Now both thorium and Uranium-238 can become nuclear fuels by absorbing a neutron.
There's a few steps thorium goes through on this way.
It first absorbs a neutron and becomes Thorium-233 going from 232 to 233.
And then that Thorium-233 will decay over a period of about half an hour into another
element, Protactinium-233, and Protactinium-233 have a half life of about 30 days.
In terms of reactors that's pretty long, and it drives a lot of what I'm going to talk
about with the chemical processing.
But ultimately it will decay to Uranium-233 - so long as it does not absorb a neutron
- and it has a very quality fission.
About 91% of the time it's going to fission rather than absorb.
And that makes U-233 the best fuel in the thermal spectrum, it outperforms everything
else, and it's one of the reasons we really get a kick out of thorium.
The process by which we would use thorium in the reactor involves introducing thorium
into an outer region of the reactor called the blanket.
And in the blanket the thorium would absorb the neutron- It would take that first step
number, remember, 232 to 233?
It's going to absorb a neutron and it's going to begin the process of becoming Uranium-233.
Now, as it takes those steps of decay, turning into other elements, protactinium and then
uranium we can employ a chemical separation to remove those new materials from the blanket,
and then introduce them into the salt that is going to go in the reactor core.
And that's the place where the fission reaction is going to take place.
That's the place where it's going to generate additional energy.
This is the machine that we would like to design, this is the Liquid Fluoride Thorium
Reactor.
It has a reactor vessel made of Hastelloy-N.
We know that we have to protect this material from the difficult environment it's going
to encounter inside the reactor.
And so that's why the overwhelming majority of the interior of the reactor is composed
of graphite structures.
Graphite structures that separate the fuel that flows through these recursive tubes from
the blanket.
And the blanket fluid surround the entire core the reactor.
It's hard to see the boundary between the blanket and the core, but that blanket protects
the metallic structures from the radiation damage.
It protects from neutron flux.
It basically keeps that nuclear reaction bottled up in a region of the reactor where it's not
going to cause nearly the damage to material that would otherwise cause.
For instance, in a one-fluid reactor where you could have fission occurring right up
to the very edge of the metallic structure.
In a two-fluid reactor there's a lot of thorium containing fluid between the edge of the core
and the reactor wall that absorbs neutrons, gammas, and radiation flux and prevent it
from damaging the material.
Because we know that metal does have some severe issues when it's close to the nuclear
reaction.
But once this fuel leaves the reactor structure, fission stops.
And so there's not an appreciable neutron or radiation flux outside the reactor to nearly
to the degree that there is inside the reactor.
So graphite is a very important structural material in this design.
It has two different fluids.
The primary fuel salt is a highly depleted lithium-fluoride beryllium-fluoride and uranium-tetrafluoride.
The blanket fluid is highly depleted lithium, beryllium and thorium tetrafluorides and that's
where that nuclear absorption of neutrons taking place in the formation of new fuel.
The coolant salt is highly depleted lithium beryllium.
Simply call it "Bare FLiBe".
And that coolant salt then is very chemically compatible in the event of this ever an in-leakage
into the fuel or into the blanket.
Because it's essentially the same solvent of which the blanket and the fuel are composed.
This is an overall view of the LFTR facility.
There's the reactor vessel.
Drain tank.
Pump.
Primary heat exchanger.
This is the gas heater, it heats a carbon dioxide.There's the carbon dioxide gas turbine.
These are chemical processing facilities for the fuel salt and the blanket salt.
And then these are off-gas processing facilities for the xenon and krypton to come out of the
fuel salt during operation.
If you're not a chemist this next part may be hard to follow.
It isn't important to understand every step of the process, only that you consider how
much more challenging these steps would be if the fuel was in solid, rather than liquid
form.
Here's the reactor.
Got a lot of graphite and the core.
The green fluid is the fuel salt.
So this is the material that's undergoing nuclear fission.
The uranium in this is undergoing nuclear fission and generating energy.
As you can see, the two regions here.
There's the fuel salt region and surrounding it in kind of the turquoise blue is the blanket
salt region.
In the blanket salt, protactinium is formed from neutron absorption on thorium in the
blanket.
That blanket salt proceeds to a reductive extraction column where it's contacted with
metallic bismuth that will remove protactinium in any uranium that's present there and return
a cleaned up blanket salt back to the reactor vessel.
That metallic bismuth stream then proceeds through a series of additional reductive extraction
cells and electrolytic cells before ending up in a decay tank.
In the decay tank we give the protactinium time to decay to Uranium-233.
And, actually there's several other protactinium isotopes in there as well- 231 and 232.
232 will also decay to Uranium-232.
Uranium-232 is still present in the decay tank here because of its formation in the
blanket.
So protactinium goes into the decay salt.
Decayed uranium comes out.
And this is also where we add thorium-tetrafluoride as a makeup material.
This is where thorium actually enters the chemical processing system.
As uranium begins to grow in, in the decay salt, it is removed through fluorination and
then it's added to a stream.
And, let me pick up from the fuel-salt's perspective- Fuel salt is taken out and it's fluorinated
also to remove uranium and other gaseous hexafluoride products.
Those two streams are joined at that point.
The remaining fuel salt, now stripped of its uranium, goes to another reductive extraction
column, where metallic bismuth is used to remove lanthanides and long-lived fission
products.
And then, that stream is returned to a reductive extraction unit where the UF6, the fuel salt
and hydrogen gas is used to reduce UF6 back to UF4, bringing it back into solution and
essentially refueling the fuel salt, and sending it back to the reactor vessel.
The HF that is produced by this reaction goes to an electrolytic cell where it is split
back into H2 which is used again for the reduction, and F2 which is used for the fluorination
steps.
So all of this forms a closed-cycle.
The upshot of the whole thing is you're going to move these new nuclear fuels out of the
blanket into a decay salt.
And the reason for this is that one month period.
It takes a month for Protactinium-233 to decay to Uranium-233.
You want this to happen outside of the reactor.
And the reason you want that to do it, is because it has a propensity to absorb a neutron
inside the reactor, if you leave it there.
You do not want your protactinium to absorb a neutron, become Protactinium-234, which
then decays to U-234, which is not a fuel.
But where do all the fission products go?
They come out here, as a stream.
Stream 54.
And if we've done this right there is no actinides in there.
This is kind of like a kidney for the nuclear reactor.
You know if you imagine that these fluids are like blood- Your body does very, very
complicated chemical processes all the time in order to keep you alive.
It's changing the pH of your blood.
It's adding glucose.
It's taking out waste product.
If you use thorium with this kind of efficiency, something really amazing becomes possible.
Every cubic meter of the Earth has got a certain amount of uranium and thorium in it.
About two cubic centimeters of thorium, and half a cubic centimeter of uranium.
If you can use thorium to the kind of efficiencies that we're talking about today, this has the
energy equivalent of more than 30 cubic meters of the finest crude oil or anthracite coal.
This is like taking any worthless piece of dirt, anywhere in the world, and turning it
into multiples of the finest chemical energy resources we have.
I mean that's absolutely amazing.
That's something that- that just completely changes our paradigm about relative national
wealth and resources and so forth.
That means worthless pieces of dirt become potential energy mines.
Now, good news is we don't have to mine average continental crust for thorium.
There's lots of places where nature has already pre-concentrated thorium in much greater concentrations
than this.
Thorium is so common in the earth's crust, that an average American's yearly energy demand-
including industry, and transportation- could be met by a half barrel of everyday rock.
But the key is to very efficiently convert thorium into energy.