The application of four gas-turbine, modular helium cooled reactors and an accelerator unit (GT/AD-MHR) has been proposed for burning transuranics recycled from LWR waste. The recycled LWR discharged transuranics encapsulated in TRISO coated particles are first loaded into the outer thermal spectrum zone of the GT/AD-MHR for burning in the critical mode for about three years. Previously burned fuel is in a central fast zone. In the fourth year, the same unit is configured as an accelerator-driven system, containing a centrally located spallation target. The three-year, thermal-zone burned fuel and the inner fast-zone fuel from the critical mode operation are used in this subcritical cycle, and remain in their respective zones. At the end of this one-year subcritical irradiation, the outer thermal-zone fuel is reconstituted and used as fast-zone fuel in another critical mode operation. As the fuel in the fast-zone has reached its end of life it is discharged, with very low transuranics content. The critical mode operation is staggered, and each GT/AD-MGR unit undergoes the subcritical burn in one out of four year. The physics performance of the GT/AD-MHR has been evaluated using independent deterministic and Monte Carlo codes and the results of the study are presented in the current paper. A companion paper discussing the verification of the codes is also presented at this meeting. Single-batch and three-batch fuel loading schemes for the GT/AD-MHR have been evaluated using the REBUS-3/DIF3D fuel cycle code, to determine the feasibility of achieving very high burnup without exceeding reactivity and power density limits. The reactor physics of the GT-MHR is complicated by the presence of the low-lying plutonium and Er-167 resonances (0.2--1.1 eV) and by the fact that the neutron spectrum has a low-energy peak about this energy range. This peak can change depending on the core state or material loading. The location of the peak and the direction of the spectral shift greatly affect both the resonance fission and capture rates and dictate the core or element criticality state and the magnitude and sign of reactivity coefficients. For these reasons, 23-energy-group, burnup-dependent microscopic cross sections are employed in the REBUS-3/DIF3D model used for evaluating the system. These cross sections were generated with the DRAGON codes using ENDF/B-VI data
