This paper describes a new reactor concept: the Salt-cooled Modular Innovative THorium HEavy water-moderated Reactor System (SMITHERS), which addresses the goals of (a) evolving deployment needs, (b) increasing overall fuel burnup, (c) reducing proliferation risk, and (d) providing high-efficiency power generation. The reactor is modular and thus scalable from a few to hundreds of megawatts(thermal). The concept further burns used fuel from light water reactors (LWRs) without aqueous separations, reducing costs and proliferation pathways relative to current reprocessing plants. The additional burning of LWR fuel reduces proliferation risk by reducing global inventories of plutonium from used fuel in a way that does not isolate weapons-useable material and that increases the amount of power produced per ton of mined uranium. Improved fuel utilization through the potential use of thorium provides cost benefits by increasing neutron economy and enabling operation at higher efficiencies. Neutron economy is increased by using the lower neutron energies associated with large quantities of heavy water moderation and/or thorium for innovative reactor control and constant long-term power generation (i.e., sustainability). Finally, the proposed reactor also generates high-temperature coolant discharge in the form of liquid salt without coolant pressurization for external process heat applications such as oil extraction. Salt offers significant improvement over existing coolants such as light water and heavy water, which require pressurization to operate at high temperatures, adding to the cost and complexity of reactor operation. SMITHERS designs discussed in this paper either burned a full core of used fuel, ThO2 with 1.2 wt% PuO2 or other fissile material, or a combination of the two.