nuclear physics question

[rdking647]

I have a question about how nuke plants operate,especially for any physicists here
its been a long time since i had physics in college so even if I knew the answer I forgot it.

I know nukes plants are usually powered by uranium oxide or something similar.
I know the half life of u-238 is a long time and even u-235 is millions of years.

so why do nuke plants need to be refueled and why do they create nuclear waste. given that the uranium slowly decays into assorted byproducts,why does the fuel "run out" it seems to me that given a chunk of fuel say 100 pounds worth,after 50 years it would still be mostly the same with just small amounts of decay products. It would still be a heat source to generate electricity.
what am

[AngryAmish]

1. not really an expert but here goes

Uranium in a reaction is not decaying, it is so close together there is a critical mass that allows the neutrons to create the chain reaction. Many orders of magnitude faster than decay.

[FBaggins]

2. It's the difference between "decay" and "fission".

It would take a really long time to decay to the point where the activity level declined substantially if you were just talking about the uranium, but remember that there's a fission reaction going on. A significantly higher number of atoms are splitting because of impacts.

Add to that the fact it isn't just uranium in there. When a uranium atom splits, you end up with a different element. Some of those are much more active than just uranium, but when they decay, you're eventually left with elements that start to hinder the reaction.

[nebenaube]

3. It drops to u-235 and then the reaction slows down.

No longer efficient to use because it won't react. Then we make armor piercing rounds out if and shed it elsewhere.

[FBaggins]

4. You're on the right track, but U-235 is actually MORE active.

It's the daughter elements that further down that slow things down.

[nebenaube]

13. Thanks

It didn't take long for me to realize there are safer things to study and work with... Can't retain it all.

[Teaser]

5. not decay

this is more like what's going on, using mousetraps as the potential energy well holding onto nuclear neutrons:

http://www.youtube.com/watch?v=Pmy5fivI_4U


neutron hits nuclei, splits it, spits out some particles, which hit other nuclei and split them and so on...

eventually you've released all the potential energy, and no more reaction. Need new uranium (loaded traps).

[Statistical]

7. Not exactly there is still billions of MW of potential energy in so called "spent fuel". See below.

[Teaser]

10. just trying to give a mental picture

There's pretty much always potential energy energy one can scavenge down to the thermodynamic efficiency of the system. But I think the OP just wanted a simple picture of what's going on.

[Statistical]

11. Even in the simple picture less than 5% of fuel is "used up" in spent fuel.

In your example above 95% of the mousetraps have never be touched in "spent" fuel.

1 ton of nuclear fuel has about 1 million Megawatt hours (MWt) of energy.
We convert about 5% of that into heat and the other 950,000 MWh remains in the untouched fuel.

The reason fuel needs to be swapped out is not because it is used up, some of it is but only a tiny fraction. It is because it is "contaminated" with neutron poisons. The car equivalent would be like filling your car, and when the fuel guage drops to 95%, pouring all the fuel out of the tank into cans, and then filling the car up again.

There are some experimental reactor designs (molten salt) that would continually scrub nuetron poisons from the fuel. The same fuel would instead last decades as a result.

[Teaser]

12. Yes.

I'm aware. But thanks for the precision.

[Statistical]

6. Spent fuel is still 96%+ U-235 & U-238.

The problem is some of the fission products are neutron poisons. In a reactor neutrons strike Uranium atoms splitting them into new isotopes and releasing energy and neutrons. Those neutrons strike other Uranium atoms splitting them and a chain reactor is formed.

The key thing is neutrons. You stop a reactor (SCRAM) by absorbing nuetrons. Fission requires the right amount of neutrons, called a neutron economy. Too little nuetrons and you can't sustain a reaction. Too many nuetrons and the reaction will speed up (too much heat).

Some of the fission products of uranium (what uranium splits into) are nuetron absorbers. They steal from the neutron economy.

When a reactor is first loaded it has LOTS of neutrons. Too many in fact so control rods (which have neutron absorbers) are insered significantly to balance the neutron economy. Giving the reactor the right rate of fission for the desired output. As the reaction goes on there are less and less "spare" neutrons so the control rods are used less and less. Finallly after about 3 years the neutron absorbers build up so much that maintaining criticality becomes difficult so the fuel is swapped out and process starts over.

Now the "spent fuel" is probably not an accurate name. Better term is contaminated fuel. The fuel is contaminated with isotopes that steal neutrons. In reprocessing the 5% that is non U-235/U-238 is removed and the remaining U-235/U-238 blend (95% of the spent fuel) is formed into new fuel rods.

[rdking647]

8. thats what i was missing

the neutron absorbers in the remaining fuel..

[Statistical]

9. Yup that is the major difference between "new fuel" and "spent fuel".

[rdking647]

14. anyone know what the neutron absorbers are?

[somone]

15. Boron, cadmium, solid graphite, regular water, heavy water, ...

Graphite has a bad side: It's flammable (like in Chernobyl)

[Statistical]

16. The ones in reactor fuel? Or the ones use to intentionally absorb neutrons (control poisons)?

The fission products in fuel that require it to to be changed are:
Xenon-135
samarium-149
gadolinium-157

These block a significant number of neutrons and eventually prevent criticality. Reactor fuel is changer "early" before the buildup makes it difficult to sustain a reaction.

These fission products also absorb nuetrons not enough to prevent criticality but they change the power charecteristics of the reactor making power control more difficult:
83Kr, 95Mo, 143Nd, 147Pm

More info (and includes control poisons)
http://en.wikipedia.org/wiki/Nuclear_poison

[rdking647]

17. thanks,that was what i was looking for