Personality traits have been recognized as an important means to mitigate psychological and physiological effects on human performance. In the nuclear industry, personality traits have been utilized only to screen out potential employees of mental illnesses. Although cognitive characteristics are considered in second generation HRA methods, there is no or superficial consideration about personality itself. Thus, an experiment was conducted to examine the relationship between personality traits and diagnosis performance in unexpected plant conditions of NPPs. As a result, five personality facets correlated with diagnosis performance were identified. The empirical data were used to derive an equation for diagnosis performance and a multiplier of personality factor through a regression analysis. This study proved that personality traits can be utilized in human resource management to maximize operators' performance, and the results can be utilized as a useful empirical source for HRA experts to judge effects of personality traits on human performance.