Thursday, September 5, 2019
Why nuclear fusion research is bullshit
Note: this is an evolving blog post that periodically undergoes changes, so come back later to see updates.
Nuclear fusion... I've seen this touted as a promising future source of "clean and unlimited energy" for most of my life. I'm no expert on the subject, but I've always been skeptical about the various claims being made.
I won't go into details of what nuclear fusion is and how it's being made to work, because I know you're all smart and know that already. So I'll cut straight to the chase.
Here is a list of past, present, and near-future nuclear fusion experiments:
Here's an article in Forbes magazine that covers some of the recent developments in nuclear fusion research. Note that this is written for the average layman and is not a deeply scientific article.
Below are my various points regarding the problems of sustaining a nuclear fusion reaction that produces "clean unlimited energy".
Fuel type and quantity
The best fuel for starting a fusion reaction is a 50:50 mix of the hydrogen isotopes tritium and deuterium. This fusion reaction produces the largest amount of energy at the lowest temperature compared to other fusion fuels. Deuterium makes up about 1 in 5000 atoms of hydrogen in ordinary water and so is relatively plentiful. Tritium is produced only in small amounts in nature, about 4kg per year, by cosmic ray interactions in the upper atmosphere. Manmade tritium is a byproduct of CANDU type nuclear fission reactors used to produce electricity.
Neutron capture by deuterium in the cooling water of CANDU reactors produces small amounts of tritium that can be extracted from the coolant water. The total global reserve of tritium is hard to know since many countries keep some of their nuclear information secret, but it's estimated that about 25 kilograms (55 pounds) current exist, and this and only grows by about half a kilogram (~1 pound) each year. [more research needed]
Tritium shortage is such a critical issue for fusion that research studies have been performed to look into the problem. The following link gives good information on tritium production and future supply problems, and I'll be adding more info from this study to this blog at a later date.
Consider also that tritium decays to Helium-3 and a low energy beta particle (electron) with a half-life of 12.3 years, so it's actually radioactive on its own and is a mild radiation hazard if it leaks into the environment. The radioactivity of tritum is actually not too bad, and can't penerate the surface layer of human skin or through air for more than about 6 millimeters, but it's certainly not perfectly "clean". Furthermore, the deuterium-tritium reaction produces highly energetic neutron radiation, definitely not clean at all. Now consider that for electricity production to occur, the neutrons from fusion must collide with the reactor walls so their energy can be dissipated and converted to heat to boil water, to run a steam turbine, to turn a generator, to make electricity. So there is a relatively low efficiency in converting neutron energy to electricity. Consider also the unavoidable production of radioactive materials as fusion neutrons bombard the reactor vessel, which in turn creates radioactive waste.
The International Thermonuclear Experimental Reactor (ITER) is currently being built in France and is scheduled to come online in 2025 (now delayed), with full tritium-deuterium reactions not taking place until 2035. It will be the biggest nuclear fusion research facility in the world. ITER's current plan is to use 1 kilogram of tritium per year, and will be producing it's own tritium supplies using traditional nuclear fission reactions such as we find in reactors that use Uranium as fuel. As you can see in the link above, ITER also plans to test the concept of using lithium "blankets" inside the reaction chamber to regenerate tritium. ITER's eventual goal is to use 125 kilograms of fuel per year, half of which will be tritium.
125kg of tritium. O... K. Currently ITER is only planning to create 1kg of its own tritium per year at the facility. It doesn't sound very sustainable to me just yet.
With more experimental fusion reactors coming online in the near future, the world's tritum supplies will be used up within a few decades. A 3 gigawatt nuclear fusion power plant would use 167kg of trtium per year. Obviously not sustainable.
So far the longest fusion reaction has been for 6 minutes 30 seconds at France's WEST fusion reactor. How much fuel did this use? It's hard to get answers on this but I'll try to find out and put the answer here later.
So far the best we've had is 16MW of energy produced for 24MW of input, or 67% return. That's the best we've done in 50+ years that this research has been going on around the world, and there has not been a single watt of electricity produced by any fusion reactor so far.
For nuclear fusion to become a self-sustaining chain reaction it must produce enough excess energy to cause more fuel to undergo the fusion reaction. We have never achieved energy break even so far, let alone producing sufficient excess energy to sustain the reaction. Will we be able to produce a sustainable reaction? Well, imagine creating a small, continuously detonating hydrogen bomb. This is the goal that nuclear fusion research needs to achieve to become a viable power source.
If ITER ever reached its goal of 500MW energy production, it would need a cooling water supply of about 12 cubic meters per second (180,000 gallons per minute), That's a reasonable amount considering it's exactly what you need for any 500MW generating plant. Around 56MW would be needed to drive the pumps for this cooling.
My issue here is the quantity of water, and particularly the warmed water released from the plant back into the environment, and the environmental effects of this warm water, at least close to the plant. Existing chemical and nuclear fission fueled power plants are known to release large quantities of warm water into the environment and this is known to be an environmental concern. The same holds true for a nuclear fusion fueled power plant.
Even once ITER gets going, it will have taken years and gigawatts of energy, and will require ongoing hundreds of megawatts of electricity and massive water supplies to sustain. Who knows at what point it will truly produce more energy than it took to build and develop. It will also have some impact on the environment. With the radioactive tritium, and the amount of warm water generated, it's certainly not as "clean" and environmentally friendly as scientists are trying to make us think it is.
Let's not forget that material properties change over time when exposed to high amounts of radiation. The high energy neutrons from the Tritium-Deuterium reaction will also cause the production of isotopes in the reactor wall, many of which will be radioactive.
What will happen to the physical containment structure over time? Well, initially experiments on ITER and other fusion experiments will run for such short periods that there won't be much damage to the reactor vessel. However, over the long term the materials of the vessel walls will swell, become brittle, fatigue, and some radioactive isotopes will be formed. In the long term this will potentially result in the entire fusion reactor, some 30,000 tons of material, becoming nuclear waste.
Transitioning to other fusion fuels and reactions
People often say that the Tritium-Deuterium (T-D) reaction is just the start, and that ideally we will transition to the safer and more available Deuterium-Deuterium (D-D) reaction, and eventually to the Hydrogen-Hydrogen reaction that provides most of the power inside stars such as our sun. However the D-D reaction still produces neutron radiation and some tritium, although the neutrons are less energetic and produce less radioactive isotopes on the vessel walls.
This site has good information about the various fusion reactions, their reaction products, and how much energy they produce.
The D-D reaction also requires vastly higher temperatures to initiate (need to supply values and references). This results in much higher energy densities required, around 30 times as much (ref), to sustain the fusion reaction. Another major problem is that the D-D reaction has a much lower energy output, around 1/68th that of the T-D reaction (refs).
In short, as you step up to a safer and more available fuel, you also require higher temperatures and confinement (i.e. pressures) to initiate and maintain the reaction. At the same time, you also step down to a lower power output, making it harder to produce a self-sustaining chain reaction and requiring more fuel to produce the same amount of power. In addition, the D-D reaction still produces some radioactive tritium, as well as neutrons which bombard the reaction vessel and produce nuclear isotopes, so it's not entirely as "clean" as it's being made out to be.
There are some combinations of fusion fuels that I haven't mentioned that produce less radiation but once again they require much higher pressures and temperatures to initiate and sustain, and they also produce less energy than the T-D reaction. For example, the next fusion reaction "up" from T-D would be deuterium and helium-3 but helium-3 is much too rare to use as a fuel. The next potential reaction would be Hydrogen-Boron-11 fusion, which requires a 500 times greater combined temperature-pressure, and produces only 1/2500th as much energy as the T-D reaction (references).
If you delve into the media, scientists are always talking about one day achieving Hydrogen-Hydrogen fusion, and since more than 99% of all matter in the universe is hydrogen, it will therefore be an unlimited supply of clean energy. However, if you delve into the scientific literature surrounding fusion research, no one is talking about using the Hydrogen-Hydrogen reaction as a fusion fuel because the pressures and temperatures required for it are far beyond the current capabilities of fusions reactors, and the scientists involved know this. It might not even be possible to create the enormous temperature and pressure required to sustain a H-H fusion reaction here on the Earth. Also, the H-H reaction produces much less energy than the T-D reaction (how much, reference). There are also other fusion reactions possible that I haven't mentioned, but like the H-H reaction, they aren't being talked about among fusion researchers, unless of course it's to the public, or to politicians that make the funding decisions for nuclear research projects, and then once again we are given the usual marketing hype about "unlimited clen energy".
So all this talk of fusion being clean and unlimited energy is wishful thinking at best. My guess is that ITER and other fusion reactors will generate some impressive examples of decades-long international cooperation among nations both friendly and semi-hostile, but in the end will only serve as a way to keep physicists employed, chew up vast energy and other resources, while producing absolutely nothing.
And that is why all this talk about nuclear fusion as an unlimited clean source of energy is bullshit.
References (this needs much expanding)
Posted by Anonymous at 11:47 AM