In 2010, Holtec International introduced their entry into the SMR competition: the 140 MWe Small Modular Underground Reactor (HI-SMUR). Although not a traditional reactor vendor, Holtec assembled a diverse team led by the newly created subsidiary, SMR LLC, to develop the HI-SMUR design. In late 2011, several design changes were made: the external steam generator system was changed substantially from a two-stage horizontal arrangement to a vertical arrangement, the reactor vessel was shortened by approximately 40%, and the power was increased to 160 MWe.

Figure 2.12 SMR-160 (United States) — SMR, LLC (HOLTEC International) © HOLTEC International SMR, LLC.

Still identified as HI-SMUR, it is more frequently being referred to as SMR-160 in recent press releases.

The SMR-160 is a compact loop configuration since the steam generator, and hence a portion of the primary coolant, is external to the reactor vessel. As an additional modification to a classic loop-type design, the SMR-160 pressurizer is located at the top of the steam generator vessel. The reactor pressure vessel has a significantly higher height-to-diameter aspect ratio compared to other SMRs to enhance the natural circulation of the primary coolant. The core uses full-height traditional 17 X 17 pin array PWR fuel assemblies and the core is expected to be refueled as a single cartridge that will be discharged to a below-grade spent fuel pool for cooling before being moved to a dry storage facility. A SMR-160 plant is currently planned to include a single reactor unit. Like most contemporary SMR designs, the reactor vessel and spent fuel pool will be entirely below grade level. Key parameters and a representative graphic for the SMR-160 design are given in Figure 2.12. [10]