The Great East Japan Earthquake and tsunami on March 11, 2011, mark the start of the nuclear accident at the Fukushima Daiichi nuclear power plant. Progression of the accident has been analyzed with the SAMPSON code. SAMPSON was originally designed as a large-scale simulation system with the maximum use of mechanistic models and theoretically based equations. In the progression analysis done for Unit 2, SAMPSON could reproduce the pressure transient of the reactor pressure vessel (RPV) reasonably well by assuming partial load operation of the reactor core isolation cooling system (RCIC). The pressure transient of the primary containment vessel was reproduced reasonably well by assuming torus room flooding. After the RCIC trip and manual opening of the steam relief valve, SAMPSON predicted the damage to the upper part of the fuel assemblies near the core center and RPV failure due to creep rupture. More than 91 wt% of the core debris relocated to the lower plenum was as particles, and the major constituents were UO2, Zr, and ZrO2 by SAMPSON analysis.