Kansai Electric Power Co. admitted Tuesday it failed to check a reactor pipe in its Mihama nuclear plant in Fukui Prefecture that burst and scalded four workers to death with steam, even though it knew for months that it needed inspection.

The steam leak from the carbon steel pipe at Mihama's No. 3 reactor Monday afternoon also injured seven other workers. The pipe had not been changed in 27 years of operation.

It was Japan's worst nuclear plant accident in terms of the death toll, but there was no radiation leak.