Chapter 29
New tools for radon progeny hazard prevention in mines
J. Skowronek |
R. Kajdasz |
Laboratory of Radiometry Central Mining Institute Katowice, Poland |
Z. Strzesniewicz Ziemowit Colliery Ledziny, Poland |
ABSTRACT
The described project is an example of prevention actions performed under real conditions in one of Polish coal mine. This project, titled Prevention Of The Short Lived Radon Progeny Hazard (Skowronek et al., 1998), was made by “Nadwiślańska Spółka Węglowa” S.A., “Ziemowit” Colliery and Central Mining Institute. The scientific part of the research was financed by Scientific Research Committee in Warsaw and the institutional part by Mining Plant “Ziemowit”. The aim of the project was to give the mining industry a set of instruments what would enable it to influence the level of the radon progeny hazard on every stage of work, starting from planning the exploitation, prevention of radon secretion from orogene to the elimination of radon from the air by means of filtration. The project was completed successfully and its results have been instituted in Nadwiślańska Spółka Węglowa S.A. and Mining Plant “Ziemowit”. The research has been made during 1994-6, and investments during 1996-7.
KEYWORDS
Air, radioactivity, radon, radon progeny, prevention
INTRODUCTION
In 1989 the new regulations on radiological prevention of miners were introduced in Poland, covering the prevention from all natural radioactive sources such as: short-lived radon progeny, radium bearing waters, radioactive sediments and the gamma radiation. The limit of the annual dose, set in the Polish standard PN-88/Z-70071 The Radiological Prevention In Underground Mining Plants (Norma, 1989) was 35 mSv.
In 1994-1995 new Geological and Mining Law (1994) was issued together with the executory acts. It was the first time in the history of the Polish and world's mining industry that problems of natural radioactive substances hazard in non-uranium mines were separated as one of the natural dangers, with all the consequences concerning the control and prevention. The regulations on radiological prevention were set together with the President of the State Atomic Energy Agency and international standards were included, especially regulations obligatory in European Union.
According to The Decree of the Chief Mining Authority, dated 3 August 1994, on the classification of workplaces…(Decree, 1994) the workplaces classified as radiological endangered are those where the risk of exposure on a dose higher than 5 mSv per year is probable. When the only source of danger is the short-lived radon progeny, it equals the potential alpha energy concentration of the short-lived radon progeny higher than 2 μJ/m3 in a workplace (average yearly). If such potential alpha energy concentration of the short-lived radon progeny is overstepped, it makes the diminution of the hazard necessary.
However, even in workplaces where the dose of 2 mSv per year may be overstepped (what equals the potential alpha energy concentration of 0.8 μJ/m3), an intensive control of this source of danger in requested (Decree of the Ministry of Industry and Trade, dated 14 April 1995). As far as the limits previously set are concerned, there is a change of value of working limits (potential alpha energy concentration of the short-lived radon progeny). It is due to the use of new conversion rate of equivalent dose, harmonised with the President of the State Atomic Energy Agency. So, the working value of the intervention level has been raised from 0.4 to 2 μJ/m3. The working value of the inspection level, as far the same as for the intervention level, has been raised to 0.4 μJ/m3. The yearly made analysis on the radiation hazard of miners showed that the short-lived radon progeny give the highest rate of the individual and collective dose.
It was proved that the conditions of exploitation and ventilation of workings have the highest influence on the level of radon progeny concentrations in mining plants, and gobs after the longwall exploitation are the main source of radon. Statistically, the highest concentration of radon progeny is present in areas of roof caving and aired in “Z” way (diagonally) what was reported by Skowronek (1992). For many years the highest attention has been called to prevention methods of such danger.
An example of such activity is a project made by Nadwiślańska Spółka Węglowa S.A., Mining Plant “Ziemowit” and Central Mining Institute titled Prevention Of The Short Lived Radon Progeny Hazard, (Skowronek et. al. 1998). The scientific part of the research was financed by Scientific Research Committee in Warsaw and the institutional part - by Mining Plant “Ziemowit”.
The aim of the project was to give the mining industry a set of instruments that would enable it to influence the level of the radon progeny hazard on every stage of work, starting from planning the exploitation, prevention of radon secretion from orogene to the elimination of radon from the air by means of filtration.
The project was completed successfully and its results have been instituted in Nadwiślańska Spółka Węglowa S.A. and Mining Plant “Ziemowit”. The research has been made during 1994-6, and investments during 1996-7.
DESCRIPTION OF RESULTS
The realisation of the project was to assure such working conditions that an excavation, where a workplace is, would not be classified as radiological endangered. In Mining Plant “Ziemowit” such requirement was fulfilled, with a significant reserve. The application of subjects of institution: technology of gob isolation, installations for radioactive aerosols filtration and principles of planning and exploitation of excavations - should assure work in the conditions of the lack of natural radioactive substances hazard.
Standard technology of gob isolation
In Mining Plant “Ziemowit” gob isolation has been used only to fire prevention, as far. Gobs were isolated with chemical foams. Also, on this material all the research for the project was made. The results of that research can be summarised as follows:
In longwall areas, the isolation of gobs from air penetration is an important element of prevention from radon daughters' hazard. Measurement showed that when isolation had been used, the potential alpha energy concentration decreased from the level higher than 6 μJ/m3 to below 1 μJ/m3. When the continuous isolation was applied, the concentration actually did not exceed 1 μJ/m3.
Different materials can be used as isolation, however material of a significant aerodynamic resistance are recommended.
Isolation of gobs for radon progeny hazard prevention can be accomplished both from the stream of fresh air - to prevent the diffusion of the air to gob areas - and from flows of escaping air to oppose the exhalation of gases from gobs to air flow.
As far as the selection of materials for gob isolation is concerned (to prevent exhalation of radon and its daughters from gobs to air flow), it is necessary to consider the possibility of diffusion of radon through some substances. It may cause the increase of radiation hazard on the further way of air flow.
Finally, the standard technology of gob isolation (Kajdasz et. al., 1997) has been elaborated; it includes as follows:
measurements in endangered area
transportation and production of IZOPIANA®
the sequence of works
the necessity of documents elaboration, required by the mining law (instructions, technologies) and its sanction by responsible authorities.
The technology of gob isolation for radon daughters' hazard prevention has been initiated in Mining Plant “Ziemowit” in the area of three longwalls. After its initiation the intervention level has not been exceeded in any point of those ventilation areas, and the inspection level at workplaces has not been surpassed for longer periods of time.
The device for filtration of radioactive aerosols ufar-300
In conditions of underground mining, it is possible that air already contaminated -because of prior contact with gobs or other sources of radon - reaches workplaces. In this case the contaminated air needs purification. Such situation is most frequent when dust concentration is high in the air.
Table 1. The main parameters of the device
Nominal output |
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Power supply tension |
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Purification efficiency of initial filter for particles <10 μm |
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Purification efficiency of thorough filter for the certified aerosol TSI DMPS of particles <0.4 μm |
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Dedusting devices, used in mining plants, are to eliminate a breathe-in dust fraction from the air. However, aerosols bearing the short-lived radon progeny are much smaller than breathe-in dust fraction particles: they do not exceed 1 micrometer. This is why dedusting devices used in mining plants are not able to eliminate the short-lived radon progeny from the air.
Filter-materials, able to filter the smallest dust fractions, were examined and tested in laboratories. When a material of a high efficiency (98%) in filtering of the short-lived radon progeny had been chosen, a filter, able to co-operate with typical ventilators used in mines, was constructed. When the filter has been examined in conditions of underground mining, a construction process of a complete device to be used in underground mining plants.
As a result of the construction process, a device UFAR-300 has been created, constructed in a co-operation of CMG KOMAG and Central Mining Institute. It consists of two chambers: for an initial filtration of bigger fraction and thorough filtration to eliminate the short-lived radon progeny.
The device has been tested in the Experimental Mining Plant “Barbara” to obtain the permission for use in underground mining plants in conditions of danger of explosion. The test was completed successfully, and finally the Chief Mining Authority issued a decision permitting the use of the device within underground mining plants in methanated and non-methanated areas in rooms classified as range a, b or c of danger of explosion granting the certificate number GM-46/98.
The device has been used underground in Mining Plant “Ziemowit”. The decrease of the potential alpha energy concentration has been obtained: from more than 2 μJ/m3 to 0.5-0.7 μJ/m3.
During the process of projecting and elaborating of the prototype an additional utilisable feature has been obtained: the device can be used as a separate instrument for filtering suspended dust. In this case, only the initial filtration part may be used.
The device UFAR-300 has been used in Mining Plant “Ziemowit” in the access way of air into one of preparatory forefields bored with a combine. The fresh air coming to a forefield had contact with gobs, so the potential alpha energy concentration exceeded there 2 μJ/m3, and when the device was used it decreased to 0.5-0.7 μJ/m3, that is to say, even below the inspection level.
The guidelines for projecting ventilation network to limit radon daughters' hazard
The system of ventilation is fundamental for the safety of underground work. The correct airflow in the system assures its proper composition, climatic conditions and does not create circumstances favourable for formation of the explosive atmosphere. Also, it does not produce a fire danger. Researches made in last years show that it influences the state of radon progeny hazard as well.
In circumstances when raw materials are exploited, which do not contain increased values of natural radioactive substances, only opening of large rock surfaces allows a radon exhalation to the air in the extent causing the radiation hazard. This fact should be taken into consideration both when ledge exploitation is planned and also if changes in a ventilation system are made.
Geological and Mining Law and its executory regulations include general instructions concerning the prevention of radon daughters' hazard. Actually, problems of mining works projecting are not complied there. In regard of this fact, in this project Regulations for projecting mining gobs and exploitation in circumstances of radon daughters' hazard (1999) have been elaborated. Two stages of work are distinguished there: the stage of preparation of workings and the stage of exploitation of ledge.
For the stage of projecting exploitation areas, requirements concerning ventilation systems are given and the possibility of using different methods of prevention. For the stage of ledge exploitation in circumstances of radon progeny hazard, requirements are given concerning the identification of radon source, ventilation systems adapted and other methods of prevention. Also, the requirements have been included concerning the prevention of radon daughters' hazard due to radioactive waters and sediments. Regulations… were illustrated with the indispensable schemes of ventilation systems.
In all cases mentioned above, the latest studies of new technologies and devices were included, achieved during the realisation of the project. There are the technology of gob isolation and device UFAR-300 eliminating radioactive aerosols from the air.
Regulations… were sent to the President of Chief Mining Authority to initiate the legislation process, introducing it as compulsory in the mining industry. The President sent it to proper branch committees, which started necessary law proceedings.
Here a computer programme RADON has been created, compatible with the system of programmes analysing the VENTGRAF ventilation network; also, it helps ventilation engineer's job of radon progeny hazard prevention. It allows modelling the flow of radon and its progeny concentrations in a ventilation system of a mine. It makes possible to forecast how changes in ventilation system influence the radon and its daughters' flow in such a network, helping to estimate the efficiency of the prophylactics taken.
Currently, Ventilation Department staff within Mining Plant „Ziemowit” uses the regulations for gob projecting while planning new plots of ledge. The example is the northern part of ledge 207. In outlets of the presently exploited adjoined plot, the potential alpha energy concentration does not exceed the intervention level.
RECAPITULATION
In Mining Plant “Ziemowit” results of the current project were systematically initiated. At present, thanks to systematic actions, the danger substantially decreased. The possibility of absorption by underground workers the effective equivalent dose more than 5 mSv, which is the lower limit appointed to the class A of natural radiation danger, is not expected. The alpha potential energy concentration higher than 2 μJ/m3 may be noticed occasionally, in a limited time and place.
Some numbers:
The maximum equivalent dose was:
in 1995 4.03 mSv
in 1996 2.68 mSv
in 1997 2.07 mSv
for different workplaces it equals the exposition to the short-lived radon progeny values as follows:
in 1995 2.8 mJh/m3
in 1996 1.9 mJh/m3
in 1997 1.5 mJh/m3
The maximum equivalent dose together for 1995-1997 for one workplace was 7.31 mSv per year, this equals the short-lived radon progeny exposition of 5.2 mJh/m3, what means
average yearly 1.4 mJh/m3 (1.97 mSv)
In 1997 the potential alpha energy concentration exceeding 2 μJ/m3 was noticed only in 5 measurements
It is clear, as far as the information above is concerned, that the decrease of radiation hazard reached in Mining Plant “Ziemowit” fulfils the requirements of the compulsory mining law, and even higher demands of prior regulations.
REFERENCES
Decree of the Ministry of Industry and Trade. Dz. U. Nr 67, poz. 342. (in Polish)
Decree, 1994: Decree of the Chief Mining Authority. M.P. nr 45, poz. 368. (in Polish
Geological and Mining Law, 1994. Dz. U. Nr 27, poz. 96. (in Polish
Kajdasz R., Skowronek J., Strześniewicz Z.: Standard technology of isolation of gobs. Dok. GIG, Katowice, 1997. (in Polish
Norma, 1989: Polska Norma PN-1988/Z-70071. (in Polish
Regulations for the prevention of radon progeny hazard in mines. Główny Instytut Górnictwa, seria Instrukcje, No 11, Katowice 1999. (in Polish
Skowronek et al., 1998: Skowronek J., Meinhardt B., Michalik B., Kajdasz R., Strześniewicz Z., Wala F.: Prevention of radon hazard. Final report. GIG, KWK Ziemowit, Katowice - Tychy, 1998. (in Polish
Skowronek J., 1992: Charakterystyka zagrożenia krótkożyciowymi produktami rozpadu radonu w kopalniach węgla kamiennego. Prace Głównego Instytutu Górnictwa, Komunikat nr 771, Katowice 1992. (in Polish
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New tools for radon progeny hazard prevention in mines