The Long Road from “Fusion” to “Fusion Power Generation”

By Imanaka Tetsuji, Kyoto University Institute for Integrated Radiation and Nuclear Science

Over the last few years, I have often heard it said that “fusion power generation is likely to be realized in the 2030s.” It was 1969 when I entered a university nuclear engineering department, and in March of the following year, Tsuruga Unit 1, Japan’s first full-scale light-water reactor, started operation. In the lectures at the university, we were told that light-water reactors were a temporary stepping-stone, that by the end of the 20 century all of them would be replaced by fast breeder reactors, and that fusion power generation would reach the stage of practical application by the middle of the 21st century. The fast breeder reactor program was aborted all over the world. I had been aware of the progress of research on nuclear fusion since I was a student, but I had never heard that “fusion power generation is likely to be realized,” and I thought that as long as basic research was still being conducted there was little reason to get excited about it.

When I was told that “fusion power generation is likely to be realized in the 2030s,” I looked through the journals of the Atomic Energy Society of Japan and the Japan Society of Plasma Science and Nuclear Fusion Research, thinking that there might have been some technological breakthrough, but could not find any reports worthy of note. In this article, I will explain the principles of nuclear fission and fusion and point out that there are a great number of insurmountable hurdles between fusion and nuclear fusion power generation.

Atoms and nuclei, protons and neutrons

Excuse me if you’re already very familiar with this, but please recall the periodic table of elements, which begins with hydrogen (H), helium (He), lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), neon (Ne) … and so on. The periodic table is a list of the elements that make up matter in the order of light ones to heavier ones. At the center of an atom is an atomic nucleus, which consists of positively charged protons, and neutrons, which are about the same size as protons but carry no charge. The order of the periodic table is determined by the number of protons in the nucleus, and this is called the atomic number. For example, the nucleus of oxygen, number 8 in the periodic table, has 8 protons, so its atomic number is 8. Even though there is electric repulsion between protons, the nucleus does not fall apart because the neutrons act like a glue. An oxygen nucleus usually has 8 neutrons.

Protons and neutrons are called nucleons, and between them there is a strong attracting force, called the nuclear force, which acts only at the very short distance between nucleons. The element with the highest atomic number that is generally found in nature is uranium (U), which has an atomic number of 92. The nucleus of uranium-235 contains 92 protons and 143 neutrons, while uranium-238 contains 92 protons and 146 neutrons. The combined number of protons and neutrons is called the mass number. An atom in which the number of protons is the same but the number of neutrons is different is called an isotope. The atomic number of hydrogen (H) is 1, but hydrogen has three isotopes with mass numbers 1, 2, and 3. Hydrogen with mass number 1 (light hydrogen) has only a proton in its atomic nucleus, deuterium (D), with mass number 2, has one proton and one neutron, and tritium (T), with mass number 3, has one proton and two neutrons. Light hydrogen and deuterium are stable isotopes that do not emit radiation, while tritium, which emits beta rays, is a radioactive isotope that has a half-life of 12 years. In beta decay, the <neutron → proton + electron> reaction takes place in the nucleus, releasing a negatively charged electron (beta ray). In beta decay, the atomic number increases by one, and in the case of tritium, the atom becomes helium-3.

Discovery and Application of Nuclear Fission

In late 1938, the German chemist Otto Hahn and his colleagues, who were trying to find a new element beyond atomic number 92 by bombarding uranium with neutrons, concluded that the product of neutron irradiation contained barium, atomic number 56. Hahn, however, puzzled by his interpretation of the phenomenon, wrote to a colleague, the physicist Lise Meitner, who had fled to Sweden to escape Nazi persecution. When she received the letter, she discussed it with her nephew, Otto Robert Frisch, also a physicist, reasoning that after neutron absorption the uranium nucleus split into two to form barium. According to Meitner’s manual calculations, the amount of energy released when the 92 protons in the uranium nucleus split in two and scatter off due to electric repulsion was about 200 million electron volts (200 MeV) [see note at end of article].

News of the discovery of nuclear fission spread quickly around the world. It was discovered that the formation of nuclei with small atomic numbers as a result of nuclear fission caused the release of extra neutrons, suggesting the possibility of a fission chain reaction. As this was just before the start of World War II, physicists around the world considered the possibility of a “fission chain reaction superbomb.” The United States’ Manhattan Project created the atomic bomb that was dropped on Hiroshima by carrying out the extraordinary feat of enriching uranium-235, which is readily fissile but comprises only 0.7% of natural uranium, to 80%. The Manhattan Project also created the Nagasaki atomic bomb by building a nuclear reactor using natural uranium as fuel, and using the reaction in which uranium 238 absorbs neutrons to form uranium-239, and then undergoes two beta decays to form plutonium-239, atomic number 94.

After the end of World War II, the development of nuclear power began by using the energy released by the fission chain reaction to produce electricity. The world’s first successful nuclear power plant was the United States’ experimental reactor EBR-1 (100 kW) in 1951.

In 1954, the Soviet Union’s Ovninsk Nuclear Power Plant (5 MW) began operation as the world’s first nuclear power plant.

Discovery and Application of Nuclear Fusion

Various theories about the origin of solar energy were debated in the 19th century, but nothing was confirmed. Early in the 20th century, Einstein proposed the special theory of relativity, which showed that energy (E) and mass (m) are equivalent and interchangeable, and that energy was obtained by multiplying mass by the square of the speed of light (c), in the simple formula E = mc². In the 1920s, it was thought that the sun’s energy source was perhaps due to the nuclear fusion of protons, which are hydrogen nuclei. In the 1930s, Hans Bethe elucidated the nuclear fusion reaction taking place in stars. The main nuclear fusion reaction taking place in the sun involves four protons undergoing a series of nuclear reactions to form a helium-4 nucleus, and thus this is called the proton-proton chain reaction (P-P chain).

Let’s explain the “mass defect.” According to Einstein’s equation, when a reaction occurs in which mass is reduced, the “defect” is converted into energy. The mass of the helium-4 nucleus is 0.7% smaller than the mass of the four protons combined. In the P-P chain, this 0.7% mass loss is released as about 27 MeV of energy. In general, when nuclei of light elements coalesce by nuclear force, a mass defect occurs, and energy is released. However, as nuclei have a positive charge and have an electrical barrier that causes them to rebound from each other, nuclei must have kinetic energy to overcome this barrier for a fusion reaction to occur.

The first artificial nuclear fusion was carried out in 1932 by John Cockcroft and his colleagues in England using an accelerator in which a proton was accelerated to several 100 keV and collided with lithium. In this experiment, the following fusion reaction was produced <proton + lithium-7→ 2 helium-4 (alpha rays)>. The kinetic energy of the two alpha rays was equal to the mass deficit, confirming that Einstein’s formula was correct. However, accelerator-based fusion consumes large amounts of energy and cannot be used for energy production. The “practical application” of nuclear fusion was the hydrogen bomb. A hydrogen bomb is a device for initiating nuclear fusion by raising the thermal kinetic energy of particles to enable them to cross the electrical barrier under the high temperature and pressure generated by the atomic bomb. This is why H-bombs are also called “thermonuclear weapons.” The fusion reaction used in the hydrogen bomb is almost exclusively <deuterium (D) + tritium (T) → helium-4 + neutron>, and is called the DT reaction. The DT reaction releases 17.6 MeV of energy, of which 3.5 MeV is distributed to helium-4 (alpha rays) and 14.1 MeV to neutrons. In a megaton-class hydrogen bomb, neutrons released from the DT reaction bring about further fission.

Fusion power generation is an attempt to achieve artificially controlled fusion on Earth and use the energy to produce electricity. However, it is impossible to maintain the high-temperature and high-pressure environment of the solar interior. This led to the idea of plasma magnetic confinement, which confines an ultra-high-temperature plasma in a vacuum magnetic field. We are familiar with the three forms of matter, solid, liquid, and gas in daily life, but when a gas is raised to a higher temperature, the electrically neutral atoms separate into positive nuclei particles and negative electron particles, forming a state called a plasma. By devising a suitable arrangement of magnets, plasma can be confined in a magnetic field. The plasma temperature can thus be raised to 100 million℃, which would enable DT fusion to occur with no contact with the outer wall. Research on this idea began around 1950. In the Soviet Union, Andrei Sakharov, known as the “father of the hydrogen bomb,” and his colleagues conducted experiments using a doughnut-shaped magnetic field later named a “tokamak.”

In the summer of 1955, the first International Conference on the Peaceful Uses of Atomic Energy was held in Geneva, Switzerland. In his opening speech, the Indian physicist Homi J. Bhabha, who chaired the conference, said, “I expect that a controlled fusion reaction will be possible within the next 20 years.” Even now, 70 years later, Bhabha’s prediction has not been realized, and for the 70 years since then people have continued to say sarcastically that fusion power generation is 30 years away.

International Thermonuclear Experimental Reactor (ITER)

The joint statement issued after the Gorbachev-Reagan summit in Geneva in 1985, which led to the end of the Cold War, called for “international cooperation to advance nuclear fusion for the benefit of all mankind.” Large-scale reactors were built in various countries to achieve a fusion reaction. In Japan, a large tokamak named JT-60 was built in the 1980s at the former Japan Atomic Energy Research Institute. All these reactors suffered from instability of the high-temperature plasma. Nevertheless, in the 1990s, the EU JET and the US TFTR tokamaks achieved a small amount of DT fusion. As research progressed, the equipment became larger and more expensive, and thus the International Thermonuclear Experimental Reactor (ITER) was planned as way to build an experimental fusion reactor through international cooperation. Despite various twists and turns, ITER was officially agreed on 2006 with the participation of the seven countries and regions: Japan, EU, US, Russia, China, South Korea, and India. In 2007, the construction of ITER commenced at the Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA) Cadarache Centre in the suburbs of Marseille, southern France.

The doughnut-shaped plasma of ITER has an outer diameter of 16m and a height of 7 m. The containment in which the basic structure is confined is 30 m in diameter and 30 m in height, and the overall weight of this mammoth precision structure exceeds 30,000 tons. The initial plan was to begin plasma operation in 2019 and DT combustion operation in 2027; the construction cost was estimated at 1.7 trillion yen. The goal of ITER is to “maintain a fusion combustion power of 500MW for 400 seconds at 50MW heat input.” While this was later revised to plasma operation in 2025, DT combustion in 2035, and a construction cost of 2.5 trillion yen, I heard that the building and production of the equipment were progressing smoothly. However, in July last year, it was reported that the plasma operation had been postponed by 9 years, until 2034, and that 800 billion yen in additional costs had become necessary. It appears that defects  found in the vacuum vessel that encloses the entire doughnut-shaped plasma necessitated repairs.

The Wall between “Fusion” and “Fusion Power Generation”

I believe that even if it is delayed by 20 or 30 years, it will somehow be possible to achieve the fusion experimental reactor ITER’s goal of “500MW for 400 seconds.” However, ITER does not generate electricity, and in fact does not have the technology to generate electricity. When it comes to the next stage of nuclear fusion power generation, the more I look into it, the bleaker the outlook becomes. There are a number of technical hurdles that I am not sure can be overcome.

• There is no technology for the successful extraction of energy.

Fortunately for the development of nuclear power plants using nuclear fission, most of the energy that is released is converted into heat within the slender fuel rods. Of the 200 MeV of energy released by fission, the kinetic energy of the two fission fragments accounts for about 90% of the energy, or 180 MeV. The remaining 10% is divided up between  neutrons, gamma rays, beta rays, and neutrinos. Nuclear fission occurs in the cylindrical fuel pellets packed into fuel rods 1cm in diameter and 4m in length. Since the range of the fission fragments is at most 0.01 mm, all the energy is used up in increasing the temperature inside the fuel pellets. The energy of the neutrons, gamma rays, and beta rays is also scattered and absorbed inside the core, which is more than 100 tons in total, and almost all of that energy becomes heat. In the case of a pressurized water reactor, heat is extracted by passing cooling water through the system at high speed and high temperature and pressure, 150 atm at 320℃, generating steam in steam generators to drive turbines. Almost 100% of the energy released by fission is transferred to the cooling water, and the thermal energy carried as steam is converted to electric energy with a power generation efficiency of about one third.

On the other hand, in a fusion DT reaction, alpha rays and neutrons are generated, but the 3.5 MeV of the charged alpha rays is consumed by heating the plasma. Since neutrons have no charge, their penetrating force is extremely high. In fusion power generation, the kinetic energy of these 14.1 MeV neutrons would be adroitly collected as thermal energy to be converted to electric energy. Neutrons pop out of the fusion plasma in all directions. These neutrons are blocked by what is called the blanket, the doughnut-shaped wall surrounding the plasma. In the blanket, neutrons repeatedly collide with the blanket material, heating up coolants such as water or helium gas. The coolant is then collected from the blanket to drive a turbine. Various proposals have been made for the structure and materials of the blanket, but it is impossible to know whether they will work properly without testing. My feeling is that it will be pretty good if it can convert half of the neutron energy into heat. The experimental reactor for testing such a blanket is the ITER, and it is best to expect that it will be around 2050 at the earliest before we know whether this technology can be used for power generation. In the case of a power reactor, for example, more than 500 blankets, each 1.4 m × 0.7 m × 0.4 m in height, width, and thickness and weighing 4 tons, will be placed in the vacuum vessel.

• Can tritium be successfully produced?

It is often said that, as the fuel for fusion power generation is obtained from sea water, it is unlimited, but this is wrong. If DT fusion power generation with a thermal output of 1000 MW is realized, the amount of tritium used for combustion for a whole day is 150 g. There is no problem with the supply of deuterium because deuterium is present in ordinary hydrogen at a ratio of about 10,000:1. However, the very small amount of tritium existing in nature would simply not be enough. In fusion power generation, therefore, lithium compounds are packed into the blanket and tritium is produced by the neutron reaction of <Li + neutron → T + helium-4>. This is done by collecting and purifying tritium from the blanket, mixing it with deuterium, and then sending it back to the plasma for combustion. However, some neutrons produced by fusion in the plasma leak or are absorbed without producing tritium, so the amount of tritium recovered tapers off . Beryllium compounds, which are neutron multipliers, are therefore inserted into the blanket to increase the number of neutrons and ensure tritium reproduction. In ITER the blanket test ports have been assigned to the three participating countries/regions of Japan, the EU, and China, and the different kinds of blankets designed by each of the participants are brought in for testing. The fine tubing of the thermal energy recovery system and tritium reproduction recovery system are threaded through the blanket material like capillaries. In the case of a power reactor, more than 500 such blankets are connected together, making the whole system extremely complex.

If the reactor was operated 200 days a year at a thermal output of 1,000 MW, the tritium required for combustion would be 30 kg per year. At the Fukushima Daiichi Nuclear Power Plant, 1.3 million tons of tritium-contaminated water has accumulated and is being diluted with seawater and discharged into the ocean. 1.3 million tons of contaminated water contains only about 3 grams of tritium. The tritium production and purification process used in nuclear fusion power generation is carried out using gas as a medium. There is no experience of tritium production, extraction, and purification in blankets, and even a small leak could lead to serious environmental pollution.

• Maintenance and inspection of fusion reactors is much more difficult than for fission reactors

In nuclear fusion, as fission products do not accumulate in the reactor as they do in fission reactors, so-called high-level radioactive waste is not generated, and although neutron activation occurs in structural materials, I thought maintenance and inspection would be easy. However, after some research, it appears to be much more difficult than for fission reactors.

In connection with the restart of existing nuclear power plants in Japan, it has been argued that neutron embrittlement of pressure vessels is a factor limiting the life of these plants, but neutron damage in fusion reactors is much more severe. In the first place, as fusion plasma is almost transparent, the number of neutrons that the blanket wall of a fusion reactor receives in one year is several 100s of times greater than the number a fission reactor pressure vessel receives in 50 years. Since ITER is an experimental reactor, its operation time is short and neutron degradation may be less problematic. However, in a prototype fusion power reactor, the blanket, which is the first wall enclosing the plasma, must be replaced every 2 to 3 years. Each blanket weighs about 4 tons, and it is easy to imagine that the replacement of more than 500 blankets inside the vacuum vessel, looking like the outer shell of a doughnut, would be a daunting task.

During a regular inspection of a light water reactor plant, the lid of the pressure vessel is removed and the pit is filled with water to replace the fuel assemblies. As the water shields the radiation, the work can be done visually by workers. In a nuclear fusion reactor, on the other hand, the radiation dose inside the vacuum vessel after shutdown is 250 Gy*/hour even in the case of ITER. Since no one can enter the vacuum vessel, blanket replacement is conducted remotely using a remote camera. ITER is designed to have a robot arm inserted through the side of the doughnut-shaped vacuum vessel to perform blanket replacement, which takes two years to replace the 500 blankets if the process goes smoothly. Since ITER does not generate electricity, it uses a simple blanket, but when it comes to a power reactor, the blanket contains cooling water piping and tritium breeding piping, and the work environment becomes more severe. Thus, since water cannot be used as shielding and the work cannot be done visually, maintenance and inspection of a fusion reactor are far more difficult than for a fission reactor.

In November last year, I went to see an exhibition named Fusion Power Generation World, held at Intex Osaka. I heard in a talk by the Cabinet Office Deputy Director-General for Nuclear Fusion that a prototype power generation reactor is under consideration to follow on from ITER and that he expects bold technological innovation from fusion ventures. I then heard talks by senior venture executives who claimed that fusion power generation will probably be realized in 10 years’ time. When I walked around the exhibition after the talks, I came across the booth of a fusion venture in the United States. When I asked the person in charge how electricity would be generated, he replied, “Once nuclear fusion is realized, the rest is just the application of existing technology.”

After doing quite a bit of study, I feel that fusion power generation is first and foremost a “flawed technology” that has been overstretched. I do not deny steady basic research, but I do not expect that “bold technological innovation will occur and nuclear fusion power will be realized at a stroke,” and I think we should not be so foolish as to spend large amounts of taxpayers’ money and chant slogans in a rush toward a mirage that will never be reached.

Notes:

1) The unit electron volt (eV) is used to describe the amount of energy involved in small particles such as radiation, atoms, and nuclei. When a voltage of 100 volts is applied to the electrode of a vacuum tube, the energy of the electrons that are generated at the cathode and travel to the anode is 100eV. The binding energy of the atoms that make up our bodies, such as carbon and nitrogen, is about 5eV. keV is 1000 eV and MeV is 1 million eV. The energy of gamma rays emitted by cesium-137, a major source of radioactive contamination caused by nuclear accidents, is 662 keV (662,000 electron volts).

• For more information on “ITER,” search for “ITER project,” and for more information on “prototype power reactors,” search for “prototype fusion reactors.”

* Gy (gray) is one unit of radiation dose and is called absorbed dose. A dose of 1 Gy represents 1 kg of a material receiving 1 J (joule) of energy from radiation. Gy multiplied by a radiation weighting factor is Sv (sievert), but you can usually assume that Gy and Sv are equivalent.