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The method of control of radon progeny hazard by means of foam. Insulation of gobbins |
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The method of control of radon progeny hazard by means of foam. Insulation of gobbins Author:: J. Skowronek
Radon progeny is often a significant hazard, especially for people working or living in confined space (mines, dwellings) as a result of its inhalation. Usually, the main source of hazard are daughters products of Rn-222 decay, a member of uranium series. If isotopes from thorium series are present in elevated concentration in the strata, then also Rn-220 and its progeny are present in air. In mining industry till 70's prevention measures against radon hazard have been undertaken only in uranium or thorium mines. But during last 30 years a lot of investigations revealed, that such problem is also common in non-uranium mines. During our investigation in coal mines in area of Upper Silesia we found, that the most important and most efficient source of the radon exhalation are remains of longwall method of excavation - gobs. Also the influence of a ventilation system cannot be neglected - a part of air may go through gobs. As a result of a relatively slow flow through gobs, that air is enriched in radon and its progeny. Reintroducing of this air into ventilation air leads to the situation, that in galleries, where such problem occurs, we can observe a very significant increase of radon and radon daughters concentrations. A control of radon hazard can be done in a following ways: -insulation of sources of radon exhalation (rock body, gobbins, old galleries, inflows of radon-rich waters) from the contact with ventilation air; -proper method of the ventilation; -filtration of the contaminated air. Experiences, gathered in uranium mines showed, that insulation of mined out gobs and emptiness leads to the significant decrease of radon exhalation and diminishing of radiation hazard. In coal mines the isolation of gobs for the control of radon hazard is similar to the techniques of insulation made for the purposes of control of methane or fire hazard. In general - undertaken measures must in both cases focused on the decrease of an air flow through gobs. During our investigation of the effectiveness of gobs insulation for the control of radon hazard we applied a similar technique to the method widely used in Ziemowit Coal Mine for the prevention of fire hazard. In this mine, a foamed plastics are used for this purpose, mainly the material known as IZO PIANA ?. In the paper investigations of the influence of gobs' insulation on the level of radon hazard are described. Experiments were done in several flats of Ziemowit Mine, and additionally a technique of the insulation with use of foam plastics is described as well. Analysis of the results with the special attention paid on the impact of the insulation on decrease of radon and radon progeny levels in the vicinity of longwalls was performed. The best example of the effectiveness of applied measures are results of measurements done in flat of longwall 920 in a seam 209. The insulation of gobs of an exploited out longwall 919 at the distance of about 800 meters in the closest vicinity of longwall 920 was done. As a result the concentration of radon progeny decreased from the level 6 uJ/m3 below value 0,8 uJ/m3. The insulation of gobbins as a prevention measure against radon and radon progeny hazard can be applied in area of air inlet into the longwall region as well as in the outlet galleries. The main goal, achieved due to the insulation is to prevent air from ventilation tracts to enter the gobs area and washed out hazardous gases as methane or carbon dioxide. It means also, that in such case neither radon nor radon progeny can be introduced into the stream of the air. It leads to the decrease of the potential alpha energy concentration of radon progeny at workplaces located in the vicinity of isolated gobs. The discussion of an efficiency of insulation in correlation with the foam type, under different conditions was done. Some materials are not good enough to make an impenetrable barrier for gaseous radon. In case of application of foam insulation of gobs at the inlet of the air this problem isn't very important, because of the fact that such sealing stops the entrance of the air into the gobs and ventilation of that area. In case of sealing of the gobs at the outlet of the air the low permeability of foam to the radon is really important. In such system, the drop of the pressure along ventilation tracts could cause a diffusion of radon from insulated area through the barrier. The increase of radon and radon progeny concentration in the ventilation air may lead to the increase of the radiation hazard for the miners in the flat. The results of the investigations of radon transport through chosen foam plastics are described. The investigations lead to the following conclusions: -only some of chemical foams are good enough as sealing to stop the diffusion of radon from gobs, -in case of application of plastic foam from ISOSCHAUM company at the outlets from longwalls the monitoring of the radiation hazard must be done due to the fact, that this material doesn't entirely stop the radon diffusion from gobs. Therefore the sealing of gobs done with that type of foam leads to the decrease of the radiation hazard, but along the ventilation tracts the equilibrium between radon and progeny can reach high value. And in the galleries which lay further from the insulated areas it may cause the increase of radon hazard above permissible level. The general conclusion, which can be drawn from the performed investigations, is that the sealing of mined out areas (gobs) is the important tool for the control of radon hazard in the vicinity of longwalls in coal mines.
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2011