What Happened at Chernobyl?

Mathew King is a Reactor Operator at the Nuclear Radiation Center at Washington State University. When Mat isn’t safely operating a nuclear reactor, he can be found commenting and contributing here at Nuclear Fissionary. Today he will help us understand the complex array of factors that contributed to the worst nuclear disaster in human history, Chernobyl.

In the last fifty years, most of the industrial world has become increasingly reliant on safe, renewable, clean, and nearly limitless energy. I’m not talking about unreliable wind turbines, or the chemical filled solar panels touted by the ‘green’ movement. Rather it’s nuclear power that fits these descriptors as well as any, and yet it is one of the most controversial forms of energy production in the world.

The pro-nuke side usually comes to the table armed with facts, statistics, and physics, while the anti-nukes have one big poster boy for their cause: Chernobyl.

Chernobyl was an RBMK-1000 Soviet power/weapons production/testing reactor near Pripyat, Ukraine which destroyed itself in the worst nuclear power disaster in history. Some basic nuclear theory is required to fully understand the event, so feel free to click on the links to learn more about the nuclear theory.

The Fatal Flaws that led to the Chernobyl Disaster

Chernobyl’s Graphite Moderated Reactor

Most reactors use either water or graphite to moderate the reactor core. Chernobyl was a graphite moderated reactor that used water only for cooling. United States (and now Russian) reactors are all required by law to be slightly under-moderated. This way, if a catastrophic event occurred where all coolant was lost the reactor would shut itself down (i.e. Three Mile Island).

Prompt-Temperature Coefficient

Another contributor to this event was an effect called a prompt-temperature coefficient (which can be either positive or negative). In short, a positive temperature coefficient means that when a reactor gets hotter, it gets more reactive and power increases. A negative temperature coefficient means exactly the opposite. The hotter the core gets the more it wants to shut itself down. All reactors are now required by law to have an engineered negative temperature coefficient. Unfortunately, Chernobyl’s was positive.

Control Rods used in RBMK Reactors

The design of the RBMK style reactors also had a large flaw in the Control Rods. The control rods contain a poison (Boron or Hafnium) that are inserted into channels between the graphite moderator and Uranium fuel to absorb neutrons and slow down the reaction. The problem was that the tips of these control rods was not Boron poison, but graphite, a moderator. This means that control rods briefly increased power before bringing power down when they were inserted.

The Unit 4 Test that sealed Chernobyl’s Fate

On April 25th of 1986, all of these design flaws came to a head when there was a scheduled test on reactor 4 of the Chernobyl complex. They were trying to find out how long they could cool the reactor if they shut down using only the residual steam and coasting turbines for power to run the coolant pumps. United States law does not allow such experiments at nuclear power plants.

In order to run this test they had to bypass several automated safety systems which was in violation of their own protocols. This was strongly objected to by the reactor operators, but unfortunately, an engineer was in charge. An American reactor operator or engineer could be sent to jail for such behavior.

This experiment had already been done several times unsuccessfully, and the only difference this time was a delay and the disabled safety systems. The reactor was run down to a low power and held there for the experiment. But shortly after that, a call came through ordering the plant to delay the experiment for several hours because of a sudden increase in power demand on the grid. The reactor was run at this low power for nearly twelve hours.

At low power, with fewer neutrons in the core, Xenon-135 built in at an accelerated rate, and the operators noticed that power was dropping further. They began withdrawing motorized control rods further and further in an attempt to stay at power, until the control elements were in their fully withdrawn position.

To make matters worse, once the experiment began, they turned on some extra cooling pumps, making the core cooler causing power to drop even more. At that point they had a reactor held at low power by cold water and burnable poison, with all of its control elements fully removed, and they were about to turn off the cooling pumps.

Compare this scenario to a game tug of war. You are pulling as hard as you possibly can but going nowhere. Then suddenly the other team lets go and you are sent flying out of control.

When they shut off the steam supply to the turbines, the flow of water slowed within the core increasing heat and raising reactor power because of their positive temperature coefficient. The reactor power increased so rapidly that someone hit the big-red button called the SCRAM(emergency shutdown).

Now, the graphite tips of the control rods (a moderator) quickly drove into the reactor, the moderator section filled the space between the fuel and further increased the reaction. This sudden increase in power and heat, led to another increase in power and heat and there was a runaway chain reaction known as a feedback loop.

The heat and power increased so rapidly that there was a steam explosion that blew the 1,200 ton reactor head off the core and through the thin roof of the building. Without the reinforced concrete and steel containment structures of American reactors, the highly radioactive molten fuel and burning graphite was ejected into the atmosphere.

The Aftermath

The rest is just a mess, firefighters were sent in unaware of the deadly radiation levels, and the Soviet government refused to even acknowledge the incident for several days. This caused the death of all firefighters and workers sent in to put out the fires and clean up the mess. It also resulted in an impossible to count number of Thyroid Cancer cases that often result in death. The entire nearby pine forest died from radiation poising within a few weeks.

It was the inherent design flaws and human error combined with a culture at the plant that placed results ahead of safety that caused this disaster. If you remove any one of those three elements from the equation, this could not happen. The Three Mile Island scenario was similar to Chernobyl in that several problems occurred simultaneously and destroyed the reactor. However, the designed fail safes and containment structure of the Reactor functioned as designed and no one was hurt, and no one received an excessive dose of radiation.

Chernobyl was a hard lesson learned by the Soviets and indeed to the entire global nuclear industry. The US has also taken some cues from the incident and placed additional limits on safety culture and operator training. Now with more need for reliable energy than ever, I hope my future kids grow up in a nuclear world.

Image Credit

Chernobyl photo courtesy of Flickr user Kamil Porembinski published under the CC license.