Indoor radon survey in a uranium area from Romania

In the Bihor uranium district there are an operating underground mine and the closed down open pit mine. The open pit mine was operated by the Soviets between 1952-1964, the ore being "exported" to the Soviet Union. The mean content of the uranium ore was 2%. The "waste rocks" from the open pit mine were used by the natives in the foundations of detached houses in rural areas. The goal of the survey was to identify this houses and to make radon and gamma dose rate measurements to assess the annual effective doses incured by the public. In the impact area of the uranium mine 114 houses built up with uranium waste rocks were found out. In direct contact with foundations of the houses gamma dose rates up to 30 ?Sv/h were measured. Also, 76 houses, chosen randomly, built up with conventional building materials were used as reference ones to avoid the influence of geology. Gamma doses rates were measured using portable dose rate meters, with NaI(Tl) detectors, having a sensitivity of 0.01?Sv/h. Radon concentrations were measured using Lucas cells [1] with a longer counting period (MDC=37 Bq. m-3) and makrofol track-etched detectors delivered by the University of Gent [2] and red there. In "waste rock" houses 221 gamma dose rate measurements were made resulting a mean of 0.13 ?Sv/h and in reference houses 151 measurements resulting a mean of 0.12 ?Sv/h with ranges of 0.07-0.18 ?Sv/h and 0.09-0.20?Sv/h, respectively. It can be seen that there is no a significant difference between the two means. Radon measurements were made in 190 houses, 114 "waste rock" houses and 76 reference ones. From the 221 radon measurements performed in the "waste rock" houses resulted a mean of 57155 Bq. m-3 , with a range of < 37-6,335 Bq. m-3 and from the 151 ones performed in the reference houses resulted a mean of 450 35 Bq. m-3 with a range < 37-2,149 Bq. m-3. By all, resulted a mean 52036 Bq. m-3. If the means of the two groups are compared using a Student test it results that there is no significant difference between them. In 81 "waste rock" houses radon concentration measurements were made using both Lucas flasks and track-etched detectors. The track-etched detectors were exposed in "waste rock" houses for periods of 3-4 months. Using a Student test some values from the two series were eliminated. If the two means are compared it results, with a confidence level of 0.998, that there is a significant difference between them. One can also see that, in this case, the grab sampling method overestimates the indoor radon level. The average effective annual dose to an individual living in houses from this area, using the recommandations of ICRP 65 [3], was estimated as being 10.060.63 mSv, from which 9.15 mSv due to inhalation of radon daughters. After UNSCEAR 93 Report [4] this area can be considered as one with elevated exposure to radon.