Radioactive particles (mainly Cs137) were dispersed inside and outside premises at a decontamination, assessment and packaging workshop. Two downwind monitoring stations noted a slight transient increase in atmospheric radioactivity. On 10/11, two subcontractors wearing overalls with hoods and equipped with filtering breathing apparatuses, drilled 8 holes (Ø 24 mm, depth 170 mm) in the plug of a radioactive waste package in order to install handling lugs. The package, showing a high dose rate, was placed in the non-ventilated room 8E (usually on the north dock) to reduce the external exposure of the employees. The drilling was followed by various contamination inspections and a light cleaning of the package and holes with compressed air. On 15/11, while the door of the room was open, the employees were working on removing dust from the holes and checking their depth by inserting a metal rod. During a depth check, one of the 2 employees was pressing lightly on the metal rod to make sure that it was indeed in the void. While a team manager was giving 6 people a tour of the premises, the rod was tested positive in another room. External contamination was detected for one of the employees and one of the visitors by the zone exit controls. The internal dose received by 2 of the employees was between 0.2 and 2 mSv (1/10th of the annual limit). Additional examinations have been planned for the staff. The radioactivity had dispersed by the open door and the orifices in the roof, contaminating the soil in an area of 4 m². This incident was related to the fact that the drilling depth specified by the procedure for using these anchors (no preliminary test carried out) is greater than the minimum thickness, free of radioactivity, required for these plugs. The measures taken included: containment and decontamination of room 8E, assessment of the package, installation of an airlock on the entrance door to this room, installation of a breathing air beacon and two atmospheric sampling devices (instantaneous and delayed measurement), drafting of a specific procedure on the installation of attachment lugs (drilling depth, etc.) and preventive measures (intervention room, clothing, etc.),a request for the characteristics of packages destined for destructive testing, and a ban on carrying out operations that may alter the containment in room 8E.