Chernobyl: The Graphite Tip Betrayal

By the end of this episode, you will be able to answer three questions. Why did the reactor power drop so low during that safety test? How did the shutdown button actually trigger the explosion? And what were the immediate and long-term consequences? Welcome to Pod-This and Study Session. It is April twenty-sixth, nineteen eighty-six.

We'll begin with the safety test, which established the specific conditions for the accident. On April 26, 1986, the crew at Reactor 4 was conducting a test on their RBMK-1000 reactor. This was a Soviet-designed model that used blocks of graphite to help manage the nuclear reaction. The crew wanted to perform what's called a turbine rundown test. This test aimed to see if the momentum of a spinning turbine could generate enough electricity to run the coolant pumps during a power outage. These pumps are vital because they move water through the core to prevent the nuclear fuel from overheating. As the operators prepared for the test, the reactor's power level did not stay where they wanted it. It unexpectedly crashed to about 30 megawatts thermal. This was a dangerously low level, as the test was supposed to occur at a much higher power. This sudden collapse was caused by a phenomenon called xenon poisoning. Xenon poisoning involves the buildup of an isotope known as Xenon-135. This isotope is created as a byproduct when uranium atoms split during fission. It's a very effective neutron absorber. In a functioning reactor, neutrons are the particles that strike atoms to keep the chain reaction going. Xenon-135 acts like a sponge that captures these neutrons before they can hit the fuel. When the reactor power is low, the xenon isn't "burned off" by the reaction, so it accumulates in the core. This effectively poisons the process and makes it nearly impossible for operators to regain a stable power level. What isotope built up in the reactor core to cause the sudden drop in power to 30 MW? The isotope was Xenon-135, which acts as a neutron absorber.

Next, we have to look at the control rods and the flaw in the A-Z five button. These show exactly how the reactor became unstable after that power drop. The operators wanted to overcome the xenon poisoning we talked about earlier. So, they withdrew almost all the control rods to get the power back up to two hundred megawatts. These rods are the main tools for absorbing neutrons and keeping the nuclear reaction under control. By pulling them out, the operators left fewer than the minimum number required for safety. This left the reactor vulnerable to something called the positive void coefficient. A void is just a steam bubble that forms in the cooling water. In this specific reactor design, as water turns to steam, the nuclear reaction actually speeds up. It creates a dangerous feedback loop. More heat leads to more steam, which leads to even higher power levels. At one twenty-three and forty seconds, the operators realized the danger. They pressed the emergency shutdown button, known as A-Z five. This button is supposed to drop all the control rods into the core to stop the reaction instantly. But the R-B-M-K control rods had tips made of graphite. As the rods moved in, those graphite tips pushed aside the liquid water that was absorbing neutrons. Since graphite helps the reaction along, this move caused an immediate, massive power spike instead of a shutdown. That surge in energy was the final step toward the physical destruction of the core. Why did pressing the A-Z five emergency shutdown button cause a power spike instead of stopping the reactor? The control rods had graphite tips. As they entered the core, they pushed the water out of the way, which caused the reaction rate to jump for a moment.

Next, we look at the explosion and its aftermath. This part details the destruction that followed the power spike. The blasts happened just seconds after the failure of the A-Z-5 system. There were two massive explosions. They were strong enough to lift the upper shield, which weighed one thousand tons. That shield was a heavy circular lid that covered the reactor core. The destruction threw about fifty tons of radioactive fuel into the atmosphere. This fuel also settled across the plant grounds and the surrounding facility. Firefighters arrived on the roof by one forty-five in the morning to deal with the fires caused by the debris. They were the first people to encounter the intense radiation coming from the exposed core. They faced extreme heat while they tried to put out the flames. This exposure led to thirty-one acute deaths among the plant workers and the emergency crews. While the site was in chaos, life in the nearby city of Pripyat went on as normal for a short time. The city had forty-nine thousand residents, but they weren't evacuated until thirty-six hours after the first explosion. Containment efforts started quickly to stop the spread of radioactive material. By November nineteen eighty-six, workers had finished the Sarcophagus. That was a concrete shell built to seal the ruins of Reactor Four. The structure was always meant to be temporary because the conditions during construction were so difficult. In twenty-sixteen, a more permanent solution was finished. It's called the New Safe Confinement. This is a massive steel arch weighing thirty-six thousand tons. It was engineered to slide over the original Sarcophagus to keep the site safe for the next one hundred years. How long did it take for the forty-nine thousand residents of Pripyat to be evacuated after the explosion? They were evacuated thirty-six hours later.

It is time for the recap quiz. We just looked at the explosion and what happened after. Now we will check what you remember about the technical side of things. Try to answer each question in your head before I give you the answer. Question one. What was the goal of the safety test they were running on Reactor Four? They wanted to see if the spinning turbine could generate enough electricity to keep the coolant pumps running during a power outage. Question two. Why did the reactor power fall all the way down to thirty megawatts during the test? The power dropped because xenon one-thirty-five built up in the core. This gas absorbed the neutrons. It is a process known as xenon poisoning. Question three. Which specific safety rule did the operators break to get the power back up to two hundred megawatts? They pulled the control rods out of the reactor core. They went past the mandatory minimum limit to try and make up for the low power. Question four. What was the design flaw that made the A-Z five emergency button cause an explosion instead of a shutdown? The control rods had tips made of graphite. When they were first pushed in, that graphite displaced the water and caused a massive spike in power. Question five. What was the name of the structure finished in twenty-sixteen to replace the original nineteen-eighty-six Sarcophagus? Engineers built the New Safe Confinement arch. They slid it into place over the reactor to keep it sealed for the next hundred years.

We have looked deep into the mechanics of that night. It is striking to think about the emergency shutdown button. The ultimate safety measure actually caused a massive power spike. It all came down to those graphite tips. You can now answer our three opening questions about the safety test, the design flaw, and the aftermath. To keep this fresh, try the recap quiz again in a few days. Answer the questions out loud before the solutions come. That is the session. Try the quiz again in a few days.