Fort de Châtillon ( Paris ) The Fort de Châtillon was a fortification located about south of Paris in the communes of Châtillon - sous - Bagneux and Fontenay - aux - Roses . It was built in 1874 and was razed beginning in 1957 , making way for expansion of the French nuclear research facility at Fontenay - aux - Roses . The fort is named for the town it was designed to protect , Châtillon - sous - Bagneux , but the greater portion of the site is actually within Fontenay - aux - Roses . The main entry and the portions in Châtillon were destroyed at the end of the Second World War . History . The site was first fortified during the Franco - Prussian War in 1870 , when the redoubt of Châtillon was built by the inhabitants of the town . The works allowed the French to slow the Prussian advance on the capital and was the scene of violent combat . After the war the site was selected for further fortification as part of the Séré de Rivières system ring of fortifications around Paris . Construction on the new pentagonal fort started in 1874 , not far from the old redoubt . Unlike other Parisian fortifications , the new fort was designed for action in all directions , as the location commanded much of the surrounding region . The barracks , believed to have been in three levels , were located at the center , surrounded by walls defended by a ditch and caponier s . The fort featured a number of internal traverses , mounded dikes intended as shelters against low - angle shellfire from a variety of directions . Nearby was the " Coupure de Châtillon " , a fortified ditch about to the northeast of the Fort de Châtillon , defended by artillery batteries . With the Fort de Châtillon , the defenses denied the use of the Châtillon hill to an enemy who might dominate the nearby Forts de Montrouge , Issy and Vanves . The " coupure " consisted of eighteen traverses covered by artillery and defended by caponier s in the ditch . The entire fortification has vanished with the encroachment of urbanization . The fort saw no particular action during the Second World War . After the French Liberation , accused collaborators Joseph Darnand and Jean Hérold - Paquis were executed by firing squad at the fort . After the war the fort changed vocation to serve the Commissariat à l ' énergie atomique , the French atomic energy agency ( CEA ) , and was the site of Zoé , the first French nuclear reactor . The Châtillon site was quickly superseded by a new atomic research facility at Saclay , which opened in 1952 . Centre d ' Études Nucléaires de Fontenay - aux - Roses ( CEN - FAR ) . In 1957 the Fort de Châtillon changed its name to the " Centre d ' Études Nucléaires de Fontenay - aux - Roses " . Only the entry and some buildings of the old fort were saved . The rest was demolished between 1957 and 1970 . From 1974 the site housed a tokamak fusion reactor prototype , the TFR . It became difficult to maintain such activities in the densely populated area , and Zoé was shut down in 1977 . The site became more oriented to other areas of nuclear research , particularly in the areas of radiation protection and nuclear waste management . The site also accommodates administrative services of the CEA . A 1996 fire in the administration building precipitated a change in use . A portion of the building that housed Zoé became the Museum of the Atom , while the remainder was dedicated to research into the life sciences . Several buildings on the site remain dedicated to the " Autorité de Sûreté Nucléaire " ( Nuclear Safety Authority ) ( ASN ) and the " Institut de Radioprotection et de Sûreté Nucléaire " ( Institute for Protection and Safety against Nuclear Radiation ) ( IRSN ) . The site is no longer spoken of as the " Fort " or " Annex " , but as CEN - FAR or IRSN Fontenay - aux - Roses . Other Forts de Châtillon . Other Forts de Châtillon include one at Châtillon - le - Duc near Besançon , as well as a Fort de Châtillon at Boulogne - sur - Mer built in the 16th century .