The suit will probably take ten years.
I doubt that any suit would be successful. No power provider I know gives more than a “best possible” commitment to provide power, not any firm guarantee. Pretty sure that the earthquake and tsunami would count as an accident of nature beyond Tepco’s control.
Sad thing is the loss of power along with the massive destruction of its farmland, fishing, and manufacturing facilities, along with its decimation of its people and their neighbourhoods because of the tsunami will take years to recover.