Poignant and bleak, the critically acclaimed HBO series Chernobyl revisits a difficult chapter in history to tell an important story about the role of science in society. While portions of the plot and characters have been embellished for TV, its an exceptional portrayal of what can happen when a community ignores the signs of an impending disaster (i.e. climate change) and includes surprisingly accurate and accessible explanations of nuclear physics.
On twitter , the show has also reignited an important discussion on nuclear power and its associated hazards. It is necessary to acknowledge that while some risks will always be there, nuclear power is actually safer than ever, and importantly, it could help curb our insatiable appetite for fossil fuels.
To learn more about this funky world of radioactive physics, we’re going to answer some questions about how our nuclear landscape has changed since 1986 (I’d say spoiler alert, but that’s not really applicable to historical events that happened decades ago).
As of July 2019, 416 nuclear reactors are operating in 34 countries, and they’re producing about 10.3% of the world’s electricity. That’s a marked drop from their peak share of 17.5% of worldwide electricity in 1996. Excluding China, the world’s nuclear production has actually been slowly, but steadily declining, perhaps due to increased contributions from wind, solar, and other renewable energy sectors. So, nuclear power still contributes a significant portion of power to more developed countries, but it is definitely not growing at the rate it was in the 80s, partially because countries are not investing in building more reactors. With pushes towards fossil-free energy, perhaps nuclear power will become more popular, but the global energy landscape is growing increasingly complex.
After 1986, all remaining RBMK-1000 reactors underwent design updates to fix these flaws, finding 58 individual issues with the system. All RBMK-1000 reactors have graphite cores, which contain mazes of cavities that hold the radioactive uranium fuel rods. The majority of changes made focus on the reactor’s graphite core. They added more absorbers so that reactions would remain stable at low power, they increased the number of control rods in the core, and increased the fuel enrichment of uranium required at power plants. These reactors aren’t built anymore, but 10 are still operating in Russia.
The type of reactor used at the Chernobyl plant is called an RBMK-1000 reactor (it stands for Reaktor Bolshoy Moshchnosti Kanalnyy/High Power Channel-type Reactor). While human error, policy failures, and a lack of knowledge all played a role in the disaster, design flaws in the RBMK reactor are also to blame. One of the main issues with these types of reactors is they have what’s referred to as a “positive void coefficient” . This means that when the coolant is lost, the rate of fission would increase. During the accident, an overproduction of steam led to more fission, a huge temperature and pressure increase, which eventually led to an explosion.
In the HBO series, one of the first harbingers of disaster post-explosion were small chunks of graphite shown strewn along the ground while firefighters worked to extinguish a blazing inferno. If the material that’s supposed to separate people from highly radioactive uranium fuel is no longer there, that’s a pretty big problem.
So, why graphite? Graphite is a cheap material, and it allowed the Russians to use lower-grade uranium as fuel. The control rods, which are supposed to slow down the rate of fission inside the core, were made of boron tipped with graphite. The boron slowed down the reaction as planned, but when the graphite interacted with the fuel as the rods are removed, it actually increased the rate of fission. So when the workers tried to slow down the reaction before the disaster, they inadvertently sped it up.