Generation IV reactors raise many hopes and expectations, in terms of optimised use of resources, reduced wastes, better safety factors. They are still on the drawing-board today, but may replace, somewhere in the future, Generation III reactors (the EPRs) considered as more efficient than today’s PWRs (pressurised water). Physicist Dr Daniel Heuer is currently studying one of the 6 concepts pre-selected in 2008 by the yearly Generation IV International Forum (that set the priority orientations), viz., the concept of molten salt reactors (MSRs) associated with the thorium cycle. What exactly are the advantages of this new technology? Will MSRs earn their place in nuclear power production?
Just like uranium, thorium is an element in the actinides, i.e., heavy, radioactive metals. Thorium is naturally present in the Earth’s surface layers, in quantities estimated recently at three times that of uranium. India, Turkey and Brazil have large reserves. In its natural state, it is not a fissile element, i.e., does not undergo a nuclear fission process. Nonetheless, if a thorium atom absorbs a neutron, it transmutes to U-233, which is an excellent fissile fuel. This is the process that underpins design of thorium nuclear power stations; it has been known since the 1950s.
The reactor design studied at Grenoble is still theoretical with its specific feature, being able tot use a liquid state fuel: the thorium is dissolved in molten salts that circulate in the reactor core.
ParisTech Review: The concept of molten salt reactors associated with the thorium cycle, sometimes referred to as Molten Salt Fast Reactors (MSFRs) is a newcomer among Generation IV reactor designs. What are their advantages?
Daniel Heuer: The main interest in MSFRs lies in their intrinsically high safety factor resulting from use of a liquid fuel phase. Reactors embodying this design prove extremely stable. Stability here relates to liquid expansion phenomenon and in the case of a liquid fuelled reactor, this is a self-regulating process, i.e., when the core begins to overheat, the fuel expands and exits the nuclear reaction zone, with the surplus fluid moving to a specific holding area. Stabilisation of reactor heating and fuel surplus shift takes a maximum of only several tens of seconds.
A second inherent advantage, in terms of safety factors, is that in a potentially dangerous situation, the core can be totally emptied, i.e., the entire liquid fuel load can be removed and placed in protected areas. We know that at Fukushima the problem was that the cooling systems were down and the (solid) fuels remaining in the reactor cores continued to heat up owing to residual core nuclear reaction heat. Should this happen with a liquid fuel – and if such a situation should ever occur – you only need to empty the fuel from the core vessel and store it in specific tanks. These tanks are designed to evacuate excess heat in a totally passive way. Emptying can be facilitated using what we call cold plugs: when the electricity supply is cut, these plugs melt and the liquid is immediately released. No special manoeuvres are needed – the reactor vessel will empty in a few minutes only.
These advantages accrue from the fuel’s liquid phase, whatever the fuel actually used. So, why was thorium chosen, in particular?
Thorium allows you to assemble a “breeder” reactor, i.e., once the nuclear reaction is underway, the reactor regenerates the fissile material ‘spent’ during the fission process.
The breeding function is only possible by combining two fuels, uranium and thorium. If, moreover, you want to use a liquid phase fuel, then thorium proves best. On the other hand, if you do not want to avail of the breeder function, you can in fact use any fuel in an MSFR design. This represents another advantage, offered by the inherent stability of such reactors.
This means that various fuels can be used as time goes past, depending on their availability and on choice of specific fuels. In particular, an MSFR can be loaded with all the actinides produced by today’s reactors, such as plutonium.
To be specific, does this mean that we shall be able to ‘burn’ some of the nuclear wastes that we must store in repositories today?
Absolutely, excluding totally spent fission products. Let me recall for your readers that fission products are atoms that result from fission of uranium and plutonium atoms. The other forms of nuclear reaction waste are mainly actinides. These are high toxic and have very long half-lives; for these two reasons it is specially interesting to be able to recycle them as fuels rather than have to store them.
Let’s take MOX as an example. This fuel combines a mix of plutonium and depleted uranium and was designed as a means to recycle plutonium, but it proves very difficult to retreat once spent, i.e., once it has been irradiated in the reactor core. We do not, as yet, know what to do with irradiated MOX but it could be used as fuel in an MSFR. In this configuration, the MSFR acts as an industrial incinerator, in addition to producing electricity.
Earlier on, you mentioned the breeder characteristic of this the MSFRs. We take this as meaning that this factor, common to other generation IV designs, is very attractive in terms of resource management …
The fact remains, that when you resort to this sort of reactor, the issue of resource procurement is no longer relevant. The main fuel in conventional pressurised water reactors (PWRs) is fissile matter, present at less than 1% content in natural uranium. In contradistinction, the fuel in a breeder reactor is a fertile matter, capable of transmuting into a fissile form of matter by neutron capture.
This fertile matter has 99% natural uranium and 100% natural thorium. So, instead of just using 1% of the natural resource, you can now use 100% and still have the same heat generation efficiency. In this perspective, the estimated reserves of the fuel source moves from 100 years to 10 000 years; in other words, we are talking about a quasi-infinite resource.
I may add here that the fact that thorium reserves in the Earth’s surface layers are estimated to be three times higher than uranium is not a decisive factor when it comes to choosing this fuel rather than others. Whether the reserve horizon is 10 000 or 30 000 years does not really make any difference.
Another sizeable advantage of installing MSFRs is the dramatic reduction you obtain of the level of waste matter produced during operations. In this kind of reactor, the irradiated, spent fuel is removed from the core, retreated and then replaced in the core to use the fissile matter produced again. In particular, the actinides and among them, notably the plutonium, are reinjected into the reactor core. In this way, you no longer produce this sort of wastes, the most hazardous to store.
But MSFRs do have some constraints, don’t they, among which the fact that you need fuel from a conventional reactor to trigger the chain reaction.
Correct; what we call the fertile matter, including thorium, cannot undergo fission in its natural form. Thorium atoms only transmute into fissile material by gaining an electron each. In order to produce and use this neutron, you must initially trigger the chain with some fissile matter. You either need some plutonium as produced in conventional reactors, or use enriched uranium which means starting from natural uranium. But we actually produce quite a lot in our conventional reactors and so for, in our studies for an MSFR, this is not at all a constraint.
Are there some technical problems that remain to be solved?
So far, we have not identified any major technical blocks, i.e., problems that we consider insurmountable? So the a priori answer is that if we want to assemble such a reactor, there would be no major technical difficulties. Naturally, we are talking here about a very new form of technology, so we need to spend money and take the time needed to optimise the design, and to validate a number of points, sequentially, before we reach the point of launching a full-scale MSFR nuclear reactor assembly programme.
Firstly, we have to validate some design-demonstration points, to allow us to build a low powered operational demonstrator. In China, research scientists have already begun to do this and they assert that they will have a demonstrator up and running in 2017.
So, China is also studying the MSFR concept?
No, as a first step, the Chinese want to reproduce what the Americans achieved in the 1960-1970s, viz., assemble a liquid fuel reactor using a different design, called MSBE (Molten Salt Reactor Experiment), with a wide thermal spectrum. The problem is that is very difficult to imagine building a scale one breeder reactor based on this concept.
Is there any advantage, in this instance, in using this other concept?
The Chinese have taken onboard the advantages of the new concept (MSFRs) but they wish to progress to this stage only after 2017. They have chosen this policy on purpose: it is indeed easier for them to begin with the wide heat spectrum model, inasmuch as that approach enables them to gain much needed know-how: about pump design specific to circulating moving molten salts, materials handling equipment and processes, heat exchange units … The rapid spectrum we are envisaging in France does carry some significant advantages, albeit with high constraints, notably when it comes to strength of materials used.
But the Chinese have shown they are capable of progressing rapidly. In February 2011, the Chinese authorities, decided to earmark 250 M$US to engage in molten salt design; since that date they have erected the building and have some 400 people working on the subject.
Concerning the rapid spectrum model, their aim, apparently, is to have a demonstrator up and running in 2025, 8 years after commissioning their thermal spectrum demonstrator.
And what about India? Are they also working on thorium cycle reactors?
Over the past 50 years, the Indian nuclear power sector has been trying to modify national policy to adopt the thorium cycle, not only because India possesses abundant amounts of this element - not uranium - but also because, as a non-signatory nation to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), India was not authorized to procure uranium abroad. This is no longer the case, given that an agreement was signed with India a few years ago.India is also studying the thorium cycle, but wants to use a solid-state fuel, in Candu reactor model reactors, developed in Canada. The Indians have bought the license rights and they plan to adapt the Candu design to operate with thorium. Having said this, since we in France came forward with the MSFR concept, India has expressed interest and has approached us in this respect.
In Europe, what other research teams are working with you on this concept?
Originally there was a French group, with a dozen or so laboratories, and one or two research scientists in each unit studying this thematic. In Grenobles, we are currently studying the neutronics and the thermo-hydraulic aspects of the problem, but we are also doing some chemistry because we need to understand better how salts behave under such conditions and some materials science and engineering.
Since 2011, a European programme, called EVOL (Evaluation and Viability of Liquid Fuel Fast Reactor Systems) has been launched, federating research teams in a dozen or so countries. France will be collaborating with the Russians who are engaged in a somewhat similar programme. It is a small-scaled programme, representing a commitment of 1 Meuros over 3 years.
What horizon do you have in mind for a working demonstrator, or even industrial scale deployment?
Well, as research scientists, we are not concerned with the question of time horizons. The prime question is that of funding: in our academic research, we just do not have the means to envisage building a demonstrator, even a small one. Over and above this question, the decision is political. For the moment in France, there has been no decision taken in this direction. The French Atomic Energy Authorities (CEA) have enough axes to grind with ASTRID, another Generation IV concept; consequently they are not currently studying the MSFR concept.
A final point here is that the idea of using molten salts and thorium in a reactor design, as I described it earlier, is recent and indeed was only recognised as a viable concept in 2008. It will, I feel, take some time for the information to circulate, for decisions to be taken … for the time being, I can only take note, regretfully, that there is no project to hand to build and commission such a demonstrator or reactor.
But if we imagine that a decision was taken along these lines, what sort of time scale would be appropriate?
Industrial scale deployment requires a whole series of stages that involve a minimum 10 to 15 years work. We can begin by assembling small-scale models without fissile matter, and step by step, introduce and raise the working model’s level of radioactivity … we also have to cope and comply with all the procedures laid down by the safety authorities. We are currently working extensively on these aspects – how exactly do you proceed with a safety status study for this type of reactor design – and we do this to be ready, when our time comes, so to speak.
In an industrial deployment scenario, we can suppose that the molten salt thorium reactor would have to co-inhabit along side other reactor models?
The main problem here (for MSFRs) is that we need fissile matter to trigger the reaction. When I mentioned that you can put any fuel in an MSFR it is not exactly true. There are certain limits to the solubility of the salts we plan to use. In particular, we cannot exceed a certain percentage for the amount of plutonium loaded and this implies that we simply cannot take the fuels as and when they are removed from conventional reactor cores. We must make a fuel “mix”, with, for example, U-233 or enriched uranium.
One possibility open is to load today’s reactors not with a plutonium and enriched uranium MOX, but with a plutonium thorium MOX. The spent fuel would then contain some plutonium but also some U-233. This is a mix that is suitable for the MSFR triggering function.
All these processes take time. In the scenarios we envisage, starting with a decision to proceed taken in principle around yr 2040 – a hypothesis that takes into account the expected operational life-span of today’s reactors – then the MSFR could be launched on an industrial scale in yr. 2070.
You have examined, we understand, the possibility of changing over to thorium fuel for EPR reactors...
Yes, indeed. This can seriously be envisaged. Moreover, I can add that it is even possible to use thorium in conventional PWR reactors and this would allow operator to burn 50% less natural uranium. If you think about this, there is a sort of competition here for Generation IV units, inasmuch as if you can consume less natural uranium in today’s reactors then they can be kept operational much longer.
Nonetheless, isn’t there still an issue in regard to waste management and disposal?
The answer is yes, of course. But operators are primarily concerned about resource procurement. If you are consuming twice less amounts of natural uranium, then logically for the same expenditure, you can either consume twice as much or buy your resource for twice less the price. Taking this to the limit, if you can gain sufficiently in reactor efficiency, even extracting uranium from sea-water could prove attractive ... You could buy uranium at 1 000 dollar/kg. At this stage, resources become almost infinite. And this really is something nuclear power operators are targeting.
If we take the stance of sustainable development, management of the fuel cycle is a high priority concern …
I agree and that is precisely why we are proposing the assembly of a molten salt reactor used as an industrial incinerator which would largely help solve the question of management for nuclear wastes as produced by operating PWRs. India is on the same wavelength as we are, and an actor such as Areva could be interested, if the Areva Group can understand that there is a potential market here.
As you can see, considering the choices among various possible uses of thorium, on one hand, and the prospect of having a simple reactor-incinerator, on the other, the changeover to breeder reactors is not the only option open and its advantages are not self-evident. Notwithstanding, the reactor we have on our design-boards would prove far safer and, I add, the breeder principle, if enacted, would simply matters no end. We also entertain the hope that a breeder reactor would be less expensive than a PWR and this may prove decisive when the time is ripe for political and industrial decisions. On a more personal level, I think it is the only solution if we want to make the changeover to Generation IV reactors, viz., to propose a future reactor which will be less expensive than a pressure water reactor. Our hypothesis still remains to be validated but it is one of the reasons that encourage us to continue our studies and our research.