Within a nuclear power plant, 2 m³ of lightly-radioactive water and resins leaked onto a cistern temporarily stored within the zone for slightly active waste. Since the normal discharge circuit was blocked, the cistern containing effluent from draining the reactor’s secondary circuit had been moved to this temporary storage zone. A break in the cistern’s indicator light (most likely due to frost) caused leaking of this liquid effluent, which had frozen prior to reaching the plant’s wastewater drainage circuit. The storage zone for slightly active waste was subject to controls and access restrictions, surfaced with bitumen and drained into 2 sewer manholes. It accommodated sealed cases containing the lightly-radioactive plant’s waste. Given that this incident did not jeopardise installation security, it was not classified on the INES scale. No radioactive substance was discharged outside the site boundary or polluted the ground water table.