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I hear a lot about the LFTR, and I wonder what the problem is. Is there a major technical hurdle that prevents this from seeing widespread use?

Sometimes there is, sometimes there isn't. Sometimes a really good idea never gets used because... well... because nobody really picks it up and runs with it. It's that simple sometimes.

But sometimes there's some hidden gotcha.

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The most common 'gotcha' is that you can't make a thorium reactor critical with just thorium. It needs 'boost' neutrons. The most way that was done in the past was a small bit of uranium or plutonium that was critical to provide the neutrons. A more innovative way is to use a small particle accellerator to provide the thorium with neutrons [1].

[1] http://www.physorg.com/news/2011-06-pint-sized-particle-nucl...

It's actually U233 that fissions. The neutrons convert thorium to U233, and when U233 fissions it produces neutrons that convert more thorium. So once you get the reaction started, you don't need an external neutron source.

LFTR proponents figure the simplest approach is to just go ahead and use U233 to seed the reaction. The U.S. has one ton of U233, which it currently plans to dispose of.

Short answer: yes, you do need to maintain an external neutron source for Thorium reactors.

The advantage of Thorium is that you can maintain the reaction subcritical. That means it is much easier to shut down by simply removing the neutron source. In a critical reactor, you have to physically inject control rods containing a neutron sink (usually Halfnium or Boron). These elements can jam or be ejected under certain unlikely conditions (this is one of the things that happened at Chernobyl, and has happened at a handful of military test reactors in the US that no one has ever heard of).

In practice, however, modern reactors are designed to take this type of jamming into account. There is no known failure mechanism where it would become an issue.

FWIW, I was an engineer on a nuclear submarine.

No no no no.. you don't want to do subcritical particle accelerator reactors. Well, you don't want solid fuel reactors, solid fuel has the same cooling issues as standard uranium, since it's the fission product breakdown that caused the Fukushima accidents (not an initial nuclear excursion, like Chernobyl).

Molten salt can be passively cooled (much better thermal conductivity) and can be moved from core to containment by gravity. Also it expands when its hot, which slows the reaction, and it's chemically stable and doesn't crack like uranium fuel pellets.

There are people who advocate subcritical reactors, but that's not how the LFTR works. Fissioning U-233 produces plenty of neutrons to keep a thorium reactor going. See the last couple paragraphs here: http://energyfromthorium.com/2006/04/28/whats-the-difference...

Mainly it's historical reasons. Light-water reactors were developed first, for nuclear submarines. Once they had those working, everybody went with what they knew, and thorium advocates got fired. Then people forgot about thorium.

At this point, NRC regulations assume solid fuel, and the nuclear industry makes most of its money from selling complex fabricated fuel rods.

However, there was a small liquid-fluoride reactor actually working back in the 60s.

The main technical objection I've seen is corrosion from the liquid-salt fuel. Sorenson claims there's an alloy that makes it a non-issue.

The first reactors were actually heavy water for producing plutonium for nuclear weapons. Light water reactors were developed next because they could still produce plutonium, were cheaper, and could be put on a submarine. But their usefulness on subs was secondary. The USSR built Pb moderated reactors for some Alfa class subs that didn't require anyone working in the engine room. For lots of reasons, this is obviously superior as a submarine reactor tech. (See http://en.wikipedia.org/wiki/Alfa_class_submarine)

The real reason we never went to Thorium is the best reason to go to Thorium. It creates U233, which is not usable for nuclear weapons. (Or at least, no nuclear weapon has ever been built with it.) This would have made it great for exporting to countries without worrying about proliferation. That's the reason we're seeing it become discussed so much more often now.

The first reactors were not heavy-water, but graphite moderated (and light-water cooled). Nazi Germany intended to build heavy-water reactors, but they did not succeed.

Pb (actually Pb-Bi eutectic) isn't a moderator, just a coolant. Alfa-class submarines' reactors were fast reactors (no moderator).

I don't know anything about the German bomb program, except that it was pretty unsuccessful. (To be fair, they had people bombing their scientists the whole war.) The heavy water reactors I referred to were from the Manhattan Project. (They did have graphite moderated too, but they weren't as successful I understand.)

You're right that Pb is only the coolant. I got typing too fast.

I believe you've confused something. Almost all plutonium from the Manhattan project (e.g. 'Fat Man') was from graphite-moderated, light-water cooled reactors at Hanford site in Washington, starting with the 'B reactor'.

Plutonium: The First 50 Years (chapter 9)

http://www.doeal.gov/SWEIS/DOEDocuments/004%20DOE-DP-0137%20...

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

The US did not build a heavy-water plutonium production reactor before those at the Savannah River Site (South Carolina), which did not start until 1953. These are all the plutonium production reactors the US has built (ref. DoE): 9 graphite-moderated light-water cooled reactors at Hanford, and 5 heavy-water moderated/heavy-water cooled reactors at Savannah River.

In the Manhattan project, there were also tiny amounts of plutonium sourced from the X-10 reactor at Oak Ridge (also graphite-moderated):

http://en.wikipedia.org/wiki/X-10_Graphite_Reactor

Separately, there was highly-enriched uranium produced at Oak Ridge (Tennessee), which was used in 'Little Boy'.

And, CP-3 was a research heavy-water reactor (Chicago), built in 1944, which was not used for breeding plutonium.

http://en.wikipedia.org/wiki/Chicago_Pile_3

Hope this clears things up...

My understanding is that building the reactors are significantly more expensive. So until the price of Uranium increases enough to justify the total cost of the Thorium reactors, we won't see many of them outside research settings.

The other issue is building a new reactor of any type right now is a giant pain in the ass, which doesn't help either.

In the video he states that the cost would be 30-50% lower because the reactors are very simple. The cost for uranium based fuel is also significantly more expensive (watch the vide). He states that the reason people don't build these is because they don't know about them, or they are stuck in the existing structure of legacy reactor thinking.

The reason that anything new is a pain in the ass is simply because of bureaucracy.

It might be that you could make a thorium reactor much safer than current light-water reactors for cheaper, but the laws regulating our nuclear power were written under the assumption that all reactors were light-water reactors - so in practice you would end up with all the fancy 100% reliable control systems needed to keep the positive feedback loops in a traditional reactor from blowing themselves up as well as the special metallurgy needed by Thorium reactors.

That is the case, and it's a big problem. NRC regulations for example require "fuel integrity," which doesn't exist with liquid fuel.

Sorenson's company plans to market initially to the military, which has need of compact energy sources for remote bases and isn't constrained by the NRC.

CANDU, one of the best uranium designs, has the same problem.

It's sort of like inkjet printers. Either it's cheaper up front and more expensive in the long run, or it costs a fortune up front but the ink is cheap.

To be completely honest, though, one of the biggest obstacles to widespread CANDU implementation in the past was that it isn't a breeder.

Thorium reactors should be quite a bit cheaper. Light-water reactors operate at 160 atmospheres of pressure. They need a very strong reactor vessel, which currently can only be forged by a single facility in Japan. They need a large containment dome because if a pipe breaks, that high-pressure water will flash into steam with a thousand times the volume. They use lots of redundant emergency active cooling mechanisms. Some of them have giant slabs of ice inside the containment dome, which reduce the volume of steam if it's released but have to be constantly refrigerated, to keep them frozen in close proximity to a nuclear reactor core.

LWRs are resupplied every year with expensive fabricated fuel rods, which are non-standard and can only be purchased from the company that sold the reactor. The fuel rods are complicated because they have to withstand a thousand-degree temperature gradient, and the fuel pellets are prone to cracking from the production of xenon gas. Xenon and other reaction products prevent the use of more than one percent or so of the energy potential of the nuclear fuel, another reason the rods are frequently replaced, and the reason we have so much nasty nuclear waste.

The nuclear industry makes most of its revenue from selling those fuel rods.

LFTRs operate at atmospheric pressure. No super-strong steel, no containment dome, no ice. It's a liquid fuel, so no proprietary fuel rods. Xenon just bubbles out of it.

The fuel has a strong "negative coefficient," meaning the reaction slows down as it gets hotter. If it nevertheless gets too hot, a salt plug melts and all the fuel drains into a passive cooling tank. No need for all those active cooling systems.

On top of that, LFTRs operate at higher temperature, so the turbine is more efficient, and the waste heat can be used to desalinate seawater. They don't require water cooling. As a bonus, marketable reaction products can be separated from the liquid fuel (http://flibe-energy.com/products/).

Misguided government regulation is the main problem, but Sorenson's company plans to get around that by selling to the military first.

"Misguided government regulation is the main problem, but Sorenson's company plans to get around that by selling to the military first."

Sorry but is this subtle irony? My apologies if it's not, I don't know anything about this field. It just seems weird that the military wouldn't be subject to the same regulations that the govt. sets? Or in the US are the military able to ignore some of the regulations around this?

The latter, I'm sure. When you are a war economy, the military rules supreme.

The NRC has no authority over nuclear aircraft carriers and submarines. The military's stuff is classified, and it does its own regulation.

> Is there a major technical hurdle ... ?

That's exactly my question, too. So I tried to find some alternative view on that topic. Unfortunately, I only found some criticism of questionable quality ...

http://www.ieer.org/fctsheet/thorium2009factsheet.pdf

... as well as a rebuttal of questionable quality:

http://energyfromthorium.com/2011/03/23/sorensen-rebuttal/

Uranium reactors are children of war, much like hydrogen-bomb-shaped communications satellites. They are a byproduct of A-bomb research. From wikipedia:

1) Weapons-grade fissionable material (233U) is harder to retrieve safely and clandestinely from a thorium reactor

2)Thorium cannot sustain a nuclear chain reaction without priming, so fission stops by default.

The focus with Uranium reactors wasn't to make reaction efficient over time, just violent. What I'm trying to say is the current atomic energy industry is heavily invested in Uranium. No wonder they're resistant to change.

Just like (almost) every spaceship designed is a descendant of the V2 rocket.

War is an incredible catalyst of the discovery process, but it unearths such discoveries just to throw them into a local optimum.

The ratio of plutonium needed to seed and convert thorium into fissionable uranium-233 is very high (0.8:1)

Except for that fact that we already have plenty of U233 sitting around being useless at Oak Ridge. Certainly plenty to do all the research and prototyping needed to get a design certification for a LFTR.

Clearly, the problem is the political will to do something different. If the nuclear lobby and the anti-nuclear lobby are both against it, it's hard to get traction. Kirk is likely correct. The only way to get this through is via the DoD. I wish FLIBE Energy the best.

I just wrote a bunch of comments arguing in its favor but here's an article I just found with various criticisms I hadn't seen before: http://daryanenergyblog.wordpress.com/ca/part-8-msr-lftr/

I'm not sure how credible this guy is, because he talks about thorium-232 as a dangerous nuclear waste, with a terrible 14 billion year half-life. The problems with this are (1) such a long half-life means its hardly radioactive at all, and (2) that's the only isotope of thorium, which means it's the same stuff that exists in large quantities already in natural ore.

Also I'm not sure why he thinks U-232 has to be separated from the U-233.

Aside from that, he raises some interesting objections.

He may be exaggerating the complexity of the reprocessing system. Wikipedia describes it in detail: http://en.wikipedia.org/wiki/Molten_salt_reactor#On-line_rep...

Kinda wish I could delete that comment. Here's a bunch of people on reddit ripping that article apart: http://www.reddit.com/r/energy/comments/ifx40/molten_salt_lf...

The "advantage" that the LFTR doesn't produce fissile material was probably considered a disadvantage by most governments, at least until about a decade ago.

Now that we have decades of experience with light water reactors it simply is a challenge to propose a solution that unproven on a commercial scale. It may look good on paper but it would be a multi-billion dollar project with multi-billion dollar risks. Visionary politicians who would want to take such risks are in short supply.

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