Gamma dose rate distribution in the Unegt subbasin, a uranium deposit area in Dornogobi Province, southeastern Mongolia

Unegt subbasin in Dornogobi Province, southeastern Mongolia, contains the Dulaan Uul uranium deposit, for which development for commercial mining has been conducted as of 2015. Zuunbayan is a commune located close to the Dulaan Uul uranium deposit, and residents of Zuunbayan and their livestock can easily approach the uranium deposit area, including an aboveground dump site, which was created as a result of the mining development. The present study measured and analyzed the gamma dose rate (absorbed dose rate in air) distribution in Unegt subbasin using data collected from a car-borne measurement survey. The gamma dose rate increased from the northern (45-65 nGy/h) to the central (50-69 nGy/h, including Zuunbayan) and the southern (54-195 nGy/h, including Dulaan Uul) parts of the study area. The gamma dose rates (up to 195 nGy/h) around the dump site in Dulaan Uul were significantly higher than the background level (< 80 nGy/h) at several points. Additional in-situ measurements showed that the gamma dose rates reached up to 450 nGy/h at these locations, which was primarily attributed to the gamma radiation emitted by ²³⁸U series elements. Spatial distribution of gamma dose rates around the dump site revealed that the gamma radiation did not originate from the dump, but from the ground, at the measurement points. Analysis of collected soil samples showed that ²³⁸U and ²²⁶Ra were concentrated in deeper soil. These results indicate that the gamma dose rates higher than the background level were not associated with the aboveground mine dump; rather, they were very probably caused by presence of uranium deposits close to the ground surface.

ELEVATION OF GAMMA DOSE RATE BY CONSTRUCTION OF THE ASIAN HIGHWAY 3 (AH3) BETWEEN ULAANBAATAR AND SAINSHAND, MONGOLIA

The present study aimed at examining changes of gamma radiation level associated with road construction in Mongolia. A car-borne survey of gamma dose rate was made for a paved, ~450-km long part of the Asian Highway 3 between Ulaanbaatar and Sainshand. The gamma dose rates ranged from 48 to 173 nGy/h. Elevation of the gamma dose rates was observed only on a 86-km long segment of the survey route which was newly constructed from 2011 to 2013. The gamma dose rates over the newer paved segment were twice as high as those over the bare dirt surface alongside it. Outdoor measurements of gamma-ray pulse height distributions also indicated an abundance of natural radionuclides, especially 232Th-series elements in road materials. These findings suggest that the gamma dose rates were elevated by introduction of road materials containing large amounts of natural radionuclides.