How do Neutrons and U235 Nuclei get out of the fuel rod

[Pete123]

I understand that there are fuel rods, water control rods. I also understand that fuel rods are metal tubes filled with special U235 pellets. Finally, I understand that water slows the neutrons down enough to collide.

How do the neutrons and U235 nuclei get out of the metal encased fuel rod into the water? Why is the interior of the fuel rod so much hotter than the surrounding water?

Thansk!

 

[Janus]

You seem to be talking about a heavy water reactor. This is a reactor that uses heavy water as the moderator so that you can use non-enriched or lightly enriched uranium for the fuel. The heavy water moderator and the control rods are two separate things. The moderator slows fast neutrons down so that there is enough of them to maintain a chain reaction with the smaller percentage of U235 found in non-enriched uranium. The control rods are made of different materials( such as boron or cadmium) that absorb neutrons. Thus by lowering or raising them you can control the rate of the reaction. This allows you to shut the reaction down entirely, or maintain a constant reaction rate by raising the rods as the fuel becomes more and more depleted.
You use heavy water in this type of reactor because it slows the neutrons, but doesn't absorb them as well as light water does.
With a light water reactor, you need to use enriched uranium which has had its relative percentage of U-235 increased so that the number of neutrons produced is enough to compensate for those lost to absorption by the water. Again you will have control rods made of different neutron absorbing materials that can be lowered or raised to control the reaction rate.
The neutrons get out of the fuel rod because the casing is made from a material chosen not to react with them. the U235 nuclei don't leave the fuel rod. They just interact with the neutrons passing through the fuel rods.

The fuel rod is the hotter because that is where the energy creating fission occurs as the U235 nuclei are split by the neutrons.

 

[Pete123]

Janus, Thank you for responding.

I didn't mean to ask about heavy water reactor, though don't know enough about reactors to ask the correct question. This question started from reading about Chernobyl, which I just learned was graphite moderated. I think your response answered what I was asking and would like to see if that is the case.

So, in Chernobyl, there were fuel rods which were, enriched U235 pellets encased in metal. They are enriched, but not to weapons grade. As a non-scientist, I view the metal casing as a solid that would block the escape of anything.

I think that your answer explained that the neutrons can escape somewhat like a small fish escaping through a net meant for bigger fish. They would then bounce off the graphite wall, which would slow them. They can then pass back through the wall of the fuel rod, collide with a nucleus and release heat. In this case, it sounds like the purpose of the water is to become heated from the fission reactions, followed by leaving the core so steam can be released.

Is this correct?

 

[Janus]

With the Chernobyl graphite reactor design, the graphite was used in the same way heavy water is; to slow neutrons so that non-enriched natural uranium can be used for fuel. The water serves two purposes, it carries away energy that is used to run the turbines for power production while it carries away heat to keep the reactor from over heating. With the graphite moderator, the danger is that if the graphite gets too hot it can catch on fire.

 

[jbriggs444]

They would then bounce off the graphite wall, which would slow them. They can then pass back through the wall of the fuel rod, collide with a nucleus and release heat.

This is not an accurate picture of what goes on. The neutrons do not "bounce" off the graphite. They penetrate through it and are slowed by it. The neutrons are not the primary way in which the fuel rods heat up (though they do contribute). A quick trip to wikipedia says that most of the fission energy ends up in the two daughter nuclei which separate at very high speed. Being charged particles they do not get far. The neutrons coming from the fission event are not affected by electromagnetic forces and are able to escape.