TOKYO — Given the fierce insularity of Japan’s nuclear industry, it was perhaps fitting that an outsider exposed the most serious safety cover-up in the history of Japanese nuclear power. It took place at Fukushima Daiichi, the plant that Japan has been struggling to get under control since last month’s earthquake and tsunami.

In 2000, Kei Sugaoka, a Japanese-American nuclear inspector who had done work for General Electric at Daiichi, told Japan’s main nuclear regulator about a cracked steam dryer that he believed was being concealed. If exposed, the revelations could have forced the operator, Tokyo Electric Power, to do what utilities least want to do: undertake costly repairs.

What happened next was an example, critics have since said, of the collusive ties that bind the nation’s nuclear power companies, regulators and politicians.

Despite a new law shielding whistle-blowers, the regulator, the Nuclear and Industrial Safety Agency, divulged Mr. Sugaoka’s identity to Tokyo Electric, effectively blackballing him from the industry. Instead of immediately deploying its own investigators to Daiichi, the agency instructed the company to inspect its own reactors. Regulators allowed the company to keep operating its reactors for the next two years even though, an investigation ultimately revealed, its executives had actually hidden other, far more serious problems, including cracks in the shrouds that cover reactor cores.