Race on to stave off nuclear crisis

>>> good evening, it started with a freak of nature, the fifth largest earthquake ever recorded on the planet, but then, right then as the rubble settled and the buildings stopped swaying, the water came ashore. the tsunami in japan killed thousands. in some parts of some towns, there's no remaining evidence that anyone ever lived there. and now tonight the crisis has taken yet another turn, and we are covering a full-blown nuclear scare in japan . there are 17 nuclear power plants across that country, 54 nuclear reactors in all, but one plant in particular is in trouble. it's the fukushima plant, and if you've seen the pictures of it over the weekend, you may know. there was one explosion in one building on saturday, another just yesterday. and now a third reactor is in trouble at that same facility. we're starting to hear terms like partial meltdown. it's an urgent situation. the nuclear danger, the desperate human toll that goes on, and our team is in place to cover it. we'll begin with chief science correspondent, robert bazell , on this immediate nuclear danger there. bob, good evening.

>> reporter: hello to you, brian. on this -- it was the second explosion in three days and the discovery that a third reactor is in critical condition . japan 's nuclear industry has not had a good day in its struggle to avert what many say could be a major disaster. until today, two reactors were in serious trouble. on saturday, there was a hydrogen explosion in one. at 11:00 a.m . today, cameras recorded a similar blast in the second. shortly afterward, the chief cabinet secretary informed the weary and anxious public. a few hours later, officials revealed the cooling had failed at a third reactor, putting it into the same dangerous condition, including a high risk for a third explosion. the government and the company that owns the reactors said that neither of the two explosions had harmed the heavy steel vessels or the metal and concrete domes that contained the nuclear material . the explosions did destroy lightweight structures that enclose the entire reactors. still, evacuations continue, because the danger persists that fuel rods, which have partially melted, could leak out of any of the three vessels, causing a catastrophic release of radiation. both the government and the company say the chance is slight. even a serious critic of japan 's nuclear industry says he believes them.

>> i think they have to be as honest as they can be, but at the same time they do have to be careful not to panic people.

>> reporter: when the earthquake struck, the reactors all automatically shut off the nuclear chain reaction that drives them. but nuclear fuel remains enormously hot. normally electrically powered pumps keep water flowing to cool it. those worked until the tsunami struck, wiping out all emergency generators and batteries. now workers using fire trucks are pumping sea water into all three reactors to try to cool them. the water releases the hydrogen when it hits the metal inside, the cause of the explosions. that water also creates steam, which has to be vented, carrying small amounts of radiation. the uss ronald reagan was diverted after small amounts of radiation were detected on board and on personnel flying relief helicopters, but the pentagon said it posed little risk to the crew. the big danger, of course, is that release of the radiation from the fuel into the environment. the danger of that recedes every day because the fuel does get cooler, but until that fuel is declared safely cool, wribrian, no one concerned with this issue is going to relax.

>> robert bazell starting off our coverage on this from tokyo tonight, bob, thanks.