How efficient are the nuclear fusion reactors of today?


Answer by Robert Steinhaus, former Tiny Guy in LLNL Field Test Division at Lawrence Livermore National Laboratory (1974-2008)

There is a practical fusion technology in existence today and that fusion technology could currently form the basis of actual fusion power plants that make practical amounts of commercial electricity from fusion energy.

Fusion is not “pie in the sky”, unicorns, or B.S.

Mankind came into possession of a practical way of generating energy from fusion over 50 years ago with the Ivy-Mike fission-fusion nuclear test that produced huge amounts of fusion energy from pure Deuterium via DD fusion. Practical fusion will always be 50+ years ago (not 50 years away). Historically, this early method of producing more energy from nuclear fusion than it took to bring the fusion device to fusion conditions actually worked the very first time it was tried – fission-fusion technology actually never failed to work. Today, pure fusion device technology derived from successful early fission-fusion designs that does not require a fission primary is possible and is a better candidate for peaceful, ultra-clean, commercial electrical energy production from fusion.

Inertial Confinement Fusion of Hybrid DT-DD Pure Fusion Devices for Energy Production –

Deuterium is a very cheap and abundant fusion fuel. The deuterium in sea water is capable of completely powering planet earth at a the current level of 17 Terawatts for 29.4 billion years. Deuterium-Tritium fusion fuel is however much easier to ignite and requires a less powerful and costly fusion driver to initiate. It makes some sense to minimize the size and cost of an ICF fusion driver (laser or particle accelerator) by using some DT fuel as the first stage fusion primary (the match) while producing the majority of fusion power from ubiquitous sustainable D-D fusion of pure Deuterium.

The energy needed to ignite an inertially confined thermonuclear fusion reaction in liquid (or solid) deuterium-tritium (DT) is not that large; it is on the order of not more than 20 MJ or about the same amount of chemical energy stored in about 2.5 cups of automotive gasoline.

The problem is that this energy must be compressed in space (focused down to an area less than a 2 mm) and in time (to less than 3 nanoseconds).

Conceptual Drawing of a 2-stage DT-DD clean energy optimized pure fusion device.

How efficient would a mini-PACER power plant be?

Inertial Confinement fusion experiments produce their energy is a repeated succession of short energetic fusion bursts. A two stage DT-DD device has a significant advantage over a single stage ICF fusion target in energy gain.

At a power plant level, the fairest and best way to express the efficiency of a fusion power plant is in terms of “wall plug efficiency”. How much (electrical) power do you have to draw from the local power mains to produce a given amount of fusion generated electricity?

Modern semi-conductor lasers are between 10% to 30% efficient at converting local electrical power into ultra-violet or X-ray laser light (lets say 10% efficient for the purpose of this calculation). To deliver 20 MJ of light energy to the DT primary fusion capsule would require drawing about 200 MJ of electrical energy from the local power grid. Realistically, there are some additional inefficiencies in delivering ultra-violet or X-ray laser light to the fusion capsule –

So the best estimate for the amount of electrical energy that has to be drawn from the local power grid to deliver 20 MJ of ultra-violet or X-ray energy to the fusion capsule is 1000 MJ per shot.

An energy yield of 250 GJ is equivalent to an energy yield of 250 x 10^3 MJ

The output of the secondary of the fusion secondary of the PPFD (peaceful pure fusion device) is expected to yield 250 Gigajoules of energy per shot after fusing about 0.8 grams of pure deuterium. So the electrical to fusion energy gain of the PPFD device is

(250 x 10^3 MJ / 1000 MJ) x 100% = 25000%

(we get 250 times as much energy from the second stage of the PPFD device as is required to draw electricity from the local power grid to power the Fast Pulse Laser Driver and ignite the DT primary first stage of the PPFD device).

Good turbine-generators that can operate at temperatures that mini-PACER power plants can operate at are only about 44% efficient. The end-to-end energy gain as measured as wall plug efficiency

(input electricity drawn from the local power line to power the laser driver -> output electricity produced by the power plant)

of a practical mini-PACER power plant would be about
250 x 0.44 x 100% = 11000%

How would a Hybrid DT-DD Pure Fusion mini-PACER Power Plant Work?

Here is a quick synopsis of how a pure 2-stage DT-DD mini-PACER fusion power plant would work.

The mini-PACER cavity, which is 11 meters in diameter and 51 meters in length and of cylindrical shape vertically oriented is buried at 100 meters depth in the ground. The mini-PACER cavity is first pumped down to a soft vacuum; this reduces that pressure peak experienced inside the cavity while firing the pure fusion PPFD (peaceful pure fusion device).

The PPFD is lowered on a cable/steel sheathed optical fiber until the device is in a position about 60 meters down from the top of the cavity.

High velocity pumps flood a spray of hot molten salt from nozzles in the roof of the mini-PACER Cavity. This hot salt falls from the roof of the cavity and then collects in a pool at the bottom of the cavity.

While the molten salt is falling through the PACER cavity from the roof, a large but still commercial fast pulse laser housed on the surface above the PACER cavity fires an intense, 20 MJ energy, short, Petawatt power, femtosecond long, pulse of laser light. This intense burst of laser light compresses and ignites a micro fusion pellet of deuterium-tritium forming the first stage of the device primary. Laser compression and initiation of the small pure fusion DT primary causes approximately 10% of the atoms in the primary to fuse. This burst of fusion energy in the device primary produces a shock detonation wave in the larger deuterium-deuterium (DD) fusion secondary – thereby reliably creating the conditions for D-D fusion plasma ignition. In excess of 97% of the pure fusion energy released by the PPFD then comes from cheap D-D fusion (with 99% or greater believed possible for a PACER power plant optimized device). The controlled small 250 GJ energy PPFD fusion burst produces intense heat which is transferred to the flowing molten salt falling through the partially evacuated mini-PACER cavity. This heat collects in a molten salt pool at the bottom of the cavity following each shot. The hot salt stores the heat which is transferred through a heat exchanger and additional engineered heat matching heat reducing apparatus to ultimately be transferred to a conventional steam turbine generator that produces electricity. The stored heat in the molten salt pool permits the mini-PACER power plant to continuously produce power until the next mini-PACER shot occurs (the pacing of mini-PACER shots is a function of device size and the power generation level desired – a small 250 Gigajoule PPFD device must be ignited every minute to produce a continuous power level of 1GWe from the mini-PACER power plant.

Concept Picture of a mini-PACER power plant –

It is widely perceived that commercial forms of nuclear fusion are currently ~30 years away (and always will be) – but the reality is that such widespread and excessive pessimism about fusion is not justified.

Practical fusion will always be 50+ years ago (not 50 years away)