The accident has only magnified the problems the system allows. According to company records, contract workers at Fukushima Daiichi receive, on average, more than twice the radiation exposure of Tepco employees. The layered system, many say, also allows for relatively little oversight by Tepco.

In a recent interview, a Tepco spokeswoman said that the company regularly evaluated its contractors and required them to provide their workers with a class on the basics of radiation. (She denied charges of widespread cheating made by some workers.)

But at a news conference last month, the chief nuclear regulator, Shunichi Tanaka, said, “There is a subcontracting structure that means even workers from third- or fourth-level contractors work at the site, and Tepco does not have a clear picture of what’s happening on the ground.”

Mr. Naka, the contractor who talked of a manpower crisis, said many of his best engineers — including those who battled explosions and fires in the early days of the crisis — have either quit, or cannot work at the plant because they have reached legal radiation limits for the year.

Yoshitatsu Uechi is one of the people who has stepped in for more experienced workers. A former bus driver and construction worker, Mr. Uechi has never before worked at a nuclear plant.

He was paid about $150 a day to work on one of the plant’s most pressing jobs: building tanks to store the vast quantities of contaminated water at the site. He describes hurried days, saying he was told at one point by his contractor to continue sealing the seams of the tanks despite rain and snow that made the sealant slide off.

He believes such slipshod work eventually compromised the tanks, some of which have since leaked.

“I spoke out many times on the defects, but nobody listened,” said Mr. Uechi, a father of four who says he left Okinawa and its depressed economy for Fukushima to provide a better life for his children. He said he rarely saw Tepco managers while on the job.