Task Types and Error Types Involved in the Human-related Unplanned Reactor Trip Events

The periodic or non-periodic test and maintenance activities in nuclear power plants (NPPs) are essential for stable power production and the sustainment of the plant safety. However, due to NPP system complexity and human limitation in cognition and action, these activities may also have the potential to induce unwanted and unanticipated reactor trip events and may render critical systems unavailable. Several studies have been conducted in the arena of maintenance human factors. James Reason emphasized the importance of maintenance-related human errors as well as human errors in responding to emergency situations [1]. Dhillon addressed the research status on human errors in test and maintenance tasks in several industries including nuclear power plants, aviation, and chemical processing plants, etc. [2]. Seminara and Parson reviewed the status of human factors engineering research in the domain of power plant maintenance [3]. Pyy analyzed the types of common cause failures in maintenancerelated tasks in nuclear power plants [4], and Toriizuka presented a work improvement method using performance shaping factors in industrial plant maintenance tasks [5]. This paper is concerned with human errors in test and maintenance activities of NPPs that have the potential for inducing unplanned reactor trips. The Korean regulatory organization for nuclear and radiological systems, Korea Institute of Nuclear Safety (KINS), provides a list of the major events, including unplanned reactor trips and unplanned initiations of safety systems that have occurred in Korean nuclear power plants, on a public website, the Operational Performance Information System (OPIS), http://opis.kins.re.kr [6]. According to OPIS, about 23% of the events that occurred during 2002 ~ 2006 were caused by human error. More recently, during 2004 ~ 2005, the contribution of human error to the unplanned reactor trip events has grown to about 34%, a significant increase. Interest in analyzing and reducing the human-induced In this paper, the contribution of task types and error types involved in the human-related unplanned reactor trip events that have occurred between 1986 and 2006 in Korean nuclear power plants are analysed in order to establish a strategy for reducing the human-related unplanned reactor trips. Classification systems for the task types, error modes, and cognitive functions are developed or adopted from the currently available taxonomies, and the relevant information is extracted from the event reports or judged on the basis of an event description. According to the analyses from this study, the contributions of the task types are as follows: corrective maintenance (25.7%), planned maintenance (22.8%), planned operation (19.8%), periodic preventive maintenance (14.9%), response to a transient (9.9%), and design/manufacturing/installation (6.9%). According to the analysis of the error modes, error modes such as control failure (22.2%), wrong object (18.5%), omission (14.8%), wrong action (11.1%), and inadequate (8.3%) take up about 75% of the total unplanned trip events. The analysis of the cognitive functions involved in the events indicated that the planning function had the highest contribution (46.7%) to the human actions leading to unplanned reactor trips. This analysis concludes that in order to significantly reduce humaninduced or human-related unplanned reactor trips, an aide system (in support of maintenance personnel) for evaluating possible (negative) impacts of planned actions or erroneous actions as well as an appropriate human error prediction technique, should be developed.