Nuclear Reactors 101

This week, Japan raised its nuclear reactor accident to a level 7, the highest stage on the nuclear incident scale created by the U.N.’s International Atomic Energy Agency. That is the same level as the 1986 Chernobyl disaster in Ukraine.

One thing is certain to come out of the reactor trouble in Japan – an international dialogue on nuclear energy generation. Both Japan and the U.S. depend on nuclear energy for approximately 20% of their national energy needs. Both countries have a long history of safe nuclear energy production with a few notable exceptions.

There are 436 nuclear reactors in the world at last count, possibly 433 now. These account for 15% of the world’s electricity production. The U.S., France and Japan alone account for more than half of the electricity generated by the world’s nuclear reactors.

What is a nuclear reactor, anyway? It’s a device used to control a sustained nuclear reaction (as if that helps).

Let’s start with the fuel. Uranium is a naturally occurring element found in bedrock in certain areas, like gold or diamonds. It’s dug up in mines where it occurs as a mixture of several kinds of uranium. Uranium 235 is the stuff used in nuclear reactors, and it’s about 1% of the uranium in the world. It’s a dwindling resource – we’re slowly running out because being radioactive, it burns itself up. The half-life is 700 million years, so don’t hold your breath.

For use in a reactor, uranium is concentrated (enriched) to a mixture of 20% U235. Continuing this enrichment to an 85% concentration of U235 would give you weapons-grade uranium.

Advanced Test Reactor core, Idaho National Laboratoryphoto © 2009 Argonne National Laboratory | more info (via: Wylio)

U235 is radioactive or unstable. What does that mean? It means it’s continually throwing off little high-energy particles (neutrons). It just sits there and cooks. Some of these particles hit other U235 atoms and more neutrons are released. Picture a pool table. Line up the balls so one hits two, and each of these hits two more. That is called a chain reaction. The other name is nuclear fission. As you can guess, it increases rather quickly – exponentially, the physicists would tell you.

U235 makes reactors go. It doesn’t need to be lit with a match or a spark plug. It’s constantly throwing off neutrons and releasing energy all on its own. The trick is to keep it from going too fast.

For use in a nuclear reactor, U235 is put in hollow tubes called fuel rods. These rods are placed in a reactor so they are close enough to cause a chain reaction. These tubes are surrounded by another material that can be adjusted to capture some of those neutrons to slow down the chain reaction. These are control rods. Simply speaking, if more neutrons hit other U235 atoms, the chain reaction speeds up. If the control rod captures more neutrons, the reaction slows down. U235 wants to release more and more energy, and the control rods slow it down. Conceptually it’s a glorified motorcycle throttle. The thing that makes a nuclear reactor work is controlling that chain reaction.

So we have U235 releasing a lot of heat and energy in the center of the reactor. The reactor coolant, usually water, circulates through the reactor where it absorbs energy and heat and turns to high pressure steam. This steam spins an electrical generator called a turbine. The steam cools back to water and is pumped through the reactor again. So, the water or reactor coolant does two things: it absorbs heat to cool the reactor, and it produces electricity. Pretty nifty.

The nuclear reactor will get too hot if the coolant stops circulating. The U235 can get so hot it melts through the bottom of the nuclear reactor. That is called a “meltdown.” That releases radiation, and that’s bad. The coolant absolutely needs to keep circulating.

Pound for pound, U235 will produce three million times the energy of the same amount of coal.

When the fuel rods have used up most of the U235, they are removed. Old fuel rods are still very radioactive and are stored in water to soak up those flying neutrons. We don’t want a chain reaction in an empty swimming pool. They need to stay completely submerged.

Nuclear reactors are designed with back-up systems for their back-up systems…for their back-up systems. Think many layers of redundancy. They are designed and built to never stop circulating coolant through the reactor. Very smart people spend a great deal of time trying to think up things that can go wrong – and preventing them.

Unfortunately, the Japanese nuclear plant was hit by a combination of natural forces the designers did not plan for. The unthinkable literally happened, and coolant stopped circulating.

So they are spraying water on the reactors with fire trucks, and trying to keep them cool. They are also filling the old fuel-rod pools from a distance. If it appears a bit unscripted and hazardous, it is.

But the result will not be Chernobyl 2, 3 and 4. These reactors are in containment buildings. There will be a local mess, but more so along the lines of the BP oil spill, not a radioactive wasteland.

Take care,

Dr. B

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