I can do complete sentences:

Criticallity is what is now continuing in Japan.

A criticality accident, sometimes referred to as an excursion or a power excursion, is an accidental increasing nuclear chain reaction in a fissile material, such as enriched uranium or plutonium. This releases a surge of neutron radiation which is highly dangerous to humans and causes induced radioactivity in the surroundings.

A critical or supercritical nuclear fission (one that is sustained in power or increasing in power) normally is supposed to occur only inside reactor cores and (very occasionally) inside some test facilities. A criticality accident occurs when a critical reaction is achieved unintentionally. Although dangerous, the low densities of fissile material and the long insertion time involved in these events limit the fission yield and peak power, preventing them from becoming a large scale nuclear explosion. The heat released by the nuclear reaction will typically cause the fissile material to expand, so that the nuclear reaction becomes subcritical again within a few seconds.

In the history of atomic power development, fewer than a dozen criticality accidents have occurred in collections of fissile materials outside nuclear reactors, but most of these have resulted in death, by radiation exposure, of the nearest person(s) to the event. However, none have resulted in explosions.


The idea of a nuclear explosion has been deemed impossible at a crippled nuclear plant. I my humble opinion this is simply not true.
The factors that the nuclear fule also contains plutonium is what makes the unthinkable runaway chain reaction a possiblility, no matter how small.



They are working furiously to find a solution to cool the core," said Mark Hibbs, a senior associate at the Nuclear Policy Program for the Carnegie Endowment for International Peace.

Nuclear agency officials said Japan was injecting seawater into the core -- an indication, Hibbs said, of "how serious the problem is and how the Japanese had to resort to unusual and improvised solutions to cool the reactor core."

Officials declined to say what the temperature was inside the troubled reactor, Unit 1. At 2,200 degrees Fahrenheit (1,200 degrees Celsius), the zirconium casings of the fuel rods can react with the cooling water and create hydrogen. At 4,000 F (2,200 C), the uranium fuel pellets inside the rods start to melt, the beginning of a meltdown

Japan is using a plutonium and uranium mix inside their reactors which make claims that a nuclear style explosion is impossible an unsubstatiated claim, in my opinion.