Ikata reactor in Shikoku reaches criticality

But, as the article explains, returning to its normal mode of operation is a good thing, since the reactor had been previously shut down for a long time:

"The reactor resumed operations five years and three months after it went offline in April 2011 for routine checkups.

The Ikata No. 3 unit became the fifth reactor brought back online under the country’s stricter safety standards introduced in July 2013 based on the lessons from the triple meltdown at Tokyo Electric Power Company Holdings Inc.’s Fukushima No. 1 plant, which was damaged in the March 2011 earthquake and tsunami."