Remediation of old subsurface repositories of radioactive waste, Russia: efficiency analysis

Boundaries of densely populated areas can approach with time to old subsurface repositories of radioactive waste due to growth of settlement territories. Protective isolation of the repositories was sometimes insufficient. Penetration of groundwater into the repository and discharge of the polluted water into neighboring rivers can lead to radioactive pollution of water resources of the region. Special rehabilitation measures at the repository sites are required to prevent this. Their efficiency is considered on example of former repositories of an atomic research center in Moscow (Russia). Radioactive materials were extracted from the subsurface burials for consequent disposal in a safer repository. A limited volume of polluted enclosing sedimentary rocks was also extracted and placed back after flushing with water. Analysis of rock pollution shows that these rehabilitation measures cannot enhance substantially a forecast of radioactive pollution. Moreover, loosening of the sedimentary rocks at the site of the earthwork leads to focusing of meteoric water from the surface in the domain of the most severe pollution of the underground medium. Elevated concentration of humic and fulvic acids in the surface water could intensify highly mobile colloidal form of actinide migration after the rehabilitation procedure. Place of polluted water discharge at a river bank and time of colloid-facilitated migration of radionuclides from the repository to the discharge place were estimated by computer simulation. Sampling of river sediments after approximately this time showed that concentration of uranium in the sediments at the calculated point exceeded about twice its values at neighboring sampling points up- and downstream (bottom sediments, which were polluted before remediation, were eliminated by a foregoing channel-cleaning of the river). Distributions of uranium in the sampled columns of the bottom sediments evidence that pollution of the sediments is caused by groundwater discharge. A comparison with the results of the mathematical modeling shows that this is in good agreement with the assumption of fast colloid-facilitated migration of uranium from the repository site. Along with the analysis of limited conditioning of the polluted rocks, these results indicate that the considered remediation method is less effective than hydraulic isolation of waste disposal area like it was performed at Oak Ridge National Laboratory.