Uranium vs Thorium for Power Production (George Stanford)

Uranium vs Thorium for Power Production 1

Uranium vs Thorium for Power Production:

IFR vs LFTR—an Overview

George S. Stanford

September 2013

The possibility of using molten-salt reactors with thorium as the fertile element has captured

the imagination of a number of researchers, who see the potential for thorium-based reactors to

be superior to today’s uranium-based light-water reactors in several respects. This document is

one person’s attempt to express concisely the essence of the thorium/uranium situation.

That’s a bit of a challenge, however, since so many variations of uranium- and thorium-based

reactors have been proposed, including some hybrids, that a comprehensive discussion would get

hopelessly bogged down with esoteric, mind-numbing details. In this paper, therefore, one

prominent example of each category of reactor has been selected for discussion: the IFR1 is

considered representative of Generation-IV uranium-fueled fast reactors, and the LFTR2 is taken

as typical of Gen-IV3 thorium-fueled reactors, even though currently its technology is not nearly

as mature as that of the IFR.

Thus we will have much-simplified view of the uranium-vs-thorium picture, under the hypo-

thetical assumption that thorium-molten-salt technology has successfully achieved the maturity

that has already been reached with uranium-based fast-reactor technology.

Let’s be perfectly clear. The issue is not whether to deploy LFTRs (or some other thorium-

based technology)4 instead of IFRs or LWRs—the IFRs are almost ready, the LFTRs are not, and

LWRs will probably continue being deployed for several decades at least, regardless.

So if this exercise is only hypothetical, why do it at all? The intention is to try to fill the

public-information gap regarding the place of the IFR in the energy picture. In the publicity

being given to thorium technology’s potential, seldom, if ever is there mention of the more-

mature fast-reactor technology. This has had the effect of distracting attention from the need to

start now to deploy the next-generation reactors that can reduce the consumption of fossil fuel,

while dealing with worries about nuclear waste and weapons proliferation.

The hope is that this paper will throw light on the plusses and minuses of going with the bird

in the hand (the IFR) versus waiting for the one in the bush (the LFTR).

The conclusions reached are summarized in tabular form at the end of the document.

1 IFR: Integral Fast Reactor, developed at Argonne National Laboratory, ~1970—1994. The program was aborted,

for non-technical reasons, in1994, when it was on the verge of commercial demonstration. 2 LFTR (pronounced “lifter”): Liquid Fluoride Thorium Reactor, an extension (so far on paper only) of early work on

thorium reactors (discontinued in the 1970s). 3 The reactors currently being built are considered to belong to the third generation (Gen-III).

4 Besides the LFTR, another thorium-burning concept is attracting interest. Called the DMSR (Denatured Molten

Salt Reactor), some U-238 is included to “denature” the U-233. In one of its forms, it is visualized as a once-through, 30-year fuel cycle, with a breeding ratio of ~0.8, which makes it sort of a compromise between LWR and LFTR. For simplicity, the DMSR is not discussed further in this overview.


Definitions and Background Information

A nucleus has an atomic number and a mass number.

The atomic number (number of protons) determines the element; e.g., 90, 92, and 94 are

the atomic numbers of thorium, uranium, and plutonium, respectively.

The mass number is the number of nucleons (protons plus neutrons) in the nucleus.

For a given element, the number of neutrons determines the isotope; e.g., the two main

isotopes of uranium (U-238 and U-235) have 146 and 143 neutrons, respectively (238

and 235 being the mass numbers).

Heavy-metal nuclei (“actinides”—thorium and above) are either fissile (the ones with odd

mass numbers) or fertile (even mass numbers).

A fissile nucleus has a good probability of fissioning (splitting into two lighter nuclei)

when struck by a neutron of any energy; but sometimes it just absorbs the neutron and is

transmuted into a fertile nucleus.

A fertile nucleus is transmuted into a fissile nucleus upon absorbing one neutron. How-

ever, fertile nuclei are said to be fissionable, since they sometimes fission if struck by a

fast neutron.

The most important fissile nuclei are U-233, U-235, Pu-239, and Pu-241.

The most important fertile nuclei are Th-232 (becomes U-233 after absorbing a neutron),

U-238 (becomes Pu-239), and Pu-240 (becomes Pu-241).

A thermal reactor is one whose neutrons are moderated (slowed down) to improve their

chances of causing fissions in the fissile isotopes. Most of the thermal reactors use water

(heavy or light) as the moderator, but some use carbon.

In a fast reactor, the neutrons remain fast—i.e., the coolant is a non-moderating fluid.

Each type of reactor needs fuel with a certain minimum fissile concentration in order to

function (achieve and maintain criticality5).

Each type of reactor also has a minimum fissile loading (amount of fissile per GWe of

capacity). In some cases, as the reactor size gets small (less than ~500 MWe), the needed

fissile percentage goes up.

Today’s reactors are almost exclusively Gen-II (Generation 2) or Gen-III thermal

reactors. Most of them are fueled with low-enrichment uranium (~4% U-235) and cooled

and moderated by light water, but some (CANDUs) use heavy water and natural uranium

(which is 0.7% U-235).

Today’s thermal reactors use less than 1% of the energy in the mined uranium, and

cannot exceed 1% even when some of the plutonium is recycled, as is done in France.

To get almost all the energy out of the mined uranium, a fast reactor (whose neutrons are

not moderated) must be used, not a thermal or epithermal reactor (in the latter, the energy

of the neutrons is reduced considerably, but not all the way down to thermal levels).

With fast reactors, enough uranium has already been mined to power the world for

several centuries.

5 When the chain reaction (or power output) in a reactor is steady—neither increasing nor decreasing—the

reactor is said to be critical.


Some Facts about Uranium and Thorium

There’s enough uranium or thorium to power civilization for as long as it lasts.

There are designs for passively safe reactors using either thorium fuel or uranium fuel.

Most of the reactors built in the next decade or two will be today’s types of thermal

reactors (Gen-III), which are water-moderated and uranium-fueled.

However, Gen-IV reactors are being actively developed in several countries—notably

India, China, South Korea, Japan, and France (but no longer in the United States, which

abdicated its leadership in that area in 1994, with the aborting of the IFR development

program when it was on the verge of a commercial-scale demonstration).

Various types of thorium-based reactors are being discussed currently, with no consensus

yet in the thorium community regarding which types would be the best ones to pursue.

Thorium advocates usually cite the advantages that their reactor concept offers over

today’s thermal reactors. The problem there is that theirs is really a Gen-IV (or Gen-V)

reactor, whose competition will not be today’s variety of thermal reactors. Very similar

improvements are offered by at least one uranium-based Gen-IV concept—the IFR,

whose existence the thorium community rarely acknowledges voluntarily.

The uranium-based IFR is the Gen-IV concept that is closest to commercialization. It is

embodied in General Electric Hitachi’s PRISM concept, and that company is ready to

build a commercial demonstration plant when given the go-ahead.

The LFTR is the thorium-based reactor concept that seems to get the most attention these

days.

In 2002, a 242-person DOE task force ranked the Argonne-developed IFR (referred to as

“Na Metal Pyro”) the most promising, overall, of 19 next-generation (Gen-IV) reactor

concepts. Of the other concept categories, the closest to the LFTR was the “molten-salt

cooled” reactor (MSR), which ranked sixth.

The LFTR’s state of maturity is about where the IFR’s was in the 1970s—a promising

concept with significant technological questions to be answered before commer-

cialization can be considered.

So the hypothetical question addressed in this essay is this: What would be the pros and

cons of IFR vs LFTR technology, if the two technologies were both ready for com-

mercialization—i.e., the technical challenges still to be encountered in commercializing

the LFTR are assumed to have been satisfactorily resolved, so that LFTRs perform as

predicted by their supporters.

LFTR Claims, the IFR, and Related Facts

Thorium-fueled reactors are viable energy producers, destined, it seems likely, to be part of

the next-generation mix of reactor types in some countries (India for one). In comparing thorium

and uranium as reactor fuels, however, the advocates of thorium-based reactors almost always

emphasize their potential advantages with respect to today’s Generation-II and -III (uranium-

fueled) thermal reactors. But that is not a relevant comparison, because, by the time a thorium

cycle has been developed to commercial readiness, the main competition will not be from Gen-

III, but from Gen-IV—probably uranium-fueled fast reactors.


Although seldom mentioned by the more vocal thorium supporters, the Gen-IV IFR is now

ready for commercial demonstration,6 and its closed fuel cycle has virtually all the advantages

over Gen-III that are claimed for the thorium cycle:

passive safety

waste minimization

resource utilization

sustainability

proliferation resistance

thermal efficiency

cost of fuel

overall cost

The various claims made for the LFTR will now be considered, in the context of proven

characteristics of the IFR.

Passive Safety

Claim: LFTRs have a number of design features and physical properties that make them

exceptionally immune to accidents that could cause dangerous radioactivity to be dispersed

off-site. One of those features is the fact that the LFTR’s coolant is not pressurized, so there

can be no “blowdown”7 of pressurized coolant if a pipe breaks.

Fact: That’s true. And the same can be said of the IFR (its coolant too runs at atmospheric

pressure).

Fact: In terms of public health, the safety record of civilian nuclear power is already better

than that of any other heavy industry, and further improvement in accident resistance is icing

on the cake.

Fact: However, passive safety is of value to plant owners, to prevent the kind of expensive

power-plant damage that happened at TMI, Chernobyl and Fukushima.

Claim: The LFTR would be built with a “freeze plug” that melts without operator interven-

tion if the reactor overheats, dumping the liquid fuel into a reservoir where it would be

passively cooled and not be able to go critical.

Fact: The IFR also is designed to shut itself down if the core overheats, with the low-level

heat removed passively by convective cooling. This gives the operators plenty of time to

shut it down permanently or restart it, depending on circumstances.

Conclusion: In practical terms, the passive-safety features of LFTRs and IFRs are equivalent.

Waste Minimization

Claim: Because the LFTR does not use U-238 for its fertile isotope, the amount of long-

lived transuranics in its used fuel is considerably less than in used LWR fuel.

Fact: True. And the waste from an IFR is similar, in that it has only trace amounts of long-

lived transuranics.

6 This is General Electric’s PRISM reactor, with recycle by pyroprocessing.

7 A “blowdown" occurs when the coolant is high-pressure water and a major pipe breaks, permitting the water to

flash into steam. There was a blowdown at Chernobyl.


Fact: While LFTRs could consume the bulk of the plutonium in the used thermal-reactor

fuel, they would not use very much of the uranium that’s left (some 94% of the LWR fuel’s

original uranium). That left-over uranium, plus about ten times as much depleted uranium,

would be treated as radioactive waste (although its activity would be quite low).

Conclusion: Overall, there is less waste from an IFR than from a LFTR.

Resource Utilization

Claim: The LFTR consumes almost 100% of its thorium fuel (an advantage with respect to

LWRs).

Fact: Perhaps true, depending on the details of the fuel cycle. And the IFR definitely

consumes almost 100 percent of its uranium-plus-plutonium fuel.

Fact: With IFRs, when all the plutonium from used thermal-reactor fuel has been recycled,

the remaining uranium from that fuel plus the depleted uranium left over from the enrichment

process could fuel a global fleet of IFRs for the better part of a millennium, and such a fleet

could grow exponentially for as long as the energy demand was growing.

Conclusion: IFRs utilize resources at least as efficiently as LFTRs.

Sustainability

Claim: Once a LFTR has been fueled with its initial loading of Th-232 plus the needed

fissile (U-235 in uranium enriched to ~20%, or Pu from the weapons inventory or from used

thermal-reactor fuel), it will generate its own replacement fissile (U-233). Only a tonne or so

of thorium per GWe-year will be needed to keep the reactor running indefinitely.

Fact: Probably true. Similarly, once an IFR has been fueled with its initial loading of ura-

nium plus the needed fissile (probably Pu from the weapons inventory or from used thermal-

reactor fuel), it can generate its own replacement fissile (Pu-239, mainly), needing only a

tonne or so of uranium per GWe-year to keep the reactor running indefinitely.

Furthermore:

Fact: IFRs can be configured to have a breeding ratio8 BR as high as 1.5 or more, leading

to self-sustained doubling times of 15 years or less.

Fact: The LFTR advocates do not claim a breeding ratio significantly greater than unity,

which means that the growth of a LFTR fleet (and of nuclear power in general) would be

seriously constrained once all of the easily available fissile had been committed, unless a

prolific source of new fissile were to come into play. (Without an external supply of fissile

material, a fleet of LFTRs cannot grow.)

Claim: The fissile inventory needed to prime (start up) one IFR could prime five or more

LFTRs with the same generating capacity.

Fact: That is true. But consider the following:

Fact: There will be enough fissile plutonium to prime all the IFRs or LFTRs that will be

needed in the next 4 or 5 decades.

Fact: But that only holds if the used thermal-reactor fuel is preserved for recycle. If it

were to be disposed of irretrievably, that would (a) guarantee the continued operation and

expansion of enrichment facilities and uranium mining, and also (b) seriously limit the

8 The breeding ratio is defined to be the number of fissile nuclei created for every heavy-metal atom fissioned.


expansion rate of nuclear power by restricting the number of advanced reactors—LFTRs or

IFRs—that could be deployed.

Fact: The larger fissile inventory in an IFR is an advantage when, as currently in the U.K.,

the desire is to sequester existing plutonium inventory as quickly as possible.

Claim: There is enough accessible thorium in the world that LFTRs would never run out of

fertile material.

Fact: True, but with the caveat that the sources of readily available fissile material must

remain adequate to permit the needed growth in capacity.

Fact: The same claim is true of IFRs—but without worry about the long-term supply of

fissile material.

Claim: After the available plutonium runs out, new LFTRs could be primed with uranium

enriched to approximately 20% U-235 (80% U-238).

Fact: True, in principle. However, this approach has a number of problems, such as

mentioned in the following three facts:

Fact: These starting-up reactors would produce long-lived transuranics (plutonium and up)

until all the U-238 in the enriched uranium had been transmuted to plutonium and fissioned

(replaced by Th-232).

Fact: Therefore startup uranium with a much higher enrichment (i.e., less U-238) would be

desirable—but uranium enriched to more than 20% is currently ruled out because it is

considered a proliferation hazard.

Fact: Possession of enrichment facilities provides a nation with an easy path to fission

weapons. A fleet of IFRs could grow indefinitely with never any need for enriched uranium,

so that a nation that only used IFRs would have no legitimate need for an enrichment

capability.

Claim: With LFTRs there would be no need for enriched uranium—as long as there’s a

supply of plutonium from used fuel from thermal reactors.

Fact: True. However, once that supply ran out (some decades hence), further expansion of

nuclear power—LFTR or IFR—would be stymied without a source of fresh fissile material

that is acceptable economically, environmentally, and proliferation-wise.

Fact: To expect that source to be uranium is arguably unrealistic, since a lot of uranium

would have to be readily available (~200 tons of natural uranium for every ton of U-235).

And, importantly, a large enrichment capacity would have to be resuscitated, with its

proliferation implications.

Fact: As mentioned above, with IFRs no enrichment of uranium would ever again be

needed.

Conclusion: Both IFRs and LFTRs are sustainable—there is more uranium or thorium than

could ever be used. However, since IFRs can breed more fissile than they use, their potential

for growth is not limited by the need for fissile material from outside sources.

Proliferation Resistance

Claim: LFTRs are proliferation-resistant because the fissile U-233 in them is too contam-

inated with U-232.


Fact: That’s very misleading—and it’s not a claim that all LFTR advocates make. Pure

U-233 is an excellent material for bombs, and it can be produced by chemically separating

from a LFTR’s molten fuel some of the intermediate Pa-233 before it decays (with a 27-day

half-life) into pure U-233—and in some of the proposed thorium cycles that would be done

routinely.

Claim: But the reactor cannot spare that U-233, because it is needed to keep the fission

chain reaction going.

Fact: The extracted U-233 can be replaced with poor-quality Pu-239 from used LWR

fuel—after all, that’s what was probably used as the fissile material for starting up the LFTR

in the first place. Thus an unsafeguarded LFTR could be a good device for converting lousy

Pu into good-quality U-233 for bombs.

Claim: The LFTR eliminates the need for aqueous reprocessing (such as PUREX), which is

required if plutonium bombs are to be made.

Fact: True. And so does the IFR.

Fact: To achieve anything near their maximum fuel-use efficiency, both IFR and LFTR

have to have their fuel processed regularly to remove the fission products.

Claim: An IFR can make good-quality Pu-239 by being run with some special U-238 fuel

elements.

Fact: True. Any type of unsafeguarded reactor can be adapted to do that, including a

LFTR.9

Fact: While safeguards are needed for assurance, in reality it is “[unlikely that any]

weapons-intent nation would try to pervert either LFTR or IFR for producing weapons

material—the proven paths work well, even for impoverished nations such as North Korea,

Pakistan, and India.”10

Conclusion: There is no meaningful proliferation-related difference between the LFTR and

the IFR. To assure that they are not being diverted for weapons production, both would have

to be subject to international inspection.

Thermal Efficiency

Claim: Since a LFTR can run at a higher temperature than an IFR, it can produce more

electrical energy per unit of thermal energy developed.

Fact: True, but the significance is easy to overestimate: At about 45% for the LFTR and

40% for the IFR, the difference is about 10%—which would seem to be significant. But

there is a trade-off, because operating at higher temperatures requires more-expensive

materials that can stand the heat, so it is not clear whether the higher temperature would be a

net advantage.

Fact: With very low fuel and operating costs, an efficiency difference of 10% or 15% is a

relatively minor consideration.

9 A possible exception to this general rule would be a reactor shipped as a small, sealed “nuclear battery.”

10 Robert Hargraves, private communication.


Fact: In principle, with judicious plant siting, the low-level heat from the power plant need

not be entirely wasted, because it could be used for such tasks as desalination, process heat,

or district heating.

Claim: The LFTR’s higher thermal efficiency means that less cooling water would be

evaporated for a given electrical output.

Fact: True, and this might give the LFTR an edge in dry areas where reducing demand for

cooling water by 10% or 15% would be important, and where dry cooling towers or

constructive uses for the excess heat would be impractical.

Conclusion: The LFTR’s higher thermal efficiency might be significant for conserving water

in some arid environments.

Cost of Fuel

Claim: Thorium is produced in abundance as a waste product from the mining of rare-earth

metals, and so the raw thorium would be essentially free for the indefinite future.

Fact: True. Similarly, the IFR’s fertile material (uranium from used LWR fuel at first,

followed by existing depleted uranium for the next few centuries) will also be essentially free

for the better part of a millennium, and after that only one tonne of uranium per year per

GWe of capacity would have to be procured. (For an LWR it’s closer to 150 tonnes per

GWe-yr.)

Fact: Although U-233 is not as attractive as Pu-239 for use in an IFR in breeder mode,

there might be special applications where converting IFRs to a thorium/U-233 cycle would

become desirable, economically or otherwise.

Conclusion: The cost of raw fuel (uranium or thorium) would be a very small part of the cost

of energy from either LFTRs or IFRs, and is not a major consideration.

Overall Cost

Claim: Simplifications stemming from the liquid nature of LFTR fuel make power from the

MSR potentially cheaper than from the LWR, coal, or the IFR.

Fact: The true cost of electricity is a very slippery topic, due in part to (a) the many

external costs of burning fossil fuel that are borne by society, and (b) the fact that there is no

commercial experience with either LFTRs or IFRs. Also there are non-technical but very

important and hard-to-quantify considerations such as proliferation resistance and the future

demand for of fissile material and its availability.

Fact: To date there have been no operational, commercial-scale power plants utilizing

either LFTR or IFR technology.

Fact: However, General Electric Hitachi is confident that power from its PRISM version of

the IFR will be economically competitive.

Fact: Very high estimates of the cost of IFRs have been made, but they are irrelevant,

since they are based on experience with the very complex and expensive PUREX type of

processing that is done for oxide fuel. It’s true that on-site pyrometallurgical processing for

metallic fuel cannot take advantage of the economy of scale that applies to a large, central


PUREX plant, but pyro is technically very much simpler, and does not involve transportation

of spent fuel.

Conclusion: Since there are no empirical cost data, at this point there is no basis for predict-

ing with confidence whether electricity from commercialized IFRs or LFTRs will be more

expensive.

Summary

Provided the LFTR works as hoped, in almost every case the technical comparison of LFTR with

IFR comes out a wash or close to it. Relevant to energy policy, therefore, the important question

is whether commercial demonstration of a technology that is ready now (the IFR) should be

delayed for two decades or more while waiting for R&D to bring to maturity a technology (such

as the LFTR) that might have some minor technical advantages—especially when those technical

advantages cannot be more than marginal in any event. One should keep in mind that R&D can

proceed regardless, and new technology can always be deployed when commercially ready.

The most important differences between the two systems discussed here are three: (a) the degree

of readiness for commercialization, (b) the fissile material needed, now and in the future, and (c)

the potential breeding rate.

In category (a), currently the advantage is clearly with the IFR, since construction of a

commercial demonstration could start tomorrow. Meanwhile, R&D on advanced thorium

systems could go forward, on the way to seeing whether there are applications where thorium has

an economic advantage over uranium.

Assuming equal readiness for commercialization, considerations in categories (b) and (c) favor

one system or the other, depending on circumstances. A large fissile inventory in the plant is an

advantage for segregating a lot of plutonium in a hurry (advantage IFR), but it’s a disadvantage

if extremely rapid early growth of nuclear energy is wanted (advantage LFTR).

However, if the lion’s share of global energy by the end of the century is to be supplied by

nuclear power, the IFR can grow into that in an orderly manner, whereas deployment of a large

number of LFTRs could preempt the readily available fissile material, putting serious constraints

on continued growth of nuclear energy after mid-century.

The first table on the next page summarizes the conclusions reached above, and the second one

mentions more quantitatively some important additional considerations pertinent to comparing

the demonstrated properties of IFRs with the expected properties of LFTRs.

See below for ther conclusions in tabular form.

ACKNOWLEDGEMENTS

Thanks to Robert Hargraves, Ralph Moir, Per Peterson, Ken Kok, and Dan Meneley for con-

structive and informative comments on earlier versions of this paper.


SUMMARIZED CONCLUSIONS REGARDING THE EXPECTED

RELATIVE PERFORMANCE OF IFRS AND LFTRS

Note: With the exception of growth potential, breeding ratio, and readiness for

commercialization, all technical differences are minor or relatively unimportant.

PROPERTY COMPARISON, LFTR vs IFR Advantage

Passive safety In practical terms, the passive-safety features of LFTRs and IFRs

are equivalent. ---

Waste minimization Overall, there is less waste from an IFR than from a LFTR. IFR

Resource utilization IFRs utilize resources at least as efficiently as LFTRs. ---

Sustainability

Both IFRs and LFTRs are sustainable—there is more uranium or

thorium than could ever be used. However, since IFRs can breed

more fissile than they use, their potential for growth is not limited

by the amount of fissile material from outside sources.

---

Proliferation resis-

tance

There is no meaningful proliferation-related difference between the

LFTR and the IFR. To assure that they are not being diverted for

weapons production, both would have to be subject to international

inspection.

---

Thermal efficiency

The LFTR’s higher thermal efficiency might turn out to be impor-

tant where water is in short supply and where dry cooling towers or

constructive uses for the excess heat would be impractical.

LFTR

Cost of fuel

The cost of fuel (uranium or thorium) would be a very small part of

the cost of energy from either LFTRs or IFRs, and is not a major

consideration.

---

Overall cost

Since there is no empirical cost data yet, there is no basis for pre-

dicting with confidence whether electricity from commercialized

IFRs or LFTRs will be the more expensive.

Unknown

Additional Important Considerations

PROPERTY IFR LFTR Advantage

Fissile loading 7-10 tonnes per GWe 1 tonne per GWe ---

Pu sequestration 7-10 tonnes Pu per GWe 1 tonne Pu per GWe IFR

Growth potential,

near term

Current fissile inventory (U.S.)

sufficient for 40-50 GWe

Current fissile inventory (U.S.)

sufficient for ~400 GWe LFTR

Breeding ratio ~0.5-1.5 (designer’s choice) Enough >1 to be self-sustaining IFR

Self-sustained

growth rate 0-6% per year Expected to be zero or negative IFR

Growth potential,

long term (>90 yr)

Can grow indefinitely with doub-

ling time down to 15 yr or less

Potentially limited to ~500 GWe

(U.S.) by available fissile* IFR

Readiness for

commercialization Ready now for commercial demo

Commercial demo only

after ~2 decades of r&d IFR

* Without using enriched mined uranium or using accelerator or fusion neutrons to convert

thorium to U-233.

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Uranium vs Thorium for Power Production (George Stanford)