The task of Department of Energy (DOE) Nuclear Weapons Complex is difficult to manage because of the lack of certainties in the nature of physical problems, uncertainties in social / political problems, and the lack of consensus in goals, means, and resource requirements. Managers need tools to help analyze impacts of various decision options. It is speculated that a system dynamics (SD) model would be suitable to deal with such problems. The Waste Tank Safety and Operations program at the Hanford site is chosen to test the usefulness of SD models for the DOE cleanup tasks. The SD model utilizes a commercial software package, STELLA, and is composed of 2,577 variables including 232 levels. It occupies about 5 megabytes in a Macintosh computer, and a 20 year simulation takes about 10 minutes on a Quadra 700 machine. The simulations for various cases show very reasonable results, which match well with the interview data. The model can analyze managerial / policy options regarding changes, budget constraints, delays, etc. Examination of the system shows that the major source of the current system inefficiency is diverse external changes. The changes not only create management work but also lower the productivity of workers in various ways. Besides changes, budget constraints, delays in work approval and material procurement are also major sources for system inefficiency. The case studies suggest that the model would be more useful if it included other programs at the Hanford site. The same techniques can be applied to any of 14 DOE Nuclear Weapons Complex sites. Furthermore, an aggregated system dynamics model which represents the totality of DOE Nuclear Weapons Complex program could be practically developed if some details are sacrificed. In conclusion, system dynamics can be a useful tool for communication purposes where different interest groups are involved, and/or external factors play important roles like the DOE clean up task.