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.

“If the wind carries the emissions to sea, that will certainly minimize the human and environmental impacts in Japan,” said Timothy Mousseau of the University of South Carolina, who has spent the past decade studying the ecological consequences of the 1986 Chernobyl disaster.

Such emissions would not endanger the United States, the Nuclear Regulatory Commission (NRC) announced Sunday in a statement. Given the thousands of miles between the countries, the United States is “not expected to experience any harmful levels of radioactivity.” In other words, the danger could simply dissipate over the Pacific.

It’s impossible to know how a plume of radioactivity traveling over the ocean might affect sea life, said Edwin Lyman, a senior scientist at the Union of Concerned Scientists, which strongly opposes nuclear power. He said there has been virtually no research done into the subject.

But if luck turns south and the winds do, too, radioactive particles could be spread far across Honshu, Japan’s largest island, and beyond.

Lyman said that simulations he has run on possible nuclear disasters in the United States estimate “tens of thousands of cancer deaths” from a total meltdown, although arriving at a figure is fraught with layers of uncertainty.

A 2005 census counted 103 million people on Honshu, including the population of Tokyo, which lies 150 miles to the southwest of Fukushima Daiichi.

Lyman’s simulations, which rely on NRC computer code, show that unfavorable winds could spread radioactivity far beyond the 12.5-mile evacuation zone, much as happened at Chernobyl in 1986.

In that disaster, a reactor exploded and a fire raged for 10 days, sending radioactive particles hundreds of miles afield. That catastrophe is the only one ever to rate a 7 on a 7-point international scale of nuclear disasters. The Japanese nuclear drama has been initially rated a 4, but researchers look to the Chernobyl explosion for clues to what the impact might be.

As a debate continues over the ultimate human and ecological toll of Chernobyl, there is some scientific consensus that at least 6,000 to 7,000 excess cases of thyroid cancer have occurred in the 25 years since. Because cancer can take decades to develop, 25 years “is not enough time to see long-term impacts on human populations,” Mousseau said.

The increased thyroid cancer was the result of the kind of broad food-chain contamination that can arise from a nuclear incident. Cows ate grass exposed to iodine-131 and then produced radioactively hazardous milk that was unknowingly fed to children, who are most at risk of thyroid cancer.

About 180,000 people were permanently displaced around Chernobyl, and this “exclusion zone” within 30 kilometers of the reactor will need to be maintained for “decades to come,” concluded a 2005 report by the Chernobyl Forum, a high-level international body organized to dispense the final word on the catastrophe.

Ecologists also debate the toll on wildlife in the exclusion zone, where radiation lingers. Mousseau and colleagues found “many fewer species than you expect, the species that are there occur in much lower numbers, and there are much higher rates of genetic mutations” than in unaffected areas.

Mousseau’s latest research paper documents damage to 50 species of birds, including smaller-than-normal brains in some.

Of the dozens of radioactive elements, or isotopes, spewed from a total meltdown, four present special dangers:

lIodine-131 accumulates in the thyroid but will radioactively decay relatively quickly, so little is left in the environment after a few months.

lCesium-137, in contrast, lingers for decades and dissolves in water, meaning it can mix with rain, enter the soil and groundwater and be taken up by plants and animals.

lStrontium-90 poses a deeper health risk. It behaves like calcium in the body, so it can accumulate in bones and teeth.

lPlutonium-241 presents a more insidious threat. It is not very toxic, but as it slowly decays, it produces the much more dangerous isotope americium-241.

Mousseau said concentrations of americium are still increasing in Ukraine and Belarus, where unspent plutonium fuel from Chernobyl dispersed. “It looks like [americium-241] will peak in about 2050 in these areas,” he said.

And so, 25 years after the worst nuclear disaster in history, the ultimate cost remains unknown.