Despite of careful planning of decommissioning projects, there are often surprises when facilities are opened for dismantling purposes, or when material is removed from hot cells, etc. Unexpected incidents and findings during the decommissioning of nuclear facilities have been referred to in the past as unknowns. However, many of the problems encountered during implementation of decommissioning are well known, it is simply that they were not expected to arise. In some other cases, the problem may not have been encountered in the decommissioning team’s experience, forcing the development of new techniques, tools and procedures to address the unexpected problem, with the attendant delays and cost overruns that this often involves. Unknowns in decommissioning cannot be eliminated, regardless of the efforts applied. This is especially the case in old facilities where documentation may have been lost or where modifications were carried out without updates to reports. As a result, when planning for decommissioning, it is prudent to assume that such problems will occur, and ensure that arrangements are in place to deal with them when they arise. This approach will not only improve the efficiency of the decommissioning project, but will also improve the safety of the operations. One of the most common root causes of unexpected events in decommissioning is the lack of detailed design information or missing records of modifications, maintenance issues and incidents during operation. It is therefore necessary to check the completeness of design information in existing plants and to ensure that configuration management techniques are applied at all stages of the lifetime of a plant. In the case of a new plant, archiving samples of materials can be a valuable source of information to support decommissioning planning. During the lifetime of plants, it is likely that modifications will be carried out involving the construction of new buildings. The opportunity should be taken in these circumstances to consider the layout, the physical size and other attributes of the plant to ensure that they do not make decommissioning of existing facilities more difficult and also to optimize the potential for reuse in support of the decommissioning of the whole site, later in the life of the facility. Characterization of all aspects of a plant is essential to reduce the number of unknowns and the likelihood of unexpected events. This characterization should be extensive, but there is a limit to the level of detail that should be sought as the cost versus benefit gain may reduce. Reducing unknowns by retrospectively obtaining physical data associated with a facility is a useful means of characterization, and there are many tools in existence that can be used to carry this out accurately and effectively. Regardless of the efforts that are employed in decommissioning planning, unexpected events should be anticipated and contingency plans prepared. Although the details of the event itself may not be anticipated, its impact may affect safety and environmental discharge, and may or may not involve radiological impacts. Regardless of more serious impacts, unexpected events are likely to result in modifications to the decommissioning plan, incur delays and cost money. Finally, regardless of the status of a facility, whether at the concept stage or at the decommissioning stage of its life cycle, it is never too early to begin thinking and planning for decommissioning.

• KwanSeong Jeong(Korea Atomic Energy Research Institute (KAERI)) Corresponding author
• In-Ho Yoon(Korea Atomic Energy Research Institute (KAERI))
• JungJun Lee(Korea Institute of Nuclear Safety (KINS))