Terrific news, albeit with plenty of caveats.

“This is very important information,” says Hiroshima-based researcher Evan Douple, who has been studying the health of atomic bomb survivors for decades.

Douple says the new radiation levels, shown to him by NPR, “should be reassuring. At these levels so far, I don’t think a study would be able to measure that there would be any health effects” — even years from now…

The general picture is that radiation levels in the vicinity of the power plant are close to “background” levels at some locations. A background level is ever-present, low-level radiation given off by rocks, cosmic rays, fossil fuels and other natural sources. It accounts for about half the radiation exposure everyone gets, with medical tests accounting for the other half.

“Readings 10 times above background I don’t think one would bat an eyelash at, really,” says David Brenner, director of Columbia University’s Center for Radiation Research.

As I say, plenty of caveats. One: There are no measurements yet from within 12 miles of the plant, so an awful lot of people may still end up being told they can’t go home again. Two: There’s one particular spot beyond that radius, around 18 miles northwest of the plant, where radiation levels are high enough to raise the risk of long-term health problems — but only very slightly. Those levels could be due to many things, including local environmental variables, but it could also mean that the problem zone is expanding. Probably not, though, since the radiation at that spot actually declined from Thursday to Friday. If the “apocalypse now” scenario of spent fuel rods melting in a dry cooling pool is coming to pass, as U.S. officials have speculated, why aren’t those numbers increasing instead?

Three, and this is the big caveat: The numbers come from … the Japanese government, whose credibility has been disintegrating since the quake struck. Only this morning, days after U.S. nuke officials claimed that the situation is worse than Japan had admitted, did they finally acknowledge that this is a Level 5 nuclear crisis, not a Level 4. The White House has been collecting radiation data on the plant using its own instruments and has grown ominously quiet about what it’s seeing, although officials told NBC yesterday that some of the numbers suggest the possibility of “dire” outcomes. Whom do you trust more to be honest here? The USAF, or Japan?

They finally connected the new power line to reactor two but the cooling systems haven’t been tested yet. If they still work, there might be a way out of the crisis. If they don’t … no one knows. And no, unfortunately, “entombing” the plant in sand and concrete isn’t much of an option. That worked at Chernobyl because the reactor had already gone haywire and they needed to snuff it by any possible means. In this case, because the containment vessels are still largely intact, dropping heavy concrete or sandbags on them could bust them open. Or, in the case of the spent fuel rods, it could insulate them in a small space to the point where they get so hot that they melt straight through the concrete. Essentially, it’s a last-ditch, nothing-left-to-do solution, and we’re not there. Yet.