This standard sets forth requirements for Fire Probabilistic Risk Assessments (FPRAs) used to support risk-informed decisions for commercial light water reactor (LWR) nuclear power plants, and prescribes general requirements for FPRA practice intended to suit a wide range of applications. This standard covers fires occurring within the plant. The American National Standard, 'External-Events PRA Methodology, ' ANSI/ANS-58.21-2007, covers fires occurring outside the plant.