On 26 September 2011, after the discovery of hydrogen in a pipe leading to the containment vessel of reactor no.1, NISA instructed TEPCO to check whether hydrogen was building up in reactor no. 2 and 3 as well.
After the discovery of hydrogen concentrations between 61 and 63 percent in pipes of the containment of reactor no. 1, nitrogen injections were started on 8 October.