The Postojnska jama cave system represents a union of old, now dry, horizontal cave passages, the Pivka river active passage, and shafts or collapse dolines, which enable surface connection with some ...cave passages. The present entrance to the cave is formed at the contact between the Postojna basin impermeable Eocene flysch and the Upper Cretaceous limestone. The Pivka river sink entrance is formed along interbedded movements in the horizontal direction, and in the vertical direction it lies in a strong tectonic fault zone of NE-SW direction. The cave passages are formed in tectonic deformations which developed by the formation of the Postojna anticline, as there are interbedded movements, and in later fault deformations, where the main tectonic fault zones directions are the Dinaric NW-SE and cross-Dinaric NE-SW. Besides two main fault directions, the accompanying broken and fissured zones which are the result of the fault Dinaric tectonics are also favourable to cave formation.
Due to the specific work regime in the Postojna Cave, which depends primarily on the daily number of visitors, and on seasonal variations in air radon concentrations, an optimal methodology for radon ...and progeny measurement and dose calculation was sought. The program of measurement throughout the years was optimized, and now comprises 3-mo exposures of etched-track detectors, and twice a year, 8-10-d measurements using continuous monitors. Radon concentrations range from about 500 Bq m(-3) in winter to about 6,000 Bq m(-3) in summer, and equilibrium factors range from 0.42 to 0.69 in winter and from 0.33 to 0.86 in summer. Radiation doses from radon decay products for employees in the cave were calculated according to the ICRP 65 methodology. The basic input data are radon concentrations and equilibrium factors at two selected locations in the cave and the records of the time spent by a worker in the cave. Effective doses received by employees annually ranged from 0.02 to 8.4 mSv.
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