Amal Chakraborty
Former Director, DRDL, Hyderabad
Fusion reactors are the world's most important energy source—almost unlimited and zero-emission. They are the future of humanity. Both fission and fusion reactions are nuclear processes. Current atomic energy plants run on the fission reaction, which generates harmful radiation and has always been a significant cause of concern.
Fission Reaction: In a nuclear fission reaction, the atomic nucleus is split into two or more smaller nuclei, releasing energy and harmful radiation. The sequence of nuclear reaction: a) a neuron collides with a heavy nucleus like uranium 235 or plutonium 239, making the nucleus unstable and disintegrated; b) energy release is around 200 MeV; 3) fragmentations have high kinetic energy; c) Neutron emission triggers chain reaction: 2 to 3 neutrons that emit in the process go on to collide with other nuclei and the chain reaction starts. Typical example:
U-235+ n (neutron) Ba-141( Barium) + Kr- 92 (Krypton) + 3n
Harmful Fission Products: Alpha and Beta particles, Gamma rays, neutron, radioactive isotopes, reactive noble gases.
Fusion Reaction: Unlike splitting the nucleus in a fission reaction, a fusion reaction joins two particles to make a heavy nucleus release energy. Extremely high energy is required to join two particles. Hence, to make a fusion reactor viable, the energy released from the fusion reaction must be appreciably more than necessary to create the fusion process. All efforts are directed to maximize the resultant gain from the fusion process.
The fusion process starts by creating high--temperature plasma. Fusion reaction occurs in hot and ionized plasma at a temperature of 150 million degrees Celsius (temperature at the core of the Sun is 15 million Celsius: Oohs! We need ten Suns in a small chamber to start the fusion process).
Huge questions are: How did scientists think of creating 10 Suns in a small chamber on the Earth? Who are the masterminds?
This is a fascinating story.
To understand the story, we need to know about the four fundamental forces:
i) strong nuclear force, ii) weak nuclear force (causes radiation), iii) gravitational force, iv) electromagnetic force.
Strong nuclear force is responsible for binding atomic nuclei against the electromagnetic force. Nuclear strong force exists when the positively charged protons close in an atomic nucleus. The electromagnetic repulsive force between the two positive protons does not allow the proton to come close enough to get joined. Hence, a very high temperature is required to impart extra energy to the protons to cross the barrier, which is called the Coulomb Barrier. It requires about 150 million degrees Celsius, which is ten times hotter than the core of the Sun, to break the Coulomb Barrier.
Chronology of Events
1920s Albert Einstein and Leo Szilard proposed the concept of nuclear reaction that include fusion
1930s Werner Heisenberg and Hans Bethe studied nuclear fission, but strong nuclear force was not fully understood
1948 Enrico Fermi and Stanislaw Ulam researched the fusion reaction for using it in nuclear weapon
1950s John Lawson and Lyman Spitzer proposed the idea of nuclear reactor
1960s Soviet physicists Igor Tamm and Andrei Sakharov invented high temperature plasma source called Tokamak (Russian for Toroidal Chamber)
1970's Two other processes for fusion, named the Laser Induced Fusion and Internal Confinement of Fusion (ICF) were developed by John Nuckolls and Keith Bruckner
1980 International Thermonuclear Experimental Reactor (ITER) project was launched with 35 member countries to be situated in southern France. India is an active member of the ITER project.
Now Many organisations around the globe are working on commercialisation of Fusion process.
Leading organisations: National Ignition Facility (NIF) in California, General Fusion of Canada, US-based Helion Energy, UK-based First Light Fusion,
China National Nuclear Corporation (CNNC). China Fusion Energy Test Reactor (CFETR)
Fusion Reaction Type:
Two most potential fusion reactions are:
Type 1: Deuterium-Tritium (D-T) reaction:
D + T -- 4 He (Helium) + n (neutron) + Energy (17.6 MeV)
Type 2: Deuterium-Helium 3 reaction
D + He3 -- He4 + p (proton) + 18.3 MeV
Deuterium is a stable isotope of Hydrogen with one neutron and one proton in the nucleus. Helium 3 (He3) is a light isotope of Helium with two protons and one neutron in the nucleus.
Harmful Effects of Fusion Reaction: Fusion reactions are significantly less harmful for the following reasons:
1. No long-lived radioactive waste which remains for thousands of years in a fission reactor.
2. Minimal radiation.
3. Less likely to result in catastrophic accidents like nuclear meltdown.
4. Only neutron radiation is a concern, but the quantity of neutrons produced is relatively much lower than that in the fission process.
It is important to note that the Fusion reaction with Helium3 does not produce any neutrons and is, hence, the safest in all nuclear reactors.
Fusion Process Sequence
1. Plasma generation: Fusion fuel (a mix of Deuterium and Tritium) is ionized to create the plasma-hot ionised gas
2. Confinement: The plasma is confined in a magnetic field to maintain this density and temperature
3. Heating: The plasma is heated to achieve 150 million degrees Celsius by several means, such as Radio Frequency, Neutral Beam, Laser beam, Ion Cyclotron Resonance, Alpha Heating, Magnetic Compression
4. Fusion Reaction: Confined and heated plasma particles collide, leading to a fusion reaction
5. Energy Release: Some mass is lost in the Fusion process, which releases energy following Einstein’s famous equation E = mC2 and is absorbed by the plasma. Similarly, energy release in a fission process is also due to lost mass in the disintegration of the nucleus, which gets converted into energy.
6. Heat Transfer: Heat energy is transferred to the coolant, which generates steam to drive the turbine and electrical generator.
Type of Fusion Reactors: There are about ten different types of fusion reactors. The following three are most promising for commercialisation:
1. Tokamak: It is the most used configuration. It is a toroidal (doughnut-shaped) vessel to confine and heat plasma. ITER’s Tokamak is the largest and most advanced, aiming to generate commercial-scale electric power by 2030
2. Stellator: It is like Tokamak with a twisted 3D magnetic field. Leading examples are Wendelstein 7-x and LHD. Results are encouraging.
3. Inertial Confinement Fusion (ICF): This technique uses a high-power laser or particle beam to compress and heat a small pellet of fusion fuel. The National Ignition Facility (NIF) leads the field with a record breaking yield. Other major companies working in ICF configuration are General Fusion and Lockheed Martin.
Fusion Reaction and Moon Rush!!!
Neil Armstrong and Buzz Aldrin were the first to land on the Moon on July 20, 1969, in the Apollo 11 mission. There were six crewed missions till Dec 1972, ending with Apollo 17. The Moon mission was discontinued in 1972. Now, there is a sudden rush to the Moon. Thirty countries, organizations, and even some universities are planning the Moon's mission. Why this sudden rush???. Moon has abundant He3 in its regolith, unconsolidated material on the surface, created by solar wind over billions of years. The
quantity is enough to produce electricity for the globe for the next 1000 years!!! And Fusion with He3 is the safest, with almost negligible radioactive emission.
India is the fourth nation to land a lunar module safely on the Moon.
Reference: Fusion Physics, edited by Mitsuru Kikuchi et al. IAEA
