What role does vacuum technology play in nuclear fusion?

In recent years, nuclear fusion has become one of the focus areas of science in the world, and there are more and more reports and discussions on nuclear fusion in the media of various countries. But many people are still unfamiliar with it, equating it with nuclear power plants and atomic bombs.

what is nuclear fission?

Nuclear fission is the source of energy for nuclear power plants and the explosion of atomic bombs. It refers to a form of nuclear reaction in which a heavy atomic nucleus (mainly uranium or plutonium) is split into two or more atoms of smaller mass.

Fission is initiated by bombarding the uranium isotope U-235 with neutrons. When these neutrons hit the U-235 atoms, they release 2 to 4 neutrons, which then hit more U-235 to form a chain reaction. This process releases a lot of usable energy and can also generate electromagnetic radiation.

Fission reactors function the same as conventional power plants. Fission occurs in the reactor vessel and is controlled by control rods. The heat from the reaction is used to heat the water and generate steam. This steam then turns a turbine to generate electricity. Because used nuclear waste is radioactive and has a half-life of more than 10,000 years, nuclear waste needs to be handled safely. This not only increases the cost in the later stage but also has certain hazards.

To make nuclear fission fuel, it first needs to be enriched: increasing the concentration of U-235 from the 0.7% that occurs naturally in uranium ore to the 4-5% needed for the fission process. This requires the use of centrifuges to convert uranium ore into UF6 gas, which is then enriched into fuel pellets. Of course, the entire centrifugation process needs to be completed in a specific vacuum environment

Today, there are about 440 nuclear reactors operating in 33 countries. These reactors generate about 10 percent of the world's electricity. In addition, about 55 nuclear power reactors are under construction. Uranium ore comes from 10 countries and is 70,000 times more energy dense than fossil fuels.

What is nuclear fusion?

Nuclear fusion is a developing technology, its emergence will completely break the status quo of energy production, and belongs to the frontier field of current scientific research.

In the process of nuclear fusion, two light atoms release the electrons outside the nucleus from the shackles of the nucleus under extremely high temperature and pressure, so that the two nuclei attract and collide with each other, and the nuclei polymerize with each other to generate a new heavier mass. Atomic nuclei (such as helium). Since neutrons are uncharged, they escape the nuclei of atoms throughout the collision process, and the release of a large number of electrons and neutrons produces enormous amounts of energy. This is the source of energy for nuclear fusion.

Humans are currently studying relatively low-tech first-generation nuclear fusion. The raw materials used are the two isotopes of hydrogen, deuterium, and tritium. An ideal first-generation fusion reaction uses a 50:50 mixture of deuterium and tritium (reaction 3 below). However, tritium is very rare in nature, because tritium is a radioactive element, and its half-life is very short only 12.43 years, so it cannot be preserved in nature for a long time, and there is no condition in nature to synthesize tritium, so tritium must be Artificial production. The artificial production of tritium is very expensive. At present, nuclear fusion devices only use deuterium (reactions 1 and 2), because deuterium on the earth is very rich, and each liter of seawater contains 0.03 grams of deuterium, so the cost of fusion reactions can be greatly reduced.

In fact, human beings have achieved deuterium-tritium nuclear fusion—the explosion of a hydrogen bomb, but that is an uncontrollable instantaneous release of energy, and humans need to be controlled nuclear fusion even more. The current research on controlled nuclear fusion is devoted to the peaceful use of fusion energy.

There are two main types of research on controlled thermonuclear fusion energy: magnetic confinement fusion and inertial confinement fusion. Magnetic confinement fusion (MCF) consists of a toroidal vacuum chamber and a toroidal coil, which generates a huge helical magnetic field when energized. It uses a strong magnetic field to confine charged particles well, confining deuterium-tritium gas in a vacuum magnetic container And heating to hundreds of millions of degrees Celsius to achieve the purpose of nuclear fusion.

Tokamak is a toroidal magnetic confinement-controlled nuclear fusion experimental device invented by former Soviet scientists in the 1950s. After nearly half a century of efforts, the scientific feasibility of generating fusion energy on tokamak has been confirmed, but the relevant results are all produced in the form of short pulses, which are far away from the continuous operation of actual reactors. The two major difficulties in realizing nuclear fusion power generation are how to achieve hundreds of millions of degrees of ignition and stable long-term constraint control.

Inertial confinement fusion (ICF) uses ultra-high-intensity lasers to irradiate a deuterium-tritium target in a very short period of time to achieve fusion. A mixture of several milligrams of deuterium and tritium gas or solid is loaded into the deuterium-tritium target. The laser beam or particle beam is uniformly injected from the outside, and the spherical surface evaporates outward due to the absorption of energy. Under the reaction of it, the inner layer of the spherical surface is squeezed inward, and the deuterium and tritium in the deuterium-tritium target are squeezed and the pressure rises. accompanied by a sharp increase in temperature. When the temperature reaches the required ignition temperature, deuterium and tritium will explode in the vacuum reaction chamber, the heat of the steam generated is converted into heat energy, and the generated tritium can be recycled as new fuel. This explosion process is very short, only a few trillionths of a second. If this explosion continues continuously, the amount of energy released will be limitless.

Generally, a vacuum is used in the entire cycle of magnetic confinement, including:

1. Evacuating reaction vessel itself to UHV — needed to remove larger gas species and prevent plasma contamination.
   
   
2. Cryogenics (cryostat) — the cooling system itself requires a vacuum.
   
   
3. Tritium production — tritium is most commonly produced by colliding neutrons with lithium which requires vacuum and pumping for coolant gas. Neutrons are also produced from fission reactions or linear accelerators which both require a vacuum to operate.
   
   
4. Plasma heating — the vacuum is needed in the neutral beam injection system for heating the plasma
   
   
5. Vacuum testing — equipment is tested under a vacuum and using tools such as outgassing rigs and leak detectors.
   
   
6. Pressure gauges and instrumentation — for monitoring equipment and process performance
   
   
7. Cooling — pumping is needed for helium used to cool superconductors
   
   
8. Recirculation — unused fuel and helium are pumped from the chamber, separated, and recirculated
   
   
9. Fusion fuel — Powerful pumps are needed to inject the fusion fuels into the chamber
   
   

For inertial confinement, a vacuum is required as follows:

1. Evacuating target chamber to UHV — needed to remove larger gas species and prevent plasma contamination
   
   
2. Target positioning systems — systems used to target the lasers on the fuel require vacuum
   
   
3. Beamline vacuum — several beamlines are used for the lasers and these are all required to be under vacuum
   
   
4. Spatial filter — spatial filters are used to ‘clean up’ the laser beam and function under vacuum
   
   
5. Pressure gauges and instrumentation — for monitoring equipment and process performance
   
   

from https://www.vacuumscienceworld.com/blog/what-is-nuclear-fusion