Chernobyl was the result of a large number of factors, both in operator error and reactor design. The IAEA's final view on the matter was considers flawed reactor design the primary cause, with operator error supporting it. A better reactor wouldn't have melted down when the operators tried to scram it, wouldn't have deformed and prevented control rods from being inserted, and certainly wouldn't have blown its roof and sent contaminants into the atmosphere after the meltdown. A better reactor would have melted down like Three Mile Island -- short term panic followed by no measurable long-term health effects on the public, and resumption of operations after repairs. Of course, better operators wouldn't have lost control of the reactor in the first place, and wouldn't have disabled the automatic safety controls that would have inserted the control rods before it was too late, but the disaster we know Chernobyl to be should not have occurred even with their negligence.
Incidentally, isn't the Chernobyl reaction scheduled to hit the center of the earth in about twenty years?
No, the reaction is long dead, and stopped fairly early on. The idea of molten fuels penetrating into the soil below is the reactor building is called China Syndrome, after the tongue-in-cheek concept of the fuels burning a hole through the earth and emerging in China. Nuclear fuel burning through the bottom of the building would be a serious threat if it happened, but it never has; even Chernobyl's nuclear fuel stopped where you can see the terminus of the "lava flows", which were the molten reactor fuel mixed with anything the fuel melted through on its way down, including concrete.
The reason the reaction stopped and the molten fuel and junk cooled is that nuclear chain reactions are actually extremely hard to maintain -- a reactor meltdown is only possible because a reactor is
designed to maintain a reaction. The neutrons emitted during fission must have a high probability of fusing with another nucleus in order to destabilize that nucleus, or else the reaction will peter out quickly. This is not naturally the case for insufficiently enriched uranium reactor fuel under most conditions; the neutrons emitted are far too high-energy, and not numerous enough to sustain the reaction given the probability of any given neutron triggering fission in another atom. In order for a sustained nuclear reaction to take place in a reactor, the fuel materials are often kept in the presence of a neutron moderator that will reduce the speed of the emitted neutrons. The neutron moderator is just some kind of medium with a low atomic weight that is kept at a relatively constant temperature. As neutrons bounce off of the nuclei, they impart a lot of their energy, thus slowing the neutron and making it more likely to maintain the reaction. As the temperature of the moderator increases, the neutrons are slowed less, and the reaction slows slightly, though I believe this is generally not enough to stop the reaction. Removing the neutron moderator entirely will kill the reaction, however -- most reactors use water as a moderator in part for this reason, as if the reaction gets too hot, the water will boil into steam and the reaction will stop. Chernobyl used graphite as its neutron moderator.
A meltdown occurs when the reaction gets so hot that the nuclear fuel itself liquifies into a molten state. But graphite has an
extremely high melting point -- higher than solid diamond. The graphite did not melt, it just burned. So, when the fuel liquified and flowed out of the reaction chamber in molten form, the graphite was burning and going up into the air, or being exploded out of reactor entirely. By the time the molten fuel was out of the reactor, it was no longer in the presence of the neutron moderator, and the reaction became non-critical. It's also likely that the number of materials around once the fuel left the reactor itself caused a lot of the neutrons to be absorbed by materials that didn't immediately undergo fission, which would also slow the reaction. Put simply, the balance of conditions to sustain a nuclear reaction were no longer met, the fuel became non-critical, and the molten flow eventually cooled.
Had the reaction been sustained, and molten core even gone so far as to reach the water table below the facility, the resultant steam explosion and spread of radioactive materials could have made the Chernobyl disaster even more catastrophic and sprayed the surrounding area with vastly more radioactive material than what actually occurred. Instead, the Soviet authorities pumped liquid nitrogen into the earth, froze the water table, dumped thousands of tons of neutron absorbers and fire extinguishing materials on the crater where the reactor once was, and entombed the site in concrete, where some 95% of the radioactive materials are estimated to remain today. The rest were blasted or burned out into the atmosphere, contaminating the countryside.
If I hadn't decided to learn programming and game design, I would probably be working part-time as a reactor operator about now. Games are pretty cool, but sometimes I miss skipping out on that possibility.
Of course, I wouldn't have ever worked on LCS if that happened.
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