Following up this article..Nitrogen injection stopped and Tepco started backup unit
It turned out that the wind stopped nitrogen injection.
The strong wind caused by the storm blew out sands to stop the filter part of the nitrogen injection facility.
A nuclear plant worker found the facility stopped at 10:55 but it was stopped since 9:51 actually.
Until it was restarted, it had been stopped for 2 hours and 40 minutes.