5 of those reactors are uranium only. The worst case scenario, they will fill the air with radiation, including covering the entire island of Japan and floating on the wind to neighboring territories/countries.
1 of those reactors ( reactor number 3 ) is uranium and plutonium 239
"Plutonium-239 has a multiplication factor (k) larger than one, which means that if the metal is present in sufficient mass and with an appropriate geometry (e.g., a compressed sphere), it can form a critical mass. During fission, a fraction of the binding energy, which holds a nucleus together, is released as a large amount of thermal, electromagnetic and kinetic energy; a kilogram of plutonium-239 can produce an explosion equivalent to 20,000 tons of TNT.
 It is this energy that makes plutonium-239 useful in nuclear weapons and reactors."
Have a hydrogen explosion at that reactor, and while you may not get the full "mushroom cloud" explosive effect you would get from a purposely configured chunk of plutonium 239, you may very well watch some serious sparks fly for miles.
Edit: As of 5 hours ago: http://www.eetimes.com/electronics-news/4214120/Plutonium--fuel-rod-reactions-stoke-nuclear-tensions
"The stricken reactor No. 3 has experienced at least a partial fuel meltdown, and it contains nearly a quarter of a metric ton of plutonium
. They are venting that reactor into the air, to reduce the pressure inside, but that allows particulate matter from the melting rods--including plutonium--to be released into the environment as radioactive gas."
For now, the biggest concern at the No. 3 reactor is the uranium-plutonium fuel, called MOX (for mixed oxides of uranium and plutonium), that was loaded into the reactor last year, Moglen said. Hence, it is likely that spent fuel rods stored outside the reactor’s containment vessel are uranium-only.
"So far, the world has had no major accidents involving release of the plutonium fuel called MOX," said Ira Helfand, a member of the board of Physicians for Social Responsibility. "However, with plutonium inside reactor No. 3, if it melts down or explodes, then even microscopic quantities of particulate will cause lung cancer in anyone who inhales it. The whole area will have to be cordoned off." Edited, Mar 17th 2011 6:54am by Zorvan