ОЦЕНКА ВЛИЯНИЯ ТЕПЛОВОЙ ОБРАБОТКИ НА КОЭФФИЦИЕНТ ЭМАНИРОВАНИЯ РАДОНА-222 СТРОИТЕЛЬНЫХ МАТЕРИАЛОВ

ОЦЕНКА ВЛИЯНИЯ ТЕПЛОВОЙ ОБРАБОТКИ НА КОЭФФИЦИЕНТ ЭМАНИРОВАНИЯ РАДОНА-222 СТРОИТЕЛЬНЫХ МАТЕРИАЛОВ

Научная статья

Соколов П.Э.^1, *, Сентенберг С.А.²

¹ ORCID: 0000-0002-3960-5010,

Волгоградский государственный технический университет, Волгоград, Россия;

² Волгоградский технологический колледж, Волгоград, Россия

* Корреспондирующий автор (pr7391[at]yandex.ru)

Аннотация

Производство строительных материалов основано на переработке различного природного сырья – горных пород. Одним из видов такой переработки является тепловая обработка (обжиг) сырья. В работе предпринята попытка оценить, какое влияние оказывает тепловая обработка на изменение коэффициента эманирования радона-222. Отобранные пробы сырьевых материалов подвергались постадийному обжигу. После каждой стадии обжига проводилось определение коэффициента эманирования радона-222. Полученные результаты позволили выявить зависимости изменения этой величины. Проведен сравнительный анализ экспериментальных и расчетных данных. Сделан вывод о том, что часть радона и радия выделяется в окружающую среду в процессе обжига. Использование полученных результатов на практике позволит прогнозировать коэффициент эманирования радона-222, и дает возможность регулировать эту характеристику, изменяя температуру обжига материалов. Это позволит снизить дозы облучения населения от радона-222 и его дочерних продуктов распада.

Ключевые слова: радон-222, коэффициент эманирования радона, тепловая обработка, обжиг, строительные материалы.

HEAT TREATMENT INFLUENCE EVALUATION ON RADON-222 EMANATION FACTOR IN BUILDING MATERIAL

Research article

Sokolov P.E.^1, *, Sentenberg S.A.²

¹ ORCID: 0000-0002-3960-5010,

Volgograd state technical University, Volgograd, Russia;

² Volgograd technological College, Volgograd, Russia

* Corresponding author (pr7391[at]yandex.ru)

Abstract

Building material production is based upon processing of various natural raw material, i.e. of rock. One of the kinds of the processing is heat treatment and namely burning of raw material. In the present paper, there has been made an attempt to evaluate, what influence heat treatment exerts on changing the emanation factor of Radon-222. Selected samples of raw material were subjected to phasic burning. Determination of Radon-222 emanation factor was made after each burning phase. The obtained results allowed expose change dependability on the quantity. There has been made an analysis of experimental and calculated data. A conclusion has been drawn that a part of radon and radium is emitted into the environment while burning. The obtained results practical application will help forecast Radon-222 emanation factor and makes it possible to regulate the characteristic by means of changing the burning temperature. This will allow reduce the population irradiation dose of Radon-222 and its daughter decay product.

Keywords: Radon-222, Radon emanation factor, heat treatment, burning, building material.

Introduction

One of the main sources of the population irradiation sources is building material [1]. Between all the irradiation sources influencing man in this or that way building material takes approximately 50 % [2], [3]. The reason for that lies in the fact that people keep spending more and more time inside buildings. Man irradiation inside buildings in its turn includes two components. The first one is irradiation by natural radionuclides like ⁴⁰K, ²²⁶Ra and ²³²Th contained in building material. The second component is the irradiation emitted by radon and its isotopes and by its daughter decay products [4].

²²²Rn is the longest living among all the radon isotopes. Radon is a unique radioelement. It is generated as a result of ²²⁶Ra decay and from solid ²²⁶Ra the emanation is generated i.e. the gasiform radioactive isotope ²²²Rn, which in its turn after decay transforms again into solid state. While ²²²Rn decay there appear a number of radionuclides, such as ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi named daughter decay products (DDP). It is exactly for them man has the most amount of inner irradiation through breathing.

Inflow of radon inside a premise increases people irradiation. The inflow of radon from building material inside a premise is characterised by the emanation factor [5, 6]. Different material have different emanation factor.

Radon (²²²Rn) is a radioactive gas with a half-life period of 3.825 days. Alongside with radon while other natural radioactive families decay there appear a number of radon isotopes, such as Thoron-220 (²²⁰Tn), Actinon-219 (²¹⁹An). They have rather short half-life periods (less than a minute) and are not observed in this paper.

The emanation ability of building materials is the most important factor causing the intensity of radon release into a house. The emanation process is comparatively complex which includes radon atoms escape from the substance solid phase dew to radium radioactive recoil, diffusion in gas and liquid phases (we can ignore diffusion in solid phases due to smallness of the corresponding diffusion ratio), adsorption on walls of cracks, pores and capillaries of a substance.

To characterize the emanation process an effective parameter, and namely emanation factor is used, which determines radon fraction escaped through open pores of a substance. As follows from the definition:

   (1)

where  is the quantity of free emanation emitted by a solid substance unit mass within the time allowing define radioactive equilibrium, and  is the quantity of emanation corresponding to isotope ²²⁶Ra.

The emanation factor is used for calculation of radium effective specific activity:

   (2)

where A_Ra is ²²⁶Ra specific activity.

A significant number of papers considering radon cover estimation of radon level inside buildings, structures and premises of different function, having various number of storeys, erected of diverse building materials, situated in different areas of the world, in different countries and regions of a country. Execution of such routine, labor-consuming and money-losing works is necessary to acquire the general picture produced by radon as well as to reveal faulty or dangerous areas, materials, buildings, etc.

Another rather a large group of Research articles is dedicated to investigations about what levels of radon and levels of radon exhalation are formed by different building finishing materials inside premises [11], [12], [13]. Both natural and man-made building materials [14], as well as different measuring approaches are considered [15], [16].

The radon exhalation process can be divided into two stages: radon emanation into a material inner pores and radon atoms diffusion through these pores followed by the exit from the material. Such division is justified by the fact, that radon diffusion ratio inside a consistent substance is extremely small, and so from the material only those radon atoms are emitted, that occurred in inner pores due to aggregate recoil while Ra  α-decay.

A number of works is devoted to the application of different additives into building materials to regulate radon exhalation and emanation levels [17], [18].

Finally, to the last group, it is necessary to attribute the papers, that in this or that way are related to the influence of different technological factors on radon emanation factor. Thus, in some paper [19, 20] the influence of filler's grain size on inside a premises radon level is estimated. However, the works considering the influence of temperature processing on radon emanation factor from building materials are the most interesting. These works cover the research of certain building materials to have been subject of heating with temperatures up to 750°C [21] and up to 1200°C [22].

A great number of «traditional» building materials were and are obtained with the use of heating. We can refer to that number nearly all inorganic binding substances, such as portland cement, gypsum, lime as well as ceramic bricks and blocks, silica glass, man-made fillers for concrete, etc. Besides the said materials can be a stock for other building materials, for instance concretes. A number of researchers note, that building materials and stock they are made of, have different values of ²²²Rn emanation factor. The overwhelming majority of scientists state the fact of reducing emanation factor of building materials in comparison with raw ones [4], [23].

Considering the foregoing we have decided research the influence of heating (burning) inside the range to 1500°C.  As the materials under test we decided to use both raw materials to heat (burn), and those which usually are not subject to such processing (for comparison).

Methods and materials

To fulfil the research there was selected a group of 5 kinds of raw stock, used for building materials production. Each material was presented in three samples. The choice of the stock was random. In the group under research there occurred materials usually both subjected to heat processing and never heating processed. This was made intentionally, to compare the acquired results and to generally estimate the influence of burning on the emanation factor. As the materials under research there were taken chalk, clay, limestone, gypsum stone and sand rock.

The emanation factor, the radium specific activity and a sample mass under normal conditions were determined for all the samples.

Then the materials were subjected to a stage heat processing (burning) up to 1500°C. The experiment pitch was 150°C, which is explained by data sufficiency within a wide burning range. After each pitch determination of radon emanation factor and sample mass was made.

To fulfill the heat processing (burning) of the materials under research an electric muffle furnace was used, which allowed to heat the materials to desired temperatures, up to 1500°C. The application of the muffle furnace allowed to eliminate any contact of the researched material with fuel and its combustion products.

The materials samples selection and preparation for determination of natural radionuclides specific activities and emanation factor are made on joint hinges selected from a representative sample.

The representative sample is acquired by means of mixing and quartering of not less than 10 spot samples taken from checkpoints. The selection of samples is made according to current normative documents demands [24]. A representative sample with the size over 5 mm is grinded to a less than 5 mm size. Depending on the volume of a container used in radiometric units, sample with a mass from 1.0 to 10 kilograms is packed into a double sack, between the walls of which there is a sample`s name plate with the material name, and selection place and date.

The determination of the specific activities of natural radionuclides in building materials and Research articles of manufacture was made on the joint hinges selected from a representative sample which counted 1.0 kilogram.

The determination of the specific activities of natural radionuclides was made on a spectrometer facility with a scintillation detector based on NaI(Tl) crystal with ∅63×63 mm. The acquired spectra were treated in accordance with the standard methods [25]. We applied the said equipment due to its easy use, availability and possibility to make routine measurements with an acceptable error.

The prepared sample of the material under research was put into a Marinelli vessel (see Fig. 1). The Marinelli vessel was put on the detector of the gamma-ray spectrometer, where during a definite time (usually 30 minutes) γ-ray spectrum was gained, conditioned by natural radionuclides present in the sample. On finishing the time search of peaks formed by impulses of the same energy γ-quanta was made. The determination of nuclides composition was fulfilled by means of comparative analysis of radioactive isotopes ray characteristics and those of the sample`s registered ray [26]. Next the determination of specific activity of each the rated natural radionuclides was fulfilled: ⁴⁰K, ²²⁶Ra and ²³²Th. Then we calculated the total specific activity of the sample. Calculation of measurements error was the final stage. The observation treatment and the estimation of measurements' error were produced in accordance with measurements techniques [27], independently for each hinge and for each natural radionuclide.

Fig. 1 – Marinelli vessel: a – general view; b – open vessel

 

To measure Rn emanation and to calculate h and A_Raeff we used techniques described in [28, 29] with the equipment at our disposal considered [25].

The observed sample counting 1 kg was previously grinded up to pieces less than 5 mm (that is by a degree less than the length of radon diffusion in the given material). Such a grinding provides a complete exit from the pieces ²²²Rn atoms occurred in the material's inner pores while ²²²Rn α-decay on the one hand, and on the other hand it does not lead to any considerable increase of the total pores' surface, on which the emanation factor is dependent.

The grinded sample was put into a Marinelli vessel. A cartridge of absorbent carbon of 100 gr mass was put either. Then the vessel was hermetically closed and kept for 14-15 days. This is enough time for the accumulation of radon equilibrium amount and its daughter decay products. On the termination of the time the absorbent carbon from the cartridge in a special cuvette was put on the γ-spectrometer detector to measure the radon daughter decay products adsorbed on the carbon. On the basis of measurement of the radon daughter decay products there was calculated the radon emanation factor [28].

 

Table 1 – Average values of the 226Ra specific, effective specific activities and radon emanation factors of building materials in Volgograd region

Material	Samples’ number	ARa, Bq/kg		ARaeff, Bq/kg		η, %
		ARa	variations range	ARaeff	variations range	η	variations range
1	2	3	4	5	6	7	8
Sand rock	86	23.8	15.4-37.1	9.8	8.1-11.4	29.9	28.8-30.9
Clay	107	23.2	13.7-35.2	12.4	5.8-33.2	37.6	25.3-66.5
Sand	109	9.8	3.0-17.7	2.2	1.7-3.0	66.6	71.3-72.4
Cement	20	18.9	14.9-28.7	6.4	5.0-9.7	33.9	17.6-65.2
Chalk	50	16.2	9.1-34.1	4.9	2.7-7.4	30.3	21.6-29.5
Limestone	52	21.9	9.1-49.7	6.3	2.2-8.3	28.8	16.7-24.5
Slag	16	99.7	9.9-224.9	15.7	1.4-35.3	15.7	14.2-29.9
Lime	10	76.4	74.3-78.4	5.0	3.2-12.5	6.6	4.4-15.9

  Result

The results of measurements of the radium specific activity and radon emanation factor in building materials were cited in great number of works. However, as follows from our research [30], [31] it is necessary to take into consideration peculiarities of rock deposits, specific region, production technology and other characteristic properties influencing on the radon volumetric activity.

The results of the research of the radium specific, effective specific activity as well as of radon emanation factor for the building stock and materials frequently used in Volgograd region are given in Table 1.

It follows from the table, that the average values acquired are somewhat larger than the values drawn in [5]. This affirms our assumption of the necessity to carry out a research for every region of the country with the purpose of clarifying real values of radon emanation factor.

During the process of heat treatment (burning) raw building materials undergo a number of consecutive physicochemical transformations, and as result a building material with pre-determined properties is acquired. The research has shown that radon emanation factor change is conditioned by the processes of materials' structure transformation under temperature.

To disclose regularities governing the emanation factor change under heat treatment, there were carried out measurements of radiation characteristics of a number of building stock types. As a criterion to select materials for the research was taken the value of temperature of the materials' heat treatment. While processing into building materials raw stock exposed to high temperatures (750-1500°C) i.e. to burning, backing, alloying were chalk (1), clay (2), limestone (3). Exposed to lower temperatures (140-850°C) material was gypsum stone (5). And material usually never exposed to heat treatment was sandstone (4).

 

Table 2 – Radon emanation factor, Radium effective specific activity and materials’ mass on different stage of burning

As it is clearly seen from the table and diagrams within the process of heat treatment the sample's mass, radon emanation factor, and radium specific activity are changed. The changes of the enumerated characteristics take place due to different reasons.

According to the Table 2 data let us diagram dependencies of emanation factor, of radium effective specific activity, of the material mass on the burning temperature. As an example, we will take clay (2) and limestone (3).

When analyzing the diagrams (see Fig. 2) one can outline two areas. The first one corresponds to the temperature range of 20-750°C. In this area, we observe multidirectional changes of radon emanation factor and radium effective specific activity. The second area corresponds to the temperature range of 750-1500°C. Reduced radon emanation factor and radium affective specific activity are characteristic for this area. Throughout the whole temperature range reducing of mass of the researched materials is observed.

Fig. 2 – Dependencies of radon emanation factor, radium effective specific activity, and mass on the building temperature: a – clay; b – limestone

 

It is necessary to mention, that in our opinion the second area is more significant, because only with temperatures over 750°C it is feasible to acquire qualitative building materials with desired properties. The determining significance in the case belongs to chemical and mineralogical makeup; those being able to change not only in different deposits, but also within one deposit.

For the said reason while the analysis of observations we will use values of radon emanation factor and radium effective specific activity for the range of 750-1500°C. Let us make a graph of radon emanation factor dependence for the same materials (see above).

Fig. 3 – Correlation field of radon emanation factor on temperature: a – clay; b – limestone

 

When analyzing the graph (see Fig. 3) we can suppose the dependencies to be of linear character. The regularity governing the process looks in general like: y=ax+b, with a and b to be regression equation factor, x is burning temperature, and y is radon emanation factor. Let us determine correlation factor for radon emanation factor depending on temperature. It will make -0.9965 for clay (2), and -0.9214 – for lime stone (3). The acquired values show, that bonds between the variables is very strong and there is a linear inversely proportional dependence. Consequently, with growth of temperature in the second interval radon emanation factor falls. Let us estimate the significance of the correlation factor, and for this purpose observe two hypotheses. The basic one is H₀ : r_xy = 0, and the alternative is H¹ r_xy  ≠0. To verify H₀ hypothesis we will compute t-Student statistic, and it makes t_calc = -23.903 for clay, and t_calc = -4.7414 for lime stone. We compare the acquired value with critical value  by Student distribution (with v = 4 and fiducial probability α = 0.05)  = 2.77645 дол. There comes a conclusion that between the variables there is a dependence, and the acquired correlation factor is significant. Let us determine quantitative bonds between the dependent values. To define a dependence degree between a response and factor we will use covariation and correlation values.

To reveal the regression pattern (i.e. a and b factors) we will use graphical method, namely trend line construction on a graph in the MS Excel environment. The factors a and b for clay will be equal to -0.0202 and 43.704 correspondingly, and for lime stone (3) -0.0291 and 47.286. The regression equation will look:

   (3)

   (4)

On the diagram (see Fig. 3) there is the approximation validity value, which for clay (2) is equal - R²=0.993, and for lime stone (3) - R²=0.849. R² values are close to 1, that witnesses the trend line to be close to factual data, and consequently they correspond to reality.

Discussion

As it is obvious from Table 2 and graphs in Fig. 2 within the process of heat treatment changes of radon emanation factor, radium effective specific activity, and samples' mass under research take place. Changes of the enumerated characteristics occur due to different reasons.

Within the temperature range of 20 to 200°C physically bound water is removed from clay stock. Under 200-300°C organic and silty admixtures are burned out. Under the temperatures of 300-750°C dehydratation of clay minerals occurs, and temperatures of 750°C and over and characterized by liquid glass phase to appear, and there take place dissolution of some mineral components and new minerals' formation.

The change of the emanation factor on the first stage can be explained in the following way. While samples' preparation the material was previously grinded, so its specific surface and apparent density changed. Removal of physically bound water and burning-out of organic admixtures during initial burning period bring to both the mineral's porosity and apparent density changes. Therefore, the emanation factor change depends on and is determined by the amount of physically bound water removed while the heating and by the degree of the material's specific surface increase. A preliminary ignition of materials within the temperature range of 300 to 450°C causes significant emanation factor changes connected obviously to termination of some disturbance of mineral's crystal lattice, and to the observed material mass change. An inflation of emanation factor of rocks containing minerals of crystal structure, which do not decompose under heating, is observed with temperature corresponding the beginning of mineral's crystal lattice destruction (»700°C). With an increase of burning temperature over 700°C there begin process of release of natural materials, that contain ²²⁶Ra. This is conditioned by reducing of surface tension forces when liquid glass phase starts. Dependently on which of the two processes, and namely mass change or minerals' containing ²²⁶Ra evaporation, will be dominant, so that process will determine radon emanation factor change.

Similar processes, but of weaker intensity occur when lime stone and chalk burning. An intensive radon release by carbonate rocks is connected with their dissociation with high temperatures. Carbonate rocks already under insignificant heating release greater part of the contained radon.

For lime stones, chalk, and gypsum it is characteristic to release gaseous components (CO₂, SO₂). As a result of decarbonizing and SO₂ removal there occurs reduction of mass apparent density of materials.

Removal of gaseous products, crystal lattice destruction under high temperatures lead to chemical bounds attenuation in materials and enable removal of the distributed in the materials ²²²Ra and ²²⁶Ra. However, in comparison with clays the changes of emanation factor are not significant, because the intensity of the both above mentioned processes is approximately similar. Chalk is an exclusion, because of its loose structure and less density. Gypsum stone keeps aloof. Its main difference from the other observed materials consists in the fact, that dehydration process starts already under 55°C, and up to 180-200°C it is practically finished (i.e. 1.5H₂O released). However, when reaching 550°C and more the remaining water is released.

With increasing the temperature up to 1200°C and more abrupt decrease of emanation factor is witnessed. Burning under such high temperatures leads to complete destruction of the initial crystal lattice, to appearing of the burned material liquid glass phase and to its recrystallization and neoformations' occurrence. This process prevails over that of radon release with increase of glassy and amorphous phases in the material. Consequently, natural materials, containing radium, firmly «pressurize» in the new aggregates. Creation of fritted surface prevents emanations' release into material's pores and reduces radon emanation factor. In other words, there occurs radon atoms' redistribution. Angle of inclination of emanation factor curve points on an intensity of radon release processes. The sharper the angle the more intensive is the process.

Processes similar to those described above also occur in case if a material usually is not subjected to heat treatment, for instance sand stone.

We suppose it necessary to compare the radon emanation factor data obtained experimentally with calculated ones. For this we will make an assumption that emanation factor depends on mass change of the researched sample. Having accepted the assumption we will construct a graph for clay (2) and limestone (3).

Fig. 4 – Dependence of experimental and calculated radon emanation factor on burning temperature: a – clay; b – limestone

 

As it is seen on the graph (Fig. 4) experimental and calculated curves differ substantially for both materials. At that the difference become substantial on achieving 750°C and higher burning temperature. This indicates, that while burning a part of radon releases not for the account of emanation, but at the expense of the influence of heat treatment. The similar process occurs with radium, because otherwise, experimental and calculated curves would coincide, or would be very close (influence of measurement error).

Conclusion

The acquired results indicate that from the viewpoint of the population radiation safety it is demanded to increase the stock burning temperature to acquire materials with the lowest values of radon emanation factor. This in the end leads to reducing of irradiation dose of population. However, this statement suffers two great shortcomings. The first is economical and comes to the fact that increase of burning temperature will lead to growth of material cost for the account of additional fuel used. The second one, technological consists in the awareness that an uncontrolled increase of burning temperature of any materials will result to spoilage.

From the above said it follows that nowadays the main way to solve the problem is search of optimal technological conditions of production. Such conditions when with minimal technological fuel consumption, stock burning temperature (and as a result fuel consumption) would allow to acquire qualitative building materials (with demanded properties), including demands on values of radon emanation factor.

We are to estimate this direction, taking into consideration all current influences and distant consequences.

Конфликт интересов Не указан.

Conflict of Interest None declared.

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