Nuclear fusion (v1.0)

A key reference for H-bomb and fusion power generation is Wikipedia.

Why do stars shine?

Stars shine because of fusion taking place at their core. Nuclear physics tells us heavier nuclei want to break apart and lighter nuclei want to fuse under huge pressures and temperatures. The most stable that want to do neither is iron and nickel. The center of the sun is estimated to be 15,000,000K. The sun fusing just 4% of its hydrogen explains all the energy it has put out so far in its 5-billion-year lifetime. In that time, it has lost an extremely small proportion of mass (which converted to energy with E = M * C ** 2). 

Fusion starts with hydrogen and goes from there. The sun has a long way to go!! Hydrogen fusion will continue in our sun for another 5 billion years. But the temperature (and therefore kinetic energy) is not high enough for a hydrogen nuclei protons (which repel each other due to electromagnetic force) to have enough energy to come close enough to each other so strong nuclear force can takes over and get them to fuse! The answer is quantum tunneling. Enough hydrogen tunnels through so fusion occurs. But the probability of tunneling through the energy barrier reduces exponentially with energy barrier size and thickness and is extremely sensitive to the initial energy of the proton. There is another problem with hydrogen fusion. Helium 2 (2 protons) is unstable and falls apart very quickly putting us back to where we started. What saves the day is the weak nuclear force. There is an exceedingly small probability that one of the protons in helium 2 will spontaneously switch to a Neutron due to the weak force emitting a positron and a neutrino. This creates hydrogen 2 which is deuterium. This further fuses to give helium which is stable.

How does mankind harness this fusion capability for bombs?

A thermonuclear weapon, fusion weapon, or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lower mass, or a combination of these benefits. Characteristics of nuclear fusion reactions make possible the use of non-fissile depleted uranium as the weapon's main fuel, thus allowing more efficient use of scarce fissile material such as Uranium-235 or plutonium-239. The first full-scale thermonuclear test was carried out by the United States in 1952; the concept has since been employed by most of the world's nuclear powers in the design of their weapons. Modern fusion weapons consist essentially of two main components: a nuclear fission primary stage and a separate nuclear fusion secondary stage containing thermonuclear fuel: the heavy hydrogen isotopes deuterium and tritium, or in modern weapons, lithium deuteride. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs.

A fusion explosion begins with the detonation of the fission primary stage. Its temperature soars past approximately 100 million kelvin, causing it to glow intensely with thermal X-rays. These X-rays flood the void (the "radiation channel" often filled with polystyrene foam) between the primary and secondary assemblies placed within an enclosure called a radiation case, which confines the X-ray energy and resists its outward pressure. The distance separating the two assemblies ensures that debris fragments from the fission primary (which move much more slowly than X-ray photons) cannot disassemble the secondary before the fusion explosion runs to completion.

The secondary fusion stage—consisting of outer pusher/tamper, fusion fuel filler and central plutonium spark plug—is imploded by the X-ray energy impinging on its pusher/tamper. This compresses the entire secondary stage and drives up the density of the plutonium spark plug. The density of the plutonium fuel rises to such an extent that the spark plug is driven into a supercritical state, and it begins a nuclear fission chain reaction. The fission products of this chain reaction heat the highly compressed, and thus super dense, thermonuclear fuel surrounding the spark plug to around 300 million kelvin, igniting fusion reactions between fusion fuel nuclei. In modern weapons fueled by lithium deuteride, the fissioning plutonium spark plug also emits free neutrons that collide with lithium nuclei and supply the tritium component of the thermonuclear fuel.

The secondary's relatively massive tamper (which resists outward expansion as the explosion proceeds) also serves as a thermal barrier to keep the fusion fuel filler from becoming too hot, which would spoil the compression. If made of uranium, enriched uranium or plutonium, the tamper captures fast fusion neutrons and undergoes fission itself, increasing the overall explosive yield. Additionally, in most designs the radiation case is also constructed of a fissile material that undergoes fission driven by fast thermonuclear neutrons. Such bombs are classified as two stage weapons, and most current Teller–Ulam designs are such fission-fusion-fission weapons. Fast fission of the tamper and radiation case is the main contribution to the total yield and is the dominant process that produces radioactive fission product fallout.

How does mankind harness this fusion capability for power generation?

Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, Fusion processes require fuel and a confined environment with sufficient temperature, pressure, and confinement time to create a plasma in which fusion can occur. The combination of these figures that results in a power-producing system is known as the Lawson criterion. As a source of power, nuclear fusion has a number of potential advantages compared to fission. These include reduced radioactivity in operation, little high-level nuclear-waste, ample fuel supplies, and increased safety. However, the necessary combination of temperature, pressure, and duration has proven to be difficult to produce in a practical and economical manner. A second issue that affects common reactors is managing neutrons that are released during the reaction, which over time degrade many common materials used within the reaction chamber.

The current leading designs for confinement are the tokamak and inertial confinement (ICF) by laser. Both designs are under research at very large scales, most notably the ITER tokamak in France, and the National ignition facility (NIF) laser in the United States. Tokamak is the most well-developed and well-funded approach. This method drives hot plasma around in a magnetically confined torus, with an internal current. When completed, ITER will become the world's largest tokamak. As of September 2018, an estimated 226 experimental tokamaks were either planned, decommissioned, or operating (50) worldwide. Inertial confinement typically uses direct or indirect drive. In indirect drive, Lasers heat a structure known as a Hohlraum that becomes so hot it begins to radiate x-rays light. These x-rays heat a fuel pellet, causing it to collapse inward to compress the fuel. The largest system using this method is the National ignition facility followed closely by Laser Megajoule. In Direct drive, Lasers directly heat the fuel pellet. Notable direct drive experiments have been conducted at the Laboratory for Laser Energetics (LLE) and the GEKKO XII facilities. Good implosions require fuel pellets with close to a perfect shape in order to generate a symmetrical inward shock wave that produces high-density plasma.

Multiple approaches have been proposed to capture the energy that fusion produces. The simplest is to heat a fluid. The commonly targeted Deuterium-Tritium reaction releases much of its energy as fast-moving neutrons. Electrically neutral, the neutron is unaffected by the confinement scheme. In most designs, it is captured in a thick "blanket" of lithium surrounding the reactor core. When struck by a high-energy neutron, the blanket heats up. It is then actively cooled with a working fluid that drives a turbine to produce power. Another design proposed to use the neutrons to breed fission fuel in a blanket of nuclear waste, a concept known as a fission-fusion hybrid. In these systems, the power output is enhanced by the fission events, and power is extracted using systems like those in conventional fission reactors.

The fuels considered for fusion power have all been light elements like the isotopes of hydrogen—protium, deuterium and tritium. The deuterium and helium-3 reaction requires helium-3, an isotope of helium so scarce on Earth that it would have to be mined extra terrestrially or produced by other nuclear reactions. Ultimately, researchers hope to adopt the protium–boron-11 reaction, because it does not directly produce neutrons, although side reactions can.

In Feb 2022, UK-based JET Laboratory produced 11 megawatts of power over five seconds from nuclear fusion. In Dec 2022, government scientists at the Lawrence Livermore National Laboratory achieved a long-sought milestone in developing clean fusion energy. For the first time, the amount of energy produced by a fusion reaction exceeded the energy required to produce it. In august 2023, US scientists said they have repeated the feat—this time achieving a greater yield of energy. These are first glimmers of hope. However, there is still a long way to go before fusion is viable on an industrial scale, providing power to homes and businesses.

Is fusion power economically competitive? 

The widespread adoption of non-nuclear renewable energy has transformed the energy landscape. Such renewables are projected to supply 74% of global energy by 2050.The steady fall of renewable energy prices challenges the economic competitiveness of fusion power. Some economists suggest fusion power is unlikely to match other renewable energy costs. Fusion plants are expected to face large start up and capital costs. Moreover, operation and maintenance are likely to be costly. However, fusion power may still have a role filling energy gap left by renewables, depending on how administration priorities for energy and environmental justice influence the market. In the 2020s, socioeconomic studies of fusion that began to consider these factors emerged, and in 2022 EURO Fusion launched its Socio-Economic Studies and Prospective Research and Development strands to investigate how such factors might affect commercialization pathways and timetables. Similarly, in April 2023 Japan announced a national strategy to industrialize fusion. Thus, fusion power may work in tandem with other renewable energy sources rather than becoming the primary energy source.