Design Extension Conditions for Spent Fuel Storage

The different definitions and approaches exist for the Design Extension Conditions (DEC) concept. In 2014 the WENRA issued in the reference levels for design extension of the existing reactors including the spent fuel storage. The determination of design extension conditions for nuclear facilities strongly depends on the object itself, its design, site characteristics, relevant external and internal events and combination of events. The systematic review of the operating experience is also important to establish new or upgrade the existing design bases. After Fukushima Daiichi accident, the implementation of DEC is required in many national action plans of the EU countries for existing NPPs safety upgrades. To do this, the nuclear safety regulations, which define the nuclear safety approach and DEC requirements should be in place. For new nuclear builds, it is very important to set suitable and sufficient design conditions and to define DEC in view of new experiences, knowledge and new safety requirements. How to implement the DEC for the existing nuclear facility is more questionable and depends on original design, site specific and regulatory approaches to design extension. In Slovenia, the WENRA DEC concept for nuclear facilities was adopted in nuclear legislation in 2016. Due to stricter requirements arising from the DEC requirements and based on lessons learned from Fukushima accident, the Krsko NPP decided to implement the safety upgrade project (SUP). SUP envisages the construction of new dry storage and upgrade nuclear safety of existing spent fuel pool. Both projects shall fulfil DEC conditions in accordance with the new DEC regulations. Both projects are already in design of implementation phase. The paper will discuss and provide an outline of DEC requirements for spent fuel storages, along with some examples to articulate some of the Slovenian DEC requirements and how these are applied to Krsko NPP spent fuel storage. (author).