[en] After a short presentation of the LUCENS Experimental Nuclear Station project, a description of the plant and of the severe incident it experienced on January 21, 1969 is given. The procedures, methods and tools utilized for dismantling the reactor and simultaneously investigating the causes and consequences of the accident are described in chronological sequences. A brief review of some problems that might arise at underground nuclear plants follows. As a conclusion, main figures are given for the costs spent, the time used, the doses accumulated and the wastes handled. Some recommendations terminate this report. 4 figures