The former chief of the Tokyo Fire Department clearly remembers the day nearly 10 years ago when he ordered a team to the Fukushima No. 1 nuclear plant crippled by a massive earthquake and tsunami in the belief it "had to be done, even if it caused casualties" among its members.

The capital's former fire chief Yuji Arai, 69, and a dozen former commanding officers who were involved in the unprecedented mission in 2011 met five times between July and December last year to document their experiences in dealing with the country's worst nuclear disaster.

Most at the department did not have a strong sense of danger when the loss of power occurred at the Fukushima No. 1 nuclear plant following the earthquake and tsunami on March 11, 2011, according to documents detailing the department's response to the crisis compiled by Arai and the former officers.