Wednesday, March 25, 2015

Thermonuclear Energy

Tuesday, March 4, 2014
12:15 a.m.

Back in high school, I wrote a post on nuclear energy. It was the most-read post for a long time on this blog. It has since fallen to 2nd place behind "Mt. Rainier Weather." However, they are very close in views, so there are brief times when it reclaims first place. Although it pains me to admit it, the reason why these two posts (and many of my top posts) have so many views is not because they are popular or well-written but simply because the images on the post appear on Google Images, allowing curious websurfers to visit the source of the picture (my blog) if they so choose. Let's just pretend I'm a internationally-known weather celebrity whose posts are celebrated throughout the world without the help of Google Images, though. It sounds better.

I wrote this post soon after the Fukushima meltdown in Japan. I knew it would be a controversial post; I was actually defending fission power. Nuclear fission is the cleanest type of non-renewable power source available, and is the most feasible for large-scale electric production. Hydroelectric dams can produce a fair amount of electricity (that's why our electric bills here in Washington are so cheap), but wind and solar don't produce much. And what about solar at night or when it's foggy? And have you ever seen a wind farm full of non-rotating windmills? It's pretty pathetic. The destruction of nuclei in nuclear fission releases vast amounts of energy for very small amounts of fuel, and since no combustion is involved, no carbon dioxide is emitted. You also don't get those particulates that you often get from combustion, especially the combustion of low-quality coal. Nuclear waste can be stored safely, and the U.S. has not had an accident since Three-Mile Island. Excepting the Chernobyl meltdown in 1986, nobody has ever died as a result of radiation exposure from a nuclear power plant, disaster or not. In fact, nuclear has one of the lowest accident/failure rates of any engineered design.

As I'm sure we all know, however, the accidents, when they occur, have the potential to be very serious. This isn't McDonald's, and you're not making the mistake of giving your customer a Big Mac instead of a Quarter Pounder with Cheese. The site of the Fukushima nuclear meltdown is still highly radioactive, and cleanup will take 40+ years and tens of billions of dollars. In addition, some of the land will be unfarmable for centuries. Sure, coal power is dirty and inefficient, but there's no danger of a coal plant failure grossly contaminating the surrounding area to the same extent of a nuclear plant.

Three Mile Island, and particularly Chernobyl, occurred due to human error. The guys at Chernobyl had limited knowledge of nuclear engineering and physics, and ran the reactors with many of the safety systems turned off. In addition, there were many engineering flaws with the reactor in the first place. All these flaws have been fixed in future plants, and there is essentially no chance that an event like this could ever happen again. I thought that the Fukushima plant would have been much more advanced because it was a nuclear plant in a developed nation in the 21st century, but engineering-wise, it was found to fail the most basic of safety requirements by several nuclear safety agencies and that there was no way it could ever withstand an earthquake or tsunami.

Basically, what I'm trying to say is that all of these accidents could have been easily avoided with more care and preparation. You do NOT want to skimp on safety when it comes to nuclear energy. If you want to read my previous post on nuclear energy, you can do so here.

Alright, that's my spiel on fission power. Let's move onto fusion power, or thermonuclear energy.

Front page of the New York Times, December 7, 1960. Retrieved from Alex Wellerstein's Restricted Data Nuclear Secrecy Blog

Little Boy. A bomb so powerful, it single-handedly destroyed Hiroshima. And Fat Man, an even more powerful bomb that ended the most horrific war our civilization has faced. These nuclear weapons relied on nuclear fission just like the nuclear reactors used today. However, within a decade, a new type of bomb, a thermonuclear bomb, was developed. Whereas the previous nuclear fission bombs got their energy from the breaking apart of heavy elements, these new bombs vastly increased the efficiency of doing this and obtained additional energy by fusing light ones together. The result was a much, much more powerful bomb. The first thermonuclear bomb ever detonated was 750 times more powerful than the bomb dropped on Hiroshima. The Russians developed these weapons not long after the Americans did. It's easy to see why living in the Cold War era must have been an unsettling time.

Before we go any further, let me give you a quick debriefing on mass-energy equivalence. It might sound a little technical, but I guarantee you are at least somewhat familiar with it.

That's Einstein's mass-energy equivalence formula. It says that the amount of energy in something is equal to its mass multiplied by the speed of light squared. The formula says all types of weird things... for example, if you add energy to an object, its mass will increase by a tiny amount even though no matter has been added. Likewise, it says that when mass is lost, as it is in nuclear fission and fusion, tremendous amounts of energy are released. This is because the amount of mass lost is multiplied by the speed of light squared (c^2), and as you can see at the bottom of the picture above, c^2 is quite a large number!

Now that we've got that settled, let's take a look at how nuclear fission and nuclear fusion work.

Nuclear fission works by by bombarding an atom with a free neutron, causing it to briefly add a neutron to its nucleus. However, this bombardment renders the new nucleus unstable, and the nucleus splits into both lighter elements and releases free neutrons. 

The most common starting fuels for nuclear fission that we use are uranium and plutonium, and the below diagram shows what happens when you bombard a uranium-235 nucleus with a free neutron. It turns into a uranium-236 nucleus but soon splits into krypton and barium while releasing three free neutrons, which go on to bombard other uranium-235 nuclei. As you can see, this creates a chain reaction that grows very quickly. Massive amounts of energy are released by these free neutrons, and the reaction produces photons (the explosions create fireballs that light up the entire sky) that release energy in the form of gamma rays. In a bomb, all of this happens at once in a manner similar to the example below. In a fission reactor, this reaction is controlled so that for every 2 or 3 neutrons released, only one must be allowed to strike another uranium nucleus. If it is less than one, the reaction will fizzle, and if it is more than one, you will sizzle (it will grow into an uncontrolled reaction and you could be exposed to dangerous levels of radiation). These uncontrolled reactions cannot sustain themselves in a nuclear power plant like they can in a bomb, so you aren't in danger of setting off an explosion of that magnitude, but they can fry people nearby and deliver fatal dosages of neutrons and gamma rays.

Simple diagram of nuclear fission.

I'm a pacifist. Well, kind of. There's nothing I like more than watching nuclear test videos. The "atomic cannon" is a classic. It's only 15 kilotons, but this allowed the cameras to be placed much closer to the explosion. They also have astonishingly high resolution for 1953. Take a look at the video below!

That said, I'd be here forever if I talked about how both fission bombs and fission power work. So let me give you an overview of the latter.

Reactors and Cooling Towers at the Susquehanna Steam Electric Station: retrieved from Wikipedias' nuclear power page.

One thing I like to visualize when thinking of how much power a nuclear plant produces is the amount of steam that goes through those cooling towers. I mean, look at that stuff. It would be an interesting atmospheric science project to study if the massive amounts of heat and moisture released from these cooling towers have ever sparked convection and possibly even thunderstorms that otherwise would not be there nearby.

The general premise behind nuclear power is to control a nuclear reaction so that it heats water into steam and drives a turbine, producing electricity. To do this, enriched uranium is generally formed into 2.5 cm long pellets with a dime-size diameter. These pellets are lined up into long rods, which are then collected together into bundles. These bundles are submerged in water inside a pressure vessel to prevent them from overheating. A nuclear meltdown is when these rods overheat, melt, and create a steam explosion. It is NOT a massively uncontrolled reaction that causes runaway fission to occur and and fry the city.
In addition to the water, control rods are used to prevent a meltdown. These rods are made of elements that are non-fissionable and can absorb neutrons. Boron, silver, indium, and cadmium are a few examples. These rods are inserted into the bundles using a mechanism that can raise or lower them, and raising or lowering the rods allows the operators to control the rate of the reaction. The control rods are raised out of the uranium bundle to increase the rate of reaction by allowing fewer neutrons to be absorbed and vise versa.

Finally, the water is heated to steam and then spins a turbine. This turbine is connected to a generator, and when the turbine spins, it also spins the generator, producing power. You can't hold on to that steam forever, so you gotta send it through these huge cooling towers sooner or later. I'm sure many of us associate those cooling towers with nuclear power plants.

Alright. Now that we've got all that out of the way, let's talk about what I originally intended this post to be solely about: thermonuclear energy.

"Thermo" is a prefix for heat. Thermometer, thermodynamics, thermochemistry are just a few examples. I once told a girl she was hella thermo and asked her if she wanted to spend an evening  at Super China Buffet with me. Didn't work... she obviously wasn't cool enough for me. But thermonuclear? What's not thermo about regular nuclear? That steam is coming out at 450 degrees Celsius, and the hottest part of the reactor runs at 600 degrees. That's enough to vaporize a man in seconds. However, a successful thermonuclear reactor requires temperatures of 100 million degrees Celsius.

OK, let me repeat that last sentence, since I kind of sneaked it in there. 

A successful thermonuclear reactor requires temperatures of 100 million degrees Celsius.

Folks, that would probably even vaporize Chuck Norris. Now do you see where the "thermo" prefix comes from? At well over 150,000 times hotter than a fission reactor, you better believe a thermonuclear reactor deserves its title.

Thermonuclear energy is acquired through nuclear fusion, which in many ways is the opposite of fission and is what powers stars. Whereas fission involves the breaking apart of heavy elements such as uranium or plutonium into smaller ones and the release of energy in the process, nuclear fusion involves the "fusing" of light elements, such as hydrogen, into larger ones, such as helium. Theoretically, any element can be fused, and that's how our elements on the periodic table up to iron were created. Anything higher was created in a supernova. On Earth, we can't fuse heavier elements because we don't have the immense gravitational forces and pressure associated with stars. However, we can fuse hydrogen into helium. Let me give you a brief overview of how that works.

The most common type of hydrogen atom has one single proton and one electron. For the fusion reactors currently being tested, we take two different types of hydrogen atoms, or isotopes: deuterium and tritium. Deuterium, often known as "heavy hydrogen," is hydrogen with one neutron in addition to the one proton and electron, and tritium, which is very rare naturally but can be synthesized from lithium, has two neutrons, a proton, and an electron. Any atom that has one proton is a hydrogen atom.

Anyway, for any sort of fusion reaction to occur, the nuclei must be squeezed together. The main obstacle they have to overcome is that the protons are positively charged (neutrons have no charge). Like charges repel, so fusing an atom requires overcoming the repulsive force between the protons in the nucleus.

To overcome this, you need two things: extremely high temperatures (100 million degrees Celsius, as stated above) and incredibly high pressure (the hydrogen atoms need to be within one quadrillionth of a meter). The sun does this using the force of gravity to compress the matter into its core, which is where the fusion takes place. Since we don't have a prodigious amount of matter at our immediate disposal, we need to apply energy from magnetic fields or lasers.

I talked about deuterium-tritium reactions, but ideally we'd eventually be able to rely on deuterium-deuterium reactions. Deuterium is easier to extract from seawater than tritium is to create from lithium and is far more plentiful. The only problem is that D-D fusion requires much higher temperatures to ignite, with the absolute minimum required being 400 million degrees Celsius compared to a minimum of 45 million degrees for D-T fusion. Any engineers in the house?

Speaking of engineering, there are two types of fusion reactors that are currently being explored: magnetic confinement and inertial confinement. Let's now take a look at how those work.

Magnetic Confinement

A magnetic confinement reactor is a reactor that uses electric and magnetic fields to heat and compress electrified hydrogen gas (hydrogen plasma). The main magnetic confinement reactor that scientists from all over the world are collaborating on is located in France and is called the International Thermonuclear Experimental Reactor (ITER).

Credit: Matt Farrell, University of Illinois

The picture above shows a highly simplified schematic of a magnetic confinement chamber. Notice how the main chamber is in the shape of a big donut. This shape is called a toroid, and it happens to be the most efficient shape for confining this plasma. A magnetic confinement chamber in this shape is called a topamak. Toroids have many useful applications in electromagnetics; I know for certain that many of the alternating current transformers I have in my audio amplifiers are in the shape of toroids.

Toroidal power transformer in my Sansui G9000DB stereo receiver

The picture below gives a little more in-depth version of the parts that make up a tokamak. I'll list them below.

Credit: ITER

The vacuum vessel: confines the plasma and keeps the reaction chamber in a vacuum

Neutral beam injector (aka: ion cyclotron system): heats plasma by injecting particle beams from the accelerator into the plasma

Magnetic field coils (poloidal, toroidal): magnets that use their magnetic fields to confine the plasma and allow fusion to occur

Transformers/Central solenoid: supply the magnetic field coils with the massive amounts of electricity needed for them to maintain their extremely strong magnetic fields

Cooling equipment (cryostat, cryopump): the magnets generate a lot of heat, so the cryostat and cryopump cool them

Blanket modules: absorb free neutrons and excess heat from the fusion reaction

Diverters: remove excess helium formed from the fusion of deuterium and tritium from the chamber

The fusion reaction becomes initiated when neutral particle beams, electricity, and microwaves from various accelerators heat a mass of hydrogen gas. When this gas is heated to a sufficient temperature, it turns into plasma. Plasma is the same type of substance that stars are made out of and is regarded as the fourth state of matter (regardless of what your elementary science school teacher may have told you, there are more than just three phases of matter!). Power is supplied to the transformers to create a magnetic field (a flowing current of electricity creates a magnetic field around it), and under this extremely strong magnetic field, the plasma is compressed and fusion takes place. Well, at least that's the idea.

Inertial Confinement

While magnetic confinement works by magnetically compressing the hydrogen ions in close proximity to each other for a given amount of time, inertial confinement works by fuse them so fast that the ions are not able to overcome their inertia and move apart, leading to the name "inertial confinement." When I think of inertial confinement reactors, I often get this image of this kid shoplifting a candy bar from a store at night and running away before being surrounded by police pointing a bunch of flashlights at him. Except, in this example, the "kid" is a pea-sized pellet containing deuterium and tritium and the "flashlights" are dozens of incredibly powerful laser beams. The biggest inertial confinement reactor in the world, the National Ignition Facility (NIF) at Lawrence Livermore Laboratory in Livermore, California, contains 192 of these lasers. In the NIF, these lasers are housed within a 10-foot diameter rugby-shaped "target chamber" called a hohlfram. Much like the donut-shaped topomak, I suspect that the geometry of the hohlfram had to do more with efficiency than the engineer's favorite sport.

The lasers at the NIF will focus 1.8 million joules of energy onto the little pellet of deuterium and tritium, heating it and generating x-rays emanating from pellet. The deuterium and tritium will then turn into plasma as a result of the immense heat and radiation and compress until fusion occurs. Once this fusion occurs, the intense amount of heat and energy released from it will act to sustain fusion. We have not reached this point yet, but we are making progress; back in 2013, we generated a net gain in power produced for the first time in history.

Fusion process for an inertial confinement reactor. Credit: Lawrence Livermore National Laboratory

The general mechanism for the generation of electricity for a topamak is the same as any fission or fossil fuel reactor; a reaction produces heat that boils water, creates steam, and drives a steam turbine, creating electricity. So yes, fusion reactors will still have those awesome steam towers.

But while steam towers are nice, they don't explain the full scope of the benefits of nuclear fusion power, especially when compared to fission. Deuterium is very common in the ocean, and tritium can be easily processed from lithium. Our current deuterium/lithium reserves would last us 60 million years, but if we used just deuterium (as we hope to in the future), our fuel reserves would last 150 billion years. To put things in perspective, that's almost 11 times the age of the universe. Even if we become an incredibly power-hungry civilization and last for five billion more years (until the sun dies), we will have hardly put a ding in these reserves. So for all practical purposes, fusion, particularly deuterium-deuterium fusion, is a renewable source of energy. Uranium, on the other hand, is rare and must be mined. Fusion reactors produce less radiation than conventional fission reactors, and while waste is produced, it decays on decadal timescales and is approximately as radioactive as coal ash after 100 years. Uranium and plutonium take thousands of years to degrade to safe levels. There's no danger of a runaway meltdown like there is in a fission reactor; fusion requires incredibly specific conditions to exist, and if these conditions are disrupted due to an external factor like a massive earthquake, the fusion will cease. And let's not forget about the prodigious amounts of power produced from fusion.

Is nuclear the fusion the answer for all of our energy problems? Yes and no. I believe that it is the "holy grail" of energy and something that we should aspire to, but with greenhouse gases accumulating as fast as they are, we need to invest in proven technologies that do not emit carbon dioxide. Many of the professors I have talked to at the University of Washington believe solar will be the leading energy source in a couple decades. One thing is for sure: we need to get off coal.


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