Sunday, April 29, 2018

Nuclear Fusion: is it still worth investing on it in an age of cheap renewable energy?

A review by Giuseppe Cima of the situation with nuclear fusion. The matter is complex, but Cima identifies the crucial point: even assuming that nuclear fusion were to work as expected, it would be more expensive than the presently available renewable technologies. Consider also that it will take decades before we can have fusion reactors able to produce commercially available energy (if ever). How much better and cheaper will renewables be by that time? Considering that fusion is not a "clean" technology, as sometimes claimed, it doesn't seem to have any realistic chance to be useful for something, now or in the future. So, why are we still spending money and resources on this technology? One more example of the human blind faith in technology and its miracles (U.B.)

ITER TOKAMAK, looking carefully, at the bottom right circled in red, a human in a yellow jacket. The probable size of a magnetic confinement fusion reactor is huge and it's at the core of most of its problems.

My view on nuclear fusion, in a nutshell

 by Giuseppe Cima

Nowadays few businesses would invest in conventional nuclear power stations. In the US, subsidies of 100% or more fail to attract private investments for a nuclear fission power station, the classic form of nuclear energy. So, the perspectives for a revival of nuclear are not rosy.

But there exists another form of nuclear energy, thermonuclear fusion, the one that powers the stars. Fusion, the sticking together of light nuclei such as hydrogen, is a nuclear reaction distinct from fission, where heavy atoms, such as uranium, break apart. Fusion energy research has been pursued since the WWII years in national labs and universities all over the world. Despite all efforts, though, so far it has not provided a clear indication of being feasible. What are the current perspectives of this form of energy?

Fusion technologies

There are two ways to burn hot nuclear fusion fuel: make it react very quickly before the burning gas flies away, the way an H bomb works, or use a magnetic field to insulate the plasma from the reactor walls. The bomb method can be replicated in a series of micro-explosions in the lab, but the rate has to be high enough to produce relevant electric power and this poses huge unsolved problems. A giant laser fusion experiment in the US, the National Ignition Facility, has demonstrated how difficult and expensive is to produce a micro-explosion once a day. Imagine doing that hundreds of times per second for years. Even with a budget provided by the military for weapon development, laser fusion is far away from pointing to a credible commercial reactor.

Therefore, from the inception of fusion energy research, most efforts have been devoted to magnetic confinement of steady state hot plasmas. After 70 years of trying, almost everybody in the field has concentrated on one favorite scheme which goes under the name of TOKAMAK, a Russian invention. The tests performed so far indicate that the minimum size of a potential reactor core will be large, the size of a large building. ITER, a TOKAMAK presently under construction in France to demonstrate the feasibility of fusion, is of this size but, apart from the size, it is so expensive that its construction is requiring the financial contribution of all developed nations on earth.

The doughnut-shaped ITER reactor core is 30 meter in diameter, 20 m high. It is an extremely complex device, much more sophisticated than an equivalently powerful nuclear fission reactor and roughly 10 times the volume. Its core weights more than 30 thousand ton, just the floor of ITER uses 200 thousand cubic meters of concrete.

Size is the most obvious drawback of nuclear fusion: the large size makes it impossible to mass produce these reactors. This factor gives a considerable advantage to the competition, made of comparatively small generators: gas turbines of 50-100 MW, efficient windmills of a few MW, photovoltaic solar panels of less than 1 kW. These generators can be transported by truck and the speed of their industrial development has been inversely proportional to the power of an individual module. The cost of electricity for photovoltaics and wind originates mainly from the cost of capital invested in the generator and its ancillary equipment, just as it's the case for Deuterium-Deuterium fusion where the fuel is nearly free. Natural gas power stations burn inexpensive fuel and have the lowest generator capital cost of all, but are CO2 polluters, nowadays a serious drawback.

We must specify that the fuel for fusion reactors is nearly free only in the case of the Deuterium-Deuterium fusion. The current idea, instead, is to use the easier reaction of Deuterium with Tritium, the latter being another radioactive isotope of Hydrogen. It is a very rare isotope that can be bred in the same TOKAMAK which is burning it, but not in sufficient quantity to keep these reactions going. This is another issue of ITER-like reactors, for the time being swept under the rug.

Because of its large size and complexity, it's very hard to imagine that a TOKAMAK fusion reactor could be less expensive than a conventional fission reactor and detailed present-day estimates put the cost of the kWh to more than 12 ¢, just for the capital cost, and before knowing all the details of a working reactor.

Instead, electricity commercialized from unsubsidized photovoltaic and wind generators is presently sold at prices between 2 and 7 ¢/kWh, depending on location, and there is room for more savings. These sources are intermittent, fusion is not, but for a renewable-dominated electrical production, the additional cost of energy storage would entail a fraction of the cost of energy production. This is a purely economic consideration: renewables are already less expensive than fusion energy.

There is a second very relevant drawback linked to the large size of the fusion reactor: its development time. ITER will experiment with real fusion fuel not earlier than 2035 and will realistically carry on the experiments in the following 10 years. It implies that this experimental phase, not a prototype reactor since ITER will be incapable of producing energy, will have taken roughly 50 years.

To make a dent in the world electricity production one should implement thousands of 1 GW size reactors. How long of an experimentation phase should one consider to reach this goal from when ITER will have answered the initial round of questions? Maybe 100 years, i.e. a couple of experimental phases.

To summarize, on top of a plethora of unresolved, even unknown, design issues of technical nature, magnetic fusion poses problems linked to the huge size of the TOKAMAK reactor core: a large kWh cost and a very long development time. For the ones sensitive to the "cleanliness" of fusion I also have to mention that ITER at the end of its life will present a bill of around 30,000 tons of heavily radioactive waste without having produced a single kWh. Magnetic fusion is not clean: its fuel and the products of the reactions may be modestly radioactive, but the machinery itself is not.

Why the reactor has to be large

Why a magnetic fusion reactor has to be big, physically very large? Thermonuclear fuel has been proven to burn in the H bomb, but it can burn also non-explosively, think of the sun. For any fuel to burn in steady state, the energy released in the volume of the burning matter equals the energy escaping from it, heat produced equals heat lost, the energy balance equation. The rate at which energy is produced grows in proportion to the density of the fuel, the number of atomic nuclei per unit volume. The reactor power density increases with the density of the reacting particles.

The plasma in a reactor is a gas of atomic constituents roughly in thermal equilibrium, its kinetic energy content is characterized by a pressure. If the TOKAMAK plasma has to be contained by a magnetic field, the field pressure has to be substantially higher than the plasma pressure. The magnetic pressure produced by the external superconducting magnets at the plasma location is limited at present to less than 200 atmospheres by the mechanical strength of the magnets. Improvements are foreseeable on the magnets front and they would be helpful, but the magnet materials are themselves subject to the laws of nature of solids: these improvements will be marginal.

Like in an ordinary gas, the plasma pressure is proportional to particle temperature and density. The fusion temperature has to be in the region of hundreds of millions of deg C hence, because of the magnetic pressure limit, the particle density turns out to be pretty low, a million times less than the molecular density of the air we breathe. The result is a low power density.

On the other side of the reactor power balance equation, the energy lost by the plasma is dictated by plasma turbulent motions and the size of the device. Turbulence has been experimentally demonstrated to be present at a significant level in all magnetically confined plasmas of thermonuclear interest, just like with water in a canal.

The analogy is close, for a given incline the water flow in a canal is constrained by an irreducible turbulent drag, with negligible dependence on the canal construction details. This is the case also for energy confinement in a thermonuclear plasma, it's dominated by unavoidable turbulent fluid motions. But a reacting core large enough to reach power breakeven always exists because its volume (energy production) to surface (losses) increases with its size, a purely geometric consideration. The sun, even without a magnetic field, is certainly large enough for breakeven.

These are the reasons why the tokamak reactor has to be very large. The size required to maintain the large core temperature needed for the plasma to fuse. This is the main factor making nuclear fusion expensive and very hard.

Bottom line

As things stand, present-day renewable technologies are considerably less expensive than a potential nuclear fusion reactor - even assuming it would work as expected. My work in fusion coincided with the Reagan electric sector deregulation when something similar happened between natural gas and coal-fired power stations. The development of large aviation jet engines made possible efficient, inexpensive, factory produced, electricity generators which proved to be impossible to beat and coal power plant investors went bankrupt to allow for the American industry to take advantage of the newer, less expensive, technology. It was then too early for the wind and photovoltaic revolution but now they are here to make nuclear fusion obsolete before it has been proven to work.

The author

Giuseppe Cima has been employed in various capacities by fusion research labs and Universities in Europe and the US for most of his professional career: Euratom Culham UK, ENEA Frascati and CNR Milan, the Fusion Research Center at UT Austin. He published more than 70 peer-reviewed papers in this field, mostly about EM waves for plasma diagnostic and heating, magnetic configurations, turbulence measurements. After losing faith in a deconstructionist approach to fusion, he started an industrial automation company in Texas. He is at present retired in Venice, Italy, where he struggles to protect the environment, conserve energy and teach technology and science.


Ugo Bardi is a member of the Club of Rome, faculty member of the University of Florence, and the author of "Extracted" (Chelsea Green 2014), "The Seneca Effect" (Springer 2017), and Before the Collapse (Springer 2019)