In the popular HBO mini-series “Chernobyl,” early episodes show scientists Valery Legasov, played by Jared Harris, and Ulana Khomyuk, a composite character played by Emily Watson, expressing their alarm at what must have occurred in spring of 1986: a nuclear reactor core has exploded.

This has never happened before. But Soviet bureaucrats and politicians pose a key question to them: “How does a reactor explode?”

They can’t answer and spend the rest of the time helping clean up the mess and trying to find out what happened, even against the wishes of the state.

The show, which depicts the Chernobyl Nuclear Power Plant disaster and aftermath, uses a lot of scientific and nuclear physics terms; so many that viewers might find themselves Googling ferociously throughout the otherwise gripping drama.

Fortunately, University of Oklahoma particle physicist, researcher and professor Michael Strauss knows that terminology well. Also, he remembers how the disaster was viewed from the U.S., so he can answer many of the questions fans of the show might have.

First, how does a nuclear reactor actually work?: Atomic energy is harnessed mainly in two ways, and Strauss said it’s important understand the difference.

Fusion is when two atoms are fused together. Though we have managed to make an uncontrolled fusion reaction called a thermonuclear bomb, no one has so far been able to control fusion safely in a way that creates energy.

This happens in the Sun all of the time. If anyone ever does it on Earth, that person will be very popular and very rich.

Fission is also a process that can make an atomic bomb, but scientists have found a way to control it. This is the process in which a stable atom is made unstable, forcing it to split up and send particles called neutrons flying out, searching for another place to land.

“When it decays, it gives off a little bit of energy and it shoots out some particles called neutrons,” Strauss said. “Those neutrons are absorbed by other nuclei that are stable, but when they absorb the neutron, they become unstable. So now they break apart, sending more neutrons. So you get this thing called a chain reaction.”

Some naturally occurring elements are ripe for this kind of activity. Uranium, for instance, already has radioactive properties and, relatively speaking, gives up neutrons like they’re Christmas candy.

That’s why Uranium-235 is commonly used in nuclear power plants. The energy this process creates is given off in the form of heat, which turns water into steam that powers a turbine to create electricity, just like any other power plant.

“If this is uncontrolled, you get a bomb,” Strauss said. “If it’s controlled, you get a nice, smooth generation of energy.”

To make this controlled, nuclear power plants conduct fission inside reactors, namely the reactor core. The reactors also have things like cold water to prevent the whole thing from overheating, elements that can slow down or stop the chain reaction and moderators to ensure the process happens in a controlled space.

OK, so how does a nuclear reactor explode?: For one thing, it happens when all of that stuff isn’t used right, like at Chernobyl.

“There were not enough fail-safe safety methods, there was a lot of human error,” Strauss said. “Somehow this thing gets supercritical and they weren’t able to control it.”

Couple that with a reactor that has inherent design flaws, and suddenly a large portion of Europe becomes irradiated.

Chernobyl — officially named the Vladimir I. Lenin Nuclear Power Plant — used the Soviet designed RBMK reactor. On the night of April 26, 1986, a system test was being run on reactor No. 4 to simulate a power outage.

The test conditions had to be set just right, and they were not. The power was set too low, and the RBMK reactor became unstable.

Reactors use control rods to increase or decrease the energy output of a nuclear reaction. In this case, they were made of the element Boron, which is the opposite of Uranium in that it is good at absorbing neutrons and not going crazy.

“The control rods absorb those neutrons that, if they’re not absorbed, would otherwise cause [the reaction] to run away,” Strauss said.

The next important compound in this mix is the mineral graphite. This was used to moderate the uranium neutrons, effectively keeping everything under control.

Water was used to cool down the core and also absorb neutrons. But a sudden power spike turned the water into steam and created gaps known as “voids.”

RBMK reactors were designed to have a very high positive void coefficient, meaning when voids are created, power output increases. The reasons for making it so high are notably Soviet: it cost less money to run the reactor and it allowed for the plant to both create electricity and enrich uranium to be used in nuclear bombs.

Chernobyl personnel tried to emergency stop the test — which is called a SCRAM — by inserting the control rods, but this only caused more voids which led to more power and a subsequent steam explosion that ruptured the core and fractured one of the rods. With the reaction continuing unhindered, a much larger explosion then blew open the core, opened a giant hole in the roof and scattered graphite everywhere.

The presence of graphite outside of the building and on part of the roof led scientists to surmise the core was gone. And that meant all the highly radioactive material was exposed to the air and lifted by the ongoing graphite fire to be spread over hundreds of miles.

Plant personnel, firemen who responded to the disaster and surrounding residents died within weeks or suffered health complications for the rest of their lives due to radiation. Chernobyl and the nearby town of Pripyat, now in independent Ukraine, are still no-go zones, and will be for the foreseeable future.

Why didn’t it explode like a nuclear bomb?: To get a fission chain reaction that creates a nuclear bomb, everything needs to be compact, Strauss said.

Nuclear reactors, on the other hand, are built with enough space that an atomic bomb-type explosion can’t occur.

“You don’t have the kind of geometric configuration or concentrated material that will let it explode,” Strauss said. “When I’m building this kind of reaction, I can move things far enough apart. When you want a bomb, it has to be much tighter.”

Chernobyl suffered a meltdown, meaning the core got so hot that it turned into a “nuclear glob of radioactive material,” but that’s not in danger of exploding, Strauss said.

What did happen with Chernobyl is that meltdown almost fell into pools of coolant water below. If not for the actions of three brave volunteers who went down and drained the water — shortening their lifespans in the process — that glob would have reacted with the water, causing another steam explosion that, it is thought, would have decimated the immediate area and made about half of Europe uninhabitable for a few hundred thousand years.

So, thankfully that didn’t happen.

After all that, why would anyone still use nuclear energy?: The Soviets immediately took steps to change the RBMK reactor design, lowering its positive void coefficient to make the reactors more stable. And not every reactor type comes with a positive void coefficient; in fact, some are negative.

Strauss said American nuclear reactors have always been designed in a way that can contain accidents. For example, the U.S. came close to its own Chernobyl when the Three Mile Island plant near Harrisburg, Pennsylvania experienced a partial meltdown in 1979.

Though alarming, it was determined that no radioactive material escaped the facility. It was, however, released from the reactor core, putting plant personnel at risk.

“We build our power plants with the idea that, if something was to happen and the core was to get overheated and released radioactivity, it would stay inside a containment vessel,” Strauss said. “Chernobyl had no containment vessel, and therefore it goes all over the place.”

The technology we use to control the process that creates nuclear power is, of course, much better now than it was in 1986. Still, there are extensive safety plans in place for the 98 current nuclear power plants operating in the U.S.

“There are ways now to build nuclear power plants that are inherently safe, that human error can’t cause any problems,” Strauss said. “The default state is ‘off,’ so if anything happens, it goes off and the control rods go in.”

The closest ones to Oklahoma are Comanche Peak in Glen Rose, Texas and Wolf Creek near Burlington, Kansas.

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