OGAKI, Japan—When Masayuki Sakamoto stepped onto the grounds of the world’s most dangerous nuclear power plant in March, he had little preparation other than a half-hour briefing on protective gear.

The 56-year-old owner of a 30-person construction firm from central Japan had been hired to clear debris and shovel dirt at the Fukushima Daiichi plant at a time when its reactors were belching smoke and oozing gamma rays. He had never worn a hazmat suit or used a dosimeter. He still doesn’t have the proper paperwork that, in normal times, would be needed to work in a radioactive environment.

Mr. Sakamoto’s rare and detailed description of daily conditions at the plant reveals the extent of worker-safety concerns there. On Monday, plant operator Tokyo Electric Power Co., or Tepco, said six more workers—bringing the total to eight—have likely received larger doses of radiation than allowed, even under Japan’s loosened exposure limits.

Mr. Sakamoto, a junior-high graduate, says he feels a “mission” to help tame the national crisis, but at the same time jokes that he’s too dumb to be scared. “Smart people know about sieverts and becquerels, so they’ve really got this sense of self-preservation, fear, suspicion,” he says. “When you think about it, it’s a real plus to be uneducated and ignorant.”

To clean up the plant, Tepco is leaning heavily on people like Mr. Sakamoto: an underclass of subcontractors and laborers who often have little education, training or understanding of the hazards they face.

The issues he described were compounded by the chaos inside the plant during the first few months of the crisis. After the quake and tsunami, normal systems for monitoring radiation and overseeing workers broke down, and Tepco was slow in implementing alternatives, interviews with Mr. Sakamoto and half-dozen other workers indicate.

Tepco acknowledges the issue. “Our top priority was cooling the reactors, so people might say that our response is slow,” said Tepco spokesman Takeo Iwamoto, discussing radiation management. “We are working on it as quickly as possible.”

Some workers weren’t properly registered for working in radioactive environments, Tepco said in a May report. Many didn’t get dosimeters to monitor radiation exposure when equipment was short in the early weeks.

A few workers employed by a major construction company, who asked not to be named, say they weren’t told when they were recruited that they would be going to the plant, where radiation levels can still be many thousands of times higher than normal.

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