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Small Fusion Reactors

New Concepts & Projects for Early Fusion Reactors

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US Fusion Budgets: Office for Fusion Energy & Inertial Confinement Fusion
1. US Fusion Budget History
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Andrei Sakharov: Nobel Peace Prize for his work to cut nuclear weapons.
2.Sakharov, H-Bombs & Tokamaks
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General Atomics DIII-D: Last US Tokamak. Sized for Small Reactor.
3.General Atomics: DIII-D interior
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Japanese high power long pulse gyrotron.
5. Japanese Gyrotron for electron heating.
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Gryaznevich, Tokamak Solutions: Testing High Temperature Superconducting coil.
6. Nb-Sn Superconducting Coils
Extractable ITER module
ITER: Transport of radioactive blanket module. blanket module
7. Extractable ITER blanket module.
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Cross sections for neutron collision (red) or transmutation of Depleted Uranium-238.
8. Neutrons cross sections in Depleted Uranium.
Solid DU USA
Uranium Hexafluoride, Capenhurst UK
9. Stored Solid Depleted Uranium.
Fusion physics has been very successful in 50 years. Fusion works and has produced break-even power conditions in two Tokamak machines.

This leads towards a Fusion Power reactor to produce electricity. However, the time scale for all the engineering is long and highly susceptible to global financial conditions. Still, this is the international commitment, with the US a difficult partner (1). The UK budgets took even deeper cuts to maintain the JET contributions.

By lowering the sights to much smaller power outputs one can buid machines using existing materials and existing technologies brought up to industrial strength to produce useful fusion.

This is the basis of the current flurry of privately funded entries into the Fusion arena. The front runner is based on the Tokamak, a Russian invention by H-bomb designer, Andrei Sakharov (2) and colleagues.

The Spherical Tokamak is the most likely Small Fusion reactor to succeed in the next 15 years. The size is illustrated by the background picture (3) to this page. The plasma chamber fits in a large, two storey sitting room, but the surrounding equipment and radiation shielding fills a large hangar. The working blanket around the plasma chamber doubles the total volume and operates the fuel breeding, waste burning, and power generation for a useful plant.

Fusion experiments began with rapid flashes of plasma, lasting only 100s of microseconds. This is quite a long time in a plasma but not useful for a lightbulb. Modern large experiments run for several minutes which at least demonstrates mastery of the plasma conditions.

All the components of a Fusion Reactor have to run reliably with long pulses > 10min and a 70% duty factor, or even continuously. The list is long: magnetic coils, vacuum pumps, neutral beams, electron-cyclotron RF heating devices (4), cooling pumps and heat transfer pumps, Tritium injection, extraction and recycling equipment, and so on. They must also be energy efficient to restrain their fraction of the electric power generated.

The materials which face the plasma or have a large neutron flux through them must withstand the damage inflicted. The high voltage or high current or high frequency supply systems must also run continuously.

The ITER project has contracted groups around the world to produce the right devices and materials for the ITER project which is to run for up to an hour at a time.

It is to be expected that these teams can produce devices of a much lower specification for Small Fusion Reactors.

It has been evident from the start that magnetic fusion would need superconducting coils to produce the large magnetic fields. Ordinary copper coils would consume an embarrassingly large fraction of the net power output of a reactor and are therefore not feasible.
The first large superconducting coils (6) used a Niobium-tin alloy which worked best at liquid Helium temperatures. The reactor plasma at 100 million degrees Kelvin is surrounded by coils at 2 degrees Kelvin. The hottest stuff in the solar system next to the coldest.
High temperature superconductors have now emerged which run at liquid Nitrogen temperature. These are, in principal, well suited to Small Fusion devices.


The benefits of Small Fusion Reactors are entirely drawn from the use made of the copious Fusion neutrons. This all about the transmutation of chosen elements into something more useful or less troublesome. Fertile isotopes can become nuclear fuel, heavy isotopes can be burned for energy, others can be excited to become medical isotopes. The radioactive blanket materials need to be loaded and extracted safely, using robots and transport caskets (7).
The ITER project is developing just such machinery, as illustrated .

Neutron Collision and Absorption Cross Sections (8).
Now for something much more technical:

The 'cross section' is the effective size or probability of an interaction with a nucleus as seen by an approaching neutron. The unit of measurement is the 'barn' at 10^-28 m^2 which is like the size of a Uranium nucleus at 1.8 barns. The neutron itself is about 2% of a barn, and a pretty small projectile.

Neutrons are uncharged and are not repelled by the charge on a nucleus. They can simply bounce off or penetrate the nucleus and be absorbed by the strong nuclear force acting between all nucleons. Quantum mechanics allows for many different outcomes.

Hot fusion neutrons above 10 MeV hit a nucleus hard and can expel 1-3 neutrons, an alpha particle, or a Deuterium or Tritium nucleus from heavy nuclei. The cross sections are about 1 barn, so a direct hit is needed.

Cooler neutrons can resonate strongly with the quantum structure of a nucleus at particular energies, like the different notes of a flute. The cross section can rise to thousands of barns at precise energies. A Fast Reactor, or a carefully tuned Fusion Hybrid, can avoid cooling the neutrons too much to take advantage of these resonances.

Cold neutrons below 100eV are much more likely to be absorbed the slower their approach. The absorption cross sections for U-238 are only 1-2 barns and the collision or bounce cross section is 20 barns.
It is really important to know the neutron energy distribution in Fusion blankets especially when most of them are likely to be down at the thermal temperature of the blanket. Many blanket studies fail to do this.

Finally, the fission cross sections for Uranium and Plutonium are in hundreds of barns at reactor temperatures.

This is probably the most valuable commercial application of a Small Fusion Reactor. Uranium enrichment begins with natural Uranium which has only 0.7% of the fissile fuel isotope, U-235. A nuclear reactor is much more efficient if the Uranium fuel contains above about 5% of U-235.
Uranium Oxide, or yellowcake, from Uranium mining is converted to Uranium Hexafluoride, UF6, which is a gas above 120 degrees. This gas, spun in a centrifuge causes the heavier U-238 component diffuse towards the edge of a spinning column. The slightly richer centre is sucked out to the next centrifuge in a cascade of many such steps.
It takes 11 tonnes of natural Uranium to make one tonne of 5% enriched Uranium, leaving 10 tonnes of Depleted Uranium with only 0.2% of U-235 left in it. The DU is often stored as solid UF6.
The planet now has ten times as much DU as all the burned reactor fuel. It is a vast energy source. In principle, it can be turned into high quality Plutonium fuel by the reaction and decay sequence:
U-238 + n = U-239
= Np-239 + e + Gamma
= Pu-239 + e + Gamma.

A liquid blanket of Uranium quadrifluoride, UF4, around a 50 MW Small Fusion Reactor can produce enough Plutonium to top up a 1GW Fission reactor.

The UK owns enough DU to run an all electric Britain for up to 500 years.

Small Fusion reactors can be built as appropriate versions of Magnetic or Laser Fusion devices. They have many useful applications in the 1-50 MW range. At 100MW they can serve as Fusion cores to small 300-500MW Hybrid Fusion-Fission power reactors.

There is clearly much to be learned and much to be done.
DT Fusion reaction
How D-T Fusion works
Peng ST Power Reactor
Peng: ST Reactor
Small Fusion Overviews
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World Spherical Tokamaks
Small Fusion Library
ST Field Line
3. Voss: Power Plant
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Zakharov: NSTX Princeton