Nuclear (Energia de Fusão)

"A energia de fusão artificial assemelha-se à energia de fusão natural, i.e., à energia solar"

by Sérgio Granato de Araújo

Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions, such as those that occur in the sun.  To fuse on our sun, nuclei need to collide with each other at very high temperatures, exceeding 10 million °C, to enable them to overcome their mutual electrical repulsion [IAEA, 2021].  But making this process work smoothly & continuously in such a high-temperature condition is an immense challenge [AC, 2022].

Fusion occurs when two or more atomic nuclei approach closely enough and long enough for the attractive nuclear force to overcome the repulsive electrostatic force, merging them into heavier nuclei.  The process releases energy because the total mass of the resulting single nucleus is less than the mass of the two original nuclei: the leftover mass becomes energy [EG, 2023].  Unlike nuclear fission, nuclear fusion is a naturally occurring reaction (e.g., in the center of a star).

Figure 1 shows the Deuterium (D) &Tritium (T) fusion reaction.  Notably, D is a stable, non-radioactive isotope, whereas T is radioactive and requires specific handling.  The fusion reaction produces a helium nucleus (alpha particle), a neutron, and energy, which produces a helium nucleus (alpha particle), a neutron, and energy.

Research

Research into fusion reactors began in the 1940s, emerging alongside fission developments during the Manhattan project, although it was not its primary focus.  Early theoretical work by physicists such as Edward Teller helped lay the foundation for later fusion research.  As of 2023, no fusion device has achieved net power at the system level.  Over decades of experimentation, scientists have struggled to produce reactions that generate more energy than they consume overall.  Most designs aim to heat their fuel to around 100 million °C, which remains a major engineering challenge.

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Fusible or Fissionable?

Light elements such as Hydrogen & Helium are in general more fusible, while heavier elements, such as Uranium, Thorium, and Plutonium, are more prone to fission and are typically used in fission-based nuclear reactors.

Advantages over fission power

As a source of power, nuclear fusion has a number of potential advantages compared to fission, including i) reduced radioactivity in operation, ii) little high-level nuclear waste (the process doesn't result in byproducts like spent rods), iii) ample fuel supplies, and iv) increased safety.

Breakeven

A fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state.  Q = 1, when fusion power equals the required heating power, is called “breakeven”.

On Dec 5, 2022, a team at LLNL’s National Ignition Facility (NIF) ran a fusion experiment that produced more energy from a fusion reaction than the laser energy used to drive it: 2.05 MJ of laser energy was directed to the target, which released 3.15 MJ of fusion energy.

The burst output of 192 laser beams focused on a tiny diamond sphere the size of a peppercorn to generate a shock wave that pushed hydrogen atoms close enough together to fuse.  While the energy released from this fusion was greater than the laser energy that hit the peppercorn sized target, the overall energy balance is a net loss and is not ready to be scaled up [AC, 2022].

Source:  Energy. Department, 2022