Twenty years ago, a fission reactor at Chernobyl, Ukraine, exploded, killing 28 people by radiation poisoning and ruining the health of thousands more by contaminating the water and soil.
By all accounts, the accident resulted from a poor reactor design and ill-advised operational decisions by the crew. It was the unforgiving laws of physics, however, that led to the worst nuclear power plant disaster in history.
Nuclear power plants generate electrical power by using pressurized steam to rotate turbine blades connected to electric generators. The heat of a controlled chain reaction creates the steam.
The trick is to control that chain reaction so that the nuclear core is hot enough to create the steam, but not hot enough to melt the reactor. Loss of that control caused the Chernobyl disaster.
Nuclear fission occurs when an atomic nucleus is too unstable to maintain its structural integrity. Basically, there’s too much energy in the tiny volume of the nucleus and the nucleus quells the riot by splitting into two smaller nuclei and emitting gamma ray photons. The process inevitably leads to an increase in temperature of the radioactive material.
Uranium is a naturally occurring “fuel” for fission reactors like the one in Chernobyl. Uranium has two principal isotopes, U-238 and U-235, which have different nuclear but identical chemical properties.
U-238 can capture a passing neutron in its nucleus. The extra neutron excites the nucleus, which spits out two beta particles and two antineutrinos, leaving behind a plutonium (Pu-239) nucleus. U-238 nuclei can be forced to fission, but generally are happy to slowly bleed away their excess nuclear energy by slow radioactive decay.
U-235, which has fewer neutrons to hold things together, is more volatile and tends to spontaneously fission, releasing three neutrons in the process. Under the right conditions, these ejected neutrons can cause other U-235 nuclei to fission. Given enough U-235 (the critical mass) in a small volume, you can have a rapid, runaway chain reaction, as the zipping neutrons create a cascade of fissioning U-235 nuclei. In other words, you have an atomic bomb.
So to create a functioning fission reactor, you use U-238 and just enough U-235 as “seasoning” to maintain a chain reaction. The more placid U-238 prevents the reactor from becoming an A-bomb, while the more mercurial U-235 — usually a few percent of the total uranium used — keeps the fission process rolling.
It should be so easy.
As it turns out, fast moving neutrons tend to zip right past U-235 nuclei and are more likely to be captured by U-238 nuclei. So you have a harder time creating a chain reaction. U-235 nuclei respond better to slower moving neutrons, so fission reactors include materials to decrease — moderate — the neutrons’ kinetic energy. The Chernobyl reactor used graphite, a handy, plentiful and unfortunately flammable moderator.
If a fission reactor were a car, the uranium pile, or core, would be the engine. The inevitable product of any engine is heat, which if left unchecked can destroy the engine. So, like a car engine, the core has to be cooled. Water is the most commonly used coolant, as it can also be the source of the superheated steam used to drive the turbine generators. In the case of the Chernobyl reactor, its maximum thermal output was 3.2 gigawatts (3.2 GW), while its maximum electrical output was 1.0 GW. Most of the remaining 2.2 GW is “lost” to heating the reactor core and the water coolant.
Cooling water is kept under pressure, to raise its boiling point and thermal efficiency. Even so, high reactor temperatures can create steam bubbles in the coolant — voids — which do not absorb neutrons. Too many voids can mean higher reaction rates, more power output, more heat, and more voids, creating a dangerous feedback loop. The Chernobyl reactor design was especially susceptible to this kind of problem.
Engines also need throttles. Water is a neutron absorber, so the circulating coolant naturally controls the chain reaction by removing neutrons from the mix. A nuclear reactor balances the cooling and absorbing properties of water to ensure a sustainable chain reaction.
To further throttle or even shut down the core, reactors also include solid neutron absorbers in control rods. Pulling out the rods increases the reaction rate, dropping them shuts the reactor down. The Chernobyl reactor had 211 control rods containing boron carbide. The Chernobyl control rods also had graphite tips, a feature that would contribute to the demise of the reactor.
So, to summarize, running a nuclear reactor is a balancing act between useful heat and self-destruction. In the case of Chernobyl, the scales tipped the wrong way.
Based on chronologies of the accident, here is what happened.
Around noon on April 25, 1986, the crew began a test of the reactor’s coolant pumps. They wanted to see if, in the event of losing their normal electrical supply, the pumps could run off the reactor’s own turbine generators in spin-down mode. (Power plants only produce electricity as needed, since there is no way to store the surplus power. A loss of electrical power would mean the plant had no electrical load, and would have to shut down the generators.)
The crew planned to reduce the thermal output of reactor #4 from 3.2 GW to 1.0 GW to prepare for the coolant pump test. The thermal output instead dropped precipitously to 30 MW, far below safe reactor operating levels. The design of the Chernobyl reactor under extremely low power levels produces a fission by-product, Xenon-135, which absorbs neutrons, “poisoning” the chain reaction.
To compensate, and to “flush out” the xenon, the crew pulled out the control rods to speed up the chain reaction. Then they turned the water pumps on, increasing the coolant flow. The extra water absorbed more neutrons, threatening to kill the chain reaction. So the crew pulled the control rods out further. In all a total of 204 of the 211 rods were pulled out, more than were recommended.
As planned, external electrical power to the pumps and the steam supply to the turbines were both cut. The reduced water flow increased the temperature of the coolant, creating more voids in the water. The reaction rate as a result increased. Meanwhile, the steam pressure also increased, since the steam had basically nowhere to go.
Unaware of the reactor core’s increasing reaction rate and temperature, and the danger of a steam explosion, the crew proceeded to end the test by dropping the control rods in SCRAM (emergency shutdown) mode. As the graphite tips re-entered the core, they moderated the neutrons and the reaction rate spiked sharply upward. The even higher core temperature warped the control rod channels, and, only partially inserted, the rods jammed.
Now the core was in runaway mode — the so-called China syndrome. The thermal output jumped to 30 GW, almost 10 times the maximum rating. About 12 hours after the test procedures began, the uranium-oxide fuel rods melted. The steam pressure blew off the reactor’s heavy concrete lid and punched a hole in the roof. The sudden exposure to oxygen ignited the graphite moderators, and a 2500 degree C conflagration ensued. The fire carried radioactive contaminants to the outside, and prevailing winds transported the fallout all over northern Europe.
(Another contributing factor to the failure was the Wigner effect, in which high energy neutrons dislocate atoms from their normal crystalline arrangements, as in graphite or metals. The dislocated atoms have higher energies than their neighbors, and if sufficient dislocated atoms snap back all at once to their normal positions, the temperature of the material sharply increases.)
Since the crew had temporarily disabled the safety systems to undertake the test, they were unaware of the danger of an explosion. All indicators in the control room were normal. After the core breech, the crew may have also been unaware of the amount of radiation pouring through the control room. They stayed on duty, pumping water in an effort to cool the white-hot core down.
At the levels of radiation the crew experienced, they were probably doomed whether they had stayed or not. Gamma radiation and neutron bombardment ionize atoms, destroy molecules and disrupt cellular processes. Victims suffer nausea, internal organ damage and internal bleeding. Their bone marrow is destroyed, shutting off red blood cell production, and white blood cell production ceases, increasing the risk of infection. Death usually follows within two weeks.
In all 28 men — including the crew and firefighters from Chernobyl — died from the radiation exposure in 1986. More died from acute radiation sickness soon after. Many thousands suffer from the longterm exposure to the radioactive fallout, which contaminated the water and the soil, and still remains. The BBC has a comprehensive review of the lingering hazards of Chernobyl that is well worth reading.
Nuclear power can provide us with electrical power, but the laws of physics are unforgiving of mistakes and poor reactor design. We need to consider what risks we are willing to accept in the pursuit of our hunger for electrical power.