What's up folks? Today; lets get back to the past tragic incident of Chernobyl.
On April 26, 1986 the Soviet Union’s Chernobyl Power Complex nuclear reactor 4 exploded. The explosion was caused by major fundamental flaws in the reactor design on top of a total disregard for protocol by some of the higher ranking people in charge, despite some serious reservations from more junior members of the staff. But before we get into all of that, let’s start at the beginning, with nuclear energy.
Nuclear energy is created by splitting apart certain types of large isotopes, called fissile isotopes. Isotopes are variations of an element that have different numbers of neutrons, and some are more stable than others. A fissile isotope breaks apart when a neutron hits it, and even once in a while on its own. When it breaks apart, that’s called nuclear fission, and it’s the isotope’s way of trying to become more stable. When it does, kinetic energy that’s converted to heat is released. And that energy is ultimately what powers an electrical grid. So, why would we use nuclear energy and not just fossil fuels? Well, the energy released from one fission reaction is around a million times greater than the energy released when one fossil fuel molecule is burned.
Now back to Chernobyl. There were many reasons Chernobyl exploded. To understand them let’s first take a look at how the reactor worked and how it was controlled. The Chernobyl RBMK-1000 reactor is a model specific to the former Soviet Union--it has never been used anywhere else. Nuclear power is considered safe--as long as protocols are followed, proper materials are used, and the reactor is designed to keep itself at a stable power level. All reactors in operation today have this design feature, but the reactors at Chernobyl didn’t. Like all modern reactors, Chernobyl used uranium-dioxide fuel enriched with uranium-235, an isotope of uranium that’s really prone to undergo fission. When uranium-235 is hit by a neutron it splits into two other atoms, plus 2-3 other neutrons. One of the atoms sometimes produced by uranium-235’s fission is Xenon-135, which played an important role in the accident. So we’ll get back to that shortly. Those 2-3 neutrons released are important because some go on to hit other atoms of uranium-235, continuing the chain reaction and continuing to create energy in the reactor. The RBMK-1000 reactor used large graphite blocks to moderate the speed of the neutrons, slowing them down by reducing their energy. Although it may sound counterintuitive, fast-moving neutrons are less likely to hit and break apart uranium-235, they simply whizz by too fast. By slowing them down it increased the chance that each neutron would hit uranium-235 and cause it to undergo fission, keeping the reaction going and releasing energy needed to heat water in the reactor. And that water is essential. The whole point of uranium-235 fission is to create enough energy to boil water in the reactor into steam. That steam is what spins the turbines that drive the electricity generators to power the electrical grid.
Ok, so now we have a good idea of how we can keep a reactor generating energy, but what about if we want to slow it down or even stop it? The general way to slow down a reactor is to have fewer neutrons hitting and splitting apart uranium-235. And the way to do that is by having neutron absorbers. So, in addition to being used to create steam, water was also used in the reactor to act as a neutron absorber. Every element absorbs neutrons to some degree, even the hydrogen and oxygen in water. While each individual hydrogen and oxygen atom aren’t that likely to absorb neutrons, there is so much water in the reactor that combined, the water acts as a good absorber. If that water were to disappear, it would throw the neutron balance out of whack. Another absorber is xenon-135, the uranium-235 fission product we mentioned earlier. Xenon-135 is a very good absorber - one of the best of all the isotopes in the universe. Just a tiny bit can completely stop a nuclear chain reaction. And finally, boron carbide control rods. Boron carbide absorbs neutrons extremely well, slowing down the nuclear chain reaction. The RBMK-1000 reactor had over 200 of them, and they were moved in and out to decrease and increase the rate of fission. So, all of these things work together to keep fission going while still controlling it. Let’s, very briefly, turn to physics to explain why Chernobyl’s reactor was so different from the ones in operation today. Today’s nuclear reactors are designed to have a negative void coefficient, which might sound very complicated, but here’s the basic premise: it’s a negative feedback loop. If the water in a reactor starts to run low the reactor power drops so that it doesn’t get out of control. That was NOT the case with Chernobyl. To cut costs, the RBMK reactors were the only commercial reactors in the world designed with a positive void coefficient, which isa positive feedback loop. So, unlike other reactors, as water boiled into steam or leaked out and that void was created, instead of power decreasing, it increased, which caused more water to boil, more steam to form, and the cycle to continue. Ok, so, I’ll admit that there are a lot of things going on here, but we are about to bring them all together.
What exactly happened the night of April 26,1986? Well the irony of that night was that they were actually doing a safety test on Chernobyl’s reactor 4. They wanted to see if, in the event that the reactor lost power, it could keep its own safety systems running until the backup generator kicked in. That safety test was, as you’d imagine, never completed. The safety test required that the nuclear reactor’s power be turned down, and it was, by lowering the control rods--but it was turned down below what normal operating protocol allowed. Turning it down so low was an issue because Xenon-135 was still being created and absorbing neutrons. Some Xenon-135 is unavoidable -- but you don’t want too much of it, because it can keep the power too low. And that’s what happened on the night of April 26, 1986. That night, the power was way too low, and they knew it. But because of all the xenon built up they were in a bind--the only way to raise the power was to start removing control rods, which are there to absorb neutrons and keep uranium-235 fission from happening too often. Under orders from shift supervisor and deputy chief engineer Anatoly Dyatlov, the operators made the disastrous mistake of removing all but 8 of the over 200 control rods, in blatant violation of safety procedures. What they should have done was raise the power slowly and safely over the course of a couple days, to get it to a place where the things moderating and controlling the reaction were again balanced and there wasn’t a danger of too much pressure building. With the control rods out the power went up, but only slightly. And xenon continued to build, keeping reactor power low. The next step in the safety test was to shutdown the pumps that would normally be sending water through the reactor. Without that water, uranium fission increased, and without the control rods in, there was no stopping the next series of events. Power began to increase, and with that, water at the bottom of the reactor core boiled and turned to steam. With the water pumps shut down for the safety test no water rushed in to replace it. That positive void coefficient that we mentioned earlier was making things worse. And steam pressure was building in the reactor; too fast. As power shot up, at least one of the workers at Chernobyl responded as they were trained to--they hit the emergency shutdown button, which inserts all of the control rods at once to stop the reaction. But they were past the point of no return--too much pressure from the steam had built up. On top of that, when those control rods started to go in, they ran into another major design flaw. The tips of the control rods were made of graphite, which keeps uranium fission going. So before the control rods could help, that graphite made the power level sky-rocket. And, just moments later, the crazy amount of pressure that built up caused the reactor to explode. As it did, atmospheric oxygen rushed in and reacted with the hot graphite blocks. Graphite was now the fuel in a combustion reaction, causing a second, fiery, explosion. The damage of the immediate explosion was minor compared to what was to come. The world became aware of the events at Chernobyl when radiation was first noticed outside a reactor in Sweden--much of eastern Europe was being exposed to the radioactive cloud. The night of the explosion only two plant workers died, but over 200 people, many of whom were fire-fighters who eventually putout the fire, came down with acute radiation syndrome. Within 2 weeks, 28 of them were dead. Chernobyl serves as a reminder of what can happen when a safety culture becomes unhinged. In the words of the late Dr. Valery Legasov, a Soviet inorganic chemist and chief of the commission investigating the Chernobyl disaster, “I advocate the respect for human engineering and sound man-machine interaction. This is a lesson that Chernobyl taught us”. The ruins of the Chernobyl reactor now sit under a metal shell--but some of the radioactive isotopes under that shell and in the surrounding areas have a half-life in the tens of thousands of years, which means they’ll exist long after we, and dozens of generations after us, are gone.
Read Also: Ammonium Nitrate - Common Fertilizer or A Dynamite?
No comments:
Post a Comment