The environmental dose rate distribution of the fusion device HT-7U during operation was calculated and analyzed by using the three-dimensional Monte Carlo transport code MCNP/4C and the latest Fusion Evaluated Nuclear Data Library FENDL/2. The influence of filling materials of the double-wall vacuum vessel on the environmental dose rate was investigated. The results demonstrate that the borated water filled in the vacuum vessel significantly reduces the environmental dose level. However the shielding effect of the enriched borated water is not evident as compared with the natural borated water. In addition

the radiation skyshine was investigated and it was found that the shadow effect was clearly shown in the environmental dose rate distribution for the thickness of the concrete ceiling less than 0.5 m.