Facts are still hard to come by, but here’s a link to the Guardian article:
Here’s what TEPCO posted on its English-language site, but I’m afraid something is lost in translation:
– At approximately 2:30 pm on May 31, big sound was confirmed at the
southern side of the reactor building of Unit 4, where wireless unmanned
heavy machineries were removing rubbles. It seemed that the unmanned
heavy machinery tucked and broke some cylinder. Nobody got injured.
There was no change in the data of the monitoring post.