3 Fukushima reactor cores melted into earth during accident, still missing – 4th could explode

http://americablog.com/2013/10/fukushima-update-three-reactor-cores-melted-earth-accident-tepco-still-charge.html
As the world approaches the day when the TEPCO-controlled cleanup of more than 1500 fuel rods at the ruined Fukushima Reactor 4 [typo corrected] is started, we learn even more about the disaster that lays in wait for us. (Read our write-up here; it will get you fully oriented.)
Now there’s an excellent new summary from Kevin Zeese and Margaret Flowers at Truthout which contains these crucial points, one of which is news to me (my emphasis and reparagraphing):

The Problems of Fukushima

Fukushima Reactor 4 after earthquake and tsunami severely damaged it

There are three major problems at Fukushima:
(1) Three reactor cores are missing;
(2) Radiated water has been leaking from the plant in mass quantities for 2.5 years; and
(3) Eleven thousand spent nuclear fuel rods, perhaps the most dangerous things ever created by humans, are stored at the plant and need to be removed, 1,533 of those are in a very precarious and dangerous position.
Each of these three could result in dramatic radiation events, unlike any radiation exposure humans have ever experienced.  We’ll discuss them in order, saving the most dangerous for last.

The 1533 fuel rods in item (3) are stored in the severely damaged (and perhaps crumbling) Reactor 4 building, sitting high off the ground in racks that have been distorted. The 11,000 (or so) fuel rods mentioned in the same paragraph are the total number of fuel rods at the site.
Your bottom line — None, some, or all could go up. What does “go up” mean? They could burn (spewing nuclear debris). They could explode (spewing lots of nuclear debris). They could go critical (atom-bomb style).
The rest of this summary article is excellent; a great look at where-we-are-now. Please do click through. Each of the points above is expanded and fully explained.