Emissions Nuclear

TerraPower’s Travelling Wave Reactor – why not use an IFR?

There has been a lot of hype recently about the “Travelling Wave Reactor” (TWR), thanks largely to a very popular TED talk given recently by none other than Microsoft founder and multi-billionaire Bill Gates. In the 20 min talk, he describes the urgent need for cheap, abundant, low-CO2 energy, if we are to successfully mitigate climate change (a problem he takes very seriously). One key focus of Gates’ talk is nuclear power… new nukes. To quote:

At TED2010, Bill Gates unveils his vision for the world’s energy future, describing the need for “miracles” to avoid planetary catastrophe and explaining why he’s backing a dramatically different type of nuclear reactor. The necessary goal? Zero carbon emissions globally by 2050.

The ‘dramatically different’ type of nuclear reactor Gates refers to is the TWR, and he and Nathan Myhrvold are offering some financial backing for the concept. But is it a good bet? Here is a brief summary of the technology he’s talking about:

Wave of the future: Unlike today’s reactors, a traveling-wave reactor requires very little enriched uranium, reducing the risk of weapons proliferation. The reactor uses depleted-uranium fuel packed inside hundreds of hexagonal pillars (shown in black and green). In a “wave” that moves through the core at only a centimeter per year, this fuel is transformed (or bred) into plutonium, which then undergoes fission. The reaction requires a small amount of enriched uranium (not shown) to get started and could run for decades without refueling. The reactor uses liquid sodium as a coolant; core temperatures are extremely hot–about 550 ºC, versus the 330 ºC typical of conventional reactors.

Further details are given here at NEI Nuclear Notes, and you can watch an animated video here.

For those who are regular readers of BNC, you’d be forgiven for thinking that this tech “sounds a lot like the IFR“. Well, it is similar, in many ways, and indeed is based on many of the same principles of sodium-cooled, metal-fueled fast reactors that have been developed by Argonne Labs and others over the last five decades. But there are also some fundamental differences, and this includes some ‘features’ that may seriously limit the ultimate usefulness of the TWR. Below, in another guest post, Dr George Stanford of SCGI takes a critical eye to the TWR vs IFR comparison, and draws some interesting conclusions…
Comments on TerraPower’s Travelling Wave Reactor (printable PDF version here)

By Dr George S. Stanford. George is is a nuclear reactor physicist, part of the team that developed the Integral Fast Reactor. He is now retired from Argonne National Laboratory after a career of experimental work pertaining to power-reactor safety. He is the co-author of Nuclear Shadowboxing: Contemporary Threats from Cold War Weaponry.

We hear from time to time about the Traveling Wave Reactor (TWR) that is being developed by TerraPower, an organization sponsored by Bill Gates. The developers are keeping many of the technical details to themselves. However, from the available info about the TWR, one can make some ball-park calculations. Some assumptions are necessary, because better numbers have not, to my knowledge, been revealed. If anyone has better info, please come forward.

Fact 1: In generating 1 GWe-yr of energy, any nuclear reactor necessarily fissions about 1 tonne of heavy metal, creating 1 tonne of fission products.

Fact 2: The TWR is based on the technology of the IFR (Integral Fast Reactor), developed at Argonne National Laboratory in the ’80s and ’90s — it uses metallic fuel and is cooled by liquid sodium. In effect, the TWR is a very large IFR (in size, not in GWe) that forgoes reprocessing, storing its fission products in the used part of the core (behind the traveling wave). This pushes the disposal problem perhaps 60 or more years into the future. Unlike the IFR, the TWR does not completely burn its fuel, and leaves behind a mixture of transuranic actinides — which perhaps eventually could be recycled (not clear).

Fact 3: In commercial readiness, the TWR is at least a decade or two behind the IFR.

Assumption 1: A TWR will operate for the predicted 60 years without refueling.

At the end of its life, therefore, it will contain 60 tonnes of fission products mixed in with 240 tonnes of heavy metal (uranium and transuranics) (see below).

Assumption 2: No net breeding.

Once started, a TWR will presumably create enough fissile material (Pu-239) to sustain itself throughout its useful life, but no net breeding potential is claimed.

Assumption 3: The TWR will achieve a burnup of 25%.

This is a guess, approximately what might be achieved in an IFR in a single pass. (LWRs achieve 4-5%)

Assumption 4: The enrichment of the initial critical zone is 20% (i.e., it’s 20% fissile).

This too is a guess, based on the 20% enrichment that a normal IFR needs.

Assumption 5: The initial fissile loading is 4 tonnes per GWe.

This is still another guess, based on the approximate fissile loading of an IFR core. (An IFR plant also has another 4 tonnes of fissile in the ex-core inventory, which a TWR does not have)

The above facts and assumptions lead to the following conclusions:

1. The initial core loading will consist of 300 tonnes of heavy metal (mainly U-238—or could be Th-232): 60 tonnes destined to be burned, plus 240 tonnes that will be left over, unused, after 60 years (Assumption 3).

Note: An IFR core has about 20 tonnes of heavy metal per GWe, and another 20 tonnes or so in ex-core inventory.

2. The initial 4 tonnes of fissile could come from three sources:

(a) It can consist of excess weapons plutonium (Pu).

(b) It can be Pu recovered from Light Water Reactor (LWR) spent fuel.


(c) It can be the fissile content of 20 tonnes of uranium that has been enriched to 20% U-235.

(a) Weapons Pu

The United States has about 85 tonnes of weapons Pu, only part of which is declared to be “excess”. That would be enough to prime about 10 IFRs or 20 TWRs—a worthwhile contribution to the longer-term energy supply, but not a major one.

(b) LWR Spent Fuel

The United States is projected to have about 85,000 tonnes of heavy metal (HM) in commercial spent fuel by 2020, containing perhaps 680 tonnes of fissile Pu. That would be enough fissile to start up 170 TWRs or 85 IFRs. For talking purposes, suppose either 170 TWRs or 85 IFRs magically spring into existence in 2020, and no more fissile Pu comes from LWRs, and also assume for a
moment that enriched uranium is not available.

Now IFRs can breed, with a doubling time of less than 15 years, whereas TWRs do not breed. In the TWR case, therefore, the nuclear capacity would remain at 170 GWe from 2020 on, The IFRs, however, would catch up in 15 years, reaching 170 GWe by 1035, 340 GWe by 2050, and so on.

Fact: Every tonne of fissile invested in a non-breeding reactor is a tonne of fissile unavailable for use in a reactor type that has growth potential.

Comment: Investing fissile material in a non-breeding (break-even) reactor is like putting money under a mattress. Deliberate net burning of fissile material is analogous to throwing banknotes into a fire.

(c) Enriched uranium

When the supply of fissile from LWRs is exhausted, the growth of a non-breeding TWR fleet is over unless there is some other source of fissile material—and there would be no fissile to get a fleet of breeders going either. As of now, the only other carrier of usable fissile material is enriched uranium.

To get the twenty tonnes of 20%-enriched uranium needed to prime a TWR, one must mine 800 tonnes of natural uranium. The global uranium reserves could support a growing TWR fleet for perhaps a century or more, but that would mean intensified worldwide mining activity and an expanding enrichment capacity, to the distress of arms-control advocates. IFRs, on the other hand, would eliminate for centuries the need to mine uranium, and eliminate forever the need to enrich uranium.

Comment: Thorium could probably substitute for the U-238 part of the TWR fuel. However that would be pointless, since it would do nothing to reduce the need for the initial fissile loading, and anyway enough U-238 to last a long time has already been mined.

* * * *

Postponement of reprocessing or waste disposal is not an obvious advantage, and brings with it eventually a significant extra waste-management effort. The TWR seems to have no significant capability that is not shared by the IFR, and it has a number of inherent disadvantages. Moreover the IFR is almost ready for prime time now, whereas the TWR development is about where the IFR was in 1980. Yes, there are non-trivial technical issues.

Will TerraPower sell enough TWRs to recoup Mr. Gates’ investment? I don’t know, of course. But the TWR’s lack of breeding alone makes it look like a second-best product, even if it can be made to work as hoped. It will have no market at all unless there is official failure to permit the IFR to come to fruition — in which case the LFTR (liquid fluoride thorium reactor) would probably be a more satisfactory non-breeding technology — but that’s another story.

And that’s how I see it.

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By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

35 replies on “TerraPower’s Travelling Wave Reactor – why not use an IFR?”

Thanks Barry, I agree that IFR is a better sodium cooled fast reactor than TWR. I’m still scratching my head what the advantage to TWR is in the first place.

Just to be clear–LFTR is a breeder, it’s just not a “net breeder” or a “doubler”. It’s an “iso-breeder”, producing enough fissile to keep itself going essentially indefinitely. Only reactors based on fast spectra and plutonium will be able to achieve high breeding gains. But thermal-spectrum “isobreeders” like LFTR have low fissile inventories that allow many to be started for a given fissile supply.


Stone & Webster was working on the LMBR in the 1980’s
I was assigned to it but did not go. There are others around who know about this who would most likely agree with these comments. I do not have enough info to contribute, I was only involved very briefly. I agree with DR. Stanford’s post.


Sounds like a new term is needed there Kirk, although “iso-breeder” is suitably geeky. How about:

Burner – reduces fissile inventory
Sustainer – maintains fissile inventory
Breeder – increases fissile inventory


The TWR reminds me of Oppenheimer’s “Nuke in a Hole”. The idea was to bury it and then abandon it 30 years later when the fuel was exhausted.

Presumably the TWR is designed so that the core can be recovered.



Thank you for this excellent analysis by Dr. Stanford. Kirk, thank you for confirming my impression that LFTRs are very low gain breeders.
This would be great when there’s a large fleet of reactors but isn’t what we need for creating the fleet.

The TWR reactor sounds to me like a solution to the wrong problem. Enough said.

I like the suggestions for what to call reactors. How about also calling a burner or sustainer reactor ‘full burn’ when it burns close to 100% of the fertiles/fissiles in the fuel. Is there any theoretical limit on the maximum percentage of burn, or is it like half life, with an asymptote?

Another issue on words, too. I’d like a better way to describe what now comes out of conventional LWRs than ‘spent nuclear fuel’. Calling it ‘spent’ doesn’t cut it when less than .7% of the energy in the source uranium has been concentrated and extracted.

I’m not a nuclear expert, having only a B. Sc. (Physics) from 1970, but I’m learning a lot through Brave New Climate and all the other excellent sites linked through here. (I think I found NNadir’s posts on Daily Kos via BNC.)

Now a question for the experts. The LFTR seems to have a lot of advantages by virtue of using liquid fuel, which (among the advantages) allows continuous extraction of materials from the core. Is a molten salt or liquid fuel fast uranium breeder design possible? Are there ‘gotchas’ in physics or chemistry that make it impossible? Or is it more an engineering challenge? In my imagination, I see extracting the valuable fission products (as NNadir says) and bred fissile excess continuously rather than in batch operations at refueling times.

I tried some searches in Google Scholar for molten salt uranium reactors. The only one seems to be original ORNL molten salt reactor, which wasn’t conceived as a uranium breeder. It started with a 235U charge and then bred thorium to 233U as a sustaining reactor.

I’d love to know if there’s been work on a fluid fuel uranium breeder. In any other area of energy, a power plant that creates energy and fuel simultaneously would be big news.

And in case anybody needs another challenge – could a liquid fuel uranium breeder be later converted to run as a full burn thorium sustainer? I suspect the core designs would be too different because of the different neutron spectrum requirements, but I know I’m speaking from ignorance. I welcome enlightenment.

Thanks again to everyone for your excellent and inspiring work here!


On the terminology issue, burner reactors as defined above by Joffan, are usually referred to as ‘converters’ in the literature, for some reason, although ‘burners’ seems a better term to me. ‘Sustainer’ is problematic, as such a reactor is balanced on a knife-edge. A fractional underperformance, and it will run out of fissile and shut down, a fraction over and it is a breeder, albeit with a doubling time so long it is irrelevant. In practice, to allow for inevitable lossses in the reprocessing system, any reactor intending to be sustainable on abundant U-238 or Th-232 must be a (marginal / minimal / bare / pick-a-term) breeder.


A liquid-fuel uranium breeder is possible, using sodium chloride (enriched in Cl-37) as the carrier, with UCl4 / UCl3 / PuCl3 dissolved in it. Such a reactor needs a fissile inventory of 7-8 Te / GW(e), similar to an IFR, but it’s all in the core at once, rather than half in use and half being cooled and reprocessed. Breeding performance and doubling time should be slightly better than IFR. However, this is a ‘paper reactor’, or at least an entirely virtual one. No such reactor has ever been built, not even a small prototype, so time-to-market would be long, and uncertain.



Thanks for your reply. I think we definitely need to go with fast time-to-market reactors. There’s a fascination with fiddling with ideas and everything that can be labeled ‘new and improved’. I can see it in every news story that stretches to make everything be the next hot idea for combating global warming, ocean acidification, and the whole list of hot buttons. It’s time to do things that work.


An interesting comment from Bill Hannahan on the mirror of this post at Energy Collective:

Bill Hannahan said:
Hello George, it is great that Bill Gates recognizes the need for nuclear power to solve the world’s energy problem. It is unfortunate that he has latched onto the Rube Goldberg TWR. The advantages are hyped and the disadvantages are glossed over. In addition to the issues you raise consider these.

Imagine you run an airline and someone offers to build an airplane that only needs refueling once after 60 years. Great, you can fly it round the clock for 60 years. But wait, you still need to perform annual maintenance checks, change the tires, brake pads, hydraulic fluid, pack wheel bearings, upgrade instrumentation and control systems replace time limited components etc, so capacity factor will not increase as much as you might expect.

In the reactor world lots of work goes on during refueling outages besides refueling, so maintenance outages will still be required for the TWR. The capacity factor of the TWR will be similar to that of conventional plants.

Do you pump coolant through the entire core for 60 years? That will require much bigger pumps that will burn up a lot of kWh’s you cannot sell to customers. Or do you put in a flow control system to direct coolant to the active region. That would involve substantial mechanical complexity.

The huge core will require a massive complex control system to monitor and regulate the distribution of power in the core to generate the required amount of power while staying within fuel rod peak power limits. Any compromise in reliability will rapidly eat up any savings.

The massive core will require a massive foundation and associated building, increasing capital cost and construction time.

At this time we really do not need a breeder reactor that solves all our energy problems for millions of years to come. We really need a simple safe efficient design that can produce energy cheaper than burning coal, and be factory mass produced in unlimited numbers quickly and at low cost.

The most likely candidate is the dirt simple Molten Salt Reactor using a once through uranium fuel cycle. They are easy and cheap to mass produce; they are inherently safe and use uranium 4-6 times more efficiently than conventional reactors.

That should be our goal, a solution the entire world can implement at a cost much lower than burning fossil fuel, and we should be spending all available resources pushing R&D of all possible energy technologies that have the potential to produce reliable, safe, dispatchable energy that is far cheaper than burning fossil fuel.

Of the known technologies I think the simplest MSR has the highest probability of meeting our needs for the next few hundred years, but if something better comes along that would be great. For a quick introduction see Dr. David LeBlanc’s slides.

Click to access d_leblanc_pres.pdf

For the details see his talk.


I’ve given this some thought, and here’s my assessment.

First of all, some people view *any* form of nuclear fuel recycling as a non-starter, at least for the near future. As far as they’re concerned, the IFR is not an option.

In spite of that, improving uranium utilization is still a major incentive for using fast reactors. Current LWRs utilize less than 1% of natural uranium. By comparison, a fast reactor can achieve perhaps 20% with a single pass without recycling.

In order to achieve this utilization, extensive breeding is required. It is preferable for the breeding occur away from the immediate vicinity of the fissile fuel to slow the reactivity increase from the breeding. This is achieved using “blankets” of fertile material (as opposed to relying on local breeding in the fissile fuel rods).

However, these blankets generate plutonium with a very high concentration of Pu-239 (or U-233 if using thorium). The potential proliferation hazard of this material can be mitigated by never opening the core for refueling during operation.

Further, because the core will operate for 40 years or more, this defers the decision to recycle the fuel by at least that long. At this point the politics, economics, and technology may be different.

Put all of this together and you invariably end up with something like the TWR.


Molten Salt Reactors:

There is also an article in press by David LeBlanc,

Please cite this article in press as: LeBlanc, D., Molten salt reactors: A new beginning for an old idea. Nucl. Eng. Des. (2010), doi:10.1016/j.nucengdes.2009.12.033


Andrew, I’ve collected a fair number of papers describing the fast-spectrum, liquid-chloride reactor concept that is the molten-salt analogue to the IFR. They can be found on my document repository near the bottom of the page. The single best one to read is probably Ottewitte’s 1992 proposal to INL (INEEL at the time).

Look for the section called “liquid-chloride reactors”.


To get a better idea of what Gates thinks he is doing, people may want to listen to his own words. Your quote isn’t from Gates, its just a blurb from the website that contains the video and transcript.

His TED talk is here: The transcript in interactive, in that you can read, then click on a word, which moves the video to the point in the transcript you clicked on.

Bill defines “miracles” in a way differently than most.

“Now when I use the word ‘miracle’, I don’t mean something that’s impossible. The microprocessor is a miracle. The personal computer is a miracle. The internet and its services are a miracle. So, the people here have participated in the creation of many miracles. Usually, we don’t have a deadline, where you have to get the miracle by a certain date. Usually, you just kin dof stand by, and some come along, some don’t. This is a case where we actually have to drive full speed and get a miracle in a pretty tight time line.”

He’s calling on the innovators of the world to take on the challenge of creating zero emission energy sources that will cost less than coal power.

Obviously, many believe this miracle is already here in the form of nuclear designs that are already well advanced. The problems Bill identified in current commercial nuclear plants are:

“It also has three big problems. Cost, particularly in highly regulated countries, is high. The issue of the safety, really feeling good about nothing could go wrong, that, even though you have these human operators, that the fuel doesn’t get used for weapons. And then what do you do with the waste? And, although it’s not very large, there are a lot of concerns about that. People need to feel good about it. So three very tough problems that might be solvable, and so, should be worked on.”

Bill made his money in an industry that lives on innovation. He sees opportunity in the nuclear field:

“…innovation really stopped in this industry quite some ago, so the idea that there’s some good ideas laying around is not all that surprising.”

And so he’s putting a bit of money into the TWR. He sees his investment as in the hundreds of millions of dollars. His personal net worth this year is said to be in excess of $50 billion. His understanding of what the TWR can do:

“The idea of Terrapower is that, instead of burning a part of uranium, the one percent, which is the U235, we decided, let’s burn the 99 percent, the U238.”

He talks as if the TWR is the only design that does this, but he can’t be that “out of it”. He also talks as if the TWR just burns everything that is in it. The Q&A after the speech, also included in the video and transcript, has Gates saying more. For one thing, he believes that it is a major problem that today’s reactors have to be refueled:

“Today, you’re always refueling the reactor, so you have lots of people and lots of controls that can go wrong, that thing where you’re opening it up and moving things in and out. That’s not good.”

One questioner said: “It’s a nuclear power plant that is its own waste disposal solution”, and Bill said: “Yeah. Well, what happens with the waste, you can let it sit there — there’s a lot less waste under this approach – then you can actually take that, and put it into another one and burn that. And we start off actually by taking the waste that exists today… That’s our fuel to begin with”.

In view of the knowledgeable critical comments I’ve read here and in other places written by people very much more familiar with nuclear technology than I am, this statement of Bill’s, which sums up what he thinks the TWR is, seems incredible:

“Once you get the first one [a TWR] built, if it works as advertised, then it’s just clear as day, because the economics, the energy density, are so different than nuclear as we know it. ”

Whatever he has up his sleeve, or if he is out of his mind about the TWR, I don’t care. As he said

“we need lots of companies working on this [ not his TWR, but on creating zero emission energy that is cheaper than fossil fuel ], hundreds…. And a lot of them, you’ll look at and say they’re crazy. That’s good”

People criticize Bill for calling for research into energy “miracles” when what we need to do is to start implementing what we know how to do right now, but these critics misunderstand. Bill wants government funded research beefed up, in addition to implementing what we know how to do now, so that by 2050, we’ll be able to get to zero emissions in enough sectors and countries that the planet will remain inhabitable.

“When countries get together in places like Copenhagen, they shouldn’t just discuss the CO2. They should discuss this innovation agenda…”

And: “We do need the market incentives, CO2 tax, cap and trade, something that gets that price signal out there.”

Bill notes that military and health research dwarfs energy research, and he calls for government funded energy research to be beefed up to a more respectable level, still lower than health or military. He says its energy $4 billion now, health around $30 billion, and military $80 billion.

He wants the energy portion to move to $14 billion.

What is important about Gates is he is a leader who has recently recognized that climate is the problem of our age, he understands that we need to emit zero GHG in every sector where this is remotely possible, he understands that we need to do it as soon as possible, and he is calling out to his peers to study this issue and get on with finding solutions.


@Kirk Sorenson

Wow – that’s a lot of material. I have been at your site (it’s one of the first I was exploring) and didn’t go far enough. Thanks!


I think the take-away is that Bill Gates ought to be congratulated, and he’s putting his money where his mouth is. The fact that he’s backed the second-placed horse in this race isn’t too important, it’s not that hard to get him on the right horse I imagine, the point is, he’s on a fast horse pointing in the right direction.


David Lewis,

Thanks for that link to the Bill Gates video at TED. At first I was very discouraged to find that he has bought the idea that rising CO2 concentrations in the atmosphere are dangerous. I was beginning to wish that IBM had plumped for CP/M instead of DOS in 1980 so we would never have heard of Bill Gates.

However, after a while I realized that no matter how weird people like Bill Gates (world’s second most wealthy person), James Hansen (Storms of My Grandchildren) and James Lovelock (Gaia hypothesis) may be they all support a rapid build up in NPP capacity.



You should be thankful that us knuckle draggers are still trying to learn about energy policy from Barry Brooks even though some of us disagree with his views on “Catastrophic Anthropogenic Global Warming”.


To David Phillips:
. Your “Future Trends” class sounds very interesting. I have one minor correction to what you have written. You say,

The [TWR] technology takes a minute amount of enriched uranium (U-235) along with the far more commonplace U-238. After an initial ignition, the wave travels through and uses up the uranium for roughly 60 years.

. The adjective "minute" is not really appropriate. Being a variant of the IFR, the fissile needs of the start-up TWR core will presumably be about the same as for an IFR core — i.e., the initial zone would consist of roughly 20 tonnes uranium per GWe, of which about 20% has to be fissile. The 4 tonnes of fissile could be either U-235 or plutonium.

. The Pu would come from used LWR fuel, in which case the uranium part would probably come from the same place.

. If U-235 were used, the 20 tonnes of 20%-enriched uranium would have to be produced in an enrichment plant, starting with about 750 tonnes of natural uranium.

. The above TWR loading is just a ballpark estimate — I'm not privy to the actual design data.

. Note, BTW, that a TWR is not small. To run for 60 years, a 1-GWe TWR would have to start out with appreciably more than 80 tonnes of uranium in its core.

. — George


“Postponement of reprocessing or waste disposal is not an obvious advantage, and brings with it eventually a significant extra waste-management effort.”
1. It reduces the risk of leakage and accidents during repeated reprocessing and refabrication
2. It avoids dealing with short-halflife wastes at all since they will have decayed
3. It greatly reduces the amount of Cs-137 and Sr-90 (halflife about 30 years, tenthlife 100 years) to be dealt with as most of it decays while the reactor is still running or during the cooling period after it is no longer running.
4. It is a reasonable bet for indefinite storage if that is called for, yet easily recoverable if at some point it is preferable to recycle or move to other storage.

“The initial core loading will consist of 300 tonnes of heavy metal (mainly U-238—or could be Th-232): 60 tonnes destined to be burned, plus 240 tonnes that will be left over, unused, after 60 years”
1. The 20% usage estimate seems a bit low, but
2. There is a huge stockpile of depleted uranium; putting some in reactors is not much of an additional cost
3. 240 tons of DU is only 12 cubic meters; adding 12 cubic meters of space to the reactor is still nothing compared to the size and pressure vessels of LWRs

“Unlike the IFR, the TWR does not completely burn its fuel, and leaves behind a mixture of transuranic actinides”
The IFR only burns about 20% in a cycle before reprocessing, and also produces a mixture of transuranic actinides that have to be recycled and refabricated.

“In commercial readiness, the TWR is at least a decade or two behind the IFR.”
The IFR reactor was just a sodium-cooled fast reactor like previous ones; the “Integral” innovation was the electrolytic on-site reprocessing, which has not been developed to the production stage and would take some time.

“Investing fissile material in a non-breeding (break-even) reactor is like putting money under a mattress. Deliberate net burning of fissile material is analogous to throwing banknotes into a fire.”
1. There is no shortage of fissile material and for the last two decades there has been more interest in fast reactors for their burning capabilities than for breeding. See MIT and Charles Forsberg’s recent “Future of the Nuclear Fuel Cycle” report:
2. The TWR likely does have a somewhat positive breeding ratio in the first years of its operation.


There is no merit in mining uranium, using it in LWR and then struggling to store it away. Fast reactors and pyroprocessing will make uranium go a long way.
Sodium is an additional risk factor in a reactor. It is just too chemically active and a fire risk. Salts and liquid metals should be used inside reactors for heat transport.
TWR amounts to a fast reactor with processing put off for long times.
Fast spectrum MOS could good if the materials to handle can be managed.


Very interesting discussion. There are a number of other safety issues with TWR which I would like to hear about.

Does this reactor have control and shut down capabilities? If so how is it going to be done? Shut down capability is an absolute requirement if the sodium can possibly leak or overheat.

Once you have shut it down can you restart it?

Will it have positive void coefficient if there is boiling or loss of the sodium?

There have been almost no successful sodium cooled plants, in fact most have come to a bad end because leakage is almost impossible to repair. It starts with Rickover’s Seawolf (SSN-575) reactor which had to be replaced with a PWR because of maintenance issues.


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