Impacts of height and density of gramineous plant community on internal wind speed -Toward safety assessments of TRU waste disposal-

Radioactive carbon (14C) is the dominant radioactive nuclide in transuranic (TRU) waste. However, only a few studies to date have taken into account 14C transition in its gaseous form (14CO2). For an appropriate biosphere assessment of geological disposal, it is important to understand the degree of 14CO2 mixing with ambient air by vegetation. To evaluate the impacts of the structure of the plant community, relative to the vertical and horizontal wind speed, we characterized the structure of two gramineous plant communities (community-1, community-2) and compared the wind at three heights (middle, top, and above) of each community. The comparison of the two plant communities revealed that the biomass was the same, but the plant height of community-1 was 78% of that of community-2, and its vegetation density was approximately twice as high. Wind speeds of middle of the community with a higher vegetation density were less affected by the winds outside the community. It was assumed that the winds in the plant communities became more restricted as the community density increased. It is therefore suggested that effects of vegetation density are an important factor for the retention and the transition of 14CO2 to vegetation.