Natural Radioactivity Evaluation of Local Soil used as Building Materials in Xinchang Section of Beishan Pre-selected Area, Northwest China

To investigate the radiation background level of jijicao block and rock mass around Xinchang section in the pre-selected area of Beishan, northwest China, the relevant soil samples and data have been obtained, and the natural radioactivity of the soil samples has been measured by gamma-ray spectroscopy with hyper-pure germanium detector. The mean activity concentrations of 226Ra, 232Th and 40K in soil samples are determined as 23.72 (from 11.2 to 48.6 Bq.kg-1), 28.72 (from 11.9 to 33.0 Bq.kg-1) and 612.93 Bq.kg-1 (from 244.0 to 907.0 Bq.kg-1), respectively, which are all lower than the Jiuquan background values, among which the concentration of 40K is higher than UNSCEAR 2008 and Beishan background value. The calculated data of radium equivalent, representative level index, external standard, internal standard, and annual effective dose are less than the recommended limits. Therefore, it can be concluded that the building being constructed of the materials is safe for the inhabitants. The findings from this research will be useful to assess the radiation hazards of building materials in humans. It is valuable for the environmental impact assessment of the underground research laboratory of high-level radioactive waste and the assessment of the hazard of radioactive building materials to human body.


Introduction
Soil is the basic resource for human survival and plays an extremely important role in the ecological environment. Generally speaking, the soil environment which is not affected by human activities in the natural state will not change after being placed in the humid environment for a certain period of time. Soil environmental conditions will show relatively stable characteristics. The background level of the soil environment is mainly determined and affected by the concentration of natural radionuclides such as rock, 226 Ra, 232 Th and 40 K in the earth's crust [1]. Diversity in geological conditions, bedrock composition, soil development types and topographic distribution in varied areas will lead to significant differences in the concentration of natural radionuclides 232 Th,226 Ra and 40 K in the soil [2]. However, with the development of the industrial revolution, radionuclides caused by human activities enter the soil environment, which leads to soil radionuclide contamination and long-term radiation hazard to human health [3]. Beishan pre-selected area is characterized by convenient transportation, weak economic conditions (such as lack of farmland, mineral resources, animal, and plant resources) and sparse population in Gansu province, northwest China. It has been selected as the most suitable area for HLW repository in China because of its superior social, economic and natural conditions [4]. In July 2011, the China Atomic Energy Authority, together with the Ministry of Ecology and Environment, approved that the Beishan area is "the first priority area" for China's HLW repository [5]. In order to use radionuclide soil as the building materials, it is of great significance to investigate and evaluate the radioactive environment of Beishan area. In this research, the background levels of radionuclides such as 226 Ra, 232 Th and 40 K in the jijicao block and the rock mass around Xinchang section in the pre-selected area of Beishan have been investigated. The radiation hazard of building materials to human body have been evaluated by calculating radium equivalent, gamma index, external standard, internal standard, absorbed dose and annual effective dose, which will be the basis for the construction, operation and management of URL of HLW.

Study area
The pre-selected area of the HLW disposal repository locates in the arid desert region of Gansu Province, northwest China. The study area lies in Xinchang section of Beishan pre-selection area, as shown in Figure 1. It has the characteristics of sparse rainfall, large evaporation, cold and dry wind, low temperature, short summer and long winter. The elevation is 1,600-1,800 m. The relative height difference of the terrain is within 100 m. The soil type on the hillside is mainly aeolian sandy soil, and the soil layer is relatively thin. Precipitation is mostly concentrated in summer, with an average annual precipitation of 85.2 mm and an average annual evaporation of 3,072.9 mm.

Sampling and analysis
60 sampling points (No. 108-167) were selected around the jijicao block and the rock mass in the open uncultivated area in spring ( Figure 2). The geographic location of the sampling point is recorded using the global positioning system (GPS) [7]. 60 surface soil samples were collected by the five-point sampling method. Stones, roots and other sundries are removed from all soil samples and each sample with the weight no less than 1 kg was put into plastic bag [8]. After the air is discharged, it is sealed and stored in a polythene bag with sampling information indicated, and then sent back to the laboratory for refrigeration. The samples were weathered in a dry environment for 30 days and dried in an oven at 110 ℃ for 6 hours [9]. The activity concentrations of 226 Ra, 232 Th and 40 K for all homogenized and equilibrium samples were measured by a gamma ray spectrometry (GB/T 11743-2013) by using a high purity Germanium (HPGe) detector (25% relative efficiency) and a coaxial-type vertical dipstick cryostat [10]. The detection limits of 226 Ra, 232 Th and 40 K were 7.05, 0.80, and 0.43 Bq· kg -1 , respectively [11]. The 226 Ra specific activities were estimated from 214 Bi (609.3 keV) and 214 Pb (295.2 and 352.0 keV), and the 232 Th specific activities were estimated from 228 Ac (911.1 keV), 212 Pb (583.1 keV), and 208 Tl (238.6 keV), while the specific activity of 40 K was determined directly from its gamma emission at 1460.83 keV. The analytical results of all samples met the quality control requirements of technical criteria for radiation environmental monitoring (HJ/T 61-2001) [12].  Figure 2. Soil sampling points in the jijicao block and the rock mass around Xinchang area.

Methodology 2.3.1 Radium equivalent activity (Raeq).
To assess the radiation hazard associated with the building materials, Raeq was evaluated. Where it is assumed that all decay products of 226 Ra and 232 Th are in radioactive equilibrium with their precursors. Raeq is calculated according to the following equation [13]: where ARa, ATh and AK are the specific activities of 226 Ra, 232 Th and 40 K in Bq· kg -1 , respectively. The equation is based on the estimation that 1 Bq· kg -1 226 Ra, 0.7 Bq· kg -1 232 Th and 13 Bq· kg -1 40 K produce the same gamma-ray dose rates [14]. Raeq is related to both internal doses due to the radon and external gamma doses, and it should have the highest value of 370 Bq· kg -1 for safe use in building materials [15].

Representative level index (Ir).
Representative level index (Ir) used to estimate the standard of gamma radiation hazard associated with the natural radionuclides in specific building materials may exceed 1 mSv· y -1 . It is calculated using the following equation [16]: (2) where CRa, CTh and CK in Bq· kg -1 are the concentration of 226 Ra, 232 Th, and 40 K, respectively.

External hazard index (Hex) and internal hazard index (Hin).The two hazard indexes (Hex and Hin)
are used to characterize the external and internal hazards due to the emitted gamma radiation. The primary objectives of Hex and Hin are to limit the radiation dose to the equivalent limit of 1 mSv· y -1 , and the two equations are as follows [17]:  [20].  232 Th is showed as white soil＞stiff soil＞sand stone＞mollisol＞sand soil. And the mean value of 40 K increases in order of mollisol＜white soil＜sand stone＜sand soil＜stiff soil. Table 1 also shows that the mean radionuclide concentrations of 226 Ra and 232 Th of all soils used as building materials are lower than the background values of Beishan, Jiuquan and the value specified in UNSCEAR 2008, while the concentration of 40 K of stiff soil is slightly higher than Beishan background value. 40 K concentrations of sand stone, sand soil, stiff soil, mollisol and white soil are all higher than the value specified in UNSCEAR 2008, and there are 1.3 to 1.45 times higher than Jiuquan background value. Table 2 shows the comparison of the radioactivity concentration of different types of soil in China and other parts of the world. The ranges of the mean values of the natural radionuclide concentration in building materials differ from one country to another, depending on the soil used for their formation. Except for 40 K, the concentration of radionuclides of 226 Ra and 232 Th in the soil types used as building materials in Xinchang section around the jijicao block and the rock mass is lower than the mean value in China. 226 Ra concentration of the different types of soil from Xinchang section is higher than the mean value in Pakistan, Cuba and Greece, but is lower than that in Iran and Brazil. The concentration of 232 Th is higher than that in Cuba, Greece, and Iraq, however it is lower than that in Pakistan and Brazil. The radionuclide concentration of 40 K is 1.4 to 4.5 times higher than those in the countries of Brazil, Pakistan, Greece, Cuba and Iraq. The previous research results revealed that the concentration of 40 K exceeds the Jiuquan background value and UNSCEAR 2008 may be caused by nuclear research and radioactive waste disposal activities in nearby area [24].

Radium equivalent activity (Raeq) in building materials
The mean value of the radium equivalent activity (Raeq) for sample soils of building materials is presented in Figure 3. The maximum Raeq is 117.2 Bq· kg -1 in white soil, while the minimum Raeq found in mollisol is approximately 103.805 Bq· kg -1 . The mean value of Raeq are 111.05, 112.51, 113.89 Bq· kg -1 in sand soil, sand stone and stiff soil, respectively. The estimated mean value of Raeq in the soil samples is 111.99 Bq· kg -1 , which is significantly less than the upper limit of 370 Bq· kg -1 , and it does not pose any radiological hazard when used for the construction of buildings.

Representative level index (Ir) in building materials
The mean value of the activity utilization index for different types of soil of building materials in Xinchang section around the jijicao block and the rock mass is shown in Figure 4. The mean activity utilization of Ir are 0.43, 0.42, 0.44, 0.39 and 0.44 mSv·y -1 in sand stone, sand soil, stiff soil, mollisol and white soil, respectively. And the mean value of Ir of different types of soil used in building material is 0.43. Ir <1 implies that the building materials in Xinchang section around the jijicao block and the rock mass are less than the specified value of 1 mSv· y -1 . The result indicates that the five types of soil are safe for using as building materials.

External hazard index (Hex) and internal hazard index (Hin) in building materials
The calculated mean values of Hex and Hin for all types of building materials are presented in Figure 5. The higher Hex of 0.32 and 0.31 are found in white soil and stiff soil, respectively. The obtained mean values of sand stone and mollisol used as building materials in this study are lower than the total mean value. The mean value of Hex estimated in total is 0.3 mSv· y -1 , which is significantly less than the upper limit of 1 mSv· y -1 . While the mean values of Hin is 0.37 mSv· y -1 , and the mean value of Hin are 0.37, 0.36, 0.36, 0.34 and 0.38 mSv· y -1 in sand stone, sand soil, stiff soil, mollisol and white soil, respectively, which are all less than the recommended level of 1 mSv· y -1 . We can say that the radiation hazard is insignificant for the population. Therefore, the five types of soil in this study can be safely used in the construction of buildings.

Absorbed gamma dose rate (DG) and annual effective dose rate (DE) in building materials
The mean values of DG in air for different types of soil used for building materials are shown in Figure  6. The maximum DG is 106.19 nGy· h −1 in white soil, while the minimum DG found in mollisol is approximately 94.14 nGy· h −1 . The estimated mean value of DG in the soil samples is 102.45 nGy· h −1 , which is slightly higher than world (populated-weighted) (UNSCEAR 2008) indoor DG of 84 nGy· h −1 by about 1.2 times. The mean annual outdoor DE of different types of soil for building material are also given in Figure 6. These values vary from 0.12 mSv· y -1 for mollisol to 0.13 mSv· y -1 for white soil. The estimated mean value of the annual DE of 0.13 mSv· y -1 is less than the permissible limit of 0.48 mSv· y -1 . Figure 6 also presents the mean DG and DE of 102.89, 101.91, 104.79 nGy· h −1 and 0.13, 0.12, 0.13 mSv· y -1 in sand soil, sand stone and stiff soil, respectively. The results indicate that the various types of building materials along with the DG and the DE, respectively. According to the current research progress, the five types of soil used as building materials in Xinchang section around the jijicao block and the rock mass shall be classified and used according to limits of radionuclides in building materials (GB6566-2010) [32]. At present, there are few studies on the effects of the soil types of radiation hazard indexes used as building materials. We compared the effects of the radiation hazard indexes related to the desert areas on the granite used as building materials. Table 3 shows the comparison of the radiation hazards of the granite samples in China and other parts of the world. The mean value of Raeq, Ir, Hex, Hin, DG and DE in the soil types used as building materials in Xinchang section around the jijicao block and the rock mass is lower than the mean value of the granite samples used as building materials in China, Pakistan and Saudi Arabia, but it is higher than that in Iran, Italy and Egypt. The previous research results revealed that granite is generally considered to be a building material with high concentration of radionuclides, but its radioactive background value is still related to its type [33].

Conclusions
The radioactivity concentrations of 226 Ra, 232 Th and 40 K in Xinchang section around the jijicao block and the rock mass are all lower than the background values of Jiuquan, among which the concentration of 40 K is higher than the background values of Beishan and the specified value in UNSCEAR 2008. The soils are divided into five types, in which the concentrations of 40 K radionuclides of sand stone, sand soil, stiff soil, mollisol and white soil are all higher than the background values of Jiuquan and the specified value in UNSEAR 2008, and the mean values of stiff soil are also higher than the background values of Beishan. The excessive concentration of 40 K may be related to nuclear research and radioactive waste disposal activities in nearby areas.
The mean values of Raeq and Ir for all types of building materials are significantly lower than the upper limit of 370 Bq· kg -1 and 1 mSv· y -1 , respectively. Hex and Hin of all kinds of soils used as building materials are less than the upper limit of the recommended level of 1 mSv· y -1 , which indicated that the radiation hazard had few effect on the residence and activities of the population. DE of the soil (0.48 mSv· y -1 ) is less than the upper limit of 1 mSv· y -1 , and the DG of five types of soil used as building materials are slightly higher than world mean value (populated-weighted) (UNSCEAR 2008) indoor DG of 84 nGy· h −1 . We suggest that the five types of soil in Xinchang section around the jijicao block and the rock mass shall be classified and used according to limits of radionuclides in building materials. The works of this research can provide reference for the construction of a URL and the environmental impact assessment of HLW, and also provide a quantitative basis for the assessment of human health hazards caused by radioactive building materials.