CPV code 45251111 designates Construction work for nuclear reactors in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
45251111 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 45251110 (Nuclear-power station construction work).