Even as Japanese officials try to sort out whether jolted nuclear reactors could slip into full meltdown after Friday’s massive earthquake and tsunami, experts are weighing in on how events unfolding there compare to previous high-profile nuclear disasters.
The Fukushima Daiichi reactors in Japan are fission reactors, and known as boiling water reactors in the industry. Inside their fuel rods, uranium atoms are split apart, producing energy in the form of heat. Water gets pumped into the core of the reactor, where it boils and turns the water into steam. The steam powers a turbine and generates electricity.
Reactors were automatically shut down when the earthquake struck. Safety rods were dropped into the reactor core. They soaked up the neutrons necessary for the fission reactions, stopping the chain of events.
Yet even with the system halted, heat continues to be produced through radioactive decay. This excess heat will taper off throughout the week, but in the meantime, it’s become the source of a series of problems that engineers are grappling to control.
“If they don’t remove the heat, everything heats up and eventually melts,” said Michael Corradini, chairman of the nuclear engineering program at the University of Wisconsin-Madison.
Japan’s reactors have met a cascade of challenges since Friday, compounding problems in the already devastated region. After the area’s power grid was knocked offline by the quake, the tsunami flooded the backup diesel power generators. Engineers then turned to seawater to keep the system from overheating. Without cooling, temperatures inside the system can skyrocket to nearly 2,200 degrees Fahrenheit — hot enough to melt the fuel rods.
Venting steam to release pressure from the reactors has sparked explosions in two of the units: 1 and 3. In both cases, fires were ignited by hydrogen gas in the steam, which reacted with oxygen and combusted during the venting process.
Now, the biggest fear is that the nuclear fuel will melt through the reactor, and ultimately release radioactive elements into the atmosphere. Experts say that a well-designed containment system makes such an event unlikely. Rods are surrounded by layers and layers of steel and concrete.
Yet reports on Monday of pumps failing and water levels dropping inside the Unit 2 reactor to leave uranium fuel rods exposed, has further sparked fears of a meltdown or another explosion.
A best case scenario now is that normal water flow gets restored, says Peter Caracappa, a professor and radiation safety officer at Rensselaer Polytechnic Institute. The worst case, he says: a breach of containment.
Some experts have compared the events unfolding in Japan to the partial nuclear meltdown that occurred at the Three Mile Island nuclear plant, near Harrisburg, Pa., in 1979.
“Three Mile Island #2 was a pressurized water reactor,” Corradini said. “This one is a boiling water reactor. But in terms of approximate behavior and how the plant runs they were similar.”
Unlike Japan, the Three Mile Island accident was a mechanical problem, the result of instruments failing to read a system malfunction. The malfunction made people believe that cooling water was going into the core when it wasn’t. This caused coolant levels to drop and nuclear fuel to partially melt. The fuel melted, re-solidifying at the bottom of the core, but failed to melt through the reactor. There were no deaths or injuries.
The reactors were of similar size — thick, steel, pill-shaped containers — powered by a similar type of nuclear fuel, said Paul Wilson, a nuclear engineering professor at University of Wisconsin-Madison. And the containment systems were similar.
The 1986 Ukrainian disaster at the Chernobyl Nuclear Power Plant, on the other hand, was an entirely different story. On April 26 of that year, the plant exploded while most of the residents of the neighboring town of Pripyat were sleeping, dispersing highly radioactive material that killed dozens and put people at a high cancer risk and newborns at a higher rate of birth defects.
While both boiling water and pressurized water reactors depend on water both to power and cool the systems, the reactor in Chernobyl relied on graphite at its core.
Plus, Wilson points out, Chernobyl was initiated by human error. “The thing that’s often discussed in Chernobyl that’s important here, too, is we have robust sealed containment in this reactor, as we did at Three Mile Island,” he said. “Chernobyl had a containment system that was not well sealed.”
“I’ve been hearing a number of reports of people saying this could be like Chernobyl,” Corradini said. “This is absolutely false. It’s inappropriate and not helpful. The plants are totally different. What happened at Chernobyl couldn’t happen here.”
For now at the Japanese plants, getting a reliable supply of electricity restored is absolutely critical, Wilson said.
“The tsunami affected their ability to draw water from the Pacific Ocean,” he said. “Being able to restore that and bring cold water in and let cold water out is very important.”