A molten salt electrolysis plant contained a lower section for carrying out electrolysis and, connected by a salt pipeline, an upper unit where rail cisterns were loaded. In this loading section, two 60-m³ buffer tanks under nitrogen atmosphere and fitted with 3 level alarms had been placed inside a building. Tank filling was being controlled from the lower unit and lasted 2 hours. An upper unit operator was assigned to monitor the filling operation and inform the lower unit should the 1st alarm be tripped. Occupied by other tasks in another unit on the day of the accident, this operator did not hear the alarm. Sodium escaped from one of the tank’s nitrogen vents exiting outside the building. The sodium exploded upon contact with snow, alerting the lower unit operator, who immediately stopped the sodium transfer. The internal emergency plan was activated. The building’s cladding was damaged. In response: the sodium transfer process was hooked up to the alarms; a cold trap was installed on the given vent; and plant procedures were modified.