The success criteria (SC) for system performance and operator timing in the NRC's standardized plant analysis risk (SPAR) models are largely based on the SC used in the associated U.S. nuclear industry PRA models. PRA models have used a variety of methods to determine SC, including conservative design-basis analyses and more realistic best-estimate methods, as well as varying modeling assumptions. Consequently, in some situations plants that should behave similarly from an accident sequence standpoint have different SC for specific scenarios. In addition, concerns periodically arise when reviewing licensee sequence timing and SC analyses in the course of performing event or condition risk assessments that could be better resolved with an up-dated set of thermal-hydraulic SC calculations. The U.S. Nuclear Regulatory Commission’s (NRC’s) Office of Nuclear Regulatory Research investigated particular SC and sequence timing issues of interest for BWR/4 Mark 1 reactors, specifically for the following issues: SC for situations with degraded high-pressure injection and relief valve criterion for non-ATWS; mitigating strategies (namely FLEX support guidelines) applied to loss-of-ac-power and other scenarios; emergency core cooling system (ECCS) injection following containment failure or venting; safe and stable end-state considerations. This paper presents a summary of NRC’s work on updated SC.