Photo taken by Justin Stahlman, Found at: http://commons.wikimedia.org/wiki/File:Chernobyl_Nuclear_Power_Plant_in_2006.jpg
Chernobyl is known as one of the biggest mistakes and explosions at a nuclear power plant…but what really happened? And could it happen again?
For more information on how nuclear power plants work before reading this, check out the other post on how nuclear power works: https://foodforscientificthought.wordpress.com/2014/07/27/how-does-nuclear-power-work/
The Chernobyl reactor was an RBMK reactor, a rare type of reactor that is unique because it is cooled by coolant water and also moderated by graphite – a combination that isn’t found in any other reactors.
On the day of the explosion, the reactor was set to be shut off just for maintenance. Some electrical engineers decided to run a test to see if power was cut to the reactor, the turbine could keep running long enough to continue pumping coolant water until a generator kicked in. The test required the power to be dropped to about 700 megawatts. However, due to miscommunication and demand for electricity, the power was not dropped as fast as scheduled, so by the time that the reactor was running on enough power to run the test, it was already late. The engineers tried to drop the power too low too quickly, causing xenon poisoning of the reaction, which made them unable to raise the power to the required 700 megawatts. To speed up the power increase, they withdrew the control rods (that help control the reaction by absorbing neutrons) to less than the minimum amount. They raised the power to 200 megawatts and carried out the test at that power level. Unfortunately, due to the test, the emergency core-cooling system had been shut off.
So, they began the test by shutting off the water pumps that deliver coolant to the reactor. This caused the water at the core to boil, turning into steam. The water is supposed to be there to absorb neutrons, stabilizing the reaction. However, when it turns into steam, it does not absorb neutrons as well, meaning the reaction could runaway much easier. This caused the power to rise above 500 megawatts very quickly.
The engineers at this time realized something was wrong and immediately lowered all 200 or so control rods into the reaction at once. These control rods, however, were made with graphite at the tip, which, when inserted into the reactor, caused the reaction to increase out of control. At this point, the reactor exploded, most likely due to steam buildup. It’s important to note that this was not a nuclear explosion; it was caused by pressure of gases.
This was a tragedy created by many problems, including design flaws in the reactor itself and miscommunication and misinformation during the test and shutdown of the reactor. Could it happen again? It’s very unlikely, given that reactors are much more carefully designed now all over the world to avoid this particular type of problem.