celia hatton, fukushima, japan. >> mitchell: one big worry for officials: water levels have dropped so much inside two of the reactors as that parts of the uranium filled fuel rods are exposed. in the worst-case scenario that could contribute to a meltdown. but what exactly does that mean? even two days after they shut down, fukushima's reactors are still generating heat. >> from the nuclear fuel rods. it's much like if you have an electric stove and immediately turn it off-- if you put your hand on the stove you will not be very happy. uranium pellets are inside the long fuel rods. if the reactor is not cooled properly, the tubes can fall apart, with the radioactive material falling to the bottom. >> it's like a car accident. it can be a fender-bender all the way up to a major collision. so when you talk about fuel melting, you can have just a few pellets melt or you can have a large number of pellets to melt. >> mitchell: the key issue is whether the reactors are being adequately cooled with water. if not, melting could begin. >> when the fuel melts, it will flow like wax into the bo

first, celia hatton in fukushima >> reporter: japan's nuclear nightmare continues, a second hydrogen explosion at the fukushima dai-ichi nuclear complex in just three days injured 11 workers. like saturday's initial explosion, today's blast destroyed the reactor's exterior though officials say its core is intact. this was followed by news another reactor in the complex is also overheating so the prospect of a third explosion is looming. and all three dai-ichi reactors have nuclear fuel rods that have been exposed to some degree, so they're teetering on the edge of a meltdown. japan's nuclear authorities are scrambling to contain the mounting problems posed by fukushima's damaged reactors. still, they insist they're in control. "this will not be the same situation at as chernobyl" explains the chief cabinet secretary yukio edano. authorities say radiation levels at the dai-ichi complex are under the legal limit but the u.s.s. "ronald reagan's" crew tells a sobering story. 17 were exposed to a month's worth of radiation during a one- hour search-and-rescue mission and had to undergo de

celia hatton, cbs news, fukushima, japan. >>> u.s. naval vessels supporting japanese aid efforts have moved away from the coast near those damaged nuclear plants. the "uss ronald reagan" is one of two u.s. aircraft carrier groups off japan's coast. the "reagan" passed through a radioactive cloud from the stricken reactors. crew members on deck reportedly received a month's worth of natural radiation in about an hour. officials say none are experiencing ill effects from the exposure. >>> engineers are trying to prevent the nuclear crisis from spreading. there are already indications, though, that fuel rods inside the fukushima plant may have partially melted. russ mitchell now on what this meltdown really is, and what they're doing to prevent it. >> fukushima's reactors are still generating heat. >> from the nuclear fuel rod it's much like if you had an electric stove and you immediately turn it off, if you put your hand on the stove, you will not be very happy. >> reporter: uranium pellets are insided long fuel rods. if the reactor is

cbs news correspondent celia hatton is in fukushima, the site of that nuclear power plant. what is the latest? >> reporter: bob, it's been another day of grim news coming from the fukushima nuclear plant. just a few hours ago, authorities warned that the situation to prevent total meltdown at three faulty nuclear reactors is an ongoing challenge. they warn if something happens to one reactor, it could affect the others nearby. the generator number 3 is now the most risk for an outer container explosion like the one that took place a day ago on generator number one. that's when officials did the only thing they could to prevent the entire reactor from blasting open. they released a bit of radioactive steam to reduce pressure in that generator number one, which led to a larger than expected explosion. that's what we can look for could be coming from generator number 3 in the next day or so. it seems likely that officials are going to have to do a similar operation on generator number 3. so, unfortunately, we're expecting more explosions from that nuclear plant. >> schieffer:

we want to go now to cbs news correspondent celia hatton, who is live from the town of fukushima neartook place at one of the reactors just a few hours ago. workers released what they say was a small bit of radioactive steam to release pressure on the containers housing the overheating reactor. japanese government spokesmen say the outer shell of the reactor is damaged, but the inner container is intact. so they're insisting that there's no real danger. though the people that i'm speaking to on the ground are very, very anxious about what's going to happen next. >> as you speak to people there, what are you hearing? wh be able to go home shortly. >> i know that they evacuated initially a six-mile area from the nuclear reactor. they have doubled that to 12 miles at this point. i mean, what does that say to you? >> that says to me that the government says they're just trying to play it safe. but that says to me that they are a little bit cautious. they're worried and they want to get people out and evacuated out while they still can, before there really is a serious emergency. i'm at th