[en] The report presents the results of a study performed for the Department of Energy to initiate the concept definition phase of an HTGR-based nuclear process heat source development program. Alternate conceptual arrangements for an 842-MW(t) Very High Temperature Gas-Cooled Reactor (VHTR) are described and evaluated. Development of a VHTR parametric design computer code to guide and support nuclear system and process plant definitions and parameter selections is also discussed. The conceptual plant designs for six alternate plant configurations are presented in detail. These designs are evaluated with respect to feasibility, engineering design, economics, safety, reliability, and development considerations. The selected design is a two-loop plant with separate secondary helium loops to transport heat to externally located reformers. The plant has intermediate heat exchangers mounted in side wall cavities. Requirements and criteria are given for the major systems and components