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.

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.

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.

Clear Science sources for technical info on nuclear plants

The Clear Science staff thought our readers might want to check out our favorite sources concerning the nuclear plant accidents in Japan. We made use of these writing today’s post.

  1. The Union of Concerned Scientists Tumblr site: All Things Nuclear
  2. The informative posts on BraveNewClimate
  3. And of course Wikipedia, which we believe is the greatest scientific resource ever created in human history
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.

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.

Atoms can’t fall through each other because their electron clouds repel each other. Push them together as hard as you want, and they will resist. They have force fields around them. Atoms don’t actually ever touch—their force fields do. Those 2 tiny electrons smeared around these atoms do that, even though the empty space inside the atoms is huge.
(BS alert: It’s not actually a force, but this is the easiest way to describe it. Fair warning.)

Atoms can’t fall through each other because their electron clouds repel each other. Push them together as hard as you want, and they will resist. They have force fields around them. Atoms don’t actually ever touch—their force fields do. Those 2 tiny electrons smeared around these atoms do that, even though the empty space inside the atoms is huge.

(BS alert: It’s not actually a force, but this is the easiest way to describe it. Fair warning.)

If the early universe had pretty much just hydrogen and helium, how did the heavier elements get made?
Typical chemical reactions happen when atoms swap or share electrons. However, at very high temperature, reactions can happen that involve the particles in the nucleus of an atom (protons and neutrons). So we call these reactions nuclear reactions.
For example, if two hydrogen atoms combine, and they each have a proton, then the resulting atom can have two protons. Protons determine the element’s identity. You started with H and ended up with He. 

If the early universe had pretty much just hydrogen and helium, how did the heavier elements get made?

Typical chemical reactions happen when atoms swap or share electrons. However, at very high temperature, reactions can happen that involve the particles in the nucleus of an atom (protons and neutrons). So we call these reactions nuclear reactions.

For example, if two hydrogen atoms combine, and they each have a proton, then the resulting atom can have two protons. Protons determine the element’s identity. You started with H and ended up with He