In-situ Measurement

The quantitative estimation of the radon exhalation from the wall made by measuring the build-up of radon activity concentration in the accumulation chamber in 5 different mine locations for 4 times measurements (each season one measurement). The geological of the rocks present in the mine galleries consist different types of limestone and marl (Cherty limestone, Greenish grey calcareous marl, etc.) as it is difficult to determine pure rocks due to the mixture of above-mentioned rocks. Detectors sent to the mine manager and placed in the same location were mostly miners were working there for each measurement. At the bottom of the cap, a CR-39 detector was placed. The cap was sealed on the surface of the mine wall. At the time of sending detectors, three detectors kept in the laboratory to evaluate the background. It was letting the exhalated radon accumulated in the cap about one week before determining the surface radon exhalation; These measurements carried out 4 times for 1 year in same locations. Table 15. is summarized the surface radon exhalation from the wall per each location.

The surface radon exhalation, from mine walls, measured to be in the range of 0.7±0.1 mBq·s^-1·m^-2 and 1.5±0.2 mBq·s^-1·m^-2, with an average of 1.1±0.1 mBq·s^-1·m^-2. Also, some variation between each period of measuring for same location observed that as it could be as results of precipitation. As results show, the seasons with high precipitation might be a reason that the radon exhalation from mine wall was lower than other seasons and it can be due to high moisture content in the rock and soils that known as a deterrent factor. The historical average precipitation data, for the month that measurement occurred, obtained from the Department of Limnology of the University of Pannonia. The average precipitation

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for March, August, October, February was about 33, 62, 53, 32 mm, respectively. Likewise, the monthly average outdoor temperature obtained as 5, 22, 10 and 2 °C, respectively.

Table 15- Surface radon exhalation from 5 different walls location in the Úrkút mine Radon Exhalation (mBq·s^-1·m^-2)

Location March Aug October February Min.- Max. Mean

L-1 1.4±0.2 1.1±0.1 0.9±0.1 1.5±0.2 0.9-1.5 1.2

L-2 1.2±0.2 0.9±0.1 0.8±0.1 1.3±0.1 0.8-1.3 1.0

L-3 1.4±0.2 1±0.1 0.8±0.1 1.4±0.2 0.8-1.4 1.1

L-4 0.9±0.1 0.8±0.1 0.7±0.1 1.1±0.1 0.7-1.1 0.9

L-5 1.1±0.1 1±0.1 0.8±0.1 1.3±0.2 0.8-1.3 1.0

Regarding the results, the surface exhalation varied from each other, in some location, the radon exhalation could reach up to two times higher than the lowest exhalation value in other location. It can be due to rocks mineralization feature on the mine wall.

Therefore, to figure out the contribution of different rocks and in continues of the previous study, the author carried out second measurements (refer to section 3.1.2.2) in the laboratory of the Institute of radiochemistry and radioecology at the University of Pannonia.

Following in-situ measurement, interesting results obtained from two different age mine walls measurements; Author conducted a continues radon monitoring (real-time measurement) some days before starting new mining activity. An AlphaGUARD radon monitor device connected to an accumulation chamber to the wall about 70 cm above the floor. Radon concentration was monitored in flow mode with hourly measurement cycle this phase of measurement named as point A (aged wall mine); Another measurement carried out after mine activity on the freshly broken wall in the same the location, named as point B (Fresh broken wall or ore); Measurements continued until released radon reduced up to its origin value at point A. Figure 30. shows the measurement result in the function of time and radon concentration in each point of time marked on the graph.

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Figure 30- Released radon in function of the age of the broken wall

As shown in Figure 30., the radon concentration in the air directly depends on the broken ore (wall) age; “A” represents radon concentration near the old mine wall with average about 850±125 Bq·m^-3, exactly after mining activity the radon released from same location with fresh broken wall increase dramatically with an average about 5900±420 “B”, these increase dropped down near to 3300±365 Bq·m^-3 by passing the time and getting aged, finally at point “D” and radon concentration came back to its constant value at point “A”.

This can be due to releasing trapped or accumulated radon between pores and grains space, when the rock was broken, radon could scape to the air, however, after a period and by aging the broken wall, the radon concentration reduces to it origin value.

This graph can be used as a mitigation management tool to reduce radon concentration at manganese mine or other underground mines; however other measurements such as a recognition of high potential radon exhalation rocks can be useful.

In section 3.2.3. of the current study an explanation of how such this experiment could be used to improve mitigation system resulting a successful reduction on the radon concentration.

87 3.1.2.2. Ex-situ Measurement

As it was discussed, on section 3.1.2.1. in the current thesis, to estimate the contribution of each type rocks in the radon exhalation, it is necessary to know the radon exhalation rate from each rock. Followingof previous study which two types of rocks were examined, in current study, the author carried out an ex-situ areal radon exhalation measurement for 32 collected rock samples of total 6 different most abundant rock types (Table 16.). Additionally, the author used a gamma spectrometry to measure the main radionuclides concentration in each sample. Three main naturally occurring radionuclides (K-40, Th-232 and Ra-226 known as radon parent) considered for gamma emitted measurement using A high-resolution gamma-ray spectrometry, using an ORTEC GMX40-76 HPGe semiconductor detector (refer to section 2.3.1. at the current thesis).

In case of gamma spectrometry, a portion of samples following AQ of the laboratory of the Institute of Radiochemistry and Radioecology at the University of Pannonia, after being transferred to the laboratory, stored at room temperature for several days and dried in a ventilated oven at 90 °C for 24 hours to reach a constant weight. Samples pulverized, sieved to less than 0.3 mm to be same size as the reference material (IEA-375, Soil standard).

Then 500 grams of the homogenized prepared sample (in same by the standard weight) filled into a leak-proof and air-tight Marinelli beaker (same geometry as standard Marinelli beaker) and sealed for 29 days in order to reach secular equilibrium between Ra-226 and Rn-222 and its short-lived decay products before being counted by gamma spectrometry.

The specific peak detection efficiency, determined for K-40, Ra-226, and Th-232 as 1.2%, 2.4%, and 1.4%. Meanwhile, the minimum detectable activity of the gamma spectroscopy, based on the observed data from background measurement, calculated at 23, 0.5, and 0.7 Bq·kg^-1, respectively. Table 16. shows the concentration of natural radionuclides in the rock samples (dry weight).

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Table 16- Concentrations of natural radionuclides in the mine rock samples (Bq·kg^-1)

Ra-226 Th-232 K-40

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After statistical analysis, it observed that dogger limestone consist the lowest concentration of naturally occurred radionuclides such as Ra-226, Th-232 and K-40 among the other rocks type with an average of 2±1, 5±1 and 72±12 Bq·kg^-1, respectively; However, underlayer black shale shows the highest values between all of the samples with an average of 16±4, 19±5 and 432±74 Bq·kg^-1, for Ra-226, Th-232 and K-40, respectively.

Additionally, black shale and carbonate ore shows high values of Ra-226 compared to other samples. The concentration of Ra-226 is higher than the mean value in 47% of the samples, while Th-232 was above the average concentration in 44% of the samples. The highest activity concentrations of Ra-226 was found in underlayer black shale.

The normal distribution for the specified mean and standard deviation of the measured radionuclides' concentration among the samples is shown in Figures 31. and 32.;

According to the obtained data, most of the K-40 distribution is in the range of ~50 and ~300 Bq·kg^-1, while the values for Ra-226 and Th-232 are between ~3 and ~15 Bq·kg^-1 and ~10 and ~30 Bq·kg^-1, respectively. The highest normal distribution values for K-40, Ra-226, and Th-232 were calculated around 0.002, 0.07, and 0.05, respectively.

Figure 31- Distributions of K-40 concentration among the rock samples

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Figure 32- Distributions of R-226 and Th-232 concentrationamong the rock samples

A compression between the Ra-226 concentrations found in this study with U-238 concentrations from other study (Bíró, et al., 2015) for the same family rocks and the same mine are summarized in Table 17.

Table 17- Comparison of the Ra-226 and U-238 concentration among rock samples

Rock Type U-238

(Bq·kg^-1)

Ra-226 (Bq·kg^-1) Carbonate Ore 1-3 (1.6) 15 & 18 (16)

Black shale 5 9-18 (13)

limestone

1-2 (1.6) 2-6 (3)

Marlstone 1 & 4 3-6 (4)

Samples in different cylinder length used for measuring the aeric radon exhalation from each rock type in terms of sample thickness; The aeric radon exhalation measurement of prime importance in the determination of contribution of each type of rocks in radon exhalation. Following the accumulation period, the chamber connected to a closed loop system, where in the radon increment was measured by an ionization radon monitor device (AlphaGUARD PQ 2000). The aeric exhalation rates of each sample in terms of thickness are illustrated in Table 18. The overall average values of radon exhalation from samples in terms of their thickness are shown in Figure 33.

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Table 18- Areal radon exhalation from rock samples in terms of sample thickness

Sample

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Carbonate ore, as was expected, showed the highest aeric exhalation, as it contained highest Ra-226 concentration among the other rock types. Puce greenish marl stone, shows the lowest radon exhalation. It needs to mention that radon exhalation directly depends on the moisture in the grain's pores and has an inverse relationship with grain size (Masahiro, et al., 2007; Harb, et al., 2016).

Figure 33- areal radon exhalation from the rock samples in terms of thickness

The radon exhalation rate is going to reach a constant value by increasing the thickness of samples. The high moisture content of the samples can be a reason for this behaviour as water in the sample could stop the radon diffusion, however in dry sample, radon exhalation can reach to the constant rate from 1 to several meters of sample thickness depends on the sample material. Várhegyi, et al. found radon exhalation can reach to a constant rate after a specific thickness of sand depending on the moisture (Várhegyi, et al., 2012). In the other words, moisture can influence on the radon exhalation. The soil moisture content as a deterrent factor influence on the radon emanation coefficient directly by filling the pores and space between material's grains resulting a temporary blockage of emanated radon from grains to pores space (radon is soluble water may also aid in the release of radon trapped in pores) and/or radon diffusion from pores to surface. However, it found the small amount of moisture can increase the radon emanation but by increasing the moisture rapidly reduce the emanation (Schumann & Gundersen, 1996; Barton & Ziemer, 1986).

93 3.2. Radon Concentration in Air

In order to investigate the radon distribution, two techniques used: a passive integrated method using NRPB track detectors were located in 12 different locations including old and new galleries, to indicate the sources of entry of radon at a height of 1–2 meters from the ground; The detectors changed in a consecutive period of three months (seasonal changes) between 2013 to 2016 and results evaluated as integrated average radon concentration per cubic meters in each season. As integrated measurements giving an overall view of radon concentration in mine e.g. measured radon concentrations during working hours can be differ from the whole-day averages due to the ventilation system that just worked only during the working hours, especially in this case when the work was organized with one shift per day, therefore, active radon monitoring carried out using AlphaGUARD PQ 2000Pro (specified to measure only radon concentration) and TESLA radon monitors (a newly marketed device to measure radon and humidity with the function of automatic smart central controller) saving the hourly values of radon activity concentrations for 5 days in each month. A comparison between results of duplicate measurements obtained with both techniques carried out concurrently in the same locations and conditions is in reasonably good agreement, with variations within 4.5% to 6.0%.