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Using nuclear heat to produce hydrogen is more effective, but HTGRs are not the only option. While molten salt reactors may not be quite hot enough to drive the S-I cycle directly, the difference can be supplied by electric heat. High temperature electrolysis is another option. An attractive aspect of coupling such processes to nuclear heat, is that heat normally wasted in power conversion systems can be used productively, improving economics.

Doing so also enables such reactors to run at 100% around the clock, while adapting to variable demand and converting the difference into hydrogen. Molten salt reactors like LFTR can also produce other products and are nearly 100% fuel efficient, more than offsetting the advantage of HTGRs.

China is actually doing both though, in addition to reactors tailored for district heating. They understand that energy drives the modern world, and their people will reap the benefits as the world willingly concentrates all means of production within their own borders. Meanwhile the west wallows in the delusion that monopolies on ideas will power our "service economies" indefinitely.

Fast molten salt reactors are perfectly capable of consuming U-238, and neatly avoid the fuel shuffling problem, having liquid fuel. There are downsides though, probably the greatest being that they need a much larger inventory of fissile to overcome the cross section issue you mention, which is an expensive up front cost.

You get the best of both worlds with LFTR, which consumes thorium in a thermal spectrum: a minimum fissile inventory, no enrichment needed, and almost no generation of Pu-239. For each GW-year, <1 ton Th in, and <1 ton fission products out, including life-saving medical isotopes and Pu-238 for powering space probes. 85% of this "waste" is stable within 10 years.

It might be profitable, if it were allowed to exist, and could be monopolized. See Targeted Alpha Therapy; a means to treat the worst cancers, which may be applied to HIV and drug-resistant bacteria as well. This has proven very effective in limited trials, but we don't have the material to expand research and treatments. Bi-213 is the best isotope, and produced in quantity as a byproduct of the thorium fuel cycle.

China is now leading our clean energy future. Here is an actual link for that, and related ones:

China spending US$3.3 billion on molten salt nuclear reactors ...
SINAP T-MSR Promotional Video
Why China’s 600 fte MSR program wants to cooperate ...

It should be mentioned that China's efforts stem from Kirk Sorensen's rediscovery and publication of the brilliant MSR work done at ORNL many decades ago, which was foolishly cancelled and lost in obscurity. He has since founded a company to further that work in the US, and there is a good overview of the vision here:

The Flibe Energy LFTR49: the triple ace in nuclear GEN IV design

Faux-environmentalists love to misrepresent "spent fuel" as "nuclear waste", even though >96% of the former it is just unused fuel, with the balance rapidly decaying to stability. Readers should appreciate that nuclear is the only energy source to responsibly manage its waste, and that it is only possible because nuclear produces such a trivial amount of waste to start with. None of the resource-intensive "renewable" branded sources have even been asked to do so.

Many advanced reactors can recycle that "waste" into new fuel, but there is one approach that stands apart from the rest. LFTR49 can consume spent fuel 90 times faster than other approaches, while producing new fuel and incredibly valuable medical isotopes unique to the thorium fuel cycle. It is also the most thorough waste burner, yet has the simplest fuel reprocessing. Using thorium enables the plants to operate with a fraction of the fuel, allowing many more to be built with the given resource, and producing virtually no long-term waste.

Flibe Energy may not offer the lowest hanging fruit among advanced reactor designs, but LFTR is uniquely able to reap the full benefits of the thorium fuel cycle: breeding in the thermal spectrum and simple chemical reprocessing. This allows LFTR to truly close the nuclear fuel cycle and run efficiently and indefinitely on nothing but the thorium byproduct of existing rare-earth mining. The online chemical reprocessing allows extraction of many valuable isotopes, and even the "waste" heat from the plant can drive industrial processes like desalination or synfuel production. Revenue from such byproducts also provides an opportunity to reduce the cost of electricity produced.

For some time, Targeted alpha therapy has shown promise for treating difficult cancers, but it may also be used to kill antibiotic-resistant bacteria and pathogens like HIV. Once this capability is developed, the antibiotic arms race will end once and for all. The looming threat is very serious, and such promising research should be a high priority.

Unfortunately, there are artificial barriers that are retarding progress. The most attractive isotopes for use with TAT are Actinium-225 and Bismuth-213, which no longer exist in nature. Looking at the periodic table, one might be inclined to believe that other substitutes exist, but they simply don’t. The neptunium decay chain is unique in that it does not pass through radon or terminate in lead. Born in supernovae long ago, it was extinct in nature until relatively recently, when it was revived in the heart of nuclear reactors.

However, conventional reactors don’t produce much, and it is impractical to extract the short-lived isotopes from solid fuel rods sealed in a reactor core. Liquid Fluoride Thorium Reactors however, are the ideal machines for producing these life-saving medical isotopes. Meanwhile, LFTR safely transforms nuclear waste into abundant and inexpensive energy.

It is worth noting that Flibe Energy is the only company in the west pursuing this technology; others developing molten salt reactors are trying to take shortcuts which miss out on the greatest benefits of the thorium fuel cycle. LFTR is a comprehensive solution, which can finally close the fuel cycle, eliminating the need for uranium mining and enrichment. It is a more challenging design, but it doesn’t kick the can down the road; it fully addresses all rational concerns with nuclear technology, and offers many new opportunities.

Peter Thiel's Founders Fund is investing in the TAP molten salt reactor. This is one of many promising designs that seek to address the concerns about nuclear and allow it to displace fossil fuels through superior economics. Additionally, the republican platform supports leveling the energy market, solving the Thorium Problem, and enabling development of advanced nuclear:

We support the development of all forms of energy that are marketable in a free economy without subsidies, including coal, oil, natural gas, nuclear power, and hydropower. A federal judge has struck down the BLM’s rule on hydraulic fracturing and we support upholding this decision. We respect the states’ proven ability to regulate the use of hydraulic fracturing, methane emissions, and horizontal drilling, and we will end the Administration’s disregard of the Nuclear Waste Policy Act with respect to the long-term storage of nuclear waste. We encourage the cost-effective development of renewable energy sources — wind, solar, biomass, biofuel, geothermal, and tidal energy — by private capital. The United States is overwhelmingly dependent on China and other nations for rare earth and other hardrock minerals. These minerals are critical to advanced technology, renewable energy, and defense manufacturing. We support expediting the permitting process for mineral production on public lands. We support lifting restrictions to allow responsible development of nuclear energy, including research into alternative processes like thorium nuclear energy.

As cool as a clean burning column of fire is, a vortex engine looks like a more practical use of the phenomenon. The idea is to capture the energy of a rising warm air column as in a solar updraft tower, though without needing to construct a tower. It also offers the potential to replace cooling towers, and extract energy from the significant amount of "waste" heat available at thermal or nuclear plants. (That heat need not be wasted, and can also be used for cogeneration. The higher temperature heat produced by advanced reactors like LFTR or other MSRs can also drive industrial processes including desalination, production of carbon-neutral synthetic fuels and ammonia, etc.)

Also see the atmospheric vortex engine.

Here is what James Hansen has to say about it. Even that is probably not enough though; a fee on CO2 may have some effect in the developed world, but the rest can not afford it, and will not accept such limitations. Even the terrible consequences looming are nothing next to the abject poverty that billions are subjected to daily. As bad as burning coal is, inexpensive fossil fuels still offer a desperately needed improvement in their lives, and it is not right to deny that to anyone in such circumstances. (It is also better than burning wood, as many "environmentalists" would have us do.)

The only practical way forward that results in rapid decarbonization, is to offer the developing countries a cheaper option, before the countless gigawatts of planned coal fired capacity are actually built. We know that nuclear can rapidly displace coal, as it has done so in the past in a number of countries. China is ramping up conventional nuclear, and developing advanced reactors. Newer mass produced LFTR or Thorcon reactors will make nuclear energy even cheaper and safer yet. See also Thorium: energy cheaper than coal for details.

These summits which result in plans too cowardly to even mention the words carbon dioxide or nuclear are perverse. Until nuclear is at least acknowledged and proposals are on the table for encouraging development and deployment of advanced reactors, they are a total waste of time.

I love the idea of LFTR. Honestly. A thousand years of cheap and plentiful fuel, simplified nuclear design, smaller physical footprint, lower risk of cataclysmic meltdown & resulting fallout, waste having a much lower half-life, no CO2 emissions...

But it's still an idea. After Oak Ridge, there's been no government-led development of LFTR reactors in the states. Our only hopes at present are either with the Chinese or a private company called Flibe Energy that's trying to gather investment funds to build LFTE reactors for army bases.

All in all, I actually expect better from the Bulletin of the Atomic Scientists.

You would think. Clearly, though, this is just a hit piece on thorium, even though it has nothing to do with modern thorium reactor designs.

Thorium is well suited to molten salt reactor designs, and in fact is best used in liquid form. These LFTR (Liquid designs will fission 90%+ of the fuel, instead of the 0.5% fissioned by conventional reactors. This means a lot less waste for the amount of energy produced. Also, the waste from such reactors is dangerous for much less time than that from conventional reactors.

Thorium reactors are being developed by Russia and China. In the US, Flibe Energy is working on LFTR designs. There's lots of interesting information in their site.

Thorium power should most definitely be developed. It's a clean, safe source of baseline power - and doesn't take the vast space required for (inconstant) solar and wind. Plus, eventually it will be great for space applications.

Load following with a nuclear plant isn't difficult if you can easily control the moderator. This can be controlled by computer. In designs with large negative temperature coefficients (such as LFTR) the reaction speed can be controlled by the rate heat is removed from the reactor, making load following is as simple as controlling the speed of a pump in a coolant loop. Most (all?) current commercial reactors are not designed to habitually operate this way.

Commercial reactors are usually run full power for capital cost recovery reasons. The cost of fuel for nuclear versus the capital cost of current reactors is such that it is always cheaper than the fuel (or storage) for alternative power generation, so in periods of low demand, nuclear wins. Capital costs are high because it is difficult to handle high pressure water (and the 1700 fold expansion in volume if containment is lost) in current commercial designs. Designs using molten salts operate at atmospheric pressure and will be dramatically cheaper to construct. Companies such as Terrestrial Energy and Flibe Energy are working on commercialization of molten salt reactors, which are feasible from megawatts to gigawatts. Such a reactor would be ideal for a remote research base.

Westinghouse Nuclear and the other vendor (GE? I don't recall) make a lot of money making fuel rods. It may not be that much compared to the cost of building the plant, but it's a very significant motivator for them to lobby against any other system.

I'd like your take on Kirk Sorenson's paperwork advocating LTFRs - from memory, Energy from Thorium and FLiBe Energy. He seems to be the most active proponent in the US of Thorium based MSRs. He and other advocates argue that because of the major differences in the entire system technology (not least the difference in proliferation risk), it would not be necessary to build Gigawatt scale reactors - instead they could be scaled to neighborhoods.

It's only an indicator not proof of anything, but IIRC the MSR run at Oak Ridge was a 10 MW reactor (the electricity produced was just sent to big resistors outside), fit into a single room, and was turned off every night and on in the morning. I think they were not using Thorium - it's been a year or so since I explored any of this and I'm too lazy/busy to retrace my steps.

... liquid salt and/or sodium remain possibilities (though the corrosive effects of molten sodium on a whole host of piping make using it as a heat exchange fluid a challenging engineering problem. That, and if it should ever cool, i.e. freeze, remelting it is going to be a solid gold nightmare.) We won't even discuss the problems involving a red hot sodium leak into a second stage steam turbine system.

So don't use Sodium, use FLiBe in a Thorium fast reactor. A LFTR. They are quite a bit safer intrinsically than current reactors are with all their safety systems designed in. As a plus, you don't need water at all for a LFTR (secondary coolant can be helium or other working fluid) - you can build them anywhere, not just near a body of water. Please watch this video it's a great intro to LFTR's and why they're so different than existing PWRs.

ok, I'll bite. The fact is, there is no "clean" energy that can be built anywhere, and many have major flaws.
Wind Turbines supposedly kill eagles and often requires long transmission lines that make them inefficient in the best of cases. Not viable everywhere.
Solar is inefficient both in land and energy generated and also generally requires long transmission lines. Energy output varies by season in many areas.
Hydroelectric Dams have a horrible safety record, especially during construction, mess up the earth's spin, and can affect wildlife that depend on rivers. Some of the better power generating models (ie pumped storage) depend on high elevation drops and some other power source (like Coal) to pump
Tidal (wave) energy - many of the same construction dangers as Hydroelectric, only works for coastal cities
Peat (mostly in Russia) - large CO2 producer, kills fish with runoff
Biofuel - corn absolutely rapes soil nutrients, and other sources aren't much better. Most sources are subsidized because they aren't economical
Geothermal - great if you live near steaming hot springs and are basically sitting on an inactive volcano, not so great if you aren't

did I miss anything?

There's nothing inherently wrong with nuclear fission, Fukishima was just using a dangerous reactor design without the failsafes built into later designs. I personally feel LWRs are dumb to build on an earthquake and tsunami prone island, but passively safe designs like the MSRE were never developed and only are being looked into again now by companies like FLiBe energy. This technology was successfully developed in the 1960s and then subsequently abandoned, and the official reason was to avoid fragmenting the industry (but we damn well know it was all about the money - the nuclear lobby existed to protect LWR patents and these were threatened by any other nuclear power technology).

Fusion will require a very expensive containment vessel, and it will be a long time before it becomes efficient in any way (when and if they manage to get more energy out than they put in).

The best demonstration of Liquid Fuel Thorium Reactor (LFTR) was by ORNL in the 60's. They had a prototype molten salt reactor using U-233. This is the fissile component of the Th-232/U-233 fuel cycle. The breeding of TH-232 into U-233 was simply omitted as unneeded complication for this prototype. This was intended to prove / debug the molten salt reactor, it was very successful in key ways.

India has been working on solid fuel thorium reactors, this is an attempt to re-use our experience with U-235 reactors technologies. It is doubtful that this would ever be competitive with a clean LFTR design.

In the US, the regulatory hurdles for LFTR are very high, unless you bypass them by selling your design to the military, which has the option to bypass these regs. This is why Flibe Energy is planning to sell their LFTR to the military first. It is a lot easier to change the regulatory environment if there is clearly functional and safe product being used by the military.

can we stop calling pressurized water and boiling water reactors "modern"?

Don't LFTR designs still call for a heat exchanger to vaporize water and drive turbines? Even by your definition of modern (MSRs), there's still a lot of boiling of water being done. So much so, that desalinized water is listed as one of the salable products by Flibe.

And before you tout China, it's not like they're actually building them yet. Their goal is to complete one within the next 20 years. Flibe is aiming for about half that. Don't get me wrong, it's still great that China is working on the issues associated with that type of reactor, but "seeing" them "built" is going a tad overboard.

This may be a joke, but it is worth pointing out that the Plutonium used in RTGs is not fissile, and can't be used to make bombs. Pu-238 is only useful for RTGs. The isotope used in bombs is Pu-239, which is a common product of Uranium based reactors.

Producing Pu-238 is actually very difficult, as described in the above link. Unfortunately, the worlds supply is dwindling, and this endangers many upcoming space missions. One attractive option for creating more is to use Liquid fluoride thorium reactors, where Pu-238 is one of many useful products created.

It's also worth noting that you're talking about nuclear weapons. It can be used to make "dirty" bombs, however.

Not if the mermen militarise the plutonium and use it against the land people.

They're vicious SOBs down there.

This may be a joke, but it is worth pointing out that the Plutonium used in RTGs is not fissile, and can't be used to make bombs. Pu-238 is only useful for RTGs. The isotope used in bombs is Pu-239, which is a common product of Uranium based reactors.

Producing Pu-238 is actually very difficult, as described in the above link. Unfortunately, the worlds supply is dwindling, and this endangers many upcoming space missions. One attractive option for creating more is to use Liquid fluoride thorium reactors, where Pu-238 is one of many useful products created.

One suggestion to worrying about melt-down is to have the fissile material already melted. Flibe Energy is suggesting the use of molten thorium flouride salt reactors, which have the interesting property of not generating plutonium in the waste stream.