To understand nuclear disasters one must first understand how a nuclear power plant generates electricity. In a conventional power generating reactor, nuclear materials are used to generate electrical energy. At the most basic level the nuclear material is arranged in a way that generates heat. The heat can be used to generate electrical energy through turbines. Simple enough right?

Fission is the main power generating process in a nuclear reactor. Fission happens when a heavy radioactive element decays into a more stable element. When an atom undergoes fission it releases heat, fission products, and a few neutrons. Fission can happen spontaneously as part of the standard radioactive decay of the radioactive element. Fission can also be accomplished in a consistent way by arranging the correct type of fuel in the correct orientation. Uranium-235 and plutonium-239 are the most common isotopes used for fuel in power generating nuclear reactors. Modern pressurized water reactors have a space for water between each of the fuel rods. Water moderates neutrons produced by fission, slowing them so they can cause fission reactions in other fuel rods. The combination of the arrangement of the fuel and water slowing neutrons allows the fission reaction to become self-sustaining or critical.

Operating a nuclear reactor is more complex than operating a conventional coal fired power plant. To increase power generation in a coal fired power plant, more coal is added to the boiler which burns and generates more heat. The operator of the coal plant can easily see the increase in power as the additional coal is added to the fire. In a nuclear power plant the relationship between actual operating output of the reactor and what the sensors monitoring the reactor are indicating is much less clear. To increase power generation in a nuclear reactor neutron absorbing control rods are withdrawn from the reactor core. Withdrawing control rods allows more neutrons within the reactor core which in turn increases the amount of fission reactions. Iodine-135, a fission product of uranium-235 with a half-life of 6.7 hours, decays into xenon-135 with a half-life of 9.2 hours. This isotope of xenon is extremely good at absorbing neutrons, decreasing the number of neutrons available for fission. The effect of xenon slowing the reaction by absorbing neutrons is called xenon poisoning. To overcome xenon poisoning and reach a new equilibrium, the reactor operator will have to wait until the xenon decays. Imagine how difficult driving would be if one had to wait for 9 hours after stepping on the gas pedal. Nuclear reactor operators need to constantly keep xenon poisoning in mind when modifying reactor output.

Nuclear reactors are built to be extremely safe. Numerous safety systems protect every aspect of the nuclear material in the reactor. Redundant shut down systems are capable of shutting down the reactor even if the main systems fail. Multiple cooling systems are capable of removing heat from the reactor. Passive cooling systems are even capable of cooling the reactor in the event of power loss. The fuel rods are sealed and placed in a pressure vessel, which is placed into another metal tube, which then is placed into a reactor vessel, all of which are inside a containment building; all additional layers of safety taken to prevent radiation from escaping. Special buildings are constructed to capture radioactive vapors and remove them from the air to prevent them from escaping containment. Generators are designed to run cooling systems for long enough that nearly any problem could be resolved. These safety systems ensure nuclear material stays where it belongs, inside the reactor.

Why do we want to prevent radiation from escaping? What problems do radiation cause? Has anybody just played with nuclear materials before?