Nuclear Reactors: Can They Explode?

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It is impossible for any pressurized-water reactor (PWR) or light-water reactor (LWR) to explode like an atomic bomb.  This is because in order for an uncontrolled chain reaction to occur that is similar to an atom bomb, the uranium fuel must be extremely enriched, much more than the 4% 235U (uranium 235) that is present in regular, commercial nuclear reactor fuel.  So, if it can't explode, what does happen if there's a problem in a nuclear reactor?  

The answer is what is called a "meltdown."  When a meltdown occurs in a reactor, the reactor literally "melts."  That is, the temperature rises in the core to the point that the fuel rods actually turn to liquid, the same way ice turns into water when heated.  If the core continues to heat, the reactor will get so hot that the steel walls of the core will also melt.  In a complete reactor meltdown, the extremely hot (about 2700½ degrees Celsius) molten uranium fuel rods melt through the bottom of the reactor and actually sink about 50 feet into the earth beneath the power plant. The molten uranium reacts with groundwater, producing large explosions of radioactive steam and debris that affect nearby towns and population centers.

Generally, nuclear meltdowns occur if the reactor loses its coolant.  This is what occurred at Three Mile Island reactor 2 and Chernobyl reactor 4 that I will discuss in a later blog entry. For now, suffice it to say that without coolant, the core's temperature will rise, resulting in the meltdown scenario I explained above.

You may be wondering, "Why can't they just shut the reactor down (also called "scram," which is a rapid emergency shutdown to place the reactor in a safe condition)?  The answer is that they can.  The problem is that, even if the reactor is completely shutdown and the nuclear chain-reaction stops, the reactor is still extremely hot and will not cool down unless coolant is put back in. The residual heat and the heat produced from the decay of the fission products are enough to drive the core's temperature up, even if the nuclear chain reaction stops.  This is what seems to be happening at the Fukushima Daiishi plant in Japan.  I may discuss scramming a reactor more in a later blog.

More: http://www.hsengine.com/s_meltdown.html