Clear Science!

You may wonder, even though the Fukushima reactors were immediately shut down during the Sendai earthquake, why are they having a possible meltdown several days later? The figure shown above is from the website AllThingsNuclear. This plot shows the gallons per minute of cooling water boiled by a reactor of the Fukushimi type as a function of time since shutdown.

Even after control rods are inserted and a reactor is shut down, neutrons will still be produced for some time in the fuel rods. Over time the reaction will slow down and stop, but it is not instant on/off. Engineers use fairly complicated mathematics called control systems to regulate things like this that have time lags between a control input (“turn off”) and a system response (it actually turning off).

Even a week after shutdown, the reactor needs to boil off 60 gallons of cooling water per minute to stay at a steady temperature. This is all planned for. The problem in Japan is that the fossil fuel generators meant to keep cooling water flowing in an emergency failed.

Let’s consider a nuclear reaction with uranium-235 as the fuel. Inside the fuel rods, a neutron with the appropriate energy collides with a uranium-235 atom and is incorporated into this atom’s nucleus. The uranium atom now has an extra neutron and becomes uranium-236. However, uranium-236 is unstable and immediately decays to two smaller atoms—the fission products. Many different fission products are made, such as cesium-133, iodine-135, etc. Wikipedia has a nice entry explaining the fission product yield for uranium-235.

Breaking atomic bonds also releases energy in the form of heat. The purpose of a nuclear power plant is to capture this heat and turn it into electricity. This is analogous to a fossil fuel power plant, where chemical bonds are broken to release heat.

When the uranium-236 decays, extra neutrons (and some other things) are also released. These are called prompt neutrons because they come directly from the fission reaction. (They’re produced promptly.) These neutrons collide with more uranium-235 and the reaction continues. Fission products can also sit around for a while and then decay to produce neutrons, and these are called delayed neutrons. If neutrons are being produced, the fission reaction will continue, and the rate of reaction will be a function of the number of neutrons being produced.

In light of the emergencies at the Fukushima I and II power plants in Japan, we’re going to talk about nuclear power plants for a while. Nuclear power is not super-complicated: there is nuclear fuel, for example either UOX or MOX pellets (uranium oxide or mixed uranium and plutonium oxides in this case), which are packed into zircaloy ceramic rods. The job of the rods is to get hot. This is just like in a fossil fuel power plant, when it’s the coal or oil’s job to get hot.

The fuel rods are surrounded by water, which gets hot, boils, and carries the heat away. In the picture above, the red line is the heat generation profile in the rods: they generate the most heat at their center. Think of the heat as a thing that is generated there, diffuses through the solid rod to the outside, transfers to the cooling water, and exits with the water as it boils and becomes steam.

If there is an interruption in this heat transfer path, then heat will begin to build up. Think of it as a heat traffic jam. If the heat begins to collect in one spot, the temperature there will rise. The zircaloy and nuclear fuel pellets are ceramics and have extremely high melting points. However, if you get a big enough heat traffic jam, they will eventually melt. That’s called a meltdown.

We are going to talk about energy for a while. Humanity uses energy a few ways. Sometimes we make electricity first and then power things with that. Other times we burn fuel directly to power an engine or a heater. But let’s step back and ask: where does energy come from in the first place?

Shown above are several primary generation methods for making electricity. What’s actually happening is that we are taking energy from somewhere else and converting it to a more useful form.

Fossil fuels like coal or oil have energy stored in the chemical bonds inside them. When we use fossil fuels, we are claiming that energy. Nuclear generation is similar, but it’s from the atomic bonds within the nuclei of the fuel materials.

If a material is high in a gravitational field it also has energy, and that is what we’re claiming with hydroelectric generation. Water up high is being lowered, and we get the energy liberated by lowering it. Wind generation claims the kinetic energy from blowing wind. Solar generation claims the radiation energy emitted by the sun.