Natural radioactivity,radon exhalation rate and radiation dose of fly ash used as building materials in Xiangyang,China

Abstract

Natural radioactivity level, radon exhalation rate and radiation hazard of fly ash used as building materials in Xiangyang, China were determined. The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in the investigated fly ash samples range from 90.3 to 1799.4, 59.9 to 145.6 and 309.0 to 906.3 Bq kg⁻¹ with an average of 440.5, 110.3 and 510.1 Bq kg⁻¹, respectively. ²²⁶Ra and ⁴⁰K are the main contributors of total activity concentration in red fly ash and grey fly ash, respectively. Radon exhalation rates of the studied fly ash samples vary from 4.7 to 74.2 Bq m⁻² h⁻¹ with the mean of 21.1 Bq m⁻² h⁻¹. The relative hazard indexes, i.e. radium equivalent activity, external hazard index, internal hazard index, indoor air absorbed dose rate and annual effective dose, indoor radon exposure dose, outdoor air absorbed dose rate and annual effective dose were calculated to assess the radiation hazard of fly ash when it was used as building materials or piled on the ground. The assessment results show that the studied fly ashes would cause excessive radiation risks to inhabitant and they are not suitable to use as building material. The ash storage pond is a high radiation risk area and the population living around ash pond would be exposed to higher radiation.

Keywords

Fly ash Natural radioactivity Radon exhalation rate Radiation hazard Building materials

Introduction

Coal, an important fossil fuel, contains trace quantities of the primary radionuclides and their decay products, particularly ⁴⁰K and the decay series headed by ²³²Th and ²³⁸U. These natural radionuclides would enter into the combustion products of coal. Coal is a technologically important material used for power generation. The increasing demand for electricity generation for industrial development and human living standards worldwide is met by combustion of fossil fuels.¹ China depends largely on coal for her energy needs, contributing more than 70% for the total power generation.² Coal, burned as fuel material in power plants, produces energy and a large amount of solid waste, i.e. coal ash. Fly ash particles, entrained up the stack in the flue gas stream, have a greater tendency to absorb trace elements such as natural radionuclides during combustion owing to their relatively small size and large surface area.^3–6 So, the natural radionuclides may be concentrated in fly ash after coal combustion in a thermal power plant.^7–9 Fly ash from coal-fired power plants is spread and distributed in surrounding area by air and may be deposited on the soil surface, thus may increase the radioactivity level of local environment.^3,4 Since fly ash contains elevated natural radionuclides compared to raw coal, therefore fly ash is considered as a potential source of radiation exposure to man. Natural radionuclides in fly ashes may pose radiation risks externally due to their gamma-ray emissions and internally due to radon and its progeny that emit alpha particles^1,3,4,10 when fly ashes used in building materials or piled on the ground.

Radon, a naturally occurring odourless and colourless α-emitting radioactive noble gas, has three isotopes, i.e. ²²²R, ²²⁰Rn and ²¹⁹Rn, of which half-lives are 3.82 days, 55.6 and 3.96 s, respectively.^11,12 Amongst the three isotopes, ²²²Rn has the most significant impact on the environment due to its relatively long half-life, enabling it to migrate quite significant distances within the geological environment before decaying. ²²²Rn is a radioactive extremely inert gas which is derived from the radioactive decay of radium (²²⁶Ra), a decay element in ²³⁸U series. ²²²Rn, as an emitter of α-particles with energy 5.48 MeV, is the most crucial and dangerous radioactive gaseous element in the science of environmental radioactivity,^13,14 which constitutes the most important natural source of radiation in the environment. ²²²Rn and its progeny are present in all dwellings, because ²²⁶Ra is inherently present in building materials as well as in the soil.^14–16 Indoor ²²²Rn has been recognized as one of the health hazard for mankind. Building materials used for construction of houses are considered as major sources of indoor ²²²Rn,¹⁰ and thus is important to understand the generation and migration process of radon from building materials, as radon can contribute to 55% of total radiation dose received by the population from the environment.¹⁰

Fly ash is widely used in building materials, e.g. in the production of brick, cement and concrete, and as underground cavity filling material. Therefore, fly ash is of considerable economic and environmental importance. There have been increasing concerns regarding the environmental radioactivity of fly ash during past decades. A large amount of researches on natural radioactivity level and radon exhalation rate have been done all over the world.^{1,3,4,7,17–23} The relative work in China, however, is limited,^2,6 especially the radon exhalation rate of fly ash. In China, coals used in coal-fired power plants are found to have high ash contents, resulting in the production of large amount of fly ashes. In the present work, natural radioactivity, radon exhalation rate and associated radiation hazard of fly ash used in Xiangyang, China have been determined. The results could offer the basic information for safety procedure and disposal of fly ash.

Materials and methods

Sample collection and preparation

Xiangyang, the second biggest city of Hubei province, is located at the northwest of Hubei province (31°54′–32°10′N and 112°00′–112°14′E), with a population of approximately 5.5 million. Seventeen grey fly ash (GFA) samples (numbered GFA1 to GFA17) and seven red fly ash (RFA) samples (numbered RFA1 to RFA7) were collected from brick factories and ash storage ponds of Xiangyang in 2013 and 2014. All collected samples were kept in cleaned and numbered polyethylene bags.

Each sample was homogenized and dried in an oven at 105℃ for 12 h to remove moisture. The dried samples were weighed and sealed in gas-tight, radon impermeable, cylindrical polyethylene containers (7.0 cm height and 6.5 cm diameter). These samples were then left for 4–5 weeks to allow for radium, thorium and their short-lived progenies to reach radioactive equilibrium.^5,24

Radioactivity measurement

The activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K in fly ash samples was determined using a high-resolution gamma-ray spectrometric system (ORTEC, USA) with a p-type coaxial HPGe detector with a relative efficiency of 50% and an energy resolution of 1.9 keV at 1332.5 keV of ⁶⁰Co. The detector is maintained in a vertical position in a lead cylindrical shield of 12 cm thickness and 65 cm height to reduce background radiation and kept cool by electric refrigeration system. The detector was coupled to an 8192 multi-channel pulse height analyser and the system was calibrated for the gamma energy range of 40 keV to 10 MeV. The absolute efficiency calibration of the gamma spectrometer was determined using a solid nuclide mixture of gamma reference materials sealed in standard cylindrical polyethylene containers (7.0 cm height and 6.5 cm diameter).

The activity of ²³²Th was determined by 338.5, 911.1 and 968.9 keV gamma rays emitted from ²²⁸Ac and by 583.19 and 860.56 keV from ²⁰⁸Tl, respectively. The activity of ²²⁶Ra was measured by 609.4 and 1120.4 keV gamma rays emitted from ²¹⁴Bi and by 295.21 and 351.92 keV from ²¹⁴Pb, whereas ⁴⁰K activity was measured directly through its gamma ray energy peak of 1460.8 keV.^25,26 The standard sources for ²²⁶Ra and ²³²Th (in secular equilibrium with ²²⁸Th) were prepared using known activity contents and mixing with the matrix material of phthalic acid powder. Analar grade potassium chloride (KCl) of a known amount of the same geometry was used as the standard source of ⁴⁰K.²⁴ Each sample was counted for 28,800 s.^27,28 Background measurements were taken under the same conditions of sample measurements. The uncertainties of measurements for three radionuclides are all less than 10%.

Radon exhalation rate measurement

The radon exhalation rates of fly ash samples were measured with electrostatic radon sampler-2 (ERS-2) (Tracerlab company, Germany), which is an active and electrostatic monitor equipped with an accumulation chamber and a solid-state alpha detector to measure the radon from materials. The ERS-2 operates with an alpha spectroscopy detector and multi-channel analyser with 256 channels. For determination of radon exhalation rate, ERS-2 was placed on the surface of the measured sample with silicone-sealing ring to avoid air leakage. Then the diffusion mode operation was selected and high voltage was turned on to reach the stable HV 500. Each sample was counted for 6000 s and radon exhalation rate (Bq m⁻² h⁻¹) in fly ash was calculated using ERSEval software.^27,29

Results and discussion

Specific activity

The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in the investigated fly ash samples are given in Table 1. As shown in Table 1, the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in GFA samples range from 90.3 to 165.6, 83.9 to 145.6 and 309.0 to 593.2 Bq kg⁻¹ with an average of 134.0, 123.8 and 455.0 Bq kg⁻¹, respectively; while in RFA samples, these radionuclides range from 658.0 to 1799.4, 59.9 to 93.5 and 459.1 to 906.3 Bq kg⁻¹ with an average of 1185.0, 77.5 and 643.8 Bq kg⁻¹, respectively. The concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K occupy the total activity of 15.4–21.7%, 14.0–20.7% and 60.1–65.7% in GFA samples, respectively, 59.2–65.9%, 2.2–6.6% and 31.9–34.9% in RFA samples, respectively (Figure 1), which indicate that the specific activity due to ⁴⁰K is the largest contributor to the total activity for GFA samples and ²²⁶Ra is the largest contributor to the total activity for RFA samples. The concentrations of ²²⁶Ra and ⁴⁰K in RFA samples are significantly higher than those in GFA samples, while ²³²Th concentrations in RFA samples are lower than those in GFA samples. The concentrations of ²²⁶Ra in all RFA samples are 3.7–10.0 times the typical activity concentration of ²²⁶Ra in fly ash (180 Bq kg⁻¹) of European Union countries,³⁰ while in all GFA samples, the concentrations are less than the typical value given by the European Union. The mean concentration of ²²⁶Ra in RFA samples is also larger than the maximum activity concentration of ²²⁶Ra in fly ash (1100 Bq kg⁻¹) of European Union countries.³⁰ The concentrations of ²³²Th in all RFA samples are lower than, while in most GFA samples (94%) are larger than, the typical activity concentration of ²³²Th in fly ash (100 Bq kg⁻¹) of European Union countries.³⁰ The concentrations of ²³²Th in all samples are lower than the maximum activity concentration of ²³²Th in fly ash (300 Bq kg⁻¹) of European Union countries.³⁰ The concentrations of ⁴⁰K in all GFA samples and four RFA samples (57%) are lower than the typical activity concentration of ⁴⁰K in fly ash (650 Bq kg⁻¹) of European Union countries.³⁰

Figure 1.

Relative contributor of ²²⁶Ra, ²³²Th and ⁴⁰K to the total activity concentration.

Table 1.

Activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in fly ash samples and radon exhalation rate.

Sample	Activity concentration (Bq kg⁻¹)			Radon exhalation rate (E_x) (Bq m⁻² h⁻¹)
Sample	²²⁶Ra	²³²Th	⁴⁰K	Radon exhalation rate (E_x) (Bq m⁻² h⁻¹)
Grey fly ash
GFA1	141.0	145.6	431.2	7.9
GFA2	146.3	141.3	449.8	12.2
GFA3	138.7	143.8	452.3	13.0
GFA4	119.5	105.9	420.7	4.7
GFA5	123.4	112.1	442.2	6.5
GFA6	164.0	105.7	486.3	20.2
GFA7	135.0	114.3	478.7	13.3
GFA8	131.8	124.1	423.6	12.6
GFA9	90.3	83.9	309.0	5.4
GFA10	132.3	117.6	446.6	13.7
GFA11	147.7	132.2	521.6	18.7
GFA12	165.6	141.6	593.2	20.5
GFA13	134.2	120.0	433.2	6.8
GFA14	143.8	128.9	488.8	9.4
GFA15	127.1	125.2	487.8	7.6
GFA16	101.4	136.7	422.3	5.8
GFA17	135.3	126.3	447.5	8.3
Min	90.3	83.9	309.0	4.7
Max	165.6	145.6	593.2	20.5
Mean	134.0	123.8	455.0	11.0
Red fly ash
RFA1	1799.4	59.9	871.1	74.2
RFA2	1775.5	68.2	906.3	66.6
RFA3	1640.1	64.7	872.8	60.8
RFA4	745.8	80.3	474.6	30.2
RFA5	813.4	89.5	460.1	28.1
RFA6	658.0	86.6	459.1	23.4
RFA7	862.8	93.5	462.8	36.4
Min	658.0	59.9	459.1	23.4
Max	1799.4	93.5	906.3	74.2
Mean	1185.0	77.5	643.8	45.7
All investigated fly ash samples
Min	90.3	59.9	309.0	4.7
Max	1799.4	145.6	906.3	74.2
Mean	440.5	110.3	510.1	21.1

Compared to the natural radioactivity of local soil, the concentrations of ²²⁶Ra and ²³²Th in GFA and RFA samples are significantly higher than in Xiangyang soil (31.3 and 56.5 Bq kg⁻¹ for ²²⁶Ra and ²³²Th, respectively),³¹ while ⁴⁰K concentrations in two types of fly ash samples are lower than found in Xiangyang soil (675.0 Bq kg⁻¹).³¹ When fly ash was added to soil with 1:1 ratio to produce brick, the radioactivity concentrations of ²²⁶Ra and ²³²Th in the products produced by using GFA and soil were higher than bricks produced by just using soil by 164% and 60%, respectively. The radioactivities of ²²⁶Ra, ²³²Th as well as the total activity in the products manufactured by using RFA and soil were 1843%, 19% and 75%, respectively, higher compared to the bricks produced just by soil.

The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in all investigated fly ash samples of Xiangyang, China are compared with findings of earlier studies^{1,2,5,17,23,32,33} given in Table 2. The average values of ²²⁶Ra and ⁴⁰K in fly ash samples used in Xiangyang are significantly higher than those reported for National Thermal Power Corporation (NTPC) in India, Shanghai, Baoji, Baqiao and Xijiao coal-fired power plant (CFPP) samples^1,2,5,33 and are within the range given for fly ash samples from Greece and Turkey.^17,23,32 The concentration of ²³²Th in fly ash from Xiangyang was higher than values reported for the fly ash of Greece and was within the range of other compared works.^1,2,5,23,33

Table 2.

Comparison of mean activity concentrations of radionuclides (²²⁶Ra, ²³²Th and ⁴⁰K) in fly ash of Xiangyang CFPP with other CFPPs (arithmetic mean values are given in parentheses).

CFPP	²²⁶Ra	²³²Th	⁴⁰K	References
NTPC, Dadri, India	81.8–177.3 (118.6)	111.6–178.5 (147.3)	BDL-495.9 (352.0)	¹
Greece	794–1028	52–57	403–516	¹⁷
Greece	273–1377	41–65	143–661	³²
Turkey	17.0–2720.0 (360.3)	9.0–696.0 (101.7)	12.0–2974.0 (517.1)	²³
Shanghai, China	136.5–189.9 (160.3)	123.6–202.4 (159.9)	176.5–278.6 (246.2)	³³
Baoji, China	76.1–165.7 (112.2)	118.7–195.6 (147.5)	261.5–520.8 (385.6)	²
Baqiao, China	60.5–131.8 (83.2)	61.5–164.6 (87.4)	155.9–316.1 (234.8)	⁵
Xijiao, China	46.4–148.0 (69.5)	59.3–153.9 (79.3)	123.3–343.0 (233.0)	⁵
Xiangyang, China	90.3–1799.4 (440.5)	59.9–145.6 (110.3)	309.0–906.3 (510.1)	Present study

BDL: below detection limit.

Radon exhalation rate

Radon exhalation rates of all analysed fly ash samples are shown in Table 1. Table 1 shows that the radon exhalation rate values of RFA samples are higher than GFA samples. The radon exhalation rate values in all investigated fly ash samples, ranging from 4.7 to 74.2 Bq m⁻² h⁻¹ with an average of 21.1 Bq m⁻² h⁻¹, are lower than the world average value of 57.6 Bq m⁻² h⁻¹ for soil¹⁰ except for three RFA samples (RFA1, RFA2 and RFA3). The radon exhalation rate values in fly ash samples from Xiangyang are higher than those in fly ash samples from NTPC Dadri (U.P.) India ranging from 0.08 to 0.24 Bq m⁻² h⁻¹ with a mean of 0.16 Bq m⁻² h⁻¹.¹ Figure 2 shows there is a positive correlation (R² = 0.9497) between radon exhalation rate and ²²⁶Ra concentration in the analysed fly ash samples.

Figure 2.

Relationship between radon exhalation rate and ²²⁶Ra concentration in fly ash samples.

Assessment of radiation hazard

Fly ash is widely used in manufacturing cement, brick, concrete or for filling the underground cavities, construction of road/rail embankments and reinforced earth walls, mine filling and agriculture, etc.^18,22,34 To assess the radiological hazard of fly ash used in building materials or piled on the ground, radium equivalent activity (Ra_eq), external hazard index (H_ex), internal hazard index (H_in), indoor air absorbed dose rate (D_in) and annual effective dose (AED_in), indoor radon exposure dose (E_Rn), outdoor air absorbed dose rate (D_out) and annual effective dose (AED_out) were calculated in this study.

Radium equivalent activity

The distribution of ⁴⁰K, ²²⁶Ra and ²³²Th in fly ash is not uniform. To compare the activity concentrations and assess the radiological hazard of the fly ash used, the radium equivalent activity (Ra_eq) was calculated using equation (1),³⁵ defined according to the estimation that 1 Bq kg⁻¹ of ²²⁶Ra, 0.7 Bq kg⁻¹ of ²³²Th and 13 Bq kg⁻¹ of ⁴⁰K would produce the same gamma ray dose.

R a_{eq} = C_{Ra} + 1.43 C_{Th} + 0.077 C_{k}

(1)

Where C_Ra, C_Th and C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq kg⁻¹, respectively. The calculated values of Ra_eq for the studied fly ash samples are shown in Table 3. As can be seen from Table 3, the Ra_eq values for all RFA samples and some GFA samples are higher than the recommended limit (370 Bq kg⁻¹).¹⁰

Table 3.

Radium equivalent activity (Ra_eq), external hazard index (H_ex), internal hazard index (H_in), indoor air absorbed dose rate (D_in), indoor annual effective dose (AED_in), indoor radon exposure dose (E_Rn), outdoor air absorbed dose rate (D_out) and outdoor annual effective dose (AED_out) for fly ash samples.

Sample	Ra_eq (Bq kg⁻¹)	H _ex	H _in	D_in (nGy h⁻¹)	AED_in (mSv y⁻¹)	E_Rn (mSv y⁻¹)	D_out (nGy h⁻¹)	AED_out (mSv y⁻¹)
Grey fly ash
GFA1	382.4	1.03	1.41	324.4	1.591	0.548	173.6	0.213
GFA2	383.0	1.04	1.43	326.0	1.599	0.847	174.1	0.214
GFA3	379.2	1.02	1.40	322.0	1.579	0.896	172.3	0.211
GFA4	303.3	0.82	1.14	260.1	1.276	0.324	138.5	0.170
GFA5	317.8	0.86	1.19	272.2	1.335	0.448	145.0	0.178
GFA6	352.6	0.95	1.40	306.1	1.501	1.394	161.6	0.198
GFA7	335.3	0.91	1.27	288.2	1.414	0.921	153.3	0.188
GFA8	341.9	0.92	1.28	291.7	1.431	0.871	155.6	0.191
GFA9	234.1	0.63	0.88	200.1	0.982	0.373	106.7	0.131
GFA10	334.9	0.91	1.26	286.8	1.407	0.946	152.7	0.187
GFA11	376.9	1.02	1.42	323.0	1.585	1.295	172.0	0.211
GFA12	413.8	1.12	1.57	355.6	1.744	1.419	189.1	0.232
GFA13	339.2	0.92	1.28	290.1	1.423	0.473	154.6	0.190
GFA14	365.8	0.99	1.38	313.2	1.536	0.647	166.8	0.205
GFA15	343.7	0.93	1.27	293.7	1.441	0.523	156.8	0.192
GFA16	329.4	0.89	1.16	277.4	1.361	0.398	149.4	0.183
GFA17	350.4	0.95	1.31	299.2	1.468	0.573	159.6	0.196
Min	234.1	0.63	0.88	200.1	0.982	0.324	106.7	0.131
Max	413.8	1.12	1.57	355.6	1.744	1.419	189.1	0.232
Mean	346.1	0.94	1.30	295.9	1.451	0.759	157.7	0.194
Red fly ash
RFA1	1952.1	5.28	10.14	1791.0	8.786	5.129	902.9	1.107
RFA2	1942.8	5.25	10.05	1781.0	8.737	4.606	898.5	1.102
RFA3	1799.8	4.86	9.30	1649.9	8.094	4.208	832.5	1.021
RFA4	897.2	2.42	4.44	812.4	3.985	2.092	413.5	0.507
RFA5	976.8	2.64	4.84	883.6	4.335	1.942	449.8	0.552
RFA6	817.2	2.21	3.99	737.3	3.617	1.618	376.3	0.461
RFA7	1032.1	2.79	5.12	933.7	4.580	2.515	475.2	0.583
Min	817.2	2.21	3.99	737.3	3.617	1.618	376.3	0.461
Max	1952.1	5.28	10.14	1791.0	8.786	5.129	902.9	1.107
Mean	1345.4	3.64	6.84	1227.0	6.019	3.159	621.2	0.762
All investigated fly ash samples
Min	234.1	0.63	0.88	200.1	0.982	0.324	106.7	0.131
Max	1952.1	5.28	10.14	1791.0	8.786	5.129	902.9	1.107
Mean	637.6	1.72	2.91	567.4	2.784	1.459	292.9	0.359

External and internal hazard indexes

To limit the external gamma radiation dose to 1.5 mSv y⁻¹, the external hazard index (H_ex) is defined by equation (2).³⁵

H_{ex} = (C_{Ra} / 370) + (C_{Th} / 259) + (C_{K} / 4810)

(2)

Where C_Ra, C_Th and C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq kg⁻¹, respectively. The value of this index must be less than unity for the radiation hazard to be negligible. For the maximum value of H_ex to be less than unity, the maximum value of Ra_eq must be less than 370 Bq kg⁻¹. Table 3 shows that the H_ex values of the analysed fly ash samples are larger than or close to unity.

In addition to the external hazard, radon and its short-lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index (H_in) which is defined by equation (3).³⁵

H_{in} = (C_{Ra} / 185) + (C_{Th} / 259) + (C_{K} / 4810)

(3)

Where C_Ra, C_Th and C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq kg⁻¹, respectively. For the safe use of a material in the construction of dwellings, H_in should be less than unity. As seen from Table 3, the calculated values of H_in of the studied fly ash samples are higher than unity except for GFA9 sample (0.88).

Indoor external exposure due to gamma radiation

According to the report of European Commission,³⁰ the indoor air absorbed dose rate (D_in) due to gamma ray emission from the natural radionuclides in the fly ash is defined by equation (4).

D_{in} (nGy h^{- 1}) = 0.92 C_{Ra} + 1.1 C_{Th} + 0.08 C_{K}

(4)

Where C_Ra, C_Th and C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq kg⁻¹, respectively. The estimated indoor air absorbed dose rate values for the investigated fly ash samples range from 200.1 to 1791.0 nGy h⁻¹ with an average of 567.4 nGy h⁻¹ as listed in Table 3, which are significantly higher than the world population-weighted average indoor absorbed gamma dose rate of 84 nGy h⁻¹.¹⁰ The average indoor gamma radiation dose rate of China is 99.1 nGy h⁻¹.¹⁰ The average indoor gamma radiation dose rate of Xiangyang is 98.8 nGy h⁻¹.³⁶ Considering the conversion coefficient (0.7 Sv Gy⁻¹) and the indoor occupancy factor (0.8) proposed by United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR),¹⁰ the corresponding annual effective dose (AED_in) due to gamma ray emission of ²²⁶Ra, ²³²Th and ⁴⁰K from the studied samples ranges from 0.982 to 8.786 mSv y⁻¹ with an average of 2.784 mSv y⁻¹ as shown in Table 3. These are higher than the recommended limit of 1 mSv y^–1 30 except for GFA9 sample (0.982) and the average annual external effective dose rate of 0.41 mSv y⁻¹.¹⁰

Indoor internal exposure due to radon

The risk of lung cancer from domestic exposure due to radon and its daughters can be estimated directly from the indoor inhalation exposure effective dose. The contribution of indoor radon concentration from fly ash samples can be calculated using equation (5).^1,3,4

C_{Rn} = E_{x} \times S / (λ_{v} \times V)

(5)

Where C_Rn is radon concentration (Bq m⁻³), E_x is the radon (²²²Rn) exhalation rate (Bq m⁻² h⁻¹), S is the exhalation surface area (m²), V is the room volume (m³) and λ_v is the air exchange rate (h⁻¹). In the calculation, the maximum radon concentration from the building material was assessed by assuming the room as a cavity with S/V = 2.0 m⁻¹ with an air exchange rate of 0.5 h⁻¹.^1,3,4 The indoor annual exposure effective dose of ²²²Rn (E_Rn) released from fly ash was calculated using equation (6).^1,3,4,11

E_{Rn} = (DCF \times 8760 \times nF \times C_{Rn}) (170 \times 3700)

(6)

Where C_Rn is the ²²²Rn concentration (Bq m⁻³) released from fly ash, n is the fraction of time spent indoors taken as 0.8, 8760 is the number of hours per year, F is the equilibrium factor and DCF is the dose conversion factor, expressed in mSv WLM⁻¹.

A working level month (WLM) is the exposure to the radon short-lived decay products in equilibrium with a concentration of 3700 Bq m⁻³ of ²²²Rn during a working month (170 h).¹ Equilibrium factor F quantifies the state of equilibrium between radon and its daughters, taken as 0.4 as suggested by UNSCEAR.¹⁰ From radon exposure, the effective dose equivalents are estimated by using a DCF of 3.88 mSv (WLM)⁻¹.^1,37 Table 3 shows that the indoor internal exposure effective dose (E_Rn) due to radon emitted from fly ash ranges from 0.324 to 5.129 mSv y⁻¹ with an average of 1.459 mSv y⁻¹.

Outdoor external exposure due to gamma radiation

An attempt was made in the present work to evaluate the gamma radiation from the ash pond. Conversion factors were used to transform specific activities, C_Ra, C_Th and C_K of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in the air. The absorbed dose rate at 1 m above the ground (nGy h⁻¹ per Bq kg⁻¹) was calculated by using equation (7).³⁸

D_{out} = 0.461 C_{Ra} + 0.623 C_{Th} + 0.0414 C_{K}

(7)

The calculated results show that the outdoor air absorbed dose rate (D_out) ranges from 106.7 to 902.9 nGy h⁻¹ with an average of 292.9 nGy h⁻¹ (Table 3), which are significantly higher than the population-weighted average value of the global primordial radiation of 59 nGy h⁻¹.¹⁰ The average natural gamma radiation dose rate of China (62.0 nGy h⁻¹)¹⁰ and the average natural gamma radiation dose rate of Xiangyang (64 nGy h⁻¹).³⁶ The corresponding annual effective dose rate (AED_out), considering the conversion coefficient from gamma absorbed dose in air to effective dose (0.7 Sv Gy⁻¹) and outdoor occupancy factor (0.2) proposed by UNSCEAR,¹⁰ ranges from 0.131 to 1.107 mSv y⁻¹ with an average of 0.359 mSv y⁻¹ for the studied fly ash samples (Table 3), which exceed the worldwide average (0.07 mSv y⁻¹) for the outdoor annual effective dose rate¹⁰ and the mean Xiangyang outdoor annual effective dose rate of 0.078 mSv y⁻¹.

Conclusion

Comparatively high values of activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in fly ash samples from Xiangyang were found, particularly RFA samples, which contain extra high ²²⁶Ra concentration. Radon exhalation rates are also higher than other reported for fly ash and the radon inhalation exposure effective doses are also high. The radium equivalent activity values of most fly ash samples are higher than the recommended value of 370 Bq kg⁻¹. The external hazard and internal hazard index values of the studied samples are close to or higher than unity. The indoor air absorbed gamma dose rates of the fly ash are significantly higher than the world population-weighted average indoor absorbed gamma dose rate and the corresponding annual effective doses are higher than the recommended limit. Thus, the fly ash of Xiangyang is not suitable to use as building material. The gamma absorbed dose rates around ash pond and the corresponding annual effective dose rate are 2–15 times the world average. The ash storage pond is a high radiation risk area and the population living around ash pond may be exposed to a high level of radiation dose rate. The management of usage and disposal of fly ashes, especially RFAs, and their radioactivity levels measurement should be strengthened by the local government and environmental protection agency.

Footnotes

Authors' contribution

All authors contributed equally in the preparation of this manuscript.

Acknowledgments

All experiments were conducted in the Environmental Science Lab of Shaanxi Normal University. We thank Shigang Chao, Xiang Ding and Ni Zhao for their helps with the experiments. Appreciation is expressed to the Editor-in-Chief, Prof. Chuck Yu and the anonymous reviewers for their insightful suggestions and critical reviews of the manuscript.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research was supported by the National Natural Science Foundation of China through Grant 41271510 and the Fundamental Research Funds for the Central University through Grants GK 200901008.

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