Nuclear Power Plants (NPP) have complex and dynamic work environments. Nuclear safety and organizational management rely largely on human performance and teamwork. Multi-disciplinary teams work interdependently to complete cognitively demanding tasks such as outage control. The outage control period has the highest risk of core damage and radiation exposure. Thus, team coordination and communication are critically important during this period. The purpose of this thesis is to review and synthesize teamwork studies in NPPs, outage management studies, official Licensee Event Reports (LER), and Inspection Reports (IRs) to characterize team brittleness in NPP systems. Focusing on team brittleness can provide critical insights about how to increase NPP robustness and to create a resilient NPP system. For this reason, more than 900 official LERs and IRs reports were analyzed to understand human and team errors in the United States (US) nuclear power plants. The findings were evaluated by subject matter experts to create a better understanding of team cognition in US nuclear power plants. The results of analysis indicated that human errors could be caused by individual human errors, team errors, procedural errors, design errors, or organizational errors. In addition to these, some of the findings showed that number of reactors, operation year and operation mode could affect the number of reported incidents.