"Tokyo Electric Power Co said on Thursday (25.Feb.2016) its officials were unaware of a company emergency manual that defined a meltdown as damage exceeding 5 per cent of a reactor’s fuel."
link: http://www.thehindu.com/news/international/japan-utility-admits-it-delayed-report-of-fukushima-meltdown/article8281076.ece [thehindu.com]
color me a skeptical but isn't the (original) fuel always being fundamentally damaged (transmuted) in a fission reactor?