Thorium

Thorium is currently described by several nuclear proponents as a better alternative to uranium fuel. Thorium itself is, however, not a fissile material. It can only be transformed into fissile uranium-233 using breeder and reprocessing technology. It is 3 to 4 times more abundant than uranium. Concerning safety and waste disposal there are no convincing arguments in comparison to uranium fuel. A severe disadvantage is that uranium-233 bred from thorium can be used by terror organisations for the construction of simple but high-impact nuclear explosives. Thus development of a thorium fuel cycle without effective denaturation of bred fissile materials is irresponsible.

Introduction

Thorium (Th) is a heavy metal of atomic number 90 (uranium has 92). It belongs to the group of actinides, is around 3 to 4 times more abundant than uranium and is radioactive (half-life of Th-232 as starter of the thorium decay-chain is 14 billion years with alpha-decay). There are currently hardly any technical applications. Distinctive is the highly penetrating gamma radiation from its decay-chain (thallium-208 (Tl-208): 2.6 MeV; compared to gamma radiation from Cs-137: 0.66 MeV). Over the past decade, a group of globally active nuclear proponents is recommending thorium as fuel for a safe and affordable nuclear power technology without larger waste and proliferation problems. These claims should be submitted to a scientific fact check. For that reason, we examine here the claims of thorium proponents.

Claim 1: The use of thorium expands the availability of nuclear fuel by a factor 400

Thorium itself is not a fissile material. It can, however, be transformed in breeder reactors into fissile uranium-233 (U-233), just like non-fissile U-238 (99.3% of natural uranium) can be transformed in a breeder reactor to fissile plutonium. (A breeder reactor is a reactor in which more fissile material can be harvested from spent nuclear fuel than present in the original fresh fuel elements. It may be sometimes confusing that in the nuclear vocabulary every conventional reactor breeds, but less than it uses (and therefore it is not called a breeder reactor).)

For that reason, the use of thorium presupposes the use of breeder and reprocessing technology. Because these technologies have almost globally fallen into disrepute, it cannot be excluded that the more neutral term thorium is currently also used to disguise an intended reintroduction of these problematic techniques.

The claimed factor 400: A factor of 100 is due to the breeder technology. It is also achievable in the uranium-plutonium cycle. Only a factor of 3 to 4 is specific to thorium, just because it is more abundant than uranium by this factor.

Claim 2: Thorium did not get a chance in the nuclear energy development because it is not usable for military purposes

In the early stages of nuclear technology in the USA (from 1944 to the early 1950s), reprocessing technology was not yet well developed. Better developed were graphite moderated reactors that used natural uranium and bred plutonium. For the use of thorium (which, other than uranium, does not contain fissile components), enriched uranium or possibly plutonium would have been indispensable. Initially, neither pathway for thorium development was chosen because it would have automatically reduced the still limited capacity for military fissile materials production. (Thorium has a higher capture cross section for thermal (that means slow) neutrons than U-238. For that reason, it needs as fertile material in reactors a higher fissile density than U-238.)

Only when the US enrichment capacity at about 1950 delivered sufficient enriched uranium, the military and later civil entry into thorium technology started: in 1955 a bomb with U-233 from thorium was exploded, and a strategic U-233 reserve of around 2 metric tons was created. The large head-start of the plutonium bomb could not be overtaken any more, and plutonium remained globally the leading military fission material (although, according to unconfirmed sources, Indian nuclear weapons contain U-233). The US military research concluded in 1966 that U-233 is a very potent nuclear weapon material, but that it offers hardly any advantages over the already established plutonium.⁶

Because light water reactors with low-enriched uranium (LEU) were already too far developed, thorium use remained marginal also in civil nuclear engineering: for instance, the German "thorium reactor" THTR-300 in Hamm operated only for a short time, and in reality it was a uranium reactor (fuel: 10% weapon-grade 93% enriched U-235 and 90% thorium) because the amount of energy produced by thorium did not exceed 25%.

Claim 3: Thorium use has hardly any proliferation risk

The proliferation problem of Th / U-233 needs a differentiated analysis ‒ general answers are easily misleading. First of all, one has to assess the weapon capability of U-233. Criteria for good suitability are a low critical mass and a low rate of spontaneous fission. The critical mass of U-233 is only 40% of that of U-235, the critical mass of plutonium-239 is around 15% smaller than for U-233. A relatively easy to construct nuclear explosive needs around 20 to 25 kg U-233. The spontaneous fission rate is important, because the neutrons from spontaneous fission act as a starter of the chain reaction; for an efficient nuclear explosion, the fissile material needs to have a super-criticality of at least 2.5 (criticality is the amount of new fissions produced by the neutrons of each fission.)

When, because of spontaneous fissions, a noticeable chain reaction already starts during the initial conventional explosion trigger mechanism in the criticality phase between 1 and 2.5, undesired weak nuclear explosions would end the super-criticality before a significant part of the fissile material has reacted. This largely depends on how fast the criticality phase of 1 to 2.5 is passed. Weapon plutonium (largely Pu-239) and moreover reactor plutonium have – different from the mentioned uranium fission materials U-235 and U-233 – a high spontaneous fission rate, which excludes their use in easy to build bombs.

More specifically, plutonium cannot be caused to explode in a so-called gun-type fission weapon, but both uranium isotopes can. Plutonium needs the far more complex implosion bomb design, which we will not go into further here. A gun-type fission weapon was used in Hiroshima – a cannon barrel set-up, in which a fission projectile is shot into a fission block of a suitable form so that they together form a highly super-critical arrangement (see the picture in sheet 7 in reference #1). Here, the criticality phase from 1 to 2.5 is in the order of magnitude of milliseconds – a relatively long time, in which a plutonium explosive would destroy itself with weak nuclear explosions caused by spontaneous fission. One cannot find such uranium gun-type fission weapons in modern weapon arsenals any longer (South Africa's apartheid regime built 7 gun-type fission weapons using uranium-235): their efficiency (at most a few percent) is rather low, they are bulky (the Hiroshima bomb: 3.6 metric tons, 3.2 meters long), inflexible, and not really suitable for carriers like intercontinental rockets.

On the other hand, gun-type designs are highly reliable and relatively easy to build. Also, the International Atomic Energy Agency (IAEA) reckons that larger terror groups would be capable of constructing a nuclear explosive on the basis of the gun-type fission design provided they got hold of a sufficient amount of suitable fissile material.¹ Bombs with a force of at most 2 to 2.5 times that of the Hiroshima bomb (13 kt TNT) are conceivable. For that reason, the USA and Russia have tried intensively for decades to repatriate their world-wide delivered highly enriched uranium (HEU).

A draw-back of U-233 in weapon technology is that – when it is produced only for energy generation purposes – it is contaminated with maximally 250 parts per million (ppm) U-232 (half-life 70 years).² That does not impair the nuclear explosion capability, but the uranium-232 turns in the thorium decay chain, which means ‒ as mentioned above ‒ emission of the highly penetrating radiation of Tl-208. A strongly radiating bomb is undesirable in a military environment – from the point of view of handling, and because the radiation intervenes with the bomb's electronics. In the USA, there exists a limit of 50 ppm U-232 above which U-233 is no longer considered suitable for weapons.

Nevertheless, U-232 does not really diminish all proliferation problems around U-233. First of all, simple gun-type designs do not need any electronics; furthermore, radiation safety arguments during bomb construction will hardly play a role for terrorist organisations that use suicide bombers. Besides that, Tl-208 only appears in the end of the decay chain of U-232: freshly produced or purified U-233/U-232 will radiate little for weeks and is easier to handle.² It is also possible to suppress the build-up of uranium-232 to a large extent, when during the breeding process of U-233 fast neutrons with energies larger than 0.5 MeV are filtered out (for instance by arranging the thorium in the reactor behind a moderating layer) and thorium is used from ore that contains as little uranium as possible.

A very elegant way to harvest highly pure U-233 is offered by the proposed molten salt reactors with integrated reprocessing (MSR): During the breeding of U-233 from thorium, the intermediate protactinium-233 (Pa-233) is produced, which has a half-life of around one month. When this intermediate is isolated – as is intended in some molten salt reactors – and let decay outside the reactor, pure U-233 is obtained that is optimally suited for nuclear weapons.

An advantage of U-233 in comparison with Pu-239 in military use is that under neutron irradiation during the production in the reactor, it tends to turn a lot less into nuclides that negatively influence the explosion capability. U-233 can (like U-235) be made unsuitable for use in weapons by adding U-238: When depleted uranium is already mixed with thorium during the feed-in into the reactor, the resulting mix of nuclides is virtually unusable for weapons. However, for MSRs with integrated reprocessing this is not a sufficient remedy. One would have to prevent separation of protactinium-233.⁹

The conclusion has to be that the use of thorium contains severe proliferation risks. These are less in the risk that highly developed states would find it easier to lay their hands on high-tech weapons, than that the bar for the construction of simple but highly effective nuclear explosives for terror organisations or unstable states will be a lot lower.

Claim 4: Thorium reactors are safer than conventional uranium reactors

The fission of U-233 results in roughly the same amounts of the safety-relevant nuclides iodine-131, caesium-137 and strontium-90 as that of U-235. Also, the decay heat is virtually the same. The differences in produced actinides (see next claim) are of secondary importance for the risk during operation or in an accident. In this perspective, thorium use does not deliver any recognisable safety advantages.

Of greater safety relevance is the fact that uranium-233 fission produces 60% less so-called delayed neutrons than U-235 fission. Delayed neutrons are not directly created during the fission of uranium, but from some short-lived decay products. Only due to the existence of delayed neutrons, a nuclear reactor can be controlled, and the bigger their share (for instance 0.6% with U-235), the larger is the criticality range in which controllability is given (this is called delayed criticality). Above this controllable area (prompt criticality) a nuclear power excursion can happen, like during the Chernobyl accident. The fact that the delayed super-critical range is with U-233 considerably smaller than with U-235, is from a safety point of view an important technical disadvantage of thorium use.

During the design of thermal molten salt reactors (breeders), the conclusion was that the use of thorium brings problems with criticality safety that do not appear with classical uranium use in this type of reactors. For that reason, it was necessary to turn the attention to fast reactors for the use of thorium in molten salt reactors. Although this conclusion cannot be generalised, it shows that the use of thorium can lead to increased safety problems.

As mentioned, a serious safety problem is the necessity to restart breeder and reprocessing technology with thorium.

Thorium is often advertised in relation to the development of so-called advanced reactors (Generation IV). The safety advantages attributed to thorium in this context are mostly, however, not germane to thorium (the fuel) but rather due to the reactor concept. Whether or not these advanced reactor concepts bring overall increased safety falls outside the scope of this article, but that is certainly not a question with a clear "yes" as the answer.

Claim 5: Thorium decreases the waste problem

Thorium use delivers virtually the same fission products as classical uranium use. That is also true for those isotopes that are important in issues around long-term disposal.⁵ Those mobile long-lived fission products (I-129, Tc-99, etc.) determine the risk of a deep geological disposal when water intrusion is the main triggering event for accidents. Thorium therefore does not deliver an improvement for final disposal.

Proponents of thorium argue that thorium use does not produce minor actinides (MA)⁵, nor plutonium. They argue that these nuclides are highly toxic (which is correct) and they compare only the pure toxicity by intake into the body for thorium and uranium use, without taking into account that these actinides are hardly mobile in final disposal even in accidents.

It may furthermore be true that thorium use does not deliver MA, but it does produce other actinides, especially protactinium-231 (Pa-231; half-life 33,000 years), with similar features as the MA. The advantage with thorium use is that the amount of the resulting long-lived actinides is smaller than that of MA in the case of uranium use by a factor of 5. On the other hand, the high level of U-233 in the waste is not without problems ‒ its toxicity is comparable with plutonium and its long half-life (160,000 years) is aggravated by the fact that its decay product Th-229 (half-life 8,000 years) is a strong gamma-radiator (besides alpha). The maximum concentration of Th-229 is reached after around 100,000 years.

Taken together, one could argue that concerning actinides, thorium use has a limited advantage in produced waste, but certainly not concerning the safety-relevant long-lived fission products. For that reason, the claim that thorium use would considerably reduce the waste problem cannot be upheld. It also needs deep geological final disposal.

Conclusion

The arguments used by thorium proponents for a move from the use of uranium to thorium are at a closer look not convincing. The use of technology based on thorium would not be able to solve any of the known problems of current nuclear techniques, but it would require an enormous development effort and wide introduction of breeder and reprocessing technology. For those reasons, thorium technology is a dead end.

In my opinion, the proliferation aspect is a vital issue. Here we would see a severe deterioration of the current situation, because the barriers to the construction of feasible nuclear explosives by, for instance, terror groups would be seriously lowered. This aspect deserves more attention. We can hope that the IAEA, the USA and Russia would oppose uncontrolled propagation of thorium technology, when they would see its introduction thwarting their decades-long efforts to reduce the proliferation risk by repatriation of HEU.

On the other hand, the current thorium hype, partially carried by a fanaticism based on limited knowledge, could lead in a populist environment to incalculable developments. For that reason, I think it important that the environment and peace movements should insist that thorium technology without sufficient proliferation control should be outlawed in the same way as currently is the case with efforts to phase out the use of HEU. As a minimum requirement, thorium technology without U-233 denaturation with U-238 should be banned, and online reprocessing in molten salt reactors should be banned.

Epilogue: the scale of the international efforts supporting thorium technology

There still exists a large gap between the propaganda of thorium proponents and real activities for the development of thorium technology – at least in western industrialised countries. The brunt of the effort lies with smaller start-up firms. The large corporations remain passive and government support for thorium development remains small. Whereas full development of thorium technology would need investments of several billion euros or dollars, current EU support is in the range of a few million per year. This can be read as a clear sign of scepticism.

This scepticism is fed by extensive studies, for instance by the governments of the UK and Norway, that were rather pessimistic about thorium.^8,10 For that reason, I still think there are good grounds for hope that false developments towards the introduction of thorium technology may be countered with clear information. Take for example the Canadian company Terrestrial Energy, involved in the development of molten salt reactors, which in 2013 dropped thorium technology and online reprocessing for proliferation reasons, and now works on molten salt reactors based on classical uranium use (Integral Molten Salt Reactor ‒ IMSR).

In Germany, work on thorium technology continues. The research centre in Jülich jumped on the thorium hype by evaluating its previous experiences with thorium fuels⁷; and in Karlsruhe, the Joint Research Centre of the European Commission (JRC) and the Karlsruhe Institute of Technology (KIT) work on an EU-supported design for a molten salt fast reactor (MSFR) with thorium use. From the MSFR, 150 kg of U-233 would have to be extracted annually. Without denaturation that would be sufficient for several nuclear explosives. In Freiburg and Karlsruhe, new initiatives were founded against this development. They deserve support.

Translated from the German original by Jan Haverkamp. Original German version published in Strahlentelex (www.strahlentelex.de), Nr. 746-747 / 32nd Volume, 1 February 2018.

References:

1. Pablo Adelfang. Research Reactor Section / IAEA: Symposium on Progress, Challenges and Opportunities for Converting U.S. and Russian Research Reactors from Highly Enriched to Low Enriched Uranium Fuel, Moscow, 8-10 June 2011; http://dels.nas.edu/resources/static-assets/nrsb/miscellaneous/Adelfang-...

2. Jungmin Kang, Frank N. v. Hippel: U-232 and the Proliferation-Resistance of U-233 in Spent Fuel, Science & Global Security, 2001, Volume 9 pp 1-32; http://fissilematerials.org/library/sgs09kang.pdf

3. Stephen F. Ashley: Thorium fuel has risks, Nature 31, Vol. 492, 6 Dec. 2012; www.researchgate.net/publication/233880587_Nuclear_energy_Thorium_fuel_h...

4. IAEA: Average number of neutrons emitted per fission, 2008; www-nds.iaea.org/sgnucdat/a6.htm

5. Rainer Moormann: Transmutation, ein Weg aus der Atommüllfalle? Strahlentelex 744-745 v. 4.1.2018, pp. 5-7, www.strahlentelex.de/Stx_18_744-745_S05-07.pdf

6. W. K. Woods: LRL interest in U-233, Lawrence Livermore, 02.10.1966 (released from confidentiality 1994), www.osti.gov/biblio/79078

7. H.-J Allelein et. al.: Thorium fuel performance assessment in HTRs, Nuclear Engineering and Design, Vol. 271, May 2014, pp. 166-170; www.sciencedirect.com/science/article/pii/S0029549313006110

8. Reinhard Wollf: Thorium ist auch keine Lösung, Absage von Norwegens Strahlenschutzbehörde, taz, 5.1.2009; www.taz.de/!5170129

9. Engel, J.R., Bauman, H.F. et. al.: Conceptual design characteristics of a denatured molten-salt reactor with once-through fueling, Department of Energy (DOE) 1980; www.osti.gov/biblio/5352526-JYbAWb/

10. The Thorium Fuel Cycle, An independent assessment by the UK National Nuclear Laboratory, Position Paper, National Nuclear Laboratory, Aug. 2010, https://web.archive.org/web/20130126205622/www.nnl.co.uk/media/8241/nnl_...

Many Nuclear Monitor readers will be familiar with the tiresome rhetoric of thorium enthusiasts − let's call them thor-bores. Their arguments have little merit but they refuse to go away.

Here's a thor-bore in full flight − a science journalist who should know better:

"Thorium is a superior nuclear fuel to uranium in almost every conceivable way ... If there is such a thing as green nuclear power, thorium is it. ... For one, a thorium-powered nuclear reactor can never undergo a meltdown. It just can't. ... Thorium is also thoroughly useless for making nuclear weapons. ... But wait, there's more. Thorium doesn't only produce less waste, it can be used to consume existing waste."¹
Thankfully, there is a healthy degree of scepticism about thorium, even among nuclear industry insiders, experts and enthusiasts (other than the thor-bores themselves, of course). Some of that 'friendly fire' is noted here.

Readiness

The World Nuclear Association (WNA) notes that the commercialization of thorium fuels faces some "significant hurdles in terms of building an economic case to undertake the necessary development work." The WNA states:

"A great deal of testing, analysis and licensing and qualification work is required before any thorium fuel can enter into service. This is expensive and will not eventuate without a clear business case and government support. Also, uranium is abundant and cheap and forms only a small part of the cost of nuclear electricity generation, so there are no real incentives for investment in a new fuel type that may save uranium resources.

"Other impediments to the development of thorium fuel cycle are the higher cost of fuel fabrication and the cost of reprocessing to provide the fissile plutonium driver material. The high cost of fuel fabrication (for solid fuel) is due partly to the high level of radioactivity that builds up in U-233 chemically separated from the irradiated thorium fuel. Separated U-233 is always contaminated with traces of U-232 which decays (with a 69-year half-life) to daughter nuclides such as thallium-208 that are high-energy gamma emitters. Although this confers proliferation resistance to the fuel cycle by making U-233 hard to handle and easy to detect, it results in increased costs. There are similar problems in recycling thorium itself due to highly radioactive Th-228 (an alpha emitter with two-year half life) present."²

A 2012 report by the UK National Nuclear Laboratory states:

"NNL has assessed the Technology Readiness Levels (TRLs) of the thorium fuel cycle. For all of the system options more work is needed at the fundamental level to establish the basic knowledge and understanding. Thorium reprocessing and waste management are poorly understood. The thorium fuel cycle cannot be considered to be mature in any area."³

Fiona Rayment from the UK National Nuclear Laboratory states:

"It is conceivable that thorium could be introduced in current generation reactors within about 15 years, if there was a clear economic benefit to utilities. This would be a once-through fuel cycle that would partly realise the strategic benefits of thorium.

"To obtain the full strategic benefit of the thorium fuel cycle would require recycle, for which the technological development timescale is longer, probably 25 to 30 years.

"To develop radical new reactor designs, specifically designed around thorium, would take at least 30 years. It will therefore be some time before the thorium fuel cycle can realistically be expected to make a significant contribution to emissions reductions targets."⁴

Thorium is no 'silver bullet'

Do thorium reactors potentially offer significant advantages compared to conventional uranium reactors?

Nuclear physicist Prof. George Dracoulis states: "Some of the rhetoric associated with thorium gives the impression that thorium is, somehow, magical. In reality it isn't."⁵

The UK National Nuclear Laboratory report argues that thorium has "theoretical advantages regarding sustainability, reducing radiotoxicity and reducing proliferation risk" but that "while there is some justification for these benefits, they are often over stated." The report further states that the purported benefits "have yet to be demonstrated or substantiated, particularly in a commercial or regulatory environment."³

The UK National Nuclear Laboratory report is sceptical about safety claims:

"Thorium fuelled reactors have already been advocated as being inherently safer than LWRs [light water reactors], but the basis of these claims is not sufficiently substantiated and will not be for many years, if at all."³

False distinction

Thor-bores posit a sharp distinction between thorium and uranium. But there is little to distinguish the two. A much more important distinction is between conventional reactor technology and some 'Generation IV' concepts − in particular, those based on repeated (or continuous) fuel recycling and the 'breeding' of fissile isotopes from fertile isotopes (Th-232>U-233 or U-238>Pu-239).

A report by the Idaho National Laboratory states:

"For fuel type, either uranium-based or thorium-based, it is only in the case of continuous recycle where these two fuel types exhibit different characteristics, and it is important to emphasize that this difference only exists for a fissile breeder strategy. The comparison between the thorium/U-233 and uranium/Pu-239 option shows that the thorium option would have lower, but probably not significantly lower, TRU [transuranic waste] inventory and disposal requirements, both having essentially equivalent proliferation risks.

"For these reasons, the choice between uranium-based fuel and thorium-based fuels is seen basically as one of preference, with no fundamental difference in addressing the nuclear power issues.

"Since no infrastructure currently exists in the U.S. for thorium-based fuels, and processing of thorium-based fuels is at a lower level of technical maturity when compared to processing of uranium-based fuels, costs and RD&D requirements for using thorium are anticipated to be higher."⁷

George Dracoulis takes issue with the "particularly silly claim" by a science journalist (and many others) that almost all the thorium is usable as fuel compared to just 0.7% of uranium (i.e. uranium-235), and that thorium can therefore power civilization for millennia. Dracoulis states:

"In fact, in that sense, none of the thorium is usable since it is not fissile. The comparison should be with the analogous fertile isotope uranium-238, which makes up nearly 100% of natural uranium. If you wanted to go that way (breeding that is), there is already enough uranium-238 to 'power civilization for millennia'."⁵

Some Generation IV concepts promise major advantages, such as the potential to use long-lived nuclear waste and weapons-usable material (esp. plutonium) as reactor fuel. On the other hand, Generation IV concepts are generally those that face the greatest technical challenges and are the furthest away from commercial deployment; and they will gobble up a great deal of R&D funding before they gobble up any waste or weapons material.

Moreover, uranium/plutonium fast reactor technology might more accurately be described as failed Generation I technology. The first reactor to produce electricity − the EBR-I fast reactor in the US, a.k.a. Zinn's Infernal Pile − suffered a partial fuel meltdown in 1955. The subsequent history of fast reactors has largely been one of extremely expensive, underperforming and accident-prone reactors which have contributed far more to WMD proliferation problems than to the resolution of those problems.

Most importantly, whether Generation IV concepts deliver on their potential depends on a myriad of factors − not just the resolution of technical challenges. India's fast reactor / thorium program illustrates how badly things can go wrong, and it illustrates problems that can't be solved with technical innovation. John Carlson, a nuclear advocate and former Director-General of the Australian Safeguards and Non-Proliferation Office, writes:

"India has a plan to produce [weapons-grade] plutonium in fast breeder reactors for use as driver fuel in thorium reactors. This is problematic on non-proliferation and nuclear security grounds. Pakistan believes the real purpose of the fast breeder program is to produce plutonium for weapons (so this plan raises tensions between the two countries); and transport and use of weapons-grade plutonium in civil reactors presents a serious terrorism risk (weapons-grade material would be a priority target for seizure by terrorists)."⁸

Generation IV thorium concepts such as molten salt reactors (MSR) have a lengthy, uncertain R&D road ahead of them − notwithstanding the fact that there is some previous R&D to build upon.^4,9

Kirk Sorensen, founder of a US firm which aims to build a demonstration 'liquid fluoride thorium reactor' (a type of MSR), notes that "several technical hurdles" confront thorium-fuelled MSRs, including materials corrosion, reactor control and in-line processing of the fuel.⁴

George Dracoulis writes:

"MSRs are not currently available at an industrial scale, but test reactors with different configurations have operated for extended periods in the past. But there are a number of technical challenges that have been encountered along the way. One such challenge is that the hot beryllium and lithium "salts" – in which the fuel and heavy wastes are dissolved – are highly reactive and corrosive. Building a large-scale system that can operate reliably for decades is non-trivial. That said, many of the components have been the subject of extensive research programs."¹⁰

Weapons proliferation

Claims that thorium reactors would be proliferation-resistant or proliferation-proof do not stand up to scrutiny.¹¹ Irradiation of thorium-232 produces uranium-233, which can be and has been used in nuclear weapons.

The World Nuclear Association states:

"The USA produced about 2 tonnes of U-233 from thorium during the 'Cold War', at various levels of chemical and isotopic purity, in plutonium production reactors. It is possible to use U-233 in a nuclear weapon, and in 1955 the USA detonated a device with a plutonium-U-233 composite pit, in Operation Teapot. The explosive yield was less than anticipated, at 22 kilotons. In 1998 India detonated a very small device based on U-233 called Shakti V."²

According to Assoc. Prof. Nigel Marks, both the US and the USSR tested uranium-233 bombs in 1955.⁶

Uranium-233 is contaminated with uranium-232 but there are ways around that problem. Kang and von Hippel note:

"[J]ust as it is possible to produce weapon-grade plutonium in low-burnup fuel, it is also practical to use heavy-water reactors to produce U-233 containing only a few ppm of U-232 if the thorium is segregated in "target" channels and discharged a few times more frequently than the natural-uranium "driver" fuel."¹²

John Carlson discusses the proliferation risks associated with thorium:

"The thorium fuel cycle has similarities to the fast neutron fuel cycle – it depends on breeding fissile material (U-233) in the reactor, and reprocessing to recover this fissile material for recycle. ...

"Proponents argue that the thorium fuel cycle is proliferation resistant because it does not produce plutonium. Proponents claim that it is not practicable to use U-233 for nuclear weapons.

"There is no doubt that use of U-233 for nuclear weapons would present significant technical difficulties, due to the high gamma radiation and heat output arising from decay of U-232 which is unavoidably produced with U-233. Heat levels would become excessive within a few weeks, degrading the high explosive and electronic components of a weapon and making use of U‑233 impracticable for stockpiled weapons. However, it would be possible to develop strategies to deal with these drawbacks, e.g. designing weapons where the fissile "pit" (the core of the nuclear weapon) is not inserted until required, and where ongoing production and treatment of U-233 allows for pits to be continually replaced. This might not be practical for a large arsenal, but could certainly be done on a small scale.

"In addition, there are other considerations. A thorium reactor requires initial core fuel – LEU or plutonium – until it reaches the point where it is producing sufficient U-233 for self-sustainability, so the cycle is not entirely free of issues applying to the uranium fuel cycle (i.e. requirement for enrichment or reprocessing). Further, while the thorium cycle can be self-sustaining on produced U‑233, it is much more efficient if the U-233 is supplemented by additional "driver" fuel, such as LEU or plutonium. For example, India, which has spent some decades developing a comprehensive thorium fuel cycle concept, is proposing production of weapons grade plutonium in fast breeder reactors specifically for use as driver fuel for thorium reactors. This approach has obvious problems in terms of proliferation and terrorism risks.

"A concept for a liquid fuel thorium reactor is under consideration (in which the thorium/uranium fuel would be dissolved in molten fluoride salts), which would avoid the need for reprocessing to separate U-233. If it proceeds, this concept would have non-proliferation advantages.

"Finally, it cannot be excluded that a thorium reactor – as in the case of other reactors – could be used for plutonium production through irradiation of uranium targets.

"Arguments that the thorium fuel cycle is inherently proliferation resistant are overstated. In some circumstances the thorium cycle could involve significant proliferation risks."¹³

Sometimes thor-bores posit conspiracy theories. Former International Atomic Energy Agency Director-General Hans Blix said "it is almost impossible to make a bomb out of thorium" and thorium is being held back by the "vested interests" of the uranium-based nuclear industry.¹⁴

But Julian Kelly from Thor Energy, a Norwegian company developing and testing thorium-plutonium fuels for use in commercial light water reactors, states:

"Conspiracy theories about funding denials for thorium work are for the entertainment sector. A greater risk is that there will be a classic R&D bubble [that] divides R&D effort and investment into fragmented camps and feifdoms."⁴

Thor-bores and uro-sceptics

Might the considered opinions of nuclear insiders, experts and enthusiasts help to shut the thor-bores up? Perhaps not − critics are dismissed with claims that they have ideological or financial connections to the vested interests of the uranium-based nuclear industry, or they are dismissed with claims that they are ideologically opposed to all things nuclear. But we live in hope.

Thor-bores do serve one useful purpose − they sometimes serve up pointed criticisms of the uranium fuel cycle. In other words, some thor-bores are uro-sceptics. For example, thorium enthusiast and former Shell executive John Hofmeister states:

"The days of nuclear power based upon uranium-based fission are coming to a close because the fear of nuclear proliferation, the reality of nuclear waste and the difficulty of managing it have proven too difficult over time."¹⁵

References:
1. Tim Dean, 16 March 2011, 'The greener nuclear alternative', www.abc.net.au/unleashed/45178.html
2. www.world-nuclear.org/info/Current-and-Future-Generation/Thorium/
3. UK National Nuclear Laboratory Ltd., 5 March 2012, 'Comparison of thorium and uranium fuel cycles', www.decc.gov.uk/assets/decc/11/meeting-energy-demand/nuclear/6300-compar...
4. Stephen Harris, 9 Jan 2014, 'Your questions answered: thorium-powered nuclear', www.theengineer.co.uk/energy-and-environment/in-depth/your-questions-ans...
5. George Dracoulis, 5 Aug 2011, 'Thorium is no silver bullet when it comes to nuclear energy, but it could play a role', http://theconversation.com/thorium-is-no-silver-bullet-when-it-comes-to-...
6. Nigel Marks, 2 March 2015, 'Should Australia consider thorium nuclear power?', http://theconversation.com/should-australia-consider-thorium-nuclear-pow...
7. Idaho National Laboratory, Sept 2009, 'AFCI Options Study', INL/EXT-10-17639, www.inl.gov/technicalpublications/Documents/4480296.pdf
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12. Jungmin Kang and Frank N. von Hippel, 2001, "U-232 and the Proliferation-Resistance of U-233 in Spent Fuel", Science & Global Security, Volume 9, pp.1-32, www.princeton.edu/sgs/publications/sgs/pdf/9_1kang.pdf
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The dominant type of new nuclear power plant, light-water reactors (LWRs), proved impossible to finance in the robust 2005–08 capital market, despite new U.S. subsidies approaching or exceeding their total construction cost. New LWRs are now so costly and slow that they save 2–20 times less carbon, approximately 20–40 times slower, than micro power and efficient end-use.

As this becomes evident, other kinds of reactors are being proposed instead ­novel designs claimed to solve LWRs’ problems of economics, proliferation, and waste. Even climate-protection pioneer Jim Hansen says these “Generation IV” reactors merit rapid R&D. But on closer examination, the two kinds most often promoted ­Integral Fast Reactors (IFRs) and thorium reactors­ reveal no economic, environmental, or security rationale, and the thesis is unsound for any nuclear reactor.

Integrated Fast Reactors (IFRs)
The IFR ­a pool-type, liquid-sodium cooled fast-neutron reactor plus an ambitious new nuclear fuel cycle­ was abandoned in 1994, and General Electric’s S-PRISM design in 2003, due to both proliferation concerns and dismal economics. Federal funding for fast breeder reactors halted in 1983, but in the past few years, enthusiasts got renewed Bush Administration support by portraying the IFR as a solution to proliferation and nuclear waste. It’s neither.

Fast reactors were first offered as a way to make more plutonium to augment and ultimately replace scarce uranium. Now that uranium and enrichment are known to get cheaper while reprocessing, cleanup, and nonproliferation get costlier ­destroying the economic rationale­ IFRs have been reframed as a way to destroy the plutonium (and similar transuranic elements) in long-lived radioactive waste. Two or three redesigned IFRs could in principle fission the plutonium produced by each four LWRs without making more net plutonium. However, most LWRs will have retired before even one commercial-size IFR could be built; LWRs won’t be replaced with more LWRs because they’re grossly uncompetitive; and IFRs with their fuel cycle would cost even more and probably be less reliable. It’s feasible today to “burn” plutonium in LWRs, but this isn’t done much because it’s very costly, makes each kg of spent fuel 7x hotter, enhances risks, and makes certain transuranic isotopes that complicate operation. IFRs could do the same thing with similar or greater problems, offering no advantage over LWRs in proliferation resistance, cost, or environment.

IFRs’ reprocessing plant, lately reframed a “recycling center,” would be built at or near the reactors, coupling them so neither works without the other. Its novel technology, replacing solvents and aqueous chemistry with high-temperature pyrometallurgy and electro refining, would incur different but major challenges, greater technical risks and repair problems, and speculative but probably worse economics. (Argonne National Laboratory, the world’s experts on it, contracted to pyroprocess spent fuel from the EBRII ­ a small IFR-like test reactor shut down in 1994 ­ by 2035, at a cost DOE estimated in 2006 at approximately 50× today’s cost of fresh LWR fuel.)

Reprocessing of any kind makes waste management more difficult and complex, increases the volume and diversity of waste streams, increases by several -to manifold the cost of nuclear fueling, and separates bomb-usable material that can’t be adequately measured or protected. Mainly for this last reason, all U.S. Presidents since Gerald Ford in 1976 (except G.W. Bush in 2006– 08) discouraged it. An IFR/pyroprocessing system would give any country immediate access to over a thousand bombs’ worth of plutonium to fuel it, facilities to recover that plutonium, and experts to separate and fabricate it into bomb cores ­hardly a path to a safer world.

IFRs might in principle offer some safety advantages over today’s light-water reactors, but create different safety concerns, including the sodium coolant’s chemical reactivity and radioactivity. Over the past half century, the world’s leading nuclear technologists have built about three dozen sodium-cooled fast reactors, 11 of them Naval. Of the 22 whose histories are mostly reported, over half had sodium leaks, four suffered fuel damage (including two partial meltdowns), several others had serious accidents, most were prematurely closed, and only six succeeded. Admiral Rickover canceled sodium-cooled propulsion for USS Seawolf in 1956 as “expensive to build, complex to operate, susceptible to prolonged shutdown as a result of even minor malfunctions, and difficult and time-consuming to repair.” Little has changed. As Dr. Tom Cochran of NRDC notes, fast reactor programs were tried in the US, UK, France, Germany, Italy, Japan, the USSR, and the US and Soviet Navies. All failed. After a half-century and tens of billions of dollars, the world has one operational commercial-sized fast reactor (Russia’s BN600) out of 438 commercial power reactors, and it’s not fueled with plutonium.

IFRs are often claimed to “burn up nuclear waste” and make its “time of concern . . . less than 500 years” rather than 10,000–100,000 years or more. That’s wrong: most of the radioactivity comes from fission products, including very-long-lived isotopes like iodine-129 and technicium-99, and their mix is broadly similar in any nuclear fuel cycle. IFRs’ wastes may contain less transuranic s, but at prohibitive cost and with worse occupational exposures, routine releases, accident and terrorism risks, proliferation, and disposal needs for intermediate- and low-level wastes. It’s simply a dishonest fantasy to claim that such hypothetical and uneconomic ways to recover energy or other value from spent LWR fuel mean “There is no such thing as nuclear waste.” Of course, the nuclear industry wishes this were true.

No new kind of reactor is likely to be much, if at all, cheaper than today’s LWRs, which remain grossly uncompetitive and are getting more so despite five decades of maturation. “New reactors” are precisely the “paper reactors” Admiral Rickover described in 1953:

An academic reactor or reactor plant almost always has the following basic characteristics: (1) It is simple. (2) It is small. (3) It is cheap. (4) It is light. (5) It can be built very quickly. (6) It is very flexible in purpose. (7) Very little development will be required. It will use off the shelf components. (8) The reactor is in the study phase. It is not being built now.

On the other hand a practical reactor can be distinguished by the following characteristics: (1) It is being built now. (2) It is behind schedule. (3) It requires an immense amount of development on apparently trivial items. (4) It is very expensive. (5) It takes a long time to build because of its engineering development problems. (6) It is large. (7) It is heavy. (8) It is complicated.

Every new type of reactor in history has been costlier, slower, and harder than projected. IFRs’ low pressure, different safety profile, high temperature, and potentially higher thermal efficiency (if its helium turbines didn’t misbehave as they have in all previous reactor projects) come with countervailing disadvantages and costs that advocates assume away, contrary to all experience.

Thorium reactors
Some enthusiasts prefer fueling reactors with thorium ­an element 3 times as abundant as uranium but even more uneconomic to use. India has for decades failed to commercialize breeder reactors to exploit its thorium deposits. But thorium can’t fuel a reactor by itself: rather, a uranium- or plutonium fueled reactor can convert thorium-232 into fissionable (and plutonium-like, highly bomb-usable) uranium-233. Thorium’s proliferation, waste, safety, and cost problems differ only in detail from uranium’s: e.g., thorium ore makes less mill waste, but highly radioactive U-232 makes fabricating or reprocessing U-233 fuel hard and costly. And with uranium-based nuclear power continuing its decades-long economic collapse, it’s awfully late to be thinking of developing a whole new fuel cycle whose problems differ only in detail from current versions.

Spent LWR fuel “burned” in IFRs, it’s claimed, could meet all humanity’s energy needs for centuries. But renewables and efficiency can do that forever at far lower cost, with no proliferation, nuclear wastes, or major risks. Moreover, any new type of reactor would probably cost even more than today’s models: even if the nuclear part of a new plant were free, the rest ­ two-thirds of its capital cost ­ would still be grossly uncompetitive with any efficiency and most renewables, sending out a kilowatt-hour for ~9–13¢/kWh instead of new LWRs’ ~12–18+¢. In contrast, the average U.S. wind farm completed in 2007 sold its power (net of a 1¢/ kWh subsidy that’s a small fraction of nuclear subsidies) for 4.5¢/kWh. Add ~0.4¢ to make it dispatchable whether the wind is blowing or not and you get under a nickel delivered to the grid. (1 US$ =  0.7 Euro)

Most other renewables also beat new thermal power plants too, cogeneration is often comparable or cheaper, and efficiency is cheaper than just running any nuclear- or fossil-fueled plant. Obviously these options would also easily beat proposed fusion reactors that are sometimes claimed to be comparable to today’s fission reactors in size and cost. And unlike any kind of hypothetical fusion or new fission reactor ­or LWRs, which have a market share below 2%­ efficiency and micro power now provide at least half the world’s new electrical services, adding tens of times more capacity each year than nuclear power does. It’s a far bigger gamble to assume that the nuclear market loser will become a winner than that these winners will turn to losers.

Small reactors
Toshiba claims to be about to market a 200-kWe nuclear plant (~5,000x smaller than today’s norm); a few startup firms like Hyperion Power Generation aim to make 10¢/kWh electricity from miniature reactors for which it claims over 100 firm orders. Unfortunately, 10¢ is the wrong target to beat: the real competitor is not other big and costly thermal power plants, but micro power and negawatts, whose delivered retail cost is often ~1–6¢/kWh. Can one imagine in principle that mass-production, passive operation, automation (perhaps with zero operating and security staff), and supposedly failsafe design might enable hypothetical small reactors to approach such low costs? No, for two basic reasons:

• Nuclear reactors derive their claimed advantages from highly concentrated sources of heat, and hence also of radiation. But the shielding and thermal protection needed to contain that concentrated energy and exploit it (via turbine cycles) are inherently unable to scale down as well as technologies whose different principles avoid these issues.

• By the time the new reactors could be proven, accepted by regulators and the public, financed, built, and convincingly tested, they couldn’t undercut the then prices of negawatts and micro power that are beating them by 2–20x today­ and would have gained decades of further head start on their own economies of mass production.

In short, the notion that different or smaller reactors plus wholly new fuel cycles (and, usually, new competitive conditions and political systems) could overcome nuclear energy’s inherent problems is not just decades too late, but fundamentally a fantasy. Fantasies are all right, but people should pay for their own. Investors in and advocates of small-reactor innovations will be disappointed. But in due course, the aging advocates of the half-century-old reactor concepts that never made it to market will retire and die, their credulous young devotees will relearn painful lessons lately forgotten, and the whole nuclear business will complete its slow death of an incurable attack of market forces. Meanwhile, the rest of us shouldn’t be distracted from getting on with the winning investments that make sense, make money, and really do solve the energy, climate, and proliferation problems, led by business for profit.

Source and contact: Amory B. Lovins, Rocky Mountain Institute. 2317 Snowmass Creek Road, Snowmass, Colorado 81654-9199, U.S.A.
Tel: +1 970 927-3851
Web: www.rmi.org