Nuclear Fusion Technology Dr. BC Choudhary, Professor Applied Science Department, NITTTR, Sector-26, Chandigarh-160019. Content Outlines Review of Nuclear Processes Nuclear Fusion Reactions Fusion Reactor Technologies Technological Developments Pros and Cons Foreseeable trends Four Forces of Nature Systematic Information about Nucleus . Nuclear density at the centre of the nucleus 44 3 0 10 nucleons/m Density of nucleons (r) in the inner region of the nucleus is about same for all nuclei. Surface thickness of all nuclei are very similar 1/3 Radius of nucleus, R = R0 A , small variation in radii . NOT ALL NUCLEI ARE SPHERICAL. Energies of nucleons in nucleus 10 MeV . ALL NUCLEI ARE NOT STABLE Nuclear Stability Proton unstable Stable nuclei Neutron unstable Stable n:p = 1.2 –1.4 Measure of Nuclear Stability . Binding Energy per nucleon 2 EB(Z,N) = {ZMp+NMn - M(Z,N)} c • Nuclei with the largest binding energy per nucleon are the most stable. • The largest binding energy per nucleon is 8.7 MeV, for mass number A = 60. • Beyond Bismuth, A = 209, nuclei are unstable. Nuclear Processes . We can release energy by creating nuclei that are more strongly bound (increasing Eb) Fission: Heaviest nuclei 56 Eb Fe split into smaller fragments. 238 N U Fusion: Lightest nuclei 1H combine to form heavier N nuclei. Both the processes evolve large amount of energy. Nuclear Fission and Fusion Nuclear fission: A large nucleus splits into several small nuclei when impacted by a neutron, and energy is released in this process Nuclear fusion: Several small nuclei fuse together and release energy Electricity from Nuclear Fission Nuclear power plants account ~17 percent of the worlds power. Nuclear Fusion The Phenomenon of fission is a good source of nuclear energy. However, a considerable larger amount of energy can be obtained by fusion of light elements to heavier ones. The energy yield per gram in fusion is about 8 times that in the fission. In order to effect the fusion of two or more nuclei, they must be brought so close together against the force of electrostatic repulsion that they face within the range of nuclear forces. This will occur only if the interacting nuclei have K.E. of about 0.1 MeV or more. This energy is undoubtedly provided by Accelerators, but their use is limited to small number of nuclei. To produce fusion of large mass of material, the K.E. must be due to the thermal motion of the nuclei, which could in principle result from a sufficient increase in temperature. To impart the particles energies as high as 0.1 MeV, its temperature shall be raised to 107 K. Nuclear fusion at very high temperature Thermonuclear Reactions. The SUN Sun is radiating energy at the rate of 41016 J/s for several billion years without showing any sign of cooling off Chemical reaction (combustion of carbon) cannot account for energy at above rate for long time. Fission process can also not be expected- because of small abundance of heavy nuclei in the sun Hydrogen and Helium constitute 90% of sun’s mass in almost equal proportion. Probable that certain nuclear processes involving H and He may be actual source of Sun’s energy. Thermonuclear reactions in the core of the Sun produce its energy In 1938, Bethe proposed a set of nuclear reactions to accounts for energy produced in sun and other stars. • At extremely high temperatures and pressures, 4 Hydrogen atoms combine to make 1 Helium atom and release energy in the process according to E = mc2 4H He + energy : HYDROGEN FUSION The Proton-proton chain The C-N-O cycle dominates in stars dominates in stars: the size heavier than the sun of the sun or smaller. Fusion of in Laboratory The easiest fusion reaction to attain is Deuterium + Tritium: 3H + 2H 4He + 1n D + T +n In order to overcome Coulombic repulsion, must have very energetic (~70 keV> 800,000,000 oC) D, T nuclei. At this temperature, D,T nuclei are ionized, forming a charged plasma No material can withstand this temperature How to confine? Lawson's Criterion for Fusion . Once a Critical ignition temperature for nuclear fusion has been achieved, it must be maintained at that temperature for a long enough confinement time () at a high enough ion density (n) to obtain a net yield of energy. In 1957, J. D. Lawson showed that the product of ion density and confinement time determined the minimum conditions for productive fusion, and that product is commonly called Lawson's criterion. Commonly quoted figures for this criterion are n 1014 s/cm3 Deuterium- Tritium Lawson’s criterion fusion for fusion n 1016 s/cm3 Deuterium - Deuterium fusion . The closest approach to Lawson's criterion has been at the “Tokamak Fusion Test Reactor ( TFTR)” at Princeton. Has reached ignition temperature and got in very close to Lawson's criterion, although not at the same time. Nuclear Fusion and Plasma Confinement Three Confinement Methods High-power laser confinement FUSION REACTOR Types of Reactors • Magnetic Confinement Fusion (Tokamak) • Inertial Confinement Fusion (ICF) : Laser Ignition MCF is about 20 years ahead of ICF TOKAMAK . Charged plasma can be confined by large magnetic fields, requiring super conducting electromagnets. • Fusion reaction occur, and the energy released makes the He byproduct more energetic, thus keeping the temperature of the plasma hot enough to ‘burn’ D,T Magnetic Field Configuration Tokamak Scheme of the tokamak principle: arrangement of magnetic field coils and the resulting magnetic field that confines the plasma. Trajectory of "trapped particles". Charged particles travel in tight "gyro- orbits" around magnetic field lines. In some cases, due to the gradient of the magnetic field, their trajectory traces out banana-shape orbits. Cross-section showing the toroidal, poloidal and divertor coils FUSION ENERGY-TOKAMAK Magnetic Fusion Reactor Schematic of a Fusion Reactor –Tokamak Design Tokamak Problems • Helium only carries 20% of energy, neutron escapes plasma • Energy lost from the core due to radiation, and this energy is proportional to Z2 Containment vessel = high Z material • Plasma is chronically unstable “confining a plasma using magnetic fields is like confining Jell-o with elastic bands” TOKAMAK Results: . The energy that goes into heating and confining the plasma is MORE than the energy that is produced Rate of energy loss > the rate of energy gain. Break-even: Eout > Ein • The energy recovered by burning some amount of fuel is greater than the energy required to get it to burn. • Analogy : The energy gained by eating a sandwich is greater than the energy of chewing, digesting TOKAMAK Ignition : P out, avg > Pin, avg • In steady state, the average power output is greater than the average power input • Analogy : Using a match to light a fire that gives off lots of heat as long as there is fuel, i.e. it is self- sustainable TOKAMAK . For best Fusion burn, want : High density plasma (N) Hot Plasma (T) Long burn time () Inertial Confinement Conditions for controlled fusion reactions; Extremely high temperatures (108 K) and pressures very hard to produce Possible through use of high energy Laser pulses from many directions simultaneously Inertial Confinement Laser Fusion Projects: Based on Nd:glass lasers Argus Laser System, USA, Shiva System – 20 lasers directed from 20 directions Delphin System in USSR- 256 beams launched through 256 amplifiers. Inertial Confinement Fusion Working : . Multiple high power lasers or ion beams are focused on a freeze dried pellet of D and T . Pellet absorbs energy and heats up dramatically, causing an imploding shockwave that crushes the D, T nuclei together . Rapidly shrinking nuclei with increasing temperature causes fusion reaction . Pellet and laser system must be designed for most of the D-T fuel to fuse before pellet explodes. ICF- Schematic An inertial confinement fusion implosion on the NOVA laser creates "microsun" conditions of tremendously high density and temperature rivaling even those found at the core of our Sun. Inside the main chamber of Nova ( National Ignition Facility) National Ignition Facility (NIF) America Fires the Most Powerful Laser in History (2010): United States' National Ignition Facility at Lawrence Livermore National Lab in California has fired the most powerful laser in history, a record-breaking 2MJ shot. The laser was originally designed to reach 1.875 MJ, but beat everyone's expectations set a new world record in the process. 192 laser beams (UV) combined to form the single shot, initially reaching 1.875 MJs. Better yet, the blast caused less damage to the laser optics than predicted, which allowed the facility to fire another shot just 36 hours after the 2.03 MJs one. “It's a remarkable demonstration of the laser from the standpoint of its energy, its precision, its power, and its availability.” - Ed Moses, Director, NIF NIF and ICF Target assembly for NIF's first integrated ignition experiment mounted in the cryogenic target positioning system (cryoTARPOS). The two triangle-shaped arms form a shroud around the cold target to protect it until they open five seconds before a shot. Fusion Schematic in NIF • 192 Laser beams in single shot Inertial Confinement Fusion Problems : • With many lasers requiring extreme precision and power, the firing is not possible more than a few times each day (optics needs to cool) • In order to reach power production conditions, the lasers must be fired more than once per second. Results : • Break-even is nowhere close Safety & Waste Management . It’s so hard to get going, that if you do anything wrong, it stops. Massive reactors still have very little fuel in the machine at any time • ~0.2 g of D,T produces 1.5 GW • Small amount of fuel means no chances of container melting .