A Fusion Nuclear Science Facility (FNSF) is necessary to make possible a DEMO of the Advanced Tokamak (AT) type after ITER. One candidate, Fusion Nuclear Science Facility-AT (FNSF-AT), should have neutron wall loading of 1-2 MW/m2, continuous operation for periods of up to two weeks, a duty factor goal of 0.3 on a year and neutron fluence of 3-6 MW-yr/m2 in ten years to enable development of blankets suitable for tritium and electricity production while demonstrating nearly all the critical elements necessary for the qualification and design of a DEMO. FNSF-AT, also called FDF, will be designed using conservative implementations of all elements of AT physics to produce 150-300MW fusion power with modest energy gain (Q<7) in a modest sized normal conducting coil device. It will demonstrate and its results will help in the selection of the DEMO tritium breeding blanket concept. It will demonstrate the tritium fuel cycle, the behavior of candidate plasma facing materials, and the design and cooling of the first wall chamber and divertor components. It will also provide experience in safe operation and remote maintenance necessary for the DEMO design.