For more than a week the world has watched the escalating crisis at Japan’s Fukushima Daiichi nuclear power plant slide from one catastrophic episode to a seemingly graver one, often upending assurances from the Japanese and adding to the fear and confusion about how it all might end.

Are we on a slow-motion path to a six-reactor meltdown? Or will Fukushima stop short of being the worst nuclear power disaster ever, and squeeze somewhere behind Chernobyl and alongside Three Mile Island in infamy?

While there can be no definitive answers amid a still-unfolding disaster, ProPublica spoke with seven top nuclear engineers and scientists to at least establish some boundaries for the disaster’s potential health and environmental impacts.

The rough consensus: The long-term and most severe effects from radiation at the plant, where four of six reactors are in crisis and hundreds of tons of spent fuel is a risk, will be largely contained to the area around the plant, affect a relatively limited population and will likely not spread outside Japan.

Even in the worst case, the crisis should not lead to the level of health and environmental destruction that followed the 1986 Chernobyl disaster, the experts say. Unlike Chernobyl, the potential for an explosion large enough to carry contaminants high into the atmosphere and to far away areas appears remote.

A complete loss of control of the Fukushima plant, followed by total meltdowns at multiple reactors and fires in the spent fuel stocks, would be an extraordinary development leading to very high radioactive emissions and contamination of the surrounding landscape that could last for decades.

Such a scenario is now less probable, in part because the fuel rods in the reactors are expected to continue to cool each day. Even a sustained fire in the spent fuel that sits on the top level of the reactors is unlikely to result in “criticality,” or a new nuclear chain reaction and reheating of that spent fuel.

The New York Times reported that Japanese officials remain concerned that criticality is possible in some of the troubled reactors or spent fuel. But even if it were to happen, the process can eventually be interrupted.

Experts interviewed by ProPublica said that even if a meltdown scenario unfolded unabated, the contamination would likely remain localized and would not affect a large population because evacuations have already been ordered. There remains uncertainty about whether worst-case contamination could reach as far as Tokyo, about 150 miles from the Fukushima plant, but few believe there is any chance of dangerous levels of contamination spreading offshore.

“The events that have happened, and the speculation for what could happen is not on the same scale as the release from Chernobyl,” said Peter Caracappa, a nuclear engineer at Rensselaer Polytechnic Institute, in Troy, N.Y. “Based on all the available information, the risk to any of the places far from the plant … would be too small to calculate with any confidence. We’re not talking intercontinental effects.”

Odds of Total Meltdown Diminish

There are two aspects to the ongoing risk at the Fukushima Daiichi plant in Japan: the fate of the reactors themselves, and the condition of the millions of pounds of spent fuel rods stored in open pools atop the reactor structures.

A total meltdown would occur if the fuel rods inside a reactor continue to overheat and break down, spilling the uranium or uranium-plutonium pellets inside them into a heap on the reactor floor. The core of the reactor containing the fuel rods is encased in a steel vessel that is then surrounded by a huge reinforced-concrete containment structure.

As the fuel consolidates, there is less space for cooling water to circulate among the pellets, which can heat into a molten substance. The hotter that molten slurry gets, the greater the possibility that it can burn through the fortified steel containment vessel meant to isolate whatever happens inside the reactor.

A breach of the reactor vessel would normally be the most critical danger. If a meltdown did happen, experts say the fuel could leak out and spread through cracks in the concrete containment, sear through a second metal liner and then flow out in the open air toward the perimeter of the plant.

“That’s the event that changes this situation from a horrible situation to a nightmare of unprecedented proportion,” said Kenneth Bergeron, a physicist who worked on nuclear reactor accident simulations at Sandia National Laboratories.

Officials have said they believe there has been a partial meltdown at least two of the four troubled reactors. But it has now been seven days since the reactors were shut down following a 9.0 earthquake that rocked the islands of Japan and triggered the devastating tsunami that swamped the power plant.

Tokyo Electric Power Company, which runs the plant, continues to work to control the temperature inside the reactors and has been injecting sea water laced with boron, which short-circuits the nuclear reaction, into the reactors to maintain cooling. Experts believe that by now, the reactors should have cooled substantially. And with each day that passes, they say, the temperature drops further and the possibility of a full meltdown diminishes.

“That doesn’t seem very likely now,” said Louis Lanese, a nuclear engineer who worked on the Three Mile Island crisis in 1979 and now is a partner with Panlyon Technologies, a nuclear energy-services firm in Flanders, N.J. “It’s cooled down. They have water over the core. Every day makes the consequences a little bit better.”

For those cores to melt now, Lanese said, there would have to be a complete loss of water, and fuel rods would have to sit for some time – days or even weeks. Even then, he said, “I don’t know if there is enough energy in that fuel to even get out of the reactor vessel.”

Spent Fuel Is Less Potent

The greater risk may now lie with the spent fuel sitting in storage pools on top of the reactors. Those pools contain very large quantities of old fuel, at least some of which still contains significant amounts of uranium, and they are not in containment like the reactor cores.

The spent fuel rods generate residual heat and must be cooled by water, but water levels have been precariously low in at least one pool – Unit 3 – and may have dried up altogether in the pool at Unit 4. The danger is that the zirconium cladding that contains the fuel pellets, when exposed to the air, can catch fire and burn intensely and leave the fuel pellets exposed.

Twice, reports have emerged of smoke and a possible fire in the pool atop Unit 4, but it has been difficult to confirm exactly what is taking place. Those reports have also stoked concerns that spent fuel could also melt down, and because it is not contained, release large amounts of radioactivity.

But much of the most dangerous material has already been spent, or has begun to degrade. Lanese said that if the cooling water has already evaporated from the pool in Unit 4 without a significant fire erupting, it is a sign that convection cooling from exposure to the air is enough to keep the rods stable.

Explosions remain a risk at the site. When nuclear fuel is hot enough, it can split the water molecules, releasing hydrogen, a flammable gas. Should spent fuel become molten, it could melt through the floor of the pool. When doused again with water, it could create hydrogen and an explosion that released radioactive contaminants. If reactor fuel were to melt down, it could fall into an area that contains water.

There have already been three hydrogen explosions at the Fukushima Daiichi plant – a gas buildup in the reactor buildings of Units 1, 2 and 3 destroyed the exterior walls. But unlike Chernobyl, the worst explosion believed possible at the Japanese plant would not push tens of thousands of feet into the atmosphere and would be a momentary event.

That explosive power is the key difference.

In Chernobyl, the reactor burst in a fiery ball while running at full capacity. The Chernobyl plant was also an entirely different design. It did not have a containment vessel to hold the fuel inside, and the core of the reactor contained graphite. The graphite burned like coal and sustained a roaring fire for two weeks, pushing radioactive particles miles into the atmosphere. That is how some of Chernobyl’s radioactive fallout ended up in Northern Europe.

Radiation Risk Mostly Local

If there is open-air exposure of molten fuel at Fukushima Daiichi, there does not appear to be a mechanism for carrying large quantities of radioactive byproducts over wide areas or great distances. A fire or hydrogen blast could carry contaminants into the lower atmosphere, but only for a relatively short way, scientists say.

The exposed fuel rods or molten slurry emit large amounts of radiation and present a serious health risk to workers inside the plant. But the radiation itself doesn’t extend very far. To affect people outside the Fukushima facility, radioactive material has to be spread around.

Long-term radiation risks result from people swallowing or breathing in tiny particles that continue to be radioactive inside the human body and continue to emit radiation as they break down over time. The radionuclides of most concern include cesium 137 – which has been detected around the Fukushima Daiichi plant – as well as strontium 90 and plutonium 239.

“A fuel melt doesn’t necessarily lead to a big disaster, any more than what we have,” said Gilbert Brown, a professor in the nuclear engineering program at the University of Massachussetts in Lowell. “Even if it’s a fuel melt, you have to have a mechanism to get all that radiation to people, to get hurt by it.”

Bergeron estimates that even after the worst kind of explosion at the Fukushima Daiichi plant, contamination might be detectable 200 miles away, with the most serious contamination within a 100-mile radius.

“That, although striking and horrible, is something described as manageable,” Bergeron said.

An evacuation has cleared out part of the area around the plant. Experts say the largest environmental impact, outside the facility, is potential contamination of the surrounding landscape. Fallout could affect groundwater and surface water supplies, as well as render much of the nearby farmland too dangerous for use.

Some of that environmental contamination can be cleaned up, but agriculture and food supplies could be affected for decades. Human health exposure can be limited by both evacuations and other precautions.

“I don’t think we are going to kill a lot of people,” said Victor Gilinsky, a former commissioner of the U.S. Nuclear Regulatory Commission and a former head of the physical sciences department at the Rand Corporation. “But you could have a tremendous amount of land contamination. Depending on the half life, it could be many time more than 30 years before you could go there.”

Much uncertainty remains about what will happen next at the Fukushima Daiichi plant. Experts caution that if there has been any lesson thus far, it is that assumptions can be easily proved wrong. But with every day that Japanese responders hold wholesale deterioration at bay – however tenuously – the health and environmental impacts should be less severe.

“I’ve worked almost 40 years in this business to keep anything even remotely like this from happening,” said Lanese. “But strange as it is, these situations tell me that these plants have even more resilience than I had expected.

“This is what an 9.0 earthquake and an eight-foot Tsunami does?” he asked. “It’s unprecedented. And those nuclear reactors are still there and still hanging in there.”

Michael Grabell and Nick Kusnetz of ProPublica contributed to this report.