Question about nuclear fuel rods ...

[dainthomas]

I always thought it was kind of neat how the fuel isn't very dangerous or very radioactive until it is put in water (your moderator). Then, it essentially "wakes up" and takes a long time to put it back to sleep.

And yes, it can actually happen in nature, too:

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

I think to "wake up" they need to be in the presence of other fuel rods as well as a moderator. Bombarding each other with neutrons is what causes them to heat up and become radioactive.

 

[FuzzyDunlop]

Am I the only one who thinks its ironic that the only that will prevent meltdown is the very thing that has caused it to start?

 

[QuantumPion]

UPS Ground. Man you should see them flip out when the tracking information has it going to the wrong city!

Actually they are shipped by FedEx Custom Critical (the name is just a coincidence D: ) on flatbed trucks in steel containers. Fresh fuel is barely radioactive, and its alpha particles cannot penetrate the cladding.

 

[QuantumPion]

Here is a picture of some workers inspecting new fuel. The white gloves are just to prevent skin oils or dirt getting on the rods. Judging from the picture those look like BWR assemblies but without the can, which is weird.

 

[Mark R]

Are they created on-site?
How are they transported?

Fuel rods didn't just drop from the sky and land in place.
Are they activated or something that I'm missing?

Fixed last sentence:
When a reactor is shut down. They just seal it off ... they don't pump water in there to cool it forever and ever.

Fuel rods are manufactured at a fuel fabrication facility.

Uranium oxide powder (usually 'enriched' uranium - but some reactors e.g. Canadian designs use 'natural' [unenriched] uranium) is compacted and heated in a kiln to form hard ceramic pellets.

These are then loaded into a metal pipe - made of a special corrosion and radiation resistant metal alloy (usually a zirconium containing alloy, such as zircalloy). The pipe is then flushed with an inert gas - usually helium - to make sure there is no oxygen or any other kind of chemical contaminant in side. Finally, end caps are welded on, to seal the fuel into the rod.

The rods, typically about the diameter of your pinky and about 12 feet long, are made into bundles (assemblies) of about 100. It is these that get shipped to the plant.

Uranium fuel is minimally radioactive. The rods are often, or at least used to be, hand made - without posing a significant radiation risk to the assembly staff.

 

[bruceb]

The reactors in Japan as here in the US have the control rods to moderate or stop the nuclear reaction. These rods can be moved by the control room staff or dropped down automatically in the event of a reactor scram as what happened here. This will slow and then stop the reaction. But the core is still very hot, so it still needs cooling water until it gets completely cooled down. Same for the used fuel rods. When they are removed they are still very hot. The storage pool is to cool them down and water also acts as a moderator to keep radiation from the rods down. That is one reason they are spaced a certain distance apart in the fuel storage pool. New fuel rods are perfectly safe until they are placed into a reactor core.