We're in a very near-term focused effort to work towards
the first power-generating molten-salt reactor and I
bring up power-generating because obviously with the Molten-Salt Reactor Experiment
we didn't get to this stage, and as we've been working on this for a
long time now, we realize there were many issues that needed to be confronted in
order to to to reach this, and I want to go through
how this affects materials, salts and other choices in the reactor design.
We're compelled to to look at alternatives to today's power generation
because of uncertainties both in terrestrial
production carbon emissions and as well as the existing nuclear technologies,
and we believe strongly that molten-salt reactors are a very, very compelling
alternative to the situation we find ourselves in today
with nuclear power. The heart of the value proposition
for molten salt is the fact that this is the only coolant choice that can
operate both at, really quite high temperatures as well as at low pressures
and and has a a tremendous advantage there
because of this, and you have to figure out, what are you
going to make the salts out of? And if you look at materials that have
low-absorption cross sections, you realize there's not a lot of choices out there...
the field narrows pretty quickly when you're
trying to do something neutronically efficient.
Materials like fluorine and-- I'll zoom in a little more on this---
fluorine has a low absorption cross-section; it's mono-isotopic
and lithium is a problem all by itself but if you can highly deplete lithium in
the lithium-6 isotope you get a really
attractive material as well as beryllium. Carbon is an attractive moderator, and
so it's not hard to see why the the list of materials that you would want to make
a molten salt out of is not tremendously long. There are a few
other alternatives but, as we'll go through,
taking many of those materials and then looking at them
from a melting temperature perspective. Most of these pure salts
melt at just much too high a temperature to be used as is.
Even materials like lithium, lithium fluoride are just much too
high temperature, but as we put various salt combinations
together we see this attractive feature that we
really like, which is we can push melting temperatures down.
And the reason that's so important is there's really a region of temperature
we need. It has to do with some material
constraints that I'm going to cover here in a moment, and it also has to do with
the need that our power conversion system---
whatever it is-- is going to have a fairly decent range of delta-T.
So we are going to have to be above the melting temperature by a margin,
have that that delta-T margin for the
power conversion system, and still be below material constraints.
And as you can see on this chart of various combinations with lithium fluoride,
the combination that uses beryllium,
lithium fluoride-beryllium fluoride, can really reach some
low melt temperatures at reasonable amounts of beryllium inclusion.
So it's not a a great surprise that lithium fluoride-
beryllium fluoride, which is commonly called FLiBe
(obviously we named our company after this) is a really attractive material here.
Zooming in on this graph a little more you see that melt
temperature for a two-thirds/one-third
combination of lithium fluoride- beryllium fluoride is around 456C,
so that gives us 50C of melt margin as well as 150C of delta-T
that we're gonna need for our power conversion system, while still
staying below the 700C threshold that I'll talk about just a moment.
There are some other mixtures that also get below there,
you can see with with lithium fluoride-
uranium fluoride, there's a potential mixture there, but I'm going to cover an
issue with that in just a moment. And then lithium---a three-to-one mixture
of lithium and thorium--- has a lower melting temperature but not
as much range as you would want, and I'll show you why that's not going to be a problem
in a few more slides here.
So these molten salts are very chemically stable in air and water. We like that a great deal.
We like their large liquid range.
It's not the salts that give us a problem,
it's the other materials and corrosions that that take place.
The salt, particularly FLiBe (lithium fluoride- beryllium fluoride), has an exceptionally
high volumetric heat capacity, even surpassing that of water.
And this volumetric heat capacity is really your fundamental yardstick
of how big your reactor is going to be, how you move that thermal energy
out from where it's generated in the core to the power conversion system.
So FLiBe just keeps showing up over and over again as a real winner in here.
But wait there's more! You can add nuclear fuel to FLiBe, obviously as was
done in the Molten-Salt Reactor Experiment, and it has good solubility
for that. So here's a picture of FLiBe as a fuel. Now why do we want to
use a molten salt as a fuel? Because we can eliminate an entire step
of heat transfer by depositing the energy of fission directly
in the coolant, that can then carry that
energy out of the core, to the power conversion system.
It's a remarkable simplification, and it also gives the reactor
tremendous response against transients and changes in power demand...
...makes the reactor potentially very, very responsive to changes in power demand.
There is an issue though, as we add uranium fuel
to the FLiBe salt, and that's that there is a corrosion reaction that happens
with chromium in the alloy. And this reaction will take place
initially in the surface layer, but it will
it will essentially stop there, unless the alloy is at too high a temperature.
and then chromium, it migrates from internally in the alloy structure to the
surface, and the reaction continues. So that's another reason
we want to keep operation temperature below 700C
with some margin, because of this chrome reaction. Now can we eliminate chrome
from our structural alloy? It doesn't appear that we're going to be
able to anytime soon, so in the near term, our best bet is to stick to those
thermal margins, and to operate within them.
And again, as you can see, FLiBe, with a little bit of uranium,
has the thermal margin needed to operate in this attractive region
without running into especially significant material constraints.
This is how MSRE was able to operate successfully for so long,
because they respected a lot of these constraints.
Now MSRE did have a much larger---I'm sorry---much lower
delta-T across their core. They were about 30C of delta-T.
We are looking at about 150C of delta-T across the core for our design, and I'll
tell you why in just a moment. When you look at the moderator space,
there have been many, many people getting excited about some
new moderator, and we would love for there to be a better
moderator option out there than graphite, but what we see over and over again
is that when you look at hydrogenous moderators,
it's just it's very very easy for them to lose their their hydrogen at the
high operating temperatures of an MSR. Obviously we use zirconium hydride in
TRIGA reactors but they don't run at high temperature.
Beryllium oxide was used in the first molten salt reactor, but it's not stable
in the salt---it will react---it will become beryllium fluoride, so you have to
clad it, and that's a problem too, and that's exactly what they did in the
Aircraft Reactor Experiment: they ran in Inconel channels.
Graphite is the only moderator that we have that we can run
unclad in the reactor at temperature. In fact graphite has just tremendous
performance at temperature. So we foresee a reactor that is going to
be built of prismatic graphite structures that are going to occupy
the majority of the core. And this is going to be able
to enable neutronically efficient designs. Why? Because we've eliminated
the need for a clad or a barrier. If you have to have a clad,
if you have to have something that goes between your fuel and your moderator,
then that's going to present problems
because it's going to be a loss term. You're going to lose neutrons
going each way, fast neutrons going out, slow neutrons going in,
as well as the damage to the material. And these metallic alloys have a
real problem with that. So we just don't see
metallic structures in the core--- in the neutronically active core region---
as being a viable thing for future molten-salt reactors
and especially not for near-term molten salt reactors.
Now graphite's not perfect...graphite undergoes dimensional changes,
and this is something that is a strong function with temperature.
Nevertheless, we are advancing a
configuration for our reactor: we would actually have
two reactor vessels, only one of which would be active at any given time.
And as the first reactor finishes a number of years of operation,
we would then pump the fuel over to the second reactor,
and continue operation while the first reactor core is able to cool down
and eventually have graphite reshuffled and replaced.
So in this in this manner, we believe we can work around the graphite issue,
and accommodate the peculiarities of this material.
Another important thing about the reactor design is that
we want to have two regions in the reactor:
an area where it is neutronically active, we call the core,
and a neutronically-absorptive region called the blanket, and this is important
because we need to protect the structural alloy from neutrons.
And the reason why, is that there is a reaction that happens in nickel alloys.
Now nickel is just a remarkable material, and nickel was the basis for
what they originally called INOR-8, we now know it as Hastelloy-N, or Alloy N.
and obviously we built things like this primary heat exchanger
for the MSRE out of Hastelloy-N. We were able to
build all of the primary structures from the Molten-Salt Reactor Experiment
out of Alloy N, but under neutronic flux,
you are going to have an embrittlement of this material
and that's because nickel will absorb neutrons:
it will go from 58 to 59, and then it will undergo an n-alpha reaction to iron.
And it's that...ironically it's that "alpha" that's the problem.
That helium accumulates at grain boundaries
and it degrades the mechanical structure of the material.
So you really want to keep neutrons out of your nickel.
Now there's a place where you don't have thick walls in your materials, like
you do in the reactor vessel, and that's in the thin walls of the
primary heat exchanger, but one of the beautiful things about molten salt, is if
you design your reactor properly, you can have the neutron flux in the
primary heat exchanger be just a tiny fraction of what it is in the core--
-essentially just residual, delayed neutrons---
and that's going to enable you to operate those heat
exchanger structures much longer because you've reduced the neutronic
flux it will be in there. So, takeaways from this: you want to go with FLiBe salt,
which is a proven salt, proven from operation in the MSRE,
you want to go with Hastelloy-N, and you want to go with graphite moderation.
These are three materials choices that enable a
molten-salt reactor to operate in a state of chemical equilibrium, and just
to contrast this---I mean---this is something that we don't have even with
today's light water reactors! The water is not in chemical equilibrium
with the zirconium clad, and the zirconium clad is not in
chemical equilibrium with the uranium dioxide pellet.
So the fact that we were able to build reactors over 50 years ago,
with materials in chemical equilibrium, is just absolutely remarkable.
And I think this is something we should be talking more and more about,
as a contrast with the way we do things today.
It gives us a lot of confidence to go forward, in the near term,
with safe and efficient molten-salt reactors, built around these 3 materials.
Now, it's not to say that FLiBe salt
has no problems (FLiBe salt does have problems) and one of them is
the inevitable formation of tritium, as was as was mentioned earlier.
And even if we could totally eliminate lithium-6 from the FLiBe salt,
there's still pathways through lithium-7 and beryllium
for some tritium to get made. So what do we do about tritium?
Well, we've heard information earlier today about approaches
that can be taken to mitigate tritium,
but our primary approach to mitigating tritium is to use
the supercritical carbon dioxide power conversion system
as a place that, in a worst-case scenario, even if the tritium gets all the way
through the primary heat exchanger, all the way through the gas heater, it's
going to get captured by the oxygen in that CO2, and can then be removed
as a vapor (as tritiated water) in the low temperature operation
of the the gas cooler region of the recompression cycle.
Just to put this in contrast, if you have a steam turbine,
then you want to make sure that tritium cannot get into that steam, because if it
gets in that steam, you are not going to be able to get it out.
So reactors (MSRs) that want to use steam turbines have to have some sort of
salt between them and the steam turbine; something that they can have great
confidence is going to capture the tritium. We're assuming that our CO2
cycle is going to be where tritium kind of goes to "get caught"
once and for all. The other wonderful thing about the
supercritical carbon dioxide gas turbine coupled with the molten-salt reactor,
is that it achieves very good efficiencies, on the order of the mid 40s in percent,
and this is because of the higher operating temperature
and the very compact turbomachinery
that the supercritical CO2 allows.
So we're very excited about this technical approach,
but as much as we like the efficiency, fundamentally it's about making sure
that we can capture tritium and go forward in the near term.
About 10 years ago the Department of Energy released this report
on their R&D roadmap, and they made it clear that it is the responsibility of industry
in the United States, to design, construct, and operate commercial nuclear power plants.
I bring this up because this was a big reason why Flibe Energy was started in 2011.
Only in the last 10 years or so has there been an opportunity
for companies to make their arguments
for what new nuclear should be, and what business plan
should be built around that, and what the product should be,
and some of these are very different than the classical nuclear
designs and scenarios that have been put forward in the past.
An analogy I like to make with people is the old pressurized water reactors we
have are a lot like a landline phone; it's really, really good and reliable at one thing,
and some of these newer reactors,
like NuScale (small light-water SMRs) are kind of like a flip phone.
They're they're nifty and neat and they
do that one thing in a new and kind of exciting way
but what we're after is analogous to a smartphone.
We want to not just do electricity, but we want to do many things: we want to
produce heat, water, medicine, and many more products
through the harnessing of all of the parts of the fission reaction.
And this is what we feel the new latitude and opportunities
that are being presented by the Department of Energy allow,
and so we're very very excited about this.
Ultimately our goal is to supply the world with energy and water and fuel
that have all of the attributes the public expects and demands.
We believe that through this molten salt technology we're going to be able
to get there, and we're very excited. Thank you very much!
From a kind of fuel cycle perspective, and you've been a big advocate for a
number of advanced fuel cycles, you've kind of been the leader of that
for a good number of years,
what's your biggest challenge you're seeing?
from a fuel cycle solution perspective, for either for your reactor,
or any of these advanced molten salt reactors that we should all take away?
One of the things that animates me, is to use nuclear fuel very efficiently,
and to absolutely minimize the production of nuclear waste,
does not get much appreciation or support in the nuclear community in general.
and it really kind of baffles me,
because almost any time I'm talking to regular people, at any level,
you know, non-technical, technical, leadership, etc,
they always bring up the issue of nuclear waste,
and how much they would like to see it addressed.
I describe our technological approach, and what we're doing,
and why I think it's going to be so compelling,
but it really baffles me that this is not something that is weighed
and taken more into account as we're making choices as a nation,
in the world, and in new reactor technologies.
No, that's that's a good point. Why go with a proven salt,
if you then go with an unproven power conversion cycle in nuclear?
Yeah, the sCO2 is unproven, but fundamentally it's about tritium capture.
We need to be absolutely sure that we are going to capture that tritium
in a very simple and defensible way.
There is a lot of work going on in sCO2 from many different sources
(most of it not even in nuclear at all)
but that's a valid question, a valid concern, and I think
the simple answer is tritium. Tritium is why we do it.
What are the main material (or element) constraints
to using FLiBe in large quantities, as would be required for wide deployment of LFTRs?
That's very simple: highly-depleted lithium.
Highly-depleted lithium is the limiting constraint
on making large amounts of FLiBe salt.
Yeah, yes it's interesting, so there's a really interesting
fuel-cycle-supply-chain kind of questions coming in there,
so um, something we need to be aware of, we like that you know the high assay LEU
and so on is this it's a key component, the depleted lithium inside.
Yeah, without highly-depleted lithium
we simply won't be able to do these things.
Yeah, excellent, well thank you Kirk, as always, excellent talk.