Molten Salt Fast Reactor

Our Nuclear Energy Options — An Overview by Euan Mearns

With a few exceptions [1], environmental lobbies have tended to oppose nuclear power with a vengeance similar to their opposition to coal and natural gas. In certain quarters [2] this has changed with the promise of abundant, cheap and safe electricity that may be produced using thorium (Th) fuelled molten salt reactors. This guest post by French physicist Hubert Flocard places the status of molten salt reactor technology within the historical context of how the nuclear industry has evolved and examines some of the key challenges facing the development and deployment of this magical and elusive energy source. We have both written the extended summary below based on Hubert’s article that follows on after the summary. Hubert’s impressive bio is at the end of the post.

[1] James Lovelock, The Revenge of Gaia
[2] Baroness Worthington, Why Thorium Nuclear Power Shouldn’t be Written Off

Extended Summary

The world nuclear industry currently runs on Generation II and Generation III reactor technology. The presently active reactors (whether moderated by pressurised water – PWR – or boiling water – BWR) are said to belong to the GII generation while more modern versions such as the EPR or the AP1000 correspond to GIII. At the beginning of the twenty first century a forum was convened to establish an international collaboration to prepare the next generation of reactor technology (GIV). A number of design options were on the table (see below) among them molten salt reactors.

1) Liquid Sodium Fast Reactor (SFR)
2) Helium Cooled Fast Reactor (HeFR)
3) Liquid Lead Fast Reactor (LFR)
4) Supercritical Water Fast Reactor (SCFR)
5) Molten Salt Fast Reactor (MSFR)
6) Very High Temperature Thermal Reactor (VHTR)

With the exception of the MSFR, that is specifically designed to run on Th fuel, all other technologies will run on U fuel. It is also worth noting that 5 of the 6 designs are fast breeder reactors designed to consume any nuclear waste that they may produce and to extend the life of the global inventory of U and Th that is available to us.

Periodic table from Web Elements.

To appreciate the evolution of reactor technology it is important to understand a little bit about the natural elements on Earth which can be made to fission following the capture of neutrons. They are the actinides located at the bottom of the periodic table. Everyone has heard of uranium (U), thorium (Th) and plutonium (Pu) but are less aware of elements like protactinium (Pa), americium and curium. Some of these less common actinides do exist in nature in minute quantities for brief periods as part of the natural radioactive decay of U to Pb. Others result from the nuclear reactions happening in reactors or at laboratory accelerators.

The isotopes of interest are 235U, 238U and 232Th. Presently, the 235U isotope is by far the most useful because it is the only one which can easily be made to fission, releasing a substantial amount of energy. Thus 235U is described as fissile while 238U and 232Th are described as fertile. Today, 99.3 % of natural U is 238 and only 0.7 % is 235. That is because most of the 235U has already decayed away to stable Pb.

Out of these three isotopes only fissile 235U can be used to initiate a nuclear chain reaction such as those that occur in nuclear reactors or atomic bombs. To achieve a chain reaction it is necessary to enrich the uranium in its 235 isotope. For nuclear power, enrichment is typically about 3.7 %, i.e. a five-fold uplift in concentration as compared to natural uranium. For atomic bombs, the enrichment is much higher, but the same procedure is used, hence concern over civilian nuclear programs in certain countries.

While fissile 235U is required to initiate a chain reaction, the fertile 238U that makes up 96.3 % of the fuel participates also in the energy production since some of it is converted to fissile 239Pu. In this respect all U based reactors breed fissile fuel by tapping into the fertile resource. Breeder reactors are simply designed to breed more fissile fuel than they consume.

Three important points need to be made before continuing. The first is that an MSFR can’t start by using only 232Th. The reactor will first require that either natural 235U or man-made 239Pu be added to initiate the fission chain reaction, since fertile 232Th cannot achieve criticality on its own. The second is that the MSFR is a breeder reactor and environmentalists have in the past opposed breeder technology. In a breeder of any design, fertile 238U or 232Th isotopes are converted to fissile isotopes like 239Pu (U cycle) or 233U (Th cycle). A MSFR will run exclusively on the thorium cycle (i.e. without addition of U5 or Pu9) when it will have bred enough 233U to maintain the chain reaction. It will take time. The “clean” label that some attach to MSFRs derives from the fact that ultimately they are designed to work in a closed cycle as opposed to the present open cycle strategy adopted for most of presently active reactors. In other words, the spent fuel is reprocessed and fissioned again and again until a stable regime is reached in which as many fissile isotopes are created than are destroyed. It has little to do with the fact that 232Th is used as the breeder fuel stock. A uranium cycle fast breeder will also burn its “waste”. And as already mentioned, the idea underlying breeding is to greatly expand the fissionable resource by converting the abundant fertile isotopes (238U as well as 232Th) into the fissile variety.

This leads to a misconception about the quantities of nuclear waste generated by an MSFR. An MSFR burning 232Th fuel will not produce significantly smaller amounts of “waste” than a fast reactor burning 238U. It is just that as already detailed, recycling the breeder isotopes eventually removes them from the environment and stabilises the inventory within the reactor.

A further misconception is that MSFR technology employing 232Th as the fertile proto- fuel will eliminate risks of nuclear proliferation. While it is true that the 232Th cycle does not produce plutonium that may relatively easily be enriched to weapons grade 239Pu, it does produce 233U instead which may also be weaponised. Anyhow a 232Th MSFR started today will require either 235U or 239Pu to initiate the fission reaction. Any country with the appropriate enrichment facilities could divert the use of these isotopes and convert them to weapons grade material if they so wish. Recent history has also shown that one does not really need a reactor to manufacture a bomb. It is enough to have efficient centrifuges.

In conclusion, the technical challenges of MSFR technology need to be considered. The molten fluorine based salts that are envisaged need to work at temperatures in the region 500 to 800˚C and containment vessels and pumps need to be designed which resist erosion, corrosion and the neutron flux from this high temperature salt. An MSFR requires a fuel reprocessing plant and for the Th cycle no such plant has thus far been designed built, tested and approved by safety authorities. Finally, there are well-understood safety protocols for GII and GIII reactors. The radical new approach offered by MSFR technology means that a whole new set of safe design principles needs to be developed.

At the end of the 1960s The Oak Ridge National Laboratory built and ran an experiment MSR-E designed to pave the way for the MSFR technology. The experiment ran for 4 years. Apart from that realisation, MSFR with a thorium-based fuel is a concept yet to leave the drawing board. It is worth pursuing, but the claimed virtues of near inexhaustible resource, enhanced safety, less waste and elimination of weapons proliferation still need to be demonstrated.


There are people who believe that, within this century and probably even before 2050, nuclear energy should become a major component in the energy production system, if not for the entire world at least for a large group of countries. They point to some valuable features of nuclear energy (centralized production of electricity and/or heat, reasonably low cost of final energy, a production that can easily be adjusted to society needs, low CO2 emissions, small footprint, etc.). They are also well aware of some of its disadvantages (global bad image in the public inducing significant unpredictable political interventions, limited availability of the natural resource, radiotoxicity of the waste, plant accidents with a related risk of releasing radioactivity, security against terrorist attacks, heavy capital investment only reimbursed over a long period, etc.). They just think that given the energy and climatic problems the world is facing now or is going to face in a not too distant future, the advantages more than balance the liabilities.

However, not all these people have the same nuclear energy on their mind.

For some, the basis of a sensible nuclear program for this century must rely first on the extensive experience accumulated on thermal-fission reactors for which the terms Generation II or Generation III (shortened in GII and GIII) have been coined and second on the already significant experience gained on fast-fission reactors. “Improvement and Optimisation” is their motto while Uranium (U) is their fuel. I belong to this group to which the adjective “conservative” can certainly be attached.

GII and GIII water-cooled and water-moderated reactors are the workhorses of the present nuclear-electricity production. If not stopped for political reasons, they will be performing their job for many more decades. On the other hand, the “fast” reactors cooled with liquid sodium have been tested successfully in many countries and together have already accumulated several hundred years of operation. They have reached a prototype status and even the pre-industrial stage. The main world safety authorities have already a thorough knowledge of the related safety questions. These reactors have also demonstrated their potential on issues such as electricity production, breeding of the fuel (a key to solve a future uranium resource shortage) and waste transmutation.

However, no western world safety authority – and therefore no utility – would consider today that their safety is such that they can be deployed at the industrial level. To simplify, one can say that they have not yet demonstrated the safety level achieved by GIII reactors which is now becoming the standard. Moreover, given the present very low price of natural uranium, they are not economically competitive.

For this reason, in the middle of the first decade of this century, a forum, the “Generation IV International Forum” (GIF), was launched associating the major nuclear industrial nations of the world (with the notable exception of India – a country named “Europe” allows also some nations, such as Germany, to participate in the activities of GIF without having to state explicitly that they are a GIF member). These nations gave themselves the task of defining the next generation of nuclear fission reactors (GIV).

According to GIF, the goals assigned to GIV reactors are the following: 1) Durability which involves a better usage of the natural resource and a minimisation of waste radiotoxicity 2) Economic performance 3) Safety and availability 4) Resistance to nuclear proliferation.

GIF identified six main lines of work suitable for an international cooperation: 1) liquid Sodium Fast Reactor (SFR); 2) Helium cooled Fast Reactor (HeFR); 3) liquid Lead Fast Reactor (LFR); 4) Super Critical water Fast Reactor (SCFR); 5) Molten Salt Fast Reactors (MSFR); 6) Very high temperature thermal reactor (VHTR). Except for MSFR all systems under study envisage uranium as their fuel. The MSFR will use thorium (Th) as a major component of its fuel. Option N°6, VHTR, being a thermal reactor precludes breeding from the start and thus very long term durability as far as the uranium resource is concerned. The rationale for keeping it within GIF is that working at high temperature and thus high Carnot efficiency, such systems will considerably extend the availability of the U resource. It should be added that other thermal-reactor options using uranium fuel and either supercritical water or molten salt as coolants are also being considered on the side-lines of GIF.

As a matter of fact, the selection of the GIV reactor options reflects as much the evaluation of their intrinsic interest as the willingness of at least a fraction of the international expert community to work on them (many more nuclear options do exist). Not too surprisingly, presently, the main effort is focused on the SFR (liquid Sodium Fast Reactor) which appears closer to reach the GIF stated goals than any of its competitors. Of course, since the Fukushima accident, which has set nuclear energy research and industry on the defensive and modified its priorities, activities have considerably slowed down within the GIF.

All the GIF-retained options other than SFR can certainly be called “innovative” (as opposed to my definition of “conservative”). Among them, the one using molten salt and thorium based fuel (MSFR) has gained many supporters in the public, if not necessarily within the community of experts. I believe that some of the enthusiasm for thorium and MSFR is misplaced in view of the present scientific and technical situation – keeping in mind that I am concerned with energy production for the 21st century, not for the centuries beyond. Because the text that follows tries to show that, for me, some supporters wave too simplistic arguments, I would like to make it clear that I think that, MSFR and Thorium fuel is definitely worth both consideration and intensive research.

First, the fact that the MSFR was retained by the international community of experts working within the programme of GIF is a sure sign of its viability. Second, thorium and molten salts have an old history dating almost from the end of the second world-war and some significant advances have been made. The main achievement was realised by the Oakridge National Laboratory (ORNL) with the successful MSR-E experiment (which used uranium fuel). Then, over the seventies, ORNL teams worked on the design of the MSBR, a 1GWe system which intended to have Thorium within its fuel (the B stands for “breeder” and the “e” is here to indicate the expected electric power which is of course lower than the thermal power). However, at the beginning of the eighties, in the US-breeder competition, the MSBR system lost to the SFR. The decision was a complex one but the smaller breeding capacity of the MSBR had a part in it. At that time, a review conducted by the French utility EDF and the French nuclear atomic commission (CEA) analysed the MSBR project and concluded that nothing could be identified which would eliminate the option either from the point of view of chemical or material science, or of nuclear and thermal-hydraulics technology. There were still many difficult open questions but no obvious showstoppers.

Therefore, keeping molten salts and thorium as an open research option for the future makes sense today as it did earlier. There are many good reasons to investigate it that I am not going to enumerate. Here, after this long introduction, I will make a survey of what I believe are the false justifications (myths) and the many unsolved problems which make it doubtful that MSFR and Thorium can play a significant role in the global power generation of this century.

Some myths concerning thorium and molten salt reactors

Myth 1: specificity of an “inexhaustible” Th natural resource

Only elements of the actinide region of the Mendeleyev periodic table can be made to fission following the capture of a neutron. As they fission, in turn, they emit neutrons allowing a chain reaction to be established under appropriate physical and technical conditions. Of all the actinides which existed when the Earth was formed, about 4.65 billion years ago, only two have survived in sizeable quantities: thorium and uranium. Thorium only exists today as the isotope 232 (232Th shortened as Th2). From its very large radioactive half life, one can infer that almost all the Th2 which existed at the birth of the Earth is still around us. Natural uranium contains two isotopes 238U (U8) and 235U (U5). While half of the original U8 is still there, only 1/100th of the original U5 has survived, the rest has disappeared via natural radioactive decay processes ending at a stable Pb isotope. This is reflected in the present natural uranium composition: 99.3 % U8 and 0.7 % U5. When nuclear engineers or opponents of nuclear energy talk about a limited uranium resource, what they have in mind is U5, not natural uranium (or U8) which is a hundred times more abundant.

For nuclear engineers, Th2 and U8 which have an even number of neutrons belong to the same category: the “fertile” isotopes while U5 is said to be “fissile”. To be started, any reactor needs a fissile element. As a matter of fact, for the vast majority of reactors in activity the concentration of U5 within natural U is not sufficient, hence the need for enrichment typically up to 3.7 %. Note that a few billion years ago, the ratio U5/U8 was larger than today, so that natural reactors could operate spontaneously as happened for instance at the Oklo site in Gabon. In today’s reactors, the presence of fertile U8 within the fuel pins is also important for energy production. Indeed, a small fraction of this U8 swallows one neutron and is transmuted (in two steps) into the isotope 239 of plutonium (Pu9) which because it is also fissile can contribute to the chain reaction which ultimately produces energy. In other words some small amount of “breeding” is already occurring in thermal reactors.

Th2 is the nuclear equivalent of U8 (233U or U3 plays for Th2 the role that Pu9 plays for U8). Because there is no fissile isotope present within natural thorium, in order to start a thorium-fuelled reactor one must add first some fissile material. Since it can’t be U3 which does not exist on Earth it could be U5 (from the same natural uranium which provides the fuel of today reactors) or Pu9 (coming for instance from the burnt fuel of standard reactors) or other fissile materials to be found for instance in the radioactive waste of standard reactors. In other words Th2, like U8, only acquires the status of an energy resource when breeding is envisaged. The only available natural resource to initiate breeding is U5.

In some presentations to the public, “breeding” appears to perform a sort of miracle: “producing more fuel than was present within the input”. It should rather be described as “producing more fissile material than was present within the input”. It is the energy potential of a fertile isotope (Th2 or U8), a kind of “fission-proto-fuel”, which is then exploited following an appropriate transmutation into either U3 or Pu9.

The U8 resource appears almost as inexhaustible as the Th2 resource (a factor 2, or 4 less does not really modify the issue, given the geology-related uncertainties). In addition, over the years, the U8 resource has acquired a significant advantage: it does not have to be mined anymore. It is already on the shelves in large quantities at least in countries which have a nuclear enrichment industry and its commercial value is zero, if not negative. As a matter of fact U8 is sometimes considered as a sort of “waste” extracted from natural uranium to obtain the U5-enriched fuel for standard reactors. Indeed for each U8 nucleus still kept in the fuel of a GII or GIII reactor, about four U8 nuclei have been removed from natural uranium and stored away. As an illustration, presently, the stock of U8 stored in France corresponds to about one thousand year of this country’s present energy production in fast reactors. Note that the former French rare-earth chemical industry has also left on the shelf a quantity of Th2 amounting to about 100 years of nuclear energy production in a MSFR. The “nuclear-fertile” resource, Th2 as well as U8, is plentiful.

In fact if there is to be a resource shortage preventing a future GIV breeder-reactor generation to replace the reactors of GII and GIII generations, it will certainly not be one of fertile isotopes (Th2 or U8) but rather a shortage of fissile elements and more specifically one of Pu9. It also appears that only those countries which have exploited PWR or BWR reactors for a long time will have produced enough Pu9 within the burnt fuel of their GII and GIII reactors to be in position to start reactors of the GIV generation at a significant level.
Myth 2: the waste of Th fuelled reactor waste is less dangerous.

There are few points to keep in mind when one discusses nuclear waste:

1) How to define nuclear waste is not simple when breeding and recycling is involved (always the case with Th). Indeed, the only unambiguous waste produced by a nuclear reactor consists of the fission products. All elements, Th, U, Pu or other isotopes generally classified as “minor actinides” present in the burnt-fuel when it is discharged from the reactor vessel still have potentially an energetic value if they can be made to fission. It is thus a matter of technical, safety and political decisions to consider whether they belong in the waste or whether they are a fuel to be recycled in the next stage of the operation of the nuclear system.

2) To illustrate this last sentence we can, for instance, consider how most countries today define the nuclear waste resulting from the operation of their GII or GIII reactors. These countries have opted for the “open-cycle” or “once-through” strategy: there is no reprocessing; the fuel pins and their casing discharged from the reactor are considered to be a waste and destined to an ultimate repository. A typical composition of the burnt fuel of a GII reactor is: fission products 5 %, fissile isotopes 1.5 %, and fertile isotopes 93.5 %. Thus 95 % of what is today defined as a nuclear waste has, “fissionwise”, an energetic potential. One can also note that the ratio of the mass fissile output (U5 plus Pu9) over that of the mass fissile input (only U5) is close to 40 %. The choice has thus been made to send to the waste a significant amount of fissile isotopes which on the other hand are known to be necessary to start any reactor and also cost energy to produce via enrichment of natural uranium. In a sense, the corresponding waste underground repository can also be caricatured as a “man-made plutonium mine”.

3) In a comparison of the thorium-cycle versus the uranium-cycle, the radiotoxicity of the fission fragment waste which dominates the total radiotoxicity for the first centuries can be set aside. It is roughly the same for both cycles. Thus any difference between the two cycles will only be visible after a few centuries have passed, say 500 years.

4) How to define the danger associated with a waste which has been sent to a permanent underground storage is also a matter of discussion. One can consider the total radiotoxicity of what is being stored. This is the radiotoxicity that, for instance, would be encountered by somebody, not too expert in questions of geology, searching for oil at the wrong place, who drills right into the underground nuclear waste repository. After a few centuries, this radiotoxicity is dominated by the actinide content of the waste. Since it is not the same for the two cycles, we shall return to that point later. On the other hand one may consider the small radiotoxicity – often smaller than natural radiotoxicity – which after many millenniums escapes to the surface through the geological barrier (the repository is typically few hundred meters below the Earth surface). Since the mobility of actinides in the ground is very small, the very-long-term escaping radiotoxicity will mostly correspond to some long lived isotopes of very mobile fission-fragment elements (for instance Zr or I). Here again there won’t be much difference between the thorium and uranium cycle. For this reason, from then on, I will only discuss the radiotoxicity associated with the actinide elements, namely that which would be met by somebody who breaks into an underground nuclear waste storage.

5) Full recycling means that all the actinides coming out at one stage are reinserted into the fuel of the next stage. Thus the large radiotoxicity within the burnt fuel does not vanish; it is just made to move around circularly from the reactors to the separation cells to the fuel production factories and back to the reactors within the diverse nuclear-industry components. On the other hand, if the recycling process (whether for the U-cycle or Th-cycle) is perfect there won’t be any radiotoxic waste stream other than that of the fission products.

6) After each pass through the reactor, recycling implies that a chemical separation is performed on the burnt fuel. Because no chemical process is perfect, the stream of actinides effectively going out to the waste and determining its middle-to-long term radiotoxicity (short term is governed by fission fragments) depends on the efficiency of the chemical separation techniques. For the uranium-cycle, efficiencies above 99.9 % have been demonstrated. The corresponding figures for the thorium cycle are not known.

7) Full recycling is also not currently envisaged for the U-Pu SFR technology. It is generally considered that only plutonium will be recycled while minor actinides such as americium (Am) and most certainly curium (Cm) will be sent to the waste. This strategy along with the efficiency of the chemical separation determines the time evolution of the radiotoxicity of the waste stream of SFR reactors. It is several orders of magnitude below that of the burnt-fuel stream of today’s reactors. I will come to that point later because I believe it gives a misleading image of the benefit of recycling as concerns waste reduction (see 10 below).

8) Assuming that the not-yet-known chemical separation efficiencies for the thorium cycle are the same as those already demonstrated for the uranium cycle, it can be shown that the radiotoxicity of the actinide waste stream of a MSFR reactor when it is working in its “asymptotic” regime, that is a system relying exclusively on the Th2-U3 cycle, is lower by at least an order of magnitude than that of a SFR reactor working exclusively within the U8-Pu9 cycle and sending americium and curium to the waste. However, this good point should also be taken with a grain of salt.

9) Indeed, while following a long period of production with GII and GIII reactors, one can envision extracting from their stored burnt fuel all the Pu9 necessary to start immediately an “asymptotic” SFR reactor. This is not possible for a MSFR. The U3 resource does not exist. No “asymptotic” MSFR reactor can be started today. The first MSFR reactors will have to use either U5 or Pu9 – or some other isotopes of higher elements – to be started. Via transmutation they will therefore produce the same undesirable elements (Am and Cu) and thus the same kind of waste as SFRs. It will take almost a century before a MSFR breeds enough U3 to avoid tapping into the U5 and Pu9 resource and thus become “asymptotic”.

10) Finally, many presentations on the actinide waste generated by fast reactors implicitly assume that mankind will rely on them forever. In other words, these presentations only consider the radiotoxicity of the waste stream which leaves the chemical reprocessing factories while electricity is still being produced by reactors. In such a case, the radiotoxic gain over the present situation (the nuclear burnt fuel is disposed without reprocessing into the long term repository as it leaves the reactor) is indeed large (several orders of magnitude). On the other hand, if one day, fusion becomes an economically viable option or if there is a major breakthrough on the energy storage question rescuing renewable intermittent energies, humans may decide to stop producing electricity via nuclear fission. At that point, all the radiotoxic isotopes present within the system – reactors, chemical and fuel fabrication factories – become a waste that must be added to the stream of the earlier electricity production period. If one assumes for instance that it will take 200 years of operation of breeder reactors before one reaches this “end of the game” situation, one finds that it is the addition of this “in-cycle” radiotoxicity which mostly determines the radiotoxic evolution of the waste during the following millenniums. Then, the radiotoxicity of the total MSFR waste will only be slightly lower than that of a SFR. There will also still be a small gain over the present strategy in which only GII and GIII reactors are used and their burnt-fuel is disposed without reprocessing. Typically we are talking here of decreases by one order of magnitude if everything in the complicated recycling scheme works optimally.

11) It is doubtful that such a small gain can suppress the opposition to the usage of nuclear energy of somebody whose main concern is the very-long-term radiotoxicity of the waste. It will also not enable societies to find a stable and long-term safe waste management solution if only for containing the radiotoxicity of the fission fragments. One can say that the nuclear waste issue – and the need for underground repositories – is not going to be removed by SFRs or by MSFRs. At most, it will be alleviated, which certainly is a plus. In addition, one may note that at least, it will be up to the countries which have benefitted from the associated electricity production to solve their own nuclear waste problem. This appears more ethically defendable than the fossil-fuel-powered electricity production in which the CO2 emitted by the beneficiaries of the electricity is graciously “offered” to the rest of the world.

12) To conclude this section on nuclear waste, one should not forget that volume, chemical properties and short-term heat production also play an important role when it comes to designing a repository.

Myth 3: The thorium cycle will eliminate the nuclear proliferation issue

A bomb needs fissile material. Neither U8 nor Th2 are good materials for making bombs. It is certainly the case that no bomb has been produced using U3 since there is no U3 available. Whether that will still be the case when U3 becomes plentiful and is routinely handled in reprocessing units is certainly not clear to me. Discussions about larger or smaller critical masses are essentially irrelevant here. In addition, as was discussed above, for a very long time a MSFR will be using U5 or Pu9, which means that the possibility of diverting these isotopes for a dangerous purpose will remain.

I believe one should not count on the physics (Th vs U) or the technology (MSFR vs SFR) to stop humans from doing foolish things. Non-proliferation has certainly technical aspects which require nuclear expertise to be present and heard in international discussions but, for me, proliferation is mostly a political issue.

Problems still to be solved problems for a molten salt thorium fuelled reactor

Here, I list some of the problems still faced by the MSFR technology.

Problem 1: Design and Material science.

The associated questions concern the salt, the vessel and the heat exchanger.

  • In a MSFR, the salt acts both as a heat carrier and nuclear fuel carrier. It also has some moderating (i.e. slowing down neutrons) effect which precludes for instance its usage for breeding with the uranium cycle. The salt must stay stable within a wide range of rather high temperatures (typically from 500°C to 800°C). A family of fluorine based salts is presently being considered. These salts should resist the high neutron fluxes within the vessels (their chemical structure must remain intact and their elements should not suffer transmutation). They should dissolve the actinides of the fuel at the required concentrations and keep them dissolved all along the circuits of the reactor (vessel, heat exchanger and connecting pipes) in variable temperature and fluid velocity conditions so as not to create unwanted deposits of nuclear material.
  • The material for the vessel and the heat exchanger (Ni-based alloys are being considered) should resist both mechanical and chemical corrosion by the salts on the inside surface and oxygen corrosion at high temperature on the external surface.
  • Salt is not as good a heat carrier as liquid metals. The design of heat exchangers capable of rapidly (typically less than 10s) extracting heat while resisting the mechanical corrosion by a fast moving salt is still a challenge.
  • The demonstration that valves and pumps capable of working reliably for many years with such salts under the planned temperature and fluid velocity conditions is not yet done.
  • The most harmful fission fragments that can poison the reactor must be eliminated on-line via the helium bubbling technique. This has not yet been demonstrated in situations close to those that will exist in a future MSFR. The material for the vessel and the heat exchanger (Ni-based alloys are being considered) should resist both mechanical and chemical corrosion by the salts on the inside surface and oxygen corrosion at high temperature on the external surface.

Each of these points should reach a status such as to receive an agreement from the safety authorities.

Problem 2: Chemistry of the combined uranium and thorium cycles

A thorium-fuelled molten-salt reactor has to be coupled to a highly efficient chemical unit to reprocess the fuel and the salt. The element-separation efficiencies should be as high as those which have already been reached at units designed for reprocessing within the uranium cycle. Presently, the scientific knowledge and technological knowhow needed to build a working prototype of a Th-cycle reprocessing unit with such performances does not exist.

Two reasons for this situation can be advanced. First the amount of man-year work on the thorium cycle is minuscule compared to that already spent for the uranium cycle. Second, the chemistry is different. First, some oxidation-potential properties of Th are not as favourable as those of U. Second, since U3 is not available and because the U5 resource is limited, one generally presents the first MSFRs as “nuclear waste burners” which will use (and destroy) the plutonium of the waste of GII and GIII reactors (some even mention higher actinides) as their initiating fissile isotope. This makes the chemistry more complicated since it must be able to handle simultaneously elements belonging to the Th and U cycles.

Problem 3: Design of a global strategy for safety

The general philosophy underlying the safety scheme of today’s reactors was elaborated over many years. It relies on the so-called “in depth defence” which requires the existence within the reactor of three barriers which have to be breached before some radiotoxic material is released to the outside world. Typically, in GII and GIII reactors, they correspond in succession to the metallic envelope of the fuel pins, the boundary of the primary circuit (vessel, primary heat exchanger) and finally the reactor building.

Even when reprocessing is performed (the situation in France) so that other sets of safety regulations have to be defined (approved and enforced) for the chemical separation unit, and the fuel-pin fabrication factory, this does not affect the general safety programme for the reactor itself. Indeed, there is still a clear physical separation between these three components of the global nuclear system. This means that the safety scheme as it exists today for GII and GIII reactors and is understood jointly by the designers of reactors, the electric utilities operators and the members of safety authorities can also be applied to SFRs. These three groups of experts may certainly argue over the implementation of the various safety items and their performance levels but at least they agree on the goals and they share a common safety language.

Nothing of the sort exists for the MSFR in which at least one barrier is a priori missing (the metallic envelope of the fuel) and which combines on the same site the reactor and the chemical reprocessing unit whose activities directly affect each other. It is my guess that no significant work has been done to define a safety scheme for molten salt reactors since the MSBR was abandoned in the first half of the eighties. What competence on this subject existed at that time is probably either obsolete today in view of the steady reinforcement of nuclear safety or simply lost. This competence has to be rebuilt, something which today appears rather problematic.


In my opinion, Thorium and molten salt reactors technologies belong definitely in the domain of research. They certainly have a potential which deserves scientific and technical investigation. On the other hand, given the present situation of the nuclear energy research institutes of the western world and the general decline in their enrolment of high-quality well-trained young engineers, it is improbable that much work will be invested into such an innovative, far-reaching but also risky option. Therefore if nuclear energy is to provide a significant contribution to the world energy mix of the 21st century, it is doubtful that thorium and molten salt technologies will be ready in time to take part.


[1] The adjective « thermal » refers here to the average kinetic energy of neutrons as they impact the heavy nuclei in the nuclear fuel. They are close to 1/40 eV (or 273°K or 0°C). On the contrary, in a “fast” reactor, the fission-neutrons are not slowed down by water so that their kinetic energy remains in an MeV range that is a factor of 107 above that in a thermal reactor. Without getting into nuclear physics details, it suffices to say here that only fast neutrons allow efficient breeding at least for the uranium cycle. The situation is different for the thorium cycle which can breed over a wide range of neutron kinetic energies, albeit less efficiently that in a fast U-Pu reactor.

Short Bio for Hubert Flocard

A former student of the Ecole Normale Supérieure (St Cloud) Hubert Flocard is a retired director of research at the French basic science institute CNRS. He worked mostly in the theory of Fermi liquids with a special emphasis on nuclear physics. He has taught at the French Ecole Polytechnique and at the Paris University at Orsay. He was for several years a visiting fellow of the Lawrence Berkeley Laboratory and he spent a year as visiting professor at the theory department of MIT (Cambridge). He has worked as an editor for the journals Physical Review C and Review of Modern Physics (APS, USA) and Reports on Progress in Physics (IoP, UK). He has chaired the nuclear physics scientific committee INTC at CERN (Switzerland). When the French parliament asked CNRS to get involved in research on civilian nuclear energy, he was charged to set up and to manage the corresponding CNRS interdisciplinary programme. He still acts as a referee to evaluate research projects submitted to Euratom.

EDITORS NOTE: This column originally appeared on the Energy Matters website. The featured image is of the inner working of a Molten Salt Fast Reactor.

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