About six tonnes of heavy water leaked out during inspection of a coolant channel tube at the Madras Atomic Power at the Madras Atomic Power Station(MAPS) last month, exposing some of the repair workers to excess tritium which is mildly radioactive, official confirmed.The leak, which occured on March 26 at unit two of maps which was shut down for inspection, was initially categorised as a "below scale" or "zero" event in the international nuclear event scale which ranges from zero (no safety significance) to seven (major accident).