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LFTRs

Paleophyte
Posts: 57
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1/27/2015 1:38:08 PM
Posted: 1 year ago
Here we are, 15 years into the 21st century and still plodding along with an aging fleet of second and third generation nuclear reactors.

So what does everybody think of LFTRs?

http://en.wikipedia.org...

They appear to have a lot of up sides including the potential to generate a ferocious amount of power from what will essentially be bubbling radioactive lava.
philochristos
Posts: 2,614
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1/27/2015 5:28:27 PM
Posted: 1 year ago
I don't know enough about them to say, but I can say why light water reactors have been preferred for so long (other than their simplicity). It's because of how they work.

Uranium 235 absorbs a thermal neutron, becomes Uranium 236, which is unstable and fissions, producing heat which is carried by water (the coolant) to steam generators to produce steam.

Whenever a fission occurs, the neutrons are fast, and they have to be slowed down so they can be absorbed by the next Uranium 235 atom to continue the cycle. In a steady state reaction, there is an average of 1 neutron per fission that survives to be absorbed by another U-235 atom. The rest of them leak or are absorbed somewhere else.

The hydrogen atoms in water are what slow the neutrons down. They are more efficient than other means because a hydrogen atom only has one proton which is close to the mass of a neutron.

The same water that is used to slow down the fast neutrons is also used to transfer the heat from the reactor to the steam generators. When water heats up, it expands, making it less dense. The less dense it is, the less efficiently it can slow down fast neutrons.

This makes light water reactors inherently stable. If, for whatever reason, the reactor began over-heating, that would result in fewer thermal neutrons, and fewer thermal neutrons would result in fewer fission reactions, and fewer fission reactions would result in the reactor cooling down.

Light water reactors have been in use so long because they're cheaper to build than other kinds, easier to control, and they're safer.

If it turns out that LFTR's have some inherent ability to be stable the same way light water reactors do, then maybe they're a good idea. Or, if it turns out that the fuel is cheaper, that might also be a factor. But I would think safety would be the biggest factor.
"Not to know of what things one should demand demonstration, and of what one should not, argues want of education." ~Aristotle

"It is the mark of an educated mind to be able to entertain a thought without accepting it." ~Aristotle
Paleophyte
Posts: 57
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1/28/2015 10:53:05 AM
Posted: 1 year ago
At 1/27/2015 5:28:27 PM, philochristos wrote:

The same water that is used to slow down the fast neutrons is also used to transfer the heat from the reactor to the steam generators. When water heats up, it expands, making it less dense. The less dense it is, the less efficiently it can slow down fast neutrons.

This makes light water reactors inherently stable. If, for whatever reason, the reactor began over-heating, that would result in fewer thermal neutrons, and fewer thermal neutrons would result in fewer fission reactions, and fewer fission reactions would result in the reactor cooling down.

Part of the story but not the whole story. LWRs have reasonable thermal stability so long as you can avoid boiling. The problem arises from the fact that not only does the water act as a neutron moderator but also as neutron absorber. When water boils its density drops by several orders of magnitude leading to fewer neutrons absorbed. Now, on average, each fission is producing more than one neutron that produces another fission and your reactor has gone supercritical. This can lead to a runaway whereby boiling leads to higher power output which leads to more boiling... That's more or less what happened to Chernobyl in its last moments.

LWRs also suffer from rather low efficiency since you can't run the core much above 400 C. Running anywhere above 100 C obviously requires a large and dangerous pressure vessel that has to enclose the entire core to prevent boiling.

Light water reactors have been in use so long because they're cheaper to build than other kinds, easier to control, and they're safer.

Not really. LWRs have been around so long because they were what we understood early on and they made lots of plutonium for the cold war. LWRs have serious safety, control and proliferation issues and the construction cost is high compared to more modern designs.

If it turns out that LFTR's have some inherent ability to be stable the same way light water reactors do, then maybe they're a good idea.

LFTRs have thermal regulation both through decreased fuel density with increasing temperature and increased neutron absorption with increasing temperature. The liquid fuel doesn't boil until about 1200 C so the reactors can be run much more efficiently at higher temperatures without the need for a pressure vessel.

Or, if it turns out that the fuel is cheaper, that might also be a factor. But I would think safety would be the biggest factor.

We have enough thorium already mined and on the surface to supply the world's power demands for the next millennium. It's a byproduct of rare-earth element mining and currently just hangs out in waste piles. Fuel fabrication is dirt cheap as all it needs is thorium fluoride. No elaborate fuel rod fabrication.
philochristos
Posts: 2,614
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1/28/2015 12:41:51 PM
Posted: 1 year ago
It sounds like LFTRs are the way to go. Why do you think they haven't been used more than light water reactors?
"Not to know of what things one should demand demonstration, and of what one should not, argues want of education." ~Aristotle

"It is the mark of an educated mind to be able to entertain a thought without accepting it." ~Aristotle
Paleophyte
Posts: 57
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1/31/2015 4:35:37 PM
Posted: 1 year ago
At 1/28/2015 12:41:51 PM, philochristos wrote:
It sounds like LFTRs are the way to go. Why do you think they haven't been used more than light water reactors?

Initially the LWRs were used because that was how we knew how to generate power. Connect heat source to boiler, boiler to turbine. Works well for coal but it's garbage for nuclear. The energy released is so much greater that you really need different materials for heat transfer.

LFTRs also didn't take off because the cold war demanded plutonium and LFTRs only make a tiny bit of that, most of which is worse than useless for bombs, and can be built to consume what plutonium they do produce.

Now nobody wants a nuclear power plant in their back yard. They're content with the 40 year old aging LWRs slowly ticking toward obsolescence but god help you if you suggest building a new reactor.
omanjoka
Posts: 37
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3/12/2015 2:52:48 AM
Posted: 1 year ago
At 1/27/2015 1:38:08 PM, Paleophyte wrote:
Here we are, 15 years into the 21st century and still plodding along with an aging fleet of second and third generation nuclear reactors.

So what does everybody think of LFTRs?

http://en.wikipedia.org...

They appear to have a lot of up sides including the potential to generate a ferocious amount of power from what will essentially be bubbling radioactive lava.

Well, we made one before, let's make another and keep it this time.
Is God willing to prevent evil, but not able? Then he is not omnipotent.
Is he able, but not willing? Then he is malevolent.
Is he both able and willing? Then whence cometh evil?
Is he neither able nor willing? Then why call him God?"
-Epicurus.