Purpose of this paper is to point out the security challenges that may occur during the decommissioning, based on the issues and lessons learned from the German phase out decision. Though national regulations may be different in other countries the basic problems and issues will be the same. Therefore presented solutions will be applicable in other countries as well.

The radioactive material remaining at the NPP during decommissioning has the most influence on how the security measures have to be designed. The radioactive material defines the risk potential of the plant and this determines the needed security level. The following aspects have been challenging in Germany:

• Scenarios varying from those, used for plants in operation, due to changed operating conditions

• Spent fuel will stay in the spent fuel pool for a quite long period before it can be removed from the plant. Risk potential of the plant stays high and requires a high level of security measures

• Security measures according to the existing operating license have to stay in place as they are, unless the first license for decommissioning is given respective the spent fuel is removed from the plant site. This even led to the question if improvements of security measures, planned and announced with focus on a plant remaining in operation for another couple of years, need to be done although they will not be required after removing the spent fuel from the plant.

A further important aspect for the security design is the fact that a plant under decommissioning has completely different and strongly varying operating procedures, compared to the stable ones of an operating plant. This leads to different needs concerning workspace, infrastructure on plant site, access to buildings etc. An optimized and highly flexible security concept is needed to ensure an adequate level of security as well as an efficient decommissioning. A deep analysis of the vital plant functions, depending on the different decommissioning stages, is required to determine the vital equipment, its location and its need for protection.

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