main natural sources of ionizing radiation under review are: naturally occurring radionuclides, taken up in the human body through ingestion and inhalation (radon and thoron progeny) as internal radiation sources, and, cosmic rays, and terrestrial gamma radionuclides as external radiation sources. The methods used for dose estimation were specific to each type of radiation source and exposure pathway. Internal exposure through ingestion of long-lived natural radionuclides in food and water has been evaluated from the natural radioactive content of diet using sensitive radiochemical methods and also multi-channel gamma spectrometry. Internal exposure due to inhalation of 222 Rn and 220 Rn daughters in indoor and outdoor air has been estimated from their individual concentration, measured by the active method of sucking air through filter and counting the deposited activity with an alpha scintillation counter. Exposure to cosmic radiation was calculated as a function of latitude and altitude. Exposure from gamma terrestrial radiation was derived from the average activity concentration of, and in soil measured by high-resolution gamma spectrometry techniques and the resulting absorbed dose rate in air. The dosimetric coefficients used in estimations are those published in UNSCEAR Report and the ICRP Publications and an indoor occupancy factor of 0.75 and an outdoor occupancy factor of 0.25 were used in all calculations. The new value for the annual per capita resulted from natural radiation background in Romania is 2.51 msv, with a corresponding annual collective of man Sv. The present estimate of the annual total is lower than the previous one by about 20%. 1. Introduction The natural radiation background is by far the most important source of human exposure to ionizing radiation. In Romania, according to the latest estimate, its contribution to the annual exposure of population represents about 84 percent of all radiation sources [1]. The natural radiation background includes naturally-occurring radionuclides taken up into the human body through inhalation ( 222 Rn and 220 Rn progeny) and ingestion (, 226 Ra, 210 Pb 210 Po, ) and - as internal radiation sources, and, cosmic radiation (directly and indirectly ionizing components) and terrestrial gamma radiation from radionuclides (, and series) present in the earth s crust and building materials as external radiation sources. The purpose of this study was to update figures on the magnitude of natural radiation exposure to the population on the basis of the new dosimetric models of the ICRP and measurement techniques. 2. Methods The methods used for dose estimations were specific for each type of radiation source and exposure pathway. Internal exposure due to inhalation of 222 Rn and 220 Rn daughters in indoor and outdoor air was estimated from their individual concentrations ( 218 Po, 214 Pb, 214 Bi, 212 Pb, 212 Bi). These were measured by means of an active method of sucking air through a filter and counting the deposited activity using an alpha scintillation counter [2]. A computer program, very versatile in handling multiple input parameters, was developed to solve the decay equations for obtaining the activity concentrations of daughters in air, to calculate the equilibrium equivalent concentration (EEC), the potential alpha energy concentrations and the equilibrium factor F, for radon daughters. Internal exposure through the ingestion of long-lived natural radionuclides in food and water was estimated from the natural radioactive content of diet, determined by using sensitive radiochemical methods and also by means of multi-channel gamma spectrometry. 1

2 Exposure from terrestrial gamma radiation was derived from the average activity concentration of,, and in soil, measured by means of high-resolution gamma spectrometry techniques, and the resulting absorbed dose rate in air. Exposure to cosmic radiation was calculated as a function of latitude and altitude [3]. The dosimetric coefficients used in estimations are those published in the UNSCEAR 2000 Report [3] and the ICRP Publications [4,5]. An indoor occupancy factor of 0.75 and an outdoor occupancy factor of 0.25 were applied in all estimates for a population of million inhabitants [6]. Exposures are expressed in terms of both individual and collective annual s. 3. Results and discussion (1). Radon and thoron short - lived decay products. Exposure to 222 Rn and 220 Rn daughters in dwellings makes the largest contribution to the overall population dose. Indoor concentration vary widely as is shown in Table I, which summarizes both the results of radon and thoron progeny concentration measurements made in 780 urban and rural typical dwellings throughout Romania, and the resulting annual s for adult. Table I. Average annual s from radon and thoron progeny inhalation Radionuclid Location Equilibrium Equivalent Annual Concentration EEC (Bq m -3 ) average range average range 222 Rn progeny INDOORS Detached house Block of flats OUTDOORS Rn progeny INDOORS Detached house Block of flats OUTDOORS The average values of the equilibrium equivalent concentrations (EEC) was evaluated at 11.7 Bq m -3 in block of flats and at 36.3 Bq m -3 in detached houses. The average EEC values for thoron progeny have been found to be of 1.4 Bq m -3 and 0.8 Bq m -3 in detached houses and block of flats, respectively. The resulting average annual of an individual living in a detached house was 2146 µsv and 368 µsv from radon and thoron progeny and of 692 µsv from radon progeny and 210 µsv from thoron progeny in block of flats. Taking into account that about 54% of Romanian population lives in detached houses and 46% in block of flats, the population-weighted averages of 25Bq m -3 EEC of 222 Rn and of 1.1 Bq m - 3 EEC of 220 Rn are associated with annual s of 1478 µsv and 289 µsv, respectively. Average annual doses received outdoors are much lower: 112 µsv from radon progeny, and only 26 µsv from radon progeny. Table II includes the resulting s received annually by inhalation of radon and thoron progeny. The total annual from internal exposure by inhalation was 1905 µsv and the corresponding annual collective was man Sv, with the main contribution of radon progeny (82 per cent). 2

3 Table II. Annual internal exposure from 222 Rn and 220 Rn progeny inhalation Radionuclide Annual per capita (man Sv) 222 Rn progeny indoors outdoors total 220 Rn progeny indoors outdoors total (2). Long-lived natural radionuclides. Internal exposure following the ingestion of long-lived natural radionuclides with food and drinking water was derived from the average annual activity intake for the adult and using the ICRP age-dependent per unit of activity intake coefficients. The results are included in Table III. The average annual was of 272 µsv. This value is mostly due to (165 µsv) which is metabolically controlled in the body and to 210 Pb 210 Po radionuclides (130 µsv), the others contributions representing only 3.4% of the total. The annual collective from internal exposure by ingestion was estimated at 6140 man Sv. Table III. Average annual exposure from ingestion of long-lived natural radionuclides. Radionuclides Annual activity intake (Bq) Annual per capita (man Sv) 234 U 226 Ra 210 Pb 210 Po 238 Th 5.8 ± ± ± ± ± ± ± (3). Cosmic radiation. The average annual s from both directly and indirectly ionizing components of cosmic rays are presented in Table IV. Table IV. Average annual exposure to cosmic rays in Romania Cosmic radiation Annual Ionizing component Neutron component per capita collective (man Sv) Since 57% of population live at altitudes below 0.2 km, 35% up to 0.4 km and only 8%above 0.4 km and at latitudes between degrees, the population-weighted average annual per capita effective dose arising from cosmic rays was estimated at 335µSv. The corresponding annual collective dose was 7555 man Sv. 3

4 (4). Terrestrial gamma radiation. Activity concentrations of, and have been measured in about 900 soil samples, the sampling points being deliberately chosen at places with relatively highdensity population. The average values and ranges of activity concentrations as well as the resulting absorbed dose rates in air, both outdoors and indoors, are presented in Table V. The outdoor average absorbed dose rate in air for Romania is 59 ngy h -1, ranging from 21 ngy h -1 to 122 ngy h -1. Using an average indoor to outdoor ratio of 1.4, resulted from the natural radioactive content of building materials, the indoor absorbed dose rate in air was estimated at 83 ngy h -1, with values lying between 30 ngy h -1 and 170 ngy h -1. Radionuclide or decay series Table V. Annual average exposure to terrestrial gamma radiation Activity concentration in soil Absorbed dose rate in air (ngy h -1 ) (Bq kg -1 ) outdoors indoors Average* Range Average Range Average Range *population weighted average By applying the conversion coefficient from air kerma to for terrestrial gamma radiation 0.72 Sv Gy -1 to our estimates of indoor and outdoor absorbed dose rate in air, it was possible to calculate the average annual s per capita, as well as the annual collective dose. The results are summarized in Table VI. Table VI. Average annual exposure to terrestrial gamma radiation Radionuclide or decay series Annual per capita (man Sv) Outdoors Indoors The resulting annual average per capita was 486µSv with the corresponding annual collective dose of man Sv. The relative contributions of, and to this collective dose are of 42%, 35% and 23%, respectively. 4. Summary The estimates of the average s received annually by people of Romania from natural radiation background are summarized in Table VII and the contribution of each natural radiation source to the annual collective is illustrated in FIG.1. 4

5 Table VII. Annual exposure from natural radiation background in Romania Source of exposure Inhalation 222 Rn 220 Rn Ingestion 40 K,, Annual effective dose per capita (man Sv) % Cosmic radiation Gamma terrestrial TOTAL Rn % Rn % Cosmic radiation 11.2% Gamma terrestrial Ingestion K-40, U-16.2% 238,TH % FIG.1. Contribution of the natural radiation sources to the annual collective The new value of the average per capita resulted from the annual exposure to the natural radiation background in Romania is 2512 µsv, of which: 1478 µsv is due to radon progeny inhalation, 289 µsv is due to thoron progeny inhalation, 272 µsv is due to ingestion of natural radionuclides, 355 µsv is due to cosmic radiation and 486 µsv to gamma terrestrial radiation. The total annual collective resulting from natural radiation exposures in Romania is man Sv at which, the radon progeny is the main contributor (53%). The others contributions, in order of their importance, are: 16.2% -gamma terrestrial radiation, 11.2% -cosmic rays, 10.5% -thoron progeny inhalation and 9.1% -internal exposure by ingestion. The distribution of the Romanian population with respect to total annual from natural radiation background is illustrated in FIG.2. It was estimated that about 69% of population received annual exposures between 1.5mSv and 3 msv, 29.6% of population had exposures between 3 msv and 5 msv and only 1.4% of population had exposures greater than 5 msv. The present estimate of the population exposure from the natural radiation background in Romania is lower than the previous one by about 20% [7]. 5

6 % msv (msv) FIG.2. Distribution of the Romanian population with respect to total annual From natural radiation background 5. References 1. Iacob, O., Diaconescu, C., Botezatu, E., An update of exposures from natural and artificial sources of ionizing radiation in Romania. Journal of Preventive Medicine, 6, No.3: 7-16, (1998). 2. Romanian Institute of Standardization, Air quality: Volumetric activity determination of Polonium-218, Lead-214, Bismuth-214 and Lead-212 Romanian Standard SR-13397, (1997). 3. United Nation Scientific Committee on the Effects of Atomic Radiation, Sources and Effects of Ionizing Radiation, UNSCEAR 2000 Report to General Assembly with Scientific Annexes, Vol. I: Sources, Annex B: Exposures from natural radiation sources: , United Nations, New York, (2000). 4. International Commission on Radiological Protection, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 2. Publication 67. Annals of the ICRP, 23, Nos. 2-3, Pergamon Press, Oxford and New York (1994). 5. International Commission on Radiological Protection, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 3. Publication 69. Annals of the ICRP, 25, No.1, Pergamon Press, Oxford and New York (1995). 6. National Institute of Statistics, Romanian Statistical Year Book, Bucharest, ISSN (2001). 7. Iacob, O., Botezatu, E., in Proceedings of the 5 th International Conference on HLNRRA: Radiation Dose and Health Effects, Munich, 2000, edited by BfS, (Salzgitter, 2002) vol. II, p.7. 6