The Next Generation Nuclear Plant (NGNP) project is being conducted by the U.S. Department of Energy (DOE) to demonstrate the technical and licensing viability of high-temperature gas-cooled reactor (HTGR) technology as a CO2 emission-free source of energy to displace the use of natural gas, petroleum, and coal for production of electricity and/or high-temperature process energy for a wide range of industrial applications. The DOE selected the HTGR as the reactor type for the NGNP project primarily because HTGRs can produce heat energy at much higher temperatures than other reactor types due to their use of ceramic, coated-particle fuel, helium coolant, and graphite as the core structural material. The DOE is considering a number of candidate HTGR designs for the NGNP demonstration plant; the DOE or a DOE-industry partnership will ultimately select the design to be licensed and constructed.

The HTGR design option being advanced by General Atomics for the NGNP demonstration plant, and for follow-on commercial deployment, is the Steam Cycle Modular Helium Reactor (SC-MHR). The SC-MHR, which is the subject of this paper, uses fuel elements in the form of hexagonal blocks, which are stacked together to form the reactor core. This type of HTGR is referred to as a prismatic HTGR, as opposed to a pebble bed HTGR, which uses billiard ball-size spherical fuel elements. The above-noted generic features of HTGRs coupled with the modular helium reactor design features of the SC-MHR allow for adequate removal of residual heat from the reactor by completely passive means in the event of a loss of forced cooling or loss of coolant pressure. This ensures that the fuel remains below time-at-temperature limits at which fuel damage could occur during such events, thereby ensuring radionuclide retention within the fuel particles. Thus, the safety of the SC-MHR (as well as other modular HTGR designs) is inherent to the design, and the rare, but severe, accidents postulated for light water reactors and other advanced nuclear concepts are not possible with the SC-MHR.

It is anticipated that design, licensing, and construction of the SC-MHR demonstration plant could potentially be completed to enable plant operations to begin in 2022.