[en] The Simplified Boiling Water Reactor (SBWR) is a revolutionary design of a boiling-water reactor. The reactor is based on passive safety systems such as natural circulation, gravity flow, pressurized gas, and condensation. SBWR has no active systems, and the flow in the vessel is by natural circulation. There is a large chimney section above the core to provide a buoyancy head for natural circulation. The reactor can be shut down by either of four systems; namely, scram, Fine Motion Control Rod Drive (FMCRD), Alternate Rod Insertion (ADI), and Standby Liquid Control System (SLCS). The safety injection is by gravity drain from the Gravity Driven Cooling System (GDCS) and Suppression Pool (SP). The heat sink is through two types of heat exchangers submerged in the tank of water. These heat exchangers are the Isolation Condenser (IC) and the Passive Containment Cooling System (PCCS). The unique design of SBWR imposes new requirements on the analytic methods for modeling its behavior. The close coupling between the power and flow, and also flow distribution among the parallel channels require a multidimensional power-prediction capability. The startup of the reactor has vapor generation and condensation taking place in the core requiring a model with a non-homogeneous, nonequilibrium, two-phase formulation. The instability at low flow/high power conditions requires modeling of the control systems and balance of plant, which has significant impact on the amplitude of the instability-induced power and flow oscillations. The RAMONA-4B code has been developed to simulate the normal operation, reactivity transients, and to address the instability issues for SBWR. The objective of this project is develop a Sun SPARC and IBM RISC 6000 based RAMONA-4B code for applications to SBWR safety analyses, in particular for stability and ATWS studies