How does fusion fit in the future energy landscape of society?
What are the advantages and possible critical challenges of this energy source which, while currently experimental, promises to make a decisive contribution to the large-scale production of safe and sustainable energy in the second half of the century?
Let’s go through some of the most common questions we receive from the public and try to clarify the science and implications of fusion energy.
Fusion and fission are two opposite nuclear processes.
In fission, the nuclei of heavy elements (in particular uranium) are bombarded by neutrons with a relatively low energy and break into two lighter nuclei, called “fission products”, plus some newly produced neutrons. In the process, the fission products have a combined mass that is slightly smaller than the original heavy element. This small loss in mass is called a “mass defect”, which is converted into a very large amount of energy according to the well-known Einstein equation E = mc2, where m is the loss of mass and c is the speed of light. The newly produced neutrons, slowed down by a “moderator”, hit other Uranium nuclei, which produce further fission neutrons, creating the famous “chain reaction“. The heat produced by the nuclear reaction is converted into electricity by a “thermodynamic cycle” and finally by an alternator, as in any thermal power plant.
In fusion, the nuclei of two light elements (in particular two isotopes of Hydrogen, Deuterium and Tritium) are heated to a very high temperature, about 10 times the temperature in the center of the Sun, so that they can collide and fuse together, combining to create a heavier element nucleus (in particular Helium) and a neutron with a lot of energy. A small amount of mass is also lost in the fusion process, i.e. the helium + neutron have less mass than the Deuterium + Tritium, which is converted into energy according to Einstein’s equation. The helium nuclei, which have a positive electric charge and therefore remain trapped in the magnetic field, maintain the plasma at a high temperature, which is required for the fusion reactions to be self-sustaining; the neutron, which has no electric charge and can pass trough the magnetic field, is instead absorbed by the plasma vessel walls and its energy is transformed into heat, which in the future can be converted into electricity in the same way as the current thermal power stations. There is no possibility for an unstable “chain reaction” with fusion.
Another fundamental difference between the two processes lies in the fact that the products of fission are highly radioactive, while the product of fusion is a harmless gas, Helium, not only safe for people and the environment but also very useful in many applications.
In addition to the differences, there are also important similarities: in particular, both processes produce neither carbon dioxide nor vast quantities of pollutants and therefore must be considered as having a low environmental impact.
To date, it is difficult to accurately estimate how much a fusion power plant will cost. We are talking about a technology that will enter the market in the second half of the century so it is not yet possible to define the project costs in detail. Currently, however, the choice of technical solutions is still underway that will allow the construction of a plant that is not only reliable but also competitive in the electricity market. In order for this to happen, we can say that the plant design must achieve a capital cost in line with the costs of the most innovative nuclear fission plants to date, namely those of advanced third generation (Gen III +).
This is an achievable goal if we consider that a nuclear fusion power plant requires spaces and structures very similar to those of a conventional fission plant. However, it should be noted that the so-called “fusion island” or the area of the nuclear site where the fusion reactor is located, the most complex and expensive part in terms of construction and materials used, will correspond to almost half of the capital cost of the entire plant. It is here therefore that efforts must be focused to achieve competitive costs. The gradual maturation, also by private industry, of skills in technologies and materials related to nuclear fusion will play a key role in reducing the costs of a plant for the generation of electricity from fusion.
The production of energy, in both cases, is described by Einstein’s equation. Therefore the energy produced is proportional to the “mass defect” and the mass defect is greater in the fusion process. With the same mass of fuel consumed, the fusion process produces an energy 4 times greater than fission.
However, both processes produce a huge amount of energy in comparison to fossil fuels. For example, for fusion, with a bathtub of sea water, or tap water, from which the Deuterium will be extracted, and with the few grams of Lithium contained in the battery of a laptop (Lithium can be converted into Tritium in the reactor) it will be possible to produce about 200,000 kWh of electricity, sufficient for the annual consumption of about 100 European households and equal to the electricity that can be obtained by burning about 60 tons of coal, which has the environmental consequences that we all know.
Fusion is CO2-free
The first and most evident advantage of nuclear fusion is the production of electricity without the release of polluting elements into the atmosphere such as nitrogen oxides or sulfur oxides or carbon dioxide, the main greenhouse gases.
The Paris Climate Agreement requires action faster than the time it takes to develop nuclear fusion
However, it should be reiterated that nuclear fusion cannot, strictly speaking, contribute to the achievement of the climate objectives indicated in the Paris Agreement, as it is believed that it will only be available in the second half of the century. By this stage the goal is that world energy pool will already have profoundly changed and will presumably be composed largely of power plants based on renewable sources, with a significant contribution from nuclear fission plants, so that the generation of electricity can take place without CO2 emissions.
Fusion increases the security of the electricity supply
In this context, nuclear fusion will play a key role as it will increase the level of “security of supply” of the electricity system. In fact, a fusion power plant will be able to generate high levels of power and energy per unit of surface used (a single 1 Gigawatt power plant would produce about 6.5-7 billion kWh and would use an area of 200-300 hectares) in a continuous production (indicatively online for more than 80% of the hours of the year). Therefore, it will be able to provide a “secure baseline” of electricity production with also the possibility of immediate reaction to grid demands, a role currently dominated by fossil fuel burning power plants, reducing the number of events in which national electricity generation could fall short of demand. In fact, in an electricity generation system based mainly on variable renewable sources, this situation occurs in the case of demand peaks in conjunction with a low production from renewable sources due to particular and not always predictable weather conditions.
The reserves of some of the chemicals used in fusion are limited
Deuterium is easily extractable from sea water and can be considered an almost inexhaustible resource. On the other hand, there are no appreciable reserves of tritium as it is a radioactive element with a half-life of 12 years: that is, in 12 years, tritium halves its mass because it decays (or transforms naturally) into another element. Tritium must therefore be produced through reactions with lithium, possibly within the fusion plant or otherwise in dedicated nuclear plants. There are reserves of lithium in nature, especially in South America, but they are smaller than those of deuterium. If we add to this that lithium is also used in other sectors (for example for the production of batteries), the quantity available for fusion could be reduced, placing limits on its diffusion. According to recent studies, however, it is believed that the recycling of lithium present in batteries will remove this concern, especially given the fact that a future 1 Gigawatt electric fusion power plant, running for 7000 hours per year, would consume only 3.7 tons of natural lithium per year, i.e. the amount currently present in the batteries of about 300 electric cars. We should note that as our ability to control and heat the plasma increases with scientific and technological advances over the coming century, we should be able to transition to plasmas based fully on Deuterium-Deuterium fusion reactions, which are more difficult to achieve but bypass the need of Tritium/Lithium.
Fusion reactions are safe
The fusion reaction of deuterium and tritium nuclei for the generation of heat is intrinsically safe. This is because any variation with respect to the optimal value of gas density, temperature and effectiveness of the magnetic confinement is sufficient for the reaction to be extinguished. Therefore, unlike fission plants which are based on chain reactions that amplify the energy and therefore the heat produced, if not properly controlled, a fusion plant does not run the risk of accidents related to a loss of control of the reaction. Although this is an advantage in terms of safety, it nevertheless highlights how difficult it is to keep the fusion reactions “on” for a long period, since they are so sensitive to the variation of the operating conditions.
Fusion is a nuclear reaction and as such produces some radioactive components
The neutron produced by the fusion reaction of deuterium and tritium is the key element to generate further tritium and for the transfer of 80% of the thermal fusion power produced to the heat exchange mechanism in the vessel walls which allows the generation of electricity. However, no material can remain immune to the absorption of a neutron. In particular, the materials that make up the internal part of the chamber in which the deuterium and tritium gas (the plasma) is contained, being subjected to a huge flux of high-energy neutrons, acquire a degree of radioactivity such as to be considered “medium-low level radioactive waste ”which must therefore be adequately treated. Unlike fission, the radioactive elements to be managed are in much smaller quantities and of a quality that makes management much easier, requiring only surface deposits.
International cooperation
To achieve such an ambitious goal, coordinated and close cooperation between countries at various levels is absolutely necessary. A technological challenge therefore becomes an opportunity to break down barriers to the common goal of producing electricity in balance with the environment in which we all live.
The fundamental reaction in a future fusion power plant is inherently safe. Reaching fusion conditions requires that some characteristic parameters of the gaseous mixture of deuterium and tritium (temperature, density, energy confinement time) and many other plant parameters, electrical, mechanical, thermal, etc., which determine them, have values within precise ranges. Outside of these ranges the reaction stops in a very quickly. If the gas mixture is too hot or too cold, if the quantity of gas inside the chamber where the reaction takes place is too much or too little, if the value of even one of the components of the magnetic field that serves to confine the nuclei of deuterium and tritium is too large or too small, then the possibility of obtaining fusion reactions diminishes greatly. In short, if anything goes wrong the fusion reactions shut down automatically.
In particular, the reagents, deuterium and tritium, are injected in gas form into the reaction chamber in very small quantities, in total less than 20 milligrams per second in a 1000 MW electric power plant. Therefore, even in the case of the worst possible accident, the temperature of the components closest to the gas mixture, the first wall and the vacuum chamber, would not reach the melting value of the material. Therefore, a loss of control incident involving the overheating and possibly the melting of a part of a fusion plant is not possible, as opposed to what could happen with runaway reactions in fission reactors, albeit with very low probability nowadays with 3rd and 4th generation fission plants.
It is true that the required fusion temperature of the deuterium and tritium gas mix is equal to 150 million degrees, a value that appears gigantic, but we must consider the third fusion parameter, the density of the gas, which must be a million times lower than the atmospheric density. Therefore, the energy present in the gas mixture, which according to the laws of physics is proportional to the product between temperature and density, is the same as that same gas would have if it were at atmospheric pressure and at a temperature close to ambient temperature. If this energy were accidentally deposited on a limited part of the surface of the vacuum chamber it could cause localized damage, but could never compromise the structure of the chamber.
The design of the future reactor will in nevertheless take into account all the safety requirements provided for by the current regulations in industrialized countries with regard to potentially catastrophic events of external origin: earthquakes, floods and external “attacks”, including the impact of aircraft.
In this regard, there are three concerns that are raised:
1) high-energy neutrons are produced in a fusion reactor;
2) one of the two reagents, Tritium, is radioactive;
3) the fusion reactor produces radioactive waste
Let’s break them down one by one.
What happens to the neutrons produced by fusion reactions?
The fusion reaction of a deuterium and a tritium nucleus (two isotopes of hydrogen) produces a nucleus of helium, a noble gas, and a high-energy neutron. In a future fusion reactor, a few hundred billion billion neutrons will be generated per second. But neutrons are generated in the innermost part of the reactor, therefore almost all of them are absorbed by the different materials that make up the structure of the reactor itself: these are high-density materials such as the beryllium of the first protective wall of the vacuum chamber, the steel of the vacuum chamber, the alloys containing lithium compounds of the breeding blanket and finally the borate cement (boron is an exceptional neutron absorber) of the biological shield. All these dense materials first slow down the neutrons, transforming their kinetic energy into heat in the heat-exchange mantle which is then used to generate electricity, then largely absorb them, preventing them from spreading into the surrounding environment. However, statistically, a small part of them is able to overcome all the barriers described, but the “neutron” design of the reactor structure means that the neutron flux reaching the environment is less than the natural neutron background from cosmic rays.
Can Tritium, the radioactive reagent, contaminate the environment outside the reactor?
One of the two reagents of the fusion reaction is tritium, a radioactive isotope of hydrogen that decays by beta emission (emission of an electron) transforming into helium, with a half-life of 12 years. Due to its short life, tritium is very rare in nature; for this reason, in a future reactor it will be generated during the normal operation of the machine, inside the so-called breeding blanket, where lithium-based compounds transmute into tritium by absorbing the neutrons produced by the fusion reactions. If, due to any anomaly, the stringent conditions that enable the fusion reaction are exceeded, the production of tritium would also be interrupted. A few milligrams of tritium per second is enough to generate fusion power in a future 1000 MW reactor; however, the total amount of tritium present in the plant, in every instant of its operation at full power, will depend on the actual design parameters of all the systems of the tritium “line” (separation, purification, circulation, storage, etc.) but it will of the order of a few kilograms. All the systems of the tritium “line” are designed with suitable measures to prevent release into the atmosphere during normal operations of the future reactor, taking into account the numerous barriers represented by the reactor structure itself and those specifically built in the areas where the tritium implants. The design of the containment measures against the possible release of tritium is carried out in the light of the experiences already gained in many sectors, industrial and medical, where tritium is commonly used today. In the case of the worst possible accident, namely the fire of the tritium plants, the radioactivity released into the environment would not require evacuation or other protective measures for the population residing in the area surrounding the site of the future fusion power plant.
What type of radioactive waste will be produced by a fusion power plant?
Based on the technical guidelines drawn up by the International Atomic Energy Agency in Vienna, in all industrialized countries radioactive waste is classified into categories, characterized by different levels of radioactivity and possible heat production. As a rule, radioactive waste, depending on the category to which it belongs and its state (solid or liquid), can be subjected to conditioning processes, which make it suitable for storage in temporary deposits near production plants or in places not very far from them and subsequently to disposal in surface or geological deposits, where the conditioned artifacts remain for as long as necessary for their radioactivity to decay below the threshold of relevance for the environment and the population. Highly radioactive and long-life waste requires disposal in geological repositories (layers of granite or clay or rock salt, at a depth of hundreds of meters) since they maintain a level of radioactivity above the relevant threshold for too long ( typically many thousands of years) to be disposed of in surface concrete structures. On the contrary, waste with very low to medium activity does not require disposal in a geological repository, since it decays below the threshold of radiological relevance in a sufficiently short time (from a few years up to about 200 years). In this case, storage ranges from disposal either in the deposits at the sites (where, as a rule, other waste is only temporarily stored) or in a permanent surface deposit.
Radioactive waste of all categories is produced in nuclear fission plants; in particular the uranium bars, extracted from the core after about three years of operation, contain fission products and transuranic elements which are high activity, long-life and high heat production waste, for which disposal in geological repositories is essential.
A fusion power plant, on the other hand, will not produce long-life, high-activity radioactive waste to be disposed of in a geological repository. In fact, the product of the reaction is helium, a valuable inert gas. Furthermore, all those materials of the reactor structure that are activated due to the absorption of the fast neutrons produced by the fusion reaction, suitably de-tritiated where necessary, are metal waste with very low, low and medium activity, similar to what happens when a cyclotron used in the radio-medicine departments of our hospitals for the production of radiopharmaceuticals is dismantled. Furthermore, for the future fusion reactor the use of low-activation metal alloys is envisaged, so that all these materials decay below the radiological significance threshold after a time not exceeding 100 years from the discharge from the reactor, so much so that the possibility of conditioning and disposing of them in the same site as the power plant is considered, without the need to transfer them to a permanent surface deposit. This would allow the recovery after decay and the reuse of almost all of them, for example for the construction of a new fusion plant, with the sole exception of the biological concrete screen which could be reused as inert material for civil uses.
No, at Consorzio RFX in Padova we have:
In both cases, the fusion reactions have been and will be in very limited numbers and very little radioactive material with low or very low activity will be produced, comparable to that produced in normal electromedical equipment.
Nuclear fusion is a technology of the future, which will only play a relevant role in the second half of this century and which will allow the continuous production of large quantities of energy with a very low environmental impact, generating part of the so-called “fusion fuel” (tritium) inside the plant itself, with a limited production of radioactive elements and with a medium-low level of radioactivity. The advantages that nuclear fusion offers make the technology truly attractive in the current global context, characterized by the search for a solution to the so-called “energy trilemma” which requires identifying energy policies that at the same time ensure the availability of energy for all, at affordable costs and low environmental impact. However, in order for fusion to really play a relevant role in this scenario, it is necessary not only to dedicate adequate resources to scientific and technological research, but also to plan the development of the energy system in the next decades so that it can include space for large fusion plants which offer a continuous baseline of energy, to be located near large cities or industrial areas, in support of generation from renewable sources such as solar and wind energy.
In each scientific sector, the development of new processes and technologies involves different phases, conventionally classified according to a scale of NASA origin (the TRL – Technology Readiness Level) which includes nine degrees of technological maturity, from the observation of fundamental principles (level 1) up to to tests (with verification of economic competitiveness and maturity for commercialization) in an operational environment (level 9). Research, technological development and pre-competitive demonstration programs are co-financed by governments all over the world, with public contribution rates ranging from 100% (usually levels 1 to 4) to very small percentages (levels 5 to 9). At the moment the most important research project on fusion, the ITER experiment, can be placed between levels 4 and 5, for which it is normal for activities to be financed with public money. In other words, we are still far from the more mature phases, where the involvement of private companies is justified by the prospect of commercial exploitation in the typical times of their investment plans (5-8 years). This does not mean that industry is not involved in the technological development process of fusion. Quite the contrary: the experimental plants already in operation and under construction in the world, large (such as ITER) or medium-size (such as DTT in Italy) require the design and construction of numerous components, often bordering on the knowledge acquired in many fields of physics and engineering. The specialized industries that are called upon to implement them, remunerated mainly with public funds, develop valuable know-how, which can also be reused in commercial sectors; therefore they can consider it convenient to participate in the costs, in exchange for the commercial exploitation of the results acquired. In this sense, industry is already participating in the development of fusion today. In addition, in recent years small companies have sprung up, usually spin-offs of universities and research centers, which thanks to funding from private foundations (for example the Bill & Melinda Gates Foundation) or companies (like our ENI) have started the construction of machines of a much smaller size than ITER, which aim to experiment with innovative configurations for the confinement of the gaseous mixture of deuterium and tritium, promising on paper for efficiency, yield, minimum size. For understandable needs of fundraising and compatibility with the industrial dimension of the lenders, these companies tend to declare extremely challenging time schedules, announcing the achievement of level 9 of the aforementioned scale in times of the order of 15 years, compatible with the duration of the industrial plans of long term of sponsors. That said, these smaller projects are all welcome. In fact, these could lead to very important results on some key technological developments, such as the use of superconducting materials at high temperatures, the development of innovative materials for the components directly in contact with the gaseous mixture of the reagents, the verification of configurations of more stable confinement, and much more. All results that could accelerate the path towards the production of electrical energy from fusion, that is, the stages following the experimentation on ITER. If the announcements of these small companies turned out to be spot on and they really can have commercial fusion technologies in the 15-20 years they project, and not between 40-50 years like the very large part of the international scientific community believes, absolutely no one would would regret it!
Today we are witnessing a slow but progressive change in the energy system, characterized by a growing presence of solar and wind power plants that produce energy intermittently, i.e. only when there is sun and when there is sufficient wind. However, an electricity generation system consisting exclusively of renewable sources and energy storage systems is a technically feasible option. The installed systems will inevitably produce more energy than is required in some periods and the excess will have to be stored with appropriate systems (directly via batteries, or indirectly, for example, by transforming it into hydrogen to be converted into electricity) to make it available when there is no energy generation. In this way, energy demand could be met without emitting pollutants and greenhouse gases. However, this would occur in the face of high costs, largely due to the need to radically change the electricity transmission system which should be strengthened to ensure the transport of energy from the numerous generation points distributed throughout the territory to end users. Furthermore, such a system would require the presence of a large fleet of fossil fuel-based generators inefficiently “on stand-by”, ready to intervene if the generation from renewables and storage is not able to meet the demand, which results in a non-100% renewable system. In this context, the presence of nuclear fusion plants acting as an energy baseline with flexible power generation would make it possible to reduce the overall generation power installed, the total capacity of the storage systems and the extent, number and complexity of the transmission and distribution networks. All this would result in a reduction of the surface used by the electrical infrastructures, an increase in the security of supply and a reduction in costs, compared to a renewable system with fossil fuel backup.
Older readers will recall that in 1989 two electrochemists from the University of Utah, Martin Fleischmann and Stanley Pons, announced with much enthusiasm, followed by great media hype they had produced an “excess of power” in an experiment including the electrolysis of deuterium water on the surface of Palladium. Essentially, instead of using very high collision speeds and temperatures that occur in a fusion reactor, the idea was to overcome the electromagnetic force that separates the deuterium nuclei by using chemical processes to squeeze the nuclei close enough for the nuclear strong force to take over and cause fusion. This excess of power registered by the scientists was attributed to a low-energy nuclear fusion process between Deuterium nuclei, which would develop in the crystal lattice of Palladium, without emission of radiation. The phenomenon was immediately baptized “cold fusion” and the news amplified by the media created the illusion of having definitively solved the energy problem for humanity; subsequently, the research field was renamed “study of low-energy nuclear phenomena in condensed matter” and the phenomenon of excess power production in the Palladium-Deuterium system was called the “Fleischmann and Pons effect”.
The reality, however, was that the cold fusion observed in 1989 was simply a measurement error, and the intense media hype around the subject saw it be labeled as one of the most famous pathological sciences in history. Many studies in prestigious research institutes (including the ENEA Research Centre in Frascati) have tried over the years to reproduce the findings of the 1989 study, but without success. There is nevertheless a small community of researchers that continue to study the possibilities related to cold fusion, however there has been no significant theoretical or experimental breakthrough to date.