Daiichi is not synonymous with Chernobyl in terms of the severity of contamination. The Ukrainian reactor blew up and spewed huge amounts of radiation for 10 days in 1986. But workers at the plants have a bond.

Among plant employees and firefighters at Chernobyl, many volunteered to try to tame, and then entomb, the burning reactor — although it is not clear that all were told the truth about the risks. Within three months, 28 of them died from radiation exposure. At least 19 of them were killed by infections that resulted from having large areas of their skin burned off by radiation, according to a recent report by a United Nations scientific committee. And 106 others developed radiation sickness, with nausea, vomiting, diarrhea and dropping blood counts that left them highly vulnerable to infections.

The people who had suffered radiation sickness developed other problems later, according to the report: cataracts, severe scarring from the radiation burns to their skin and an increased number of deaths from leukemia and other blood cancers.

Some of those Chernobyl workers were exposed to levels of radiation far beyond what has been measured to date at Daiichi — especially helicopter pilots who flew through radiation-laden smoke spewing from the reactor to drop fire-extinguishing chemicals on it.

Radiation close to the reactors was reported to reach 400 millisieverts per hour on Tuesday after a blast inside reactor No. 2 and fire at reactor No. 4, but has since dropped back to as low as 0.6 millisieverts at the plant gate. Tokyo Electric and Japanese regulators have not released any statistics on radiation levels inside the containment buildings where engineers are desperately trying to fix electrical systems, pumps and other gear wrecked by Friday’s earthquake and tsunami.

But nuclear experts said that indoor radiation levels were likely to be higher because the containment buildings were probably still preventing most radiation from leaving the plant.

The site is now so contaminated with radiation, experts say, that it has become difficult for employees to work near the reactors for extended periods of time. According to one expert’s account of nuclear emergency procedures, workers would be cycled in and out of the worst-hit parts of the plant.