The head of a major inquiry into Japan's 2011 nuclear disaster and a former senior Cabinet adviser have sounded the alarm over the restart of a 44-year old reactor, saying the industry and government have not taken on board the lessons from Fukushima.

Kansai Electric Power Co. (Kepco), which serves Osaka and its industrial environs — an area with roughly the same economic output as Mexico — rebooted the No. 3 reactor at its Mihama station in Fukui Prefecture on Wednesday.

The reactor is the oldest to be restarted since the 2011 disaster and needed special approval to have its lifetime extended beyond the standard 40-year limit. Most reactors in Japan remain shut after the disaster highlighted failings in regulation and oversight.