Decommissioning and Dismantling of the French Brennilis NPP

This paper deals with the dismantling of the Brennilis NPP plant located in the west of France (Finiste`re). This prototype NPP of Brennilis was the unique reactor of the heavy water developed in France during the 50’ and the 60’. The reactor diverged in December 1966 and the NPP was operated during 9 years from 1972 to 1981, then the permanent shutdown occurred in July 1985. In 2008, the operator and owner of the plant Electricite´ de France (EDF) commissioned the consortium Onet Technologies Grands Projets (France) and Nukem Technologies (Germany) with the dismantling of the reactor block of the NPP. The reactor block essentially contains the reactor vessel including built-in units and biological shields, the peripheral piping as well as systems for controlling the nuclear-related process. In addition to the complete dismantling, the scope of the contractual services also includes their proper handling in accordance with the applicable regulation: safety requirements, waste management, radioprotection optimization and management. The central element of the plant is the reactor pressure vessel filled with heavy water. Each of the 216 horizontal fuel element channels made of zircaloy is at each side connected to a pipe which directs the heat transfer gas to a header mounted in the upper part of the reactor block. The control rods are introduced vertically into the reactor. It should be pointed out that due to this reactor design, the reactor pressure vessel is equipped with a complex pipe system to all sides which makes it difficult to freely access the core area of the reactor block and thus to dismantle the reactor. In this context, the axial and lateral neutron shields should be mentioned, which are situated in close proximity to the reactor as well as the biological shield which protects from ionizing radiation originating from the pressure vessel. The access to the core area is made difficult due to a high local dose rate and the extremely high constructive complexity of the prototype, the interior of which is virtually criss-crossed by complex piping. The elevated local dose rate in the area of the reactor pressure vessel makes manual work in this zone impossible (even after 30 years), so that remote dismantling techniques have to be used. Before starting dismantling, the remote devices are determined with the help of a test stand which representatively simulates the real conditions of the reactor block in respect to dimensions and material.