In the media, you hear predictions that the events in the damaged nuclear power plants could be "the next Chernobyl", but this sounds like hyperbole to me. Are the events at those nuclear reactors comparable to Chernobyl? And what are the most likely outcomes of these events?
The information in this answer is no longer accurate, events at Fukushima have progressed further after this answer was written.
The Chernobyl incident was in a different kind of nuclear reactor, and involved a graphite fire and other problems which couldn't occur at this light water reactor in Japan. Worst case scenario seems to be a catastrophic explosion in one of the reactors housing facilities, destroying the reactor casing and exposing the radioactive material to the atmosphere (I suppose the REAL "worst case" would be a very large explosion scatters portions of the core, but I doubt that's possible in this case). That would increase atmospheric radiation in the local area, probably for a few kilometers. Radiation outside of the immediate area would likely not be immediately fatal. This would last until the reactor could be sealed. As evacuations have already begun it seems the personnel at the plant would be the most in danger.
Realistically it seems there will be a detectable, potentially even dangerous release of radiation. I'd guess people will be exposed to less than 100 milirems in a very bad case (3 mile island incident released about 80 mRem). So far I think they've measured about 10-20 mRems. Leathal dose is about 500 mRems. The reactors are very probably damaged beyond repair. I would guess that the surrounding areas will be off limits for months or even years, but that's speculation. The specter of the disaster will probably cause that even if there are no real dangers.
I think news reports take part in a natural selection process which massively favours sensationalist reports, so it's not surprising to hear a comparison to the Chernobyl power plant.
Chernobyl power plant had a positive void coefficient which is thought to have contributed to the problem. It had no containment building which the Fukushima reactor has.
For these reasons I attribute the comparison to sensationalism rather than actual high likelihood of this occurring.
No. Fundamentally they were different types of accidents.
- The Chornobyl accident was an excursion. Power within the reactor increased, and this increase caused more increase,* in an exponential spiral until the energy was so high that the reactor exploded in a BLEVE. The BLEVE smashed what little containment this type had, directly exposed the reactor core to atmosphere, and set it on fire. The flames and smoke carried the contents of the reactor fuel directly into the atmosphere. The energy for the BLEVE was nuclear fission that happened in that moment; though I wouldn't compare it to a "nuclear explosion" as one would find in a bomb. The consequences of radioactive decay were a footnote here.
- Fukushima had been shut down and stopped for 24 hours. The accident happened because of the heat of radioactive decay, which occurs in the hours and days after fission stops. Reactors have several layers of systems to handle that, and each reactor had one that was working; but a problem in unit 1's system caused a string of knock-on effects which broke the cooling systems in the other units. However, raw nuclear fuel was never exposed to atmosphere, and never burned. Steam pressure did leak out of containment, and brought with it mostly gases, along with a small amount of liquid and solid materials like I-131 and Cs-137 were also carried by the gases, but nothing like Chornobyl. Later, the decay heat damaged the floor of the containment structure also, allowing other components of nuclear fuel to leach into groundwater and out to sea.
So Fukushima's releases have been of a different nature, because it was carried by gases leaking out of containment, not by fire from the core burning.
Fukushima's output tended toward gases such as Hydrogen-3 (Tritium) and Xenon-135, which have either short half-lives and disappear quickly (H-3, Xe-135 and Iodine-131), or extremely long half-lives and thus extremely low radiation (Cesium-135, which Xe-135 decays to).
Whereas at Chornobyl, the fire carrying reactor core material into the atmosphere caused releases to be tipped heavily toward isotopes like Cesium-137 and Strontium-90, with 30-year half-lives: short enough to have a dangerously high rate of radiation, but long enough to require long-term evacuation.
One of Fukushima's leaks was plain old Hydrogen-1, featured in airships like the Hindenburg. This hydrogen went "straight up", and accumulated in the lightweight "tin roof" area over the maintenance crane at the top of the buildings (outside the containment), and blew out these lightly built areas of the buildings. These explosions were captured on film and looked spectacular, but they did not blow a hole in containment, which is in the lower parts of the buildings made of concrete many feet thick. There were no cameras on Chornobyl, so no means of comparison.
As to the similar INES rating of "7", this is a case of dynamic range clipping. By fiat, the scale stops at 7. Were we to design the scale with 20/20 hindsight, Fukushima would be a 7 and Chornobyl would be an 8 or 9.
* Reactors in the BWR/PWR/VVER families are designed to be dynamically stable; an increase in power has passive side-effects which reduce power, and vice versa. However, RBMK types are the opposite, and an increase in power may have side effects which increase power further, creating a vicious circle. This is exactly what bit Chornobyl; Fukushima was fully shut down so its inherent stability-at-power was never a factor.
The Fukushima Daiichi incident was a second Chernobyl. Both were rated as 7 on the International Nuclear Event Scale i.e. a Major Accident, the highest on the scale. By comparison, both Three Mile Island in 1979 and the Windscale fire in 1957 were rated 5, an accident with wider consequences.
At Fukushima, (according to this IAEA review) reactors 1, 2 and 3 had meltdowns and reactors 1, 3 and 4 had hydrogen explosions in the reactor buildings, releasing radioactive material into the environment and causing an evacuation eventually extending 30 kilometers in radius plus some special areas, affecting 160,000 residents.
They were both extremely serious, though the details were different, and the death toll was much lower in Fukushima, mainly because of the deaths of firefighters in Chernobyl. The 13-15,000 deaths in the Japanese earthquake and tsunami, mainly by drowning, was far higher than either.