Mitsuhiko Tanaka was working for Hitachi when the power plant was being built in 1974. He claims he helped cover up a manufacturing defect in the steel vessel that houses the reactor core.
My first reaction was, why didn't this asshole say something?!? Well, turns out, he did. When Chernobyl happened, he walked away from the nuclear power industry in Japan and, in 1988, tried to tell the Trade Ministry about the "ticking time bomb" as he called it. No one listened. Two years later, he wrote a book called, "Why Nuclear Power Is Dangerous." Guess what book is selling like hotcakes in Japan right now. The Trade Ministry can confirm or deny if they ever followed up on Tanaka's claims.
According to Tanaka, the damage occured during the final treatment process of the vessel. At the foundry where it was being finished, the vessel was heated to 1,112F (600C) in a process that would ultimately take two and a half years and cost tens of millions of dollars. It was to remove welding stress. The tank was to be supported by braces, but some either fell over or just weren't put inside. When the process was finished, they discovered the walls of the tank had warped. The height and width differed by 34 milimeters. Bye-bye, absolute symetry. Nuclear regulations in Japan required the core to be chucked into the scrap heap at that point. But that might have bankrupted the company. So Tanaka's boss asked him to reshape the vessel to disguise what happened.
Tanaka was a computer programmer. He used computer modeling to figure out a way to use pumpjacks to push the sunken wall out. He saved the company billions of yen, got a 3 million yen bonus, and got a certificate praising his extraordinary effort. Hitachi, meanwhile, distracted the inspector with golf and hot springs.
Tanaka had quit Hitachi back in 1977. In 1986, Chernobyl happened. Tanaka was asked to narrate a documentary. The film crew got really close to the ruptured core and got 30 hours of footage. The director died of radiation poisoning about a year later. Tanaka lost his shit. Two years later, he went to the Trade Ministry to confess what he had done. The Trade Ministry refused to investigate and Hitachi denied the charges.
I'm not afraid of nuclear power. I'm afraid of human nature.