And to the fearmongers sowing panic like so many meadow muffins from the backside of a manure spreader, shame on you.
Chernobyl was a graphite-moderated reactor, aka RBMK Reactor, with no containment vessel. The former design feature is very much obsolete, and the missing latter feature in conjunction with all that flammable graphite are what made Chernobyl, well, Chernobyl.
Japan's Fukushima Daiichi reactors are light-water reactors, aka LWR or Boiling Water Reactors, with proper containment vessels. There's none of that graphite stuff, and the reactor cores are well-contained inside exceptionally thick steel and concrete pressure vessels designed to keep all the bad stuff inside where it belongs.
It's an apples-to-oranges comparison in every way. Japan's broken reactors will never become a Chernobyl-style catastrophe, because it's physically impossible.
I'll skip the human-induced fumbling that started the Chernobyl catastrophe, and simply state that a hydrogen bubble ignited and burst open the reactor building when the fuel rods lost their cooling water. That's what happens in nearly all reactors using water as the working fluid when they run dry. The increasing residual heat melts and oxidizes the zirconium alloy cladding of the fuel rods, the remaining water flashes to steam, hydrogen splits from the water in the steam, and you have this big honkin' hydrogen bubble under pressure looking to recombine with oxygen at its earliest convenience. Why did it burst open the reactor building? Because there was no reactor pressure vessel, no real primary containment of the reactor's core, and no real way to prevent the hydrogen from getting intimate with the oxygen it wanted back.
In doing so, the bursting of the reactor building exposed the reactor core itself to the environment. Remember those graphite moderator rods I mentioned earlier? They're graphite. That's a very pure form of carbon, halfway between charcoal and diamond. It burns like all get out, and generates some serious heat at very high temperatures once ignited.
So now you have a reactor core exposed to the environment due to the lack of a reactor pressure vessel, no cooling water, residuals of a hydrogen/oxygen "recombination event" that lifted the lid off of that reactor building, and oh yeah, tons and tons of graphite fire starter sticks. This is a recipe, lest anybody miss what's coming up next...
And boy, howdy, did it ever come up in grand fashion! As many Soviet military conscripts and career troops gave their lives to throw sand, boron, and other agents into the gaping maw of a ruptured Chernobyl, the fire in that big old Weber Grill ran hot enough to climb way up there into the atmosphere. So high, in fact, that it intercepted the Jet Stream. That ain't good, because the witches brew of daughter isotopes created in the reactor both before and after the accident found a convenient dispersal path thanks to the lack of a pressure vessel plus the super-hot, graphite-fueled fire. You've seen the pictures of livestock, produce, and dairy products that were destroyed as the plume dropped isotopes over the region. The Jet Stream carried the physically lighter isotopes around the world, and we (crazy bastards flying USAF nuclear reconnaissance jets) found activity off the West Coast of the U.S. in short order. It wasn't a dangerous level of REMs, but it indicated the nature of the reactor accident when the owners weren't exactly forthcoming with the info. The fuel rods did undergo a total meltdown, and they later found the solidified and formerly molten blob in the basement of the facility right where they figured it would go. It didn't burn down to the earth's core, and it didn't undergo an uncontrolled fission reaction. It melted, burned through the floor, and solidified into a huge "elephant's foot" of an intensely radioactive uranium metal sculpture which will remain there for many generations if not longer.
Contrast that to what's going on in Japan this week. These reactors are contained, are being fed boron and seawater, and are "burped" on occasion to keep pressures down. Those "burps" release pressure and allow more cooling water to be pumped into the reactor pressure vessels. It's not easy pumping water into a pressurized boiler, as anybody who knows about feedwater pumps on steam locomotives can tell you. Unfortunately, those "burps" also contain hydrogen gas and radioisotopes, which are responsible for the external explosions we saw that lifted the roofs off of at least two of the reactor buildings. They didn't blow the reactor pressure vessels, but they did make some serious skylights overhead. The reactor cores are still contained, which is in stark contrast to Chernobyl, where Pandora's Box was out there in the open for all to experience.
Radiation, while a fine trigger for genetic diversity in Homo Sapiens and other species of flora and fauna, really isn't too healthy to living organisms in larger doses. I know, I absorbed more than a few beta and gamma hits when inserting the pointy end of a jet into a suspected nuclear debris cloud to get a good sample. As crazy as the mission might seem, we still stayed safe by following some basic rules.
Time, Distance, and Shielding are what saves your bacon. Time means minimal time spent in contact with radiation. Distance means that the specific energies of those alpha, beta, and gamma emitters drop off considerably with the more distance you put between yourself and them, roughly an inverse square formula. Shielding means that you put something that blocks those ionizing energies via the appropriate material for the radiation you want to stop. Alpha's easy, a sheet of paper will do. Beta needs something a bit more substantial, and Gamma's no fun at all. I'm seeing evidence of all three methods of protection being used by our Japanese friends while they're fighting the reactor problems and keeping the Japanese public safe. Somebody's got their head screwed on straight, especially after the double whammy of a 9.0 earthquake and the tsunami it spawned hit them in rapid succession.
As far as I'm concerned, the Japanese reactor crews and Japanese Self Defense Forces are doing the appropriate textbook maneuvers to keep a bad situation from getting worse. Sure, the non-pure seawater will permanently ruin those reactor cores, but it'll cool down the fuel rod assemblies over time until they're safe to extract and reprocess (save for the Tokai-Mura incident, Japan's pretty good at reprocessing spent fuel rods). The venting of excess core steam pressure prevents damage to the reactor pressure vessels, and allows the cooling water to be pumped in with considerably less resistance. The vented steam will contain radioisotopes and hydrogen, especially now that seawater is the expedient coolant of choice, but it's way less activity than were those pressure vessels to rupture and cause the fuel rods catch fire while exposed to the environment.
And that released radioactivity is much, much less than what Chernobyl scattered hither and yon, with fewer isotopes and lower specific activities in the shorter, lower-altitude dispersed plumes. In the meantime, all the scared folks buying Potassium Iodide tablets in Washington, Oregon and California are merely providing the industry with a temporary monetary boost. Those poor sods will never have occasion to use them here in the U.S., and if they do use them without actually being in contact with radionuclides, they'll booger up their thyroid glands something fierce. How's that for cruel irony? The fearmongers cause injuries by scaring people into sucking down potassium iodide tablets like candy, when the plume coming from Japan never carries enough debris to injure folks here, let alone make it across the Pacific. Wunnerful!
Instead of spending money on potassium iodide tablets, how's about writing a check to the relief fund of your choice to help the folks on the other side of the Pacific who have no electricity, no food, no water, no shelter, and need to get their lives back on an even keel after witnessing so much death and destruction? You'll feel better, honest. In the meantime, Uncle Sam is monitoring the situation probably better than anyone else in the world...