300-Tonne Contaminated Water Leak from Storage Tank — INES Level 3

On August 20, 2013, TEPCO reported that approximately 300 tonnes of highly contaminated water had leaked from a bolted storage tank at the site, containing approximately 80 million becquerels per liter of beta-emitting radionuclides including strontium-90. Some water was estimated to have reached the Pacific Ocean via a drainage ditch. The NRA upgraded the incident to INES Level 3 (Serious Incident). The leak highlighted systemic problems with TEPCO's improvised storage infrastructure and triggered demands for more rigorous oversight.