Nuclear reactors aren’t supposed to explode. The engineers who design them go to great lengths to make sure they can’t. So, how exactly did the people at the Vladimir Lenin Power Station mange to do so?
First off, I want to get some pedantry out of the way. The explosion at Chornobyl was NOT technically a nuclear explosion. It was a steam explosion caused by a run away nuclear reactor. The fissile material in a nuclear reactor cannot achieve the kind of critical mass that a nuclear weapon does.
The reactor at Chornobyl was an RBMK-1000. These are light-water boiling water reactors with a graphite moderator. The cooling water enters the bottom of the fuel channels and is boiled off as it rises to make steam to turn the turbines. The graphite moderator is used to slow down the neutrons to increase the chance that they will interact with the uranium fuel.
The night of the event, the staff was preparing to run a test. On a loss of normal power to the station, the output of the generator is supplied to the main circulating pumps that push water through the core to keep them spinning for several seconds while the reactor is shutting down. A previous test hadn’t maintained that power supply long enough and they were running it again.
Preparation for the test required the Emergency Core Cooling System to be disabled. This wouldn’t have mattered much in the end, but it is something that we would NEVER do in the US. Bypassing safety features like that is a good way to end up in prison here.
It also required power to be at 700-1000MWt (about 25% power). The crew began to reduce power, but the grid operator stopped this power reduction due to high demand on the local grid. Anytime you move power in a reactor, Xenon concentration changes. Xenon is a great absorber of neutrons. When you reduce power, this concentration rises, absorbing neutrons and “poisoning” the core.
When the grid dispatcher finally allowed them to reduce power further, the crew struggled to stabilize the reactor at the target power. Xenon was building in quickly, forcing power to drop further. At some point power fell to 30MWt, only about 1%.
In an attempt to raise power, the crews began pulling control rods out of the core. Safety procedures required a certain number of rods to be inserted in the core at all times to limit the effects of a couple of the design flaws in the reactor. The low limit was 15 rods. At the time of the accident, the crew had pulled all but approximately 8 control rods from the core.
Not satisfying this safety limit had two major effects. It caused the void coefficient to become very large. The void coefficient is a measure of how steam inside the coolant effects the neutron flux inside the core and by doing so, the power of the reactor. A positive void coefficient means that as more steam is produced in the core, more neutrons are available, so reactor power rises. As power rises, temperature rises, creating more steam. This can result in a very hard to control feedback loop.
The other reason to keep rods in the core was the positive scram effect. This was something the crews were NOT aware of. The tips of the control rods were made of graphite. Graphite is also used in RBMK reactors to moderate neutrons. This meant that, as the tips of the control rods fell into the core, power actually rose as the graphite moderated more neutrons and those neutrons hit the fuel.
The end result of having too few rods inserted in the core was a very unstable reactor. Despite being unable to return power to the 700MWt the test required, the engineer running it ordered the crew to begin. They did as good Soviets are wont to, and followed orders. At 0123 on 4/26/1986, the closed the valves supplying steam to the turbine.
The turbine immediately began to slow down. The pumps it was still supplying power to also slowed down. As flow in the core lowered, the water being pump through it stayed inside longer. More of the water flashed to steam.
The positive void coefficient now makes it’s grand entrance. As the water in the core flashed to steam, more neutrons were available for fission. This caused power to rise. As power rose, it heated up the core. More steam was created. More steam, more neutrons, more power, more steam … You get the idea. A feedback loop had started inside the core.
The operators realized something was wrong and decided to SCRAM the reactor. If you’ve watched the Chernobyl mini-series on HBO (you should, it is amazing), this is the infamous AZ-5 button. This action is intended to quickly insert all control rods and shutdown the core.
Now the positive scram effect, the one that the Soviet overlords in Moscow had decided not to tell anyone about for fear of embarrassing the Soviet Union, came into play. As the graphite tips of the control rods entered the core (remember, almost all of them were out), power rose rapidly. This fed into the feedback loop already established by the positive void coefficient.
The reactor went prompt critical. See the post below to really dig into that, but suffice it to say the reactor was now in the process of exploding. Power peaked at over 30,000MWt, more than ten times the design of the core. This caused all of the water in the reactor that wasn’t already steam to instantly flash.
Steam takes up a great deal more room than water does. Inside the pressure vessel of the reactor, there was nowhere for the steam to go. Pressure surged. Fuel channels ruptured, adding more heat to the system. The steam explosion blew the 1000 ton support plate off the top of the reactor. This jammed the control rods. They had only made it halfway into the core at that point.
The rest of this story is the more familiar part of all this. The reactor was inside a building very similar to your high school gym, not a concrete and steel-lined containment structure like everyone else’s reactors. The roof of this gym that was over the reactor was destroyed. The graphite in the reactor ignited. Massive quantities of radiation were lofted into the night sky by the hot air rising from the destroyed reactor.
By Artur Korneyev – Original publication: 1996Immediate source: https://www.atlasobscura.com/articles/the-famous-photo-of-chernobyls-most-dangerous-radioactive-material-was-a-selfie, Fair use, https://en.wikipedia.org/w/index.php?curid=62302438
In the days that followed, approximately 30 firefighters and plant workers would die from acute radiation syndrome. The UN estimates that over the lifetime of the people exposed to the accident, it is possible that 5000 of them will die of cancers.
This is the worst case nuclear event, in my opinion. Since this is my blog, my opinion wins here. A terrible reactor design, no containment structure, inept Soviet attempts at a coverup, all led to this being as bad as it is possible to be. To give that statement some weight, the Fukushima disaster melted 3 reactor cores. But, because the reactors were a western design with a containment building, the 3 melted cores only released 10% of the radiation that Chornobyl did.
This may surprise you, but there are still 7 RBMK reactors in operation in Russia. They were heavily modified following the event, and another Chornobyl is (probably) impossible.
I also want to point out that such an event is impossible in US reactors. The design of US reactors precludes the kinds of feedback loops that caused Chornobyl to destroy itself. US reactor designers don’t hide the kind of design flaws that allowed it to occur, and US regulators would never allow such a design to be built. (This is one of the many reasons I disagree with my colleagues who want the NRC dissolved.)
As always, this is all open source. Here are some really good links
https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html
https://www.iaea.org/newscenter/focus/chernobyl/faqs
https://www.un.org/en/observances/chernobyl-remembrance-day/background
Sleepy Neutron's Nuclear Knowledge 
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