Readers of Intermediate Physics for Medicine and Biology should become familiar with the Chernobyl disaster because it illustrates how exposure to radiation can affect people over different time scales, from short term acute radiation sickness to long-term radiation-induced cancer.
It turned out I could not attend the symposium. My friend Gene Surdutovich stepped in at the last minute to replace me, and because he is from Ukraine—where the disaster occurred—he provided more insight than I could have. However, I thought the readers of this blog might want to read a transcript of the talk I planned to present. It was supposed to be my “TED Talk,” aimed at a broad audience with limited scientific background. No Powerpoint, no blackboard; just a few balls and a pencil as props.
The nuclear reactor in Chernobyl had an inherently unstable design that led to the worst nuclear accident in history. To understand why the design was so unstable, we need to review some physics.
The nucleus of an atom contains protons and neutrons. The number of protons determines what element you have. For instance, a nucleus with 92 protons is uranium. The number of neutrons determines the isotope. If a nucleus has 92 protons and 146 neutrons it is uranium-238 (because 92 + 146 = 238). Uranium-238 is the most common isotope of uranium (about 99% of natural uranium is uranium-238). If the nucleus has three fewer neutrons, that is only 143 neutrons instead of 146, it’s uranium-235, a rare isotope of uranium (about 1% of natural uranium is uranium-235).
No stable isotopes of uranium exist, but both uranium-235 and uranium-238 have very long half-lives (a half-life is how long it takes for half the nuclei to decay). Their half-lives are several billion years, which is about the same as the age of the earth. So many of the atoms of uranium that originally formed with the earth have not decayed away yet, and still exist in our rocks. We can use them as nuclear fuel.
Although uranium-235 is the rarer of the two long-lived isotopes, it is the one that is the fuel for a nuclear reactor. The uranium-235 nucleus is “fissile” meaning that it is so close to being unstable that a single neutron can trigger it to break in two pieces, releasing energy and two additional neutrons. This is called nuclear fission.
A nuclear chain reaction can start with a lot of uranium-235 and a single neutron. The neutron causes a uranium-235 nucleus to fission, breaking into two pieces plus releasing two additional neutrons and energy. These two neutrons hit two other uranium-235 nuclei, causing each of them to fission, releasing a total of four neutrons plus more energy. These four neutrons hit four other uranium-235 nuclei, releasing eight neutrons….and so on. The atomic bomb dropped on Hiroshima at the end of World War Two was based on just such an uncontrolled uranium-235 chain reaction. Fortunately, there are ways to control the chain reaction, so it can be used for more peaceful purposes, such as a nuclear reactor.
One surprising feature of uranium-235 is that SLOW neutrons are more likely to split the nucleus than FAST neutrons. How this effect was discovered is an interesting story. Enrico Fermi, an Italian physicist, was studying nuclear reactions in the 1930s by bombarding different materials with neutrons. He observed more nuclear reactions if his apparatus sat on a wooden table top than if it sat on a marble table top! What? It turns out wood was better at slowing the neutrons than marble. Think how confusing this must have been for Fermi. He was so confused that he tried submerging the apparatus in a pond behind the physics building and the reactions increased even more!
A uranium-235 chain reaction triggered by neutrons works best with slow neutrons. Therefore, nuclear reactors need a “moderator”: a substance that slows the neutrons down. The moderator is the key to understanding what happened at Chernobyl.
The best moderators are materials whose nuclear mass is about the same as the mass of a neutron. If the nucleus was a lot heavier than the neutron, the neutron would not slow down after the collision. Imagine this tennis ball is the light neutron, and this big basketball is the heavy nucleus. When the neutron hits the nucleus, it just bounces off. It changes direction but doesn’t slow down. Now, imagine this neutron collides with a very light particle, represented by this ping pong ball. When the relatively heavy neutron hits the light particle, it will just push it out of the way like a train hitting a mosquito. The neutron itself won’t slow down much. To be effective at slowing the neutron down, the nucleus needs to be about the same mass as the neutron. What has a similar mass to a neutron? A proton. What nucleus contains a single proton? Hydrogen. Watch what happens when a neutron and a hydrogen nucleus collide? This ball is the neutron, and this ball is the proton: the hydrogen nucleus. Right after the collision, the neutron stops! It is like when a moving billiard ball slams into a stationary billiard ball; the one that was moving stops, and the one that was stationary starts moving. Interacting with hydrogen is a great way to slow down neutrons. Therefore, hydrogen is a great moderator. Where do you find a lot of hydrogen? Water (H2O). It was the hydrogen in the water of the wooden table top that was so effective at slowing Fermi’s neutrons. The water in the pond behind the physics building was even better; it had even more hydrogen.
Other elements that have relatively light nuclei are also good moderators, such as carbon (carbon’s nucleus has 6 protons and 6 neutrons). It’s somewhat heavier than you want in order to slow neutrons optimally, but it’s not bad, and its abundant, cheap, and dense. During the Manhattan Project, Fermi (who had fled fascist Italy and settled in the United States) built the first nuclear reactor in a squash court under the football stadium at the University of Chicago. His reactor was a “pile” of uranium balls, with each ball surrounded by blocks of graphite (almost pure carbon, like the lead in this pencil). The uranium was the fuel and the graphite was the moderator.
Before we talk more about moderators, you might be wondering why Fermi’s reactor didn’t explode, destroying Chicago? One reason was that his uranium was a mix of uranium-235 and uranium-238, and was in fact 99% uranium-238. The uranium-238 doesn’t contribute to the chain reaction; it’s not fissile. To make matters worse, uranium-238 can absorb a neutron and dampen the chain reaction. When uranium-238 captures a neutron to become uranium-239, it takes a neutron “out of action” so to speak. During the Manhattan Project the United States spent enormous amounts of time and money separating uranium-235 from uranium-238, so it could use almost pure uranium-235 in the atomic bomb. But Fermi didn’t have any such enriched uranium. Also, Fermi controlled his reactor using a super-duper neutron absorber, cadmium. Cadmium sucks up the neutrons, stopping the chain reaction. Fermi could push in or pull out cadmium control rods to keep the speed of the reaction “just right.” As an emergency backup he had one big cadmium control rod suspended over the reactor by a rope. One of Fermi’s assistants stood by with an axe. If things started to go out of control, his job was to cut the rope dropping the cadmium rod and stopping the reaction. Fortunately, Fermi took great pains to operate the reactor carefully, and no such problems occurred. Had things gone wrong, the reactor probably wouldn’t have exploded like a bomb. It would have just gotten very hot and melted, causing a “meltdown” with all sorts of radiation release, like at Chernobyl. It’s a scary thought because it was in the middle of Chicago, but we were at war against the Nazis, so people took some risks.
Now back to the moderator. Let’s consider three different moderators. First, “heavy water.” This is water containing a rare, heavy isotope of hydrogen, hydrogen-2 (its nucleus consists of one proton and one neutron). While it is not quite as good as hydrogen-1 at slowing down neutrons, it’s still very good, and it has one advantage. Hydrogen-1 (a single proton) can sometimes absorb a neutron to become hydrogen-2. It’s as if occasionally these two balls stick together when they hit. This capture of a neutron slows the chain reaction. Hydrogen-2, however, rarely absorbs a neutron to become hydrogen-3, so it’s a great moderator: it slows the neutrons without absorbing them. During World War Two, the Germans tried to construct a nuclear reactor using heavy water as the moderator. The problem was, heavy water is difficult and expensive to make. There was a plant in Norway that produced heavy water, and it was controlled by the Germans. The British sent in a commando raid that sabotaged the plant, causing all that precious heavy water to flow down the drain. Heavy water is so expensive it isn’t used nowadays in reactors, and we won’t discuss it anymore.
The second moderator we’ll consider is regular water made using hydrogen-1 (I’ll call it just “water” as opposed to “heavy water”). Nowadays most nuclear reactors in the United States use water as the moderator. They also use water as the coolant. You need a coolant to keep the reactor from getting too hot and melting. Also, the coolant is how you get the heat out of the reactor so you can use it to run a steam engine and generate power. So in the United States, water in a nuclear reactor has two purposes: it’s the moderator and the coolant. Suppose that the reactor, for some reason, gets too hot and the water starts boiling off. That will cause the moderator to boil away. No more moderator, no more slowing down the neutrons. No more slowing down the neutrons, no more chain reaction. This is a type of a negative feedback loop that makes the reactor inherently safe. It’s like the thermostat in your house: if the house gets too hot, the thermostat turns off the furnace, and the house cools down. Recall that hydrogen-1 can also absorb neutrons, and in theory that could cause the reactor to speed up when the water boils away because there is less neutron absorption. So neutron absorption and moderation are opposite effects. But a reduction of neutron absorption is less important than the disappearance of the moderator, so on the whole when water boils the reaction slows down. We say that the reactor has a “negative void coefficient.” The “void” means the water is boiling, forming bubbles. The “negative” means this negative feedback loop occurs, keeping the reaction from increasing out of control.
Now for the third moderator: carbon. The Russians built something called an RBMK reactor. This is a Russian acronym, so I won’t try to explain what the different letters mean. Suffice to say, an RBMK reactor is a nuclear reactor that uses carbon as the moderator. Chernobyl was an RBMK reactor. Like Fermi’s original reactor, the carbon was in the form of graphite. In addition, an RBMK reactor uses water as the coolant. Graphite is the moderator and water is the coolant. Now, suppose this type of reactor begins to heat up and the water starts to boil away. The hydrogen in the water is not the primary moderator, the carbon in the graphite is. So, the reaction doesn’t slow down when the water boils away; the carbon moderator is still there, slowing the neutrons. But remember, the hydrogen in water sometimes absorbs a neutron, taking it out of action. This neutron capture decreases as the water boils away, so the reaction increases. Increased heat causes water to boil, causing the reaction to speed up, causing increased heat, causing more water to boil, causing the reaction to speed up even more, causing yet more increased heat … This is a positive feedback loop; a vicious cycle. The reactor has a “positive void coefficient.” It’s as if the thermostat in your house was wired wrong, so when the house got hot the furnace turned ON, heating the house more. Normally the reactor is designed with all sorts of controls to prevent this positive feedback loop from taking off. For instance, control rods can be pushed in and out as needed. But, if for some reason these controls are not in place, the reactor will heat up dramatically and quickly, just as it did at Chernobyl.
Why do we have nuclear reactors? Nuclear reactors produce heat to power a steam engine, which in turn generates electricity. The steam needs to be at high pressure, so it can turn the turbine. Therefore, the reactor is in a pressure container. It’s like a pressure cooker. If the water boils too much, the pressure builds up until the container can’t handle it anymore and bursts, releasing steam. It’s a little like your whistling tea pot, except instead of whistling when the water boils, the reactor explodes. And unlike your teapot, the reactor releases radioactive elements along with the steam. You get a cloud of radioactivity.
Another problem with an RBMK reactor is that graphite burns. It’s pure carbon. It’s like coal. Once the pressure container bursts, oxygen can get in igniting the graphite, starting a fire. The graphite then spews radioactive smoke up into the atmosphere. Many of the people killed in the Chernobyl accident were firemen, trying to put out the fire.
Another issue, a little less important but worth mentioning, was the control rods. Chernobyl had control rods made out of boron, which like cadmium is an excellent neutron absorber. It vacuums up the neutrons and stops the chain reaction. The problem was, the control rods were tipped with graphite. As you push in a control rod, initially it would be like adding moderator, quickening the reaction. Only when the rod was completely pushed in would the boron absorb neutrons, slowing the reaction. So, the control rods would eventually suppress the chain reaction, but initially they made things worse. If, like at Chernobyl, a problem developed quickly, the control rods couldn’t keep up.
I won’t go in to all the comedy of errors that were the immediate cause of the accident at Chernobyl. The reactor was undergoing a test, and several of the controls were turned off. Some safeguards were still in place, but mistakes, poor communication, and ignorance prevented them from working. Whatever the immediate cause of the accident, the crucial point is that the reactor design itself was unstable. It’s like trying to balance this pencil on its tip. You can do it if you are careful and have some controls, but it’s inherently unstable. If you are not always vigilant, the pencil will fall over. The unstable design of the Chernobyl reactor made it a disaster waiting to happen.
If you would like to hear me give this talk (slightly modified), you can watch the YouTube video below. This winter I was teaching the second semester of introductory physics, and when the coronavirus pandemic arrived I had to switch to an online format. I recorded a lecture about Chernobyl when we were discussing nuclear energy.
My Chernobyl talk, given to my Introductory Physics class,
online from home because of Covid-19.