The Advanced Neutron Source (ANS) is a new basic and applied research facility based on a powerful steady-state research reactor that provides beams of neutrons for measurements and experiments in the fields of materials science and engineering, biology, chemistry, materials analysis, and nuclear science. The useful neutron flux for these experiments will be at least five times, and in some cases twenty times, more than is available at the world`s best existing facilities. In addition, ANS will provide irradiation capabilities for the production of radioisotopes for medical applications, research, and industry and facilities for materials irradiation testing. The need for a new steady-state neutron research facility in the United States was emphasized by the 1984 National Academy Report and confirmed by the Department of Energy`s (DOE`s) Energy Research Advisory Board in 1985. These studies defined a minimum thermal neutron flux requirement of 5 {times} 10{sup 19} m{sup {minus}2} {center_dot} s{sup {minus}1}. The National Steering Committee for an Advanced Neutron Source, with representation from the major fields of science that will use the facility, was established in 1986 and has continued to define the performance requirements and instrument layouts needed by the user community. To minimize technical risks and safety issues,more » the project adopted a policy of not relying upon new inventions to meet the minimum performance criteria, and the design presented in this report is built on technologies already used in other facilities and development programs: for example, the involute aluminum-clad fuel plates common to HFIR and ILL and the uranium silicide fuel developed in DOE`s Reduced Enrichment for Research and Test Reactors program and tested in reactors worldwide. At the same time, every state-of-the-art technique has been implemented to optimize neutron beam delivery at the experiments.« less