[en] In Japan, the concentrated storage of low-level radioactive wastes generated from nuclear power plants is expected to start at the Shimokita site in a few years. In this study, an estimation for the barrier performance on the concrete wall of a monolithic pit type disposal facility for low-level radioactive wastes was performed by using a random walk analysis model 'SMART-R' developed by CRIEPI for the radionuclide migration. The principal results are summarized as follows. 1) Among the Co-60, Sr-90, Cs-137, Ni-63, inorganic and organic C-14 and Tc-99 for which this estimation was performed, it was shown that the organic C-14 and Tc-99, that are the radionuclides with long life and low adsorptivity, have the possibility to leak from the disposal facility. 2) From these estimation, it was made clear that the concentration of Tc-99 can be reduced and those travelling time can be delayed as the available concrete wall thickness increases. 3) From the results estimated roughly on the internal radiation dose for each radionuclides subjected to this estimation in the case of the 25 cm wall thickness, it was shown that the concrete wall should has an adequate performance as the barrier against the influence of leaked radionuclides upon the human environment. 4) It was clarified that those performance estimations on the barriers of the disposal facility mentioned above should potentially be used to designate rationally a disposal facility of low level wastes. (author)