The detection of the highly radioactive elements cesium-137 and iodine-131 outside the Fukushima Daiichi nuclear plant heralds the beginning of an ecological and human tragedy. The open question is whether it will be limited, serious or catastrophic.

The two radioactive isotopes can mean only one thing: Two or more of the reactor cores are badly damaged and at least partially melted down.

In the best case, operators will pump enough seawater and other coolants into the stricken reactor cores to squelch overheating. Such a success would prevent further releases of radiation beyond the unknown amount spewed into the air by controlled venting and the explosion of reactor containment buildings Saturday and Monday.

In such a hoped-for scenario, the only casualties would probably be the handful of plant workers reported Sunday to be suffering from acute radiation sickness. But there's also the immense anxiety triggered by the incident and the toll of the subsequent evacuation on nearby residents.

The consequences of the most dire scenarios are much harder to estimate. They include the loss of the facility, an expensive local cleanup - a foregone conclusion - and a wide-scale disaster that renders the countryside around the plant uninhabitable for decades.

"There is a worst case, and then the question is, 'Is there a worst case beyond the worst case?' " said Gilbert Brown, a nuclear engineer at the University of Massachusetts at Lowell.

If the last-ditch efforts to cool the reactors fail, the heavy cylindrical cores - each containing tons of radioactive fuel - could flare to hotter than 4,000 degrees and melt through the layers of steel and cement engineered to contain them.

Such a meltdown might be underway, said Arnie Gundersen, chief engineer at the consulting firm Fairewinds Associates. Gundersen has 39 years of experience in the nuclear energy business and helps oversee the Vermont Yankee nuclear plant, whose reactors are the same vintage and design as those of the stricken Fukushima Daiichi unit 1.

Gundersen said an intense battle to cool the cores is playing out in the control rooms of the facility. Operators have "got to keep pouring in saltwater, and they're hoping they will get enough cooling going" to prevent a total meltdown.

If a full meltdown occurs, a huge molten lump of radioactive material would burn through all containment, destroy the building and fall to the ground, exposed. A toxic stew of exotic radioactive particles would then spread on the wind and rain.

The dangers posed by such a disaster rest on two factors: the amount of radioactive material released and the weather.

On Sunday, the International Atomic Energy Agency offered a spot of good news. The prevailing winds at Daiichi are blowing to the northeast, out to sea, and should continue to do so for the next three days.