Thirty years ago today, on Tuesday, April 26, the Chernobyl nuclear power plant melted down outside Pripyat, Ukraine.
The resulting explosion killed 31 people almost immediately, and the spray of radioactive fallout forced the permanent evacuation of at least 100,000 people nearby.
With “Chernobyl” now serving as shorthand for the dangers of nuclear power, it’s tempting to worry a similar tragedy could befall any of the 100 commercial nuclear power plants in the US.
But writer and photographer Andrew Leatherbarrow — whose revisits the disaster in his new non-fiction book, “Chernobyl 01:23:40” — told Tech Insider in an email that this isn’t possible.
“There’s a ton of misinformation about Chernobyl,” he wrote in a subreddit session called “IAmA,” where we first heard from Leatherbarrow.
Using “Chernobyl” as a stand-in for “any nuclear disaster” isn’t out-and-out wrong, he said, but it glosses over many of the important differences between the issues that caused Chernobyl’s infamous meltdown and those that might plague nuclear power plants in the US.
No nuclear reactor is completely immune from certain hazards, particularly as US nuclear plants approach obsolescence.
Even so, there are better case studies for US reactors than Chernobyl.
How Chernobyl worked
On the night of April 25, 1986, a few engineers began an experiment to see whether they could power a cooling pump using the plant’s own energy. First they lowered the control rods, slowing down nuclear reactions and decreasing the reactor’s energy output.
This started a series of events that eventually led to a complete meltdown. The reactions slowed too quickly, generating too little energy, so engineers raised the control rods. Then, unsatisfied with the low energy output, raised them more while shutting off important safety systems.
To understand what happened next, it helps to look at the Chernobyl Nuclear Power Plant’s reactor design. Specifically, it was a Soviet-created graphite-moderated channel tube reactor, which in Russian takes the acronym RBMK.
All reactors are powered by nuclear fission. When radioactive isotopes like uranium-235 are bombarded with neutrons, they split in two, releasing free neutrons and a ton of energy in the process. The free neutrons hit more atoms, causing a chain reaction and more energy to be released.
Because the kinetic energy of the released neutrons is so high, they need a material that can slow the speed and energy to facilitate the next collision. There are also a set of control rods that absorb some of the free neutrons, keeping the chain reaction under control.
At Chernobyl, as the name of the reactor type suggests, the moderator was made of graphite.
Leatherbarrow likened this to “oxygen in a fire” in his email to Tech Insider. “The graphite blocks constantly fan the flames, so to speak, increasing the reaction,” he wrote.
Like reactors used in the States today, water passes through the RBMK’s core and is heated up by the energy released by fission. This boils the water into steam to drive turbines that generate electricity.
Chernobyl’s reactor design isn’t used in the US
The similarities begin and end there.
US nuclear power plants use pressurised water reactors (PWR) and boiling water reactors (BWR), neither of which use graphite — instead, water works as both the coolant and the moderator for fission.
The graphite, he said, was the key to the whole meltdown.
“The problem with the RBMK is that the tips of each control rod were made out of graphite, an incredibly foolish decision,” Leatherbarrow wrote.
Basically, Chernobyl’s design put a material that promotes fission at the ends of its control rods — not a material that slows down a nuclear reaction. You might be able to see where this is going.
At 1:23 am on April 26, the nuclear reactions the engineers had been trying encourage by raising the control rods started surging out of control, and the reactor began to overheat. Too many control rods had left the core. The operators tried to shut off the plant, rapidly lowering the rest of the rods.
“That was a lot of graphite,” which again boosts fission, Leatherbarrow said. “It caused a huge power surge, creating even more heat and steam until the system couldn’t take it and it went bang.”
Not a single US reactor uses graphite. All use water, which Leatherbarrow noted “is far safer.”
“There’s absolutely no way the accident at Chernobyl could have happened at any American commercial power reactor,” he concluded.
But the US isn’t immune to nuclear meltdown
If you need a case study for what a meltdown in the US could look like, you don’t need to reach as far back as Chernobyl.
Fukushima Daiichi, the Japanese reactor that melted down after getting slammed with an earthquake-triggered tsunami in March 2011, is a more apt comparison.
Like 34 of the 100 commercial nuclear power plants in the US, the Fukushima Daiichi plant used General Electric-designed BWRs. The events that led to the meltdown weren’t caused by a test gone awry, but unforeseen natural disasters. Workers today are still trying to contain the plant’s radioactive waste, in part by building a wall of ice.
The Nuclear Regulatory Commission released recommendations in the wake of Japan’s meltdown, and has asked all nuclear facilities to submit plans for flooding and earthquake preparedness.
But as far as specific nuclear reactor failure scenarios go, the US won’t be home to the next Chernobyl.