engleski

Radon investigation in two high radon level schools in Slovenia

Among the 890 schools surveyed for radon concentrations in Slovenia, 25 buildings were found to exceed maximum permitted air concentrations for classrooms of 1000 Bqm-3. These schools have been thoroughly investigated in order to find the source of the elevated radon concentrations and undertake measures for the most effective remediation of these. This paper reports the results of complementary measurements performed in the two schools with the highest concentrations of radon. fn both cases classroom radon concentrations reached levels of 7000 Bqm-3. As with the majority of high radon level schools, these two buildings are situated on carbonate rock in the Slovene Karst, within a distance of about 100 kilometers from one another. They also have similar structural characteristics; both are one-story buildings, without a cellar, and both are over fifty years old. During a one and a half year period the following measurements were performed: average indoor radon concentration measurements in all of the rooms using track-etched detectors, electrets and continuous monitors (Genitron-AIphaGuard, Sarad-EQF30xx); soil gas radon measurements in some locations close to the building using AIphaGuard and gamma spectrometric analyses of the soil samples. In both schools the radon level is high due to geological and radiometric characteristics of the soil. The deep fissures in carbonate rocks result in easy transport of radon from deeper rock layers into overlying soils. Subsequently, radon enters the classrooms through cracks in the foundations. After a thorough examination, poor constructional quality was found in both cases. At a distance of about 2 m from the first school, soil gas radon concentrations range from 107 kBqm-3 at a depth of 20 cm, to 223 kBqm~3 at a depth of 60 cm. In the surface soil layer (0 to 15 cm), average concentrations of radium (226Ra) of 234 Bqkg-' and of uranium (238U) of 111 Bqkg-' were measured in the surrounding soil. In the second case, a soil gas radon concentration of 177 kBqm-3 was found at a depth of 25 cm as well as average concentrations of radium of 141 Bqkg', and of uranium of 69 Bqkg-' in the surface soil layer. In both cases, soil humidity was very high. It seems that in dry soils radon concentrations can be still significantly higher, especially in deeper soil layers. A maximum indoor radon concentration of about 7000 Bqm-3 was detected in the classrooms in both schools before mitigation measures were implemented. In the first school, the measures applied to reduce radon ingress via soil by means of depressurization have not been effective. More precisely, analysis showed poor communication between suction points as a result of holes and cracks under the concrete slab. Thus, the foundations were renewed and fitted with built-in ventilation tubes. In the classrooms of the second school, radon concentrations were decreased by means of the ventilation of the sub-floor tunnel. After mitigation, in both cases, the indoor radon concentrations had been reduced to 200 Bqm-3.

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