GRSAC was developed primarily to study a wide spectrum of core transient and heatup accident scenarios. It includes a detailed (~3000 nodes) 3-D hexagonal geometry T/F model for the core, plus T/F models for the RPV, SCS, and RCCS. The core T/F model consists of 163 radial nodes by 10 axial nodes for the fueled section (including the center reflector, if applicable). Axial coolant flows for the 163 channels are calculated independently; however, radial flows (which would occur more prominently in PBRs), are not accounted for. There are an additional 96 radial nodes for the side reflector, and two layers of axial nodes each for the top and bottom reflectors. There is an option to include neutronics (point kinetics), with xenon and samarium poisoning, to study accidents involving an ATWS. GRSAC also models air ingress accidents, simulating the oxidation of graphite core materials. The 3-D hexagonal geometry core thermal model allows for investigations of azimuthal temperature asymmetries in addition to axial and radial profiles. Variable core thermal properties are computed functions of temperature, and for prismatic core designs may also be dependent on block orientation and radiation damage. The annealing model for graphite can account for the increase in thermal conductivity occurring during LOFC accidents, which can have a significant effect on the predicted consequences. The primary coolant flow models cover the full ranges expected in both normal operation and accidents, including pressurized and depressurized accidents (and in between), for forced and natural circulation, for upward and downward flow, and for turbulent, laminar, and transition flow regimes. The primary loop pressure calculation can consider variable inventory (due to depressurization actions) and loop temperature changes, and use a simplified model for BOP temperatures and primary system pressure responses. The models for the RPV and the shield or RCCS are typically different for each of the various basic reactor models. FP release (for metal fuel) and Wigner stored-energy release models for graphite in the older model low-temperature gas reactors are also available.