Veterans of the Three Mile Island cleanup said that a much larger task faced the Japanese engineers who are trying contain and secure the damaged Fukushima Daiichi reactors. And Three Mile Island took 14 years.

Lake Barrett, the senior Nuclear Regulatory Commission engineer at Three Mile Island during the early phases of the cleanup said by comparison, “it was a walk in the park compared to what they’ve got.”

The Fukushima Daiichi reactors are similar to those in Pennsylvania — “the cores are probably really similar, partially melted,” Mr. Barrett said — but engineers pointed out several key differences in the aftermath of the accidents. In Japan, four separate reactors are damaged, and fixing each one is complicated by the presence of its leaking neighbors. It will also require a major infusion of equipment to replace parts far from the reactor’s core, like pumps and switchgear that were destroyed by the tsunami.

In the short term, weather is a factor: according to engineers who managed the American cleanup, which ran from 1979 to 1993, Tokyo Electric Power has only a few weeks to patch up the three smashed secondary containments before the coming rainy season, when downpours could wash more contamination into the environment. And the company will have to carefully watch that the number of workers with the necessary skills do not burn out under the size of the task, or absorb so much radiation that they have to quit.