In response to a request from Congress, the U.S. Department of Energy (DOE) asked the National Academies to evaluate its plans for managing radioactive wastes from spent nuclear fuel at sites in Idaho, South Carolina, and Washington. This interim report evaluates storage facilities at the Savannah River Site in South Carolina, with a particular focus on plans to seal the tanks with grouting. The report finds that tanks at the site do not necessarily need to be sealed shut as soon as the bulk of the waste has been removed. Postponing permanent closure buys more time for the development and application of emerging technologies to remove and better immobilize residual waste, without increasing risks to the environment or delaying final closure of the "tank farms." The report also recommends alternatives to address the lack of tank space at the site, as well as the need for focused R&D, activities to reduce the amount and improve the immobilization of residual waste in the tanks and to test some of the assumptions used in evaulating long-term risks at the site.