Q7Non-Conventional Energy Sources

(a) Nuclear Fission, Nuclear Fusion, Fusion Requirements, and Advantages

Nuclear fission is the process in which a heavy atomic nucleus (such as uranium-235 or plutonium-239) splits into two lighter nuclei upon absorbing a neutron, releasing a large amount of energy (primarily as kinetic energy of the fission fragments, converted to heat) along with additional neutrons that can sustain a chain reaction - this is the process underlying all currently operating commercial nuclear power plants. Nuclear fusion, by contrast, is the process in which two light atomic nuclei (typically isotopes of hydrogen - deuterium and tritium) combine to form a heavier nucleus (helium), also releasing a very large amount of energy, since the mass of the fusion products is slightly less than the combined mass of the original light nuclei, with this mass difference converted to energy according to Einstein's mass-energy equivalence relation.

For controlled nuclear fusion to occur and be sustained, three simultaneous requirements (often summarized as the Lawson criterion) must be satisfied: extremely high temperature (on the order of 100 million degrees Celsius or more, needed to give the positively-charged nuclei enough kinetic energy to overcome their mutual electrostatic (Coulomb) repulsion and approach close enough for the short-range strong nuclear force to bind them together), sufficiently high plasma density (to ensure a reasonable collision/fusion event rate), and sufficient energy confinement time (the plasma must be thermally insulated from its surroundings long enough, at the required temperature and density, for enough fusion reactions to occur to make the process net energy-positive).

• Advantages of nuclear fusion: uses abundant, widely available fuel (deuterium extractable from ordinary seawater, and tritium breedable from lithium, both far more abundant than mineable uranium),
• produces no long-lived high-level radioactive waste comparable to fission (fusion reaction products are primarily helium, an inert, non-radioactive gas, though the reactor structure itself becomes activated and requires eventual decommissioning),
• carries no risk of a runaway chain-reaction meltdown, since the fusion plasma is inherently difficult to sustain and simply extinguishes if confinement conditions are disturbed, rather than accelerating uncontrollably,
• and offers the potential for a very high energy density fuel source capable of meeting a substantial share of future global energy demand without the greenhouse gas emissions of fossil fuel combustion.

(b) Tokamak Reactor and Laser Fusion Reactor Advantages

A Tokamak (a Russian acronym for 'toroidal chamber with magnetic coils') is the most widely researched and developed magnetic confinement fusion reactor design, in which the hot fusion plasma is confined within a toroidal (donut-shaped) vacuum vessel by a combination of strong magnetic fields: an externally applied toroidal magnetic field (running around the long way of the torus) combined with a poloidal magnetic field (generated by a large electric current induced to flow within the plasma itself, running the short way around the torus), whose combined helical field lines confine the charged plasma particles away from the vessel walls (since charged particles are constrained to spiral along magnetic field lines rather than moving freely across them) while allowing the plasma to be heated to the extreme fusion-relevant temperatures without the plasma physically contacting and damaging (or being cooled by contact with) the vessel walls. Additional heating methods (neutral beam injection, radio-frequency heating) are typically used alongside the ohmic heating provided by the plasma current itself to reach and sustain the required fusion temperatures.

A laser fusion reactor instead uses inertial confinement: a small spherical pellet containing deuterium-tritium fuel is rapidly and symmetrically compressed and heated by an array of extremely high-power laser beams (or, in some designs, heavy-ion beams) focused simultaneously onto the pellet's surface from multiple directions, causing the outer layer to ablate explosively outward and, by Newton's third law reaction, driving the remaining fuel inward in an implosion that compresses it to extremely high density and temperature for a very brief instant (nanoseconds), long enough for a burst of fusion reactions to occur before the compressed fuel disassembles - unlike the Tokamak's continuous, steady-state magnetic confinement approach, laser (inertial) fusion relies on the fuel's own inertia to hold it together just long enough for fusion to occur, without needing any confining magnetic field at all.

The advantages of the laser fusion reactor approach include the comparatively simpler reactor geometry (no large superconducting magnet coil systems required, since confinement is provided by the pellet's own inertia rather than an external magnetic field), the pulsed rather than continuous operating mode (potentially simplifying certain aspects of heat removal and materials engineering compared to a continuously operating Tokamak plasma), and a research pathway that has demonstrated significant progress toward net energy gain in recent experimental facilities, making it a valuable complementary approach to magnetic confinement fusion research, even though significant engineering challenges (laser efficiency, target fabrication cost and rate, and overall system net energy balance) remain before either approach can be developed into a commercially viable fusion power plant.

The energy released per fusion reaction, though smaller in absolute terms than a typical fission event, is far larger per unit mass of fuel consumed: the deuterium-tritium fusion reaction releases roughly four times as much energy per unit mass of fuel as uranium-235 fission, and the fuel itself (deuterium from seawater, tritium bred from lithium) is vastly more abundant and widely distributed globally than mineable uranium ore, which is geographically concentrated in a comparatively small number of countries - this combination of energy density and fuel abundance is a central part of fusion's long-term appeal as a potential large-scale, geopolitically less contentious energy source, even though the engineering challenge of achieving sustained, net-positive fusion has proven to be extraordinarily difficult to solve.

Currently operating nuclear fission plants use a variety of reactor designs distinguished primarily by their coolant and moderator choice - pressurized water reactors (PWRs) and boiling water reactors (BWRs), which together make up the large majority of the world's operating fleet, use ordinary (light) water as both coolant and moderator; pressurized heavy water reactors (PHWRs, the dominant design used in India's domestic nuclear program) use heavy water (deuterium oxide) as moderator, allowing them to use natural (unenriched) uranium fuel rather than requiring the enrichment infrastructure needed for light-water reactors; and gas-cooled and other advanced reactor designs use alternative coolant/moderator combinations, each with different trade-offs in fuel cycle cost, safety characteristics, and achievable thermal efficiency.

The international ITER (International Thermonuclear Experimental Reactor) project, under construction in France through a collaboration of major world economies including India, is the largest and most advanced Tokamak-based experimental fusion project to date, intended to demonstrate sustained fusion plasma operation producing substantially more fusion power than the external heating power supplied to the plasma (a milestone widely referred to as scientific breakeven or Q greater than 1), representing a critical intermediate step on the long development pathway toward an eventual commercially viable fusion power plant, which most experts still project to be several decades away even under an optimistic development timeline.