My guess is that the reactor core(s) and/or spent fuel pool(s) are constantly emitting (super)heated water vapor, and that is more visible at night because the ambient air temperature is cooler (the vapor condenses). We understand that TEPCO is almost continually dumping mass quantities of water onto the messes, so that toxic vapor is constantly being carried off by the prevailing winds. Eh?

I am glad I am old.

TEPCO have admitted to a significant loss of water in the spent fuel pool located in the R4 building, which as you probably know was in shutdown for maintenance at the time of the disaster. The fuel from R4, which is relatively 'fresh' or 'hot' was removed from the RPV and placed in the SFP alongside the used fuel already stored there for long term cooling.

Like many other aspects of the crisis, this situation seems to have been largely 'off the radar' in terms of risk and crisis management. With both the fresh 'hot' and depleted 'dirty' fuel assemblies exposed to air in an uncontained environment, in a pool which appears to be leaking, they have some serious problems facing them.

Add the fact that the building itself looks in danger of collapsing, with radiation levels so high the workers cannot approach close enough even to assess the damage, and you get a sobering focal point for just one aspect of the problems at this wrecked nuclear plant.

Those interested in the latest news, and discussion of same, might care to visit ENENEWS at http://enenews.com/

"...they have some serious problems facing them."
Given the breadth of impacts from oceanic to atmospheric contamination we should substitute "we" for "they" & "us"for "them". Unfortunately radioactivity knows no boundaries.

Re: "Unfortunately radioactivity knows no boundaries."

genop,
actually, time is a boundary in that half-life decay at least reduces emissions from many isotopes; otherwise, I couldn't agree more with you.

While attention has been focused on Fukushima, NE's own Ft Calhoun nuke has shown that it, too, should be subject to greater scrutiny, even a video feed. But why stop in NE when NRC records show that tritium has leaked from at least 48 of 65 sites, including one in NJ, where it has made its way to an aquifer:

"Radioactive tritium has leaked from three-quarters of U.S. commercial nuclear power sites, often into groundwater from corroded, buried piping, an Associated Press investigation shows."

And,

"At three sites — two in Illinois and one in Minnesota — leaks have contaminated drinking wells of nearby homes, the records show, but not at levels violating the drinking water standard. At a fourth site, in New Jersey, tritium has leaked into an aquifer and a discharge canal feeding picturesque Barnegat Bay off the Atlantic Ocean."
http://news.yahoo.com/s/ap/us_aging_nukes_part2

Tritium, btw, has a half-life of approximately 12.32yrs:
http://en.wikipedia.org/wiki/Tritium

"Tritium is potentially dangerous if inhaled or ingested. It can combine with oxygen to form tritiated water molecules, and those can be absorbed through pores in the skin."

Your post's title, "Fuku in Moonlight", gave me an idea. The only way to describe the pain and magnitude of this thing is with poetry, specifically the ancient Japanese form called haiku:

Fukushima vents

the People breathe its poison

will it ever end

Bill's invented faiku!

The Nebraska reactor had me concerned when the FAA imposed a no-fly zone in the area. According to the IEAE, it's at level 4.

As somebody has mentioned, an interesting AP report notes that the NRC is merrily reducing standards and extending licenses on aging nukes. Doesn't seem especially responsible.

Fortunately, Obama's certain to be all over it.

You might go to bed one of these moonlit nights, clouded by vapors and steam steathly passing by and wake up to one of these in your neighborhood:
http://www.nuscalepower.com/ot-Scalable-Nuclear-Power-Technology.php

Brought to you by these smiling faces:
http://www.nuscalepower.com/an-Management-Leadership.php

We had a small stay from imminent irresponsibility when their felon financial backer was arrested:
http://www.oregonlive.com/business/index.ssf/2011/03/nuscale_power_still_seeking_re.html

I don't think TVA will be so lucky.

The no-fly zone over the Nebraska nukes (which are still surrounded on all sides by floodwaters) was apparently more out of a concern that swarming news helicopters would collide than out of any concern for radioactive releases.