The Nuclear Option

When I was maybe twelve years old, I read about a fourteen year old who had built a nuclear fusion reactor in his parents’ garage.  Now, this was not a break-even fusion reactor, but it nonetheless successfully fused atomic nuclei.  Since I was only twelve at the time, I naturally decided that I ought to be able to do the same, but at a younger age.  However, I never built that fusion reactor (I like to blame this on the fact that my parents wanted to use the garage for its intended purpose, and continue to keep their cars in it, but the reality is that I simply never put the time and effort into nuclear research at the expense of my other interests and activities), and even now that I own my own garage, I have yet to utilize it to experiment with nuclear physics.  I guess that makes me a slacker, but this post isn’t about my recriminations on the opportunity costs of the path I’ve so far forged in life (and really, I can’t complain, though I do sometimes wonder at the different ways life could go).

No, the entire introduction to this post is really just an attempt to make personal through an anecdote a topic that otherwise is esoteric and poorly communicated: nuclear energy.  Whether or not it has anything to do with a certain fourteen year old and his garage-built fusion reactor, I’ve been long fascinated by nuclear energy, but not unlike space, it suffers from a massive communication problem.  If you asked someone to name a job harder than the proverbial “rocket science,” you very well might be answered with “nuclear physics.”  Like I try to explain concepts from astronautical engineering in ways that are approachable to the typical reader, I intend to use this post to explain nuclear energy in similarly approachable terms.

Almost everything about nuclear energy is clouded and confused by an overriding event in history that is considered the first “harnessing of the atom.”  That event, of course, is the detonation of the first atomic bomb, and the subsequent deployment of such weapons against Japan to end World War II.  Whatever your thoughts or opinions might be on that decision, the fact remains that the average view of nuclear energy is heavily tinted by the use of these technologies in some of the most singularly destructive weapons humanity has ever created.  Losing the War argues that the effects of a nuclear blast were sufficiently horrific to shock the world from the numb stupor of atrocity that it had sunk into during the rest of World War II, and certainly the Cold War that followed cemented the equivalency of “nuclear” and “world-threatening.”

As much as the nuclear arms race of the Cold War spurred investment and increased our understanding of nuclear energy and nuclear physics, it also helped make nuclear power of the more benign sort suspect.  Understandably so: the USSR even designed a bomb that they projected would be sufficiently powerful to actually crack the planet’s crust, though they never built the weapon.  Even non-weaponized nuclear energy is strongly associated with the military, with naval vessels powered by fission reactors, and designs at one point drawn up for massive bomber fleets powered by onboard nuclear reactors (those plans were scrapped due to unsolved engineering challenges, safety concerns, and the development of the ICBM).  Incidents involving nuclear power in its early days, like Chernobyl and Three Mile Island, only enhanced this perspective that nuclear energy is by its nature a dangerous and destructive technology.

When “nuclear” makes it into the news and the popular discussion today, it is almost always in the context of danger (perhaps in part because apocalypses sell good advertisements): Iran’s accelerating uranium enrichment timeline, North Korea’s expanding missile program and test sites, lapsing or violated arms control agreements between Russia and the United States, terrorists wielding “dirty” (radiation-laced) bombs.  Even Good Omens, which I thoroughly enjoy in both its book and television series forms, features a wholly unrealistic nuclear exchange as its chosen method of Armageddon (and yes, I realize there is a certain illogic in complaining about a lack of realism in a fictional show involving miracles and a heavenly bureaucracy).  The point is: nuclear energy has abysmal public relations.

That’s a problem, because nuclear energy encompasses so much more than mere apocalypse-inducing weaponry.  “Nuclear” merely refers to anything relating to the atomic nucleus.  We did a review of atomic structure in our post about the photoelectric effect, but we’ll do another, brief overview here, since it is so fundamental to everything we will be talking about involving nuclear physics.

Atomic Structure and Behavior

Atoms can be divided into two main components: the electrons, and the nucleus.  The electrons are particles with a small amount of mass that convey a negative electrical charge.  The nucleus is, at its most basic, composed of protons.  Protons are significantly more massive particles than electrons, but they carry a positive electrical charge of precisely the same absolute value as the electron’s negative charge.  According to the Standard Model of Particle Physics, electrons are fundamental particles (they cannot be split further into constituent particles).  Protons are not fundamental particles, but the particles that compose them will not be relevant for today’s discussion.  An element is defined by the number of protons in its nucleus, which will equal the number of electrons electrostatically bound to the nucleus (if the number of protons and electrons is unequal, it is called an ion).  Because like charges repel, the protons in the nucleus are held together against this repulsion by the strong nuclear force, while because opposite charges attract, the electrons are bound to the positive nucleus by electrostatic force.  Something called the Coulomb Barrier prevents the electrons from just smashing into the protons (more on that later).

There is a third particle that can be present in an atom, called a neutron.  A neutron is almost identical to a proton in mass and composition, but carries no electrical charge – it is neutral.  Neutrons can bind into the nucleus of an atom just like protons, again by the strong nuclear force.  Two atoms with the same number of protons (they are the same element) but different numbers of neutrons are known as isotopes.  Because of the complexities of the strong nuclear force and its interactions with the repulsive electrostatic force between protons, having different numbers of neutrons in an atomic nucleus can make the atom more or less stable.

That is a convenient segue into the topic of atomic stability, which is what gives rise to radioactivity.  If you read Einstein’s Fridge, you’ll hopefully remember entropy: the tendency for a system to move from order to disorder, from variation to uniformity.  This is the actual physics at play when we say that an inanimate object like an atom “wants” to do something – it is “trying” to achieve a high entropy state.  In more rigorous terminology, it takes extra energy to maintain a state of low entropy.  Think of it like climbing a hill.  It takes a lot of effort to climb a hill, but once you’re at the top, it’s relatively easy to climb back down again.  Better yet, think of it like climbing a vertical cliff that you have to cling onto the top of once you’re there: it takes a lot of energy to stay at the top, but you’ll easily fall to the bottom if you stop putting in that extra effort.

Atoms tend to decay to a “ground state” for similar reasons (it has nothing to do with gravity, which at 10^30 times weaker than electromagnetism has no part to play on these tiny scales, but the entropic visualization is sound, especially for our purposes in today’s discussion).  It takes more and more energy to add additional protons and neutrons to an atomic nucleus, and to hold them there.  Eventually, the electrostatic repulsion in the nucleus does not balance properly with the strong nuclear attraction, and the nucleus becomes unstable, and will start ejecting particles – particles will stop trying to hold onto the cliff, and allow themselves to fall.  This is called nuclear radiation, and the process releases energy, but more on that when we talk specifically about nuclear fission.

Consider an atom with eighty protons and eighty neutrons.  If a neutron escapes the nucleus, it changes the isotope, but is the same element.  If a proton escapes, it changes the element.  We’ve found that unstable atoms decay in a predictable way that conforms to laws of quantum probability, with a most likely path, and less likely paths, that all eventually converge upon a stable “ground” state.  For instance, radon-222 (86 protons, 136 neutrons) decays with a half-life of 3.8 days into polonium-218 (3 minutes), then then lead-214 (28.6 minutes), then bismuth-214 (19.7 minutes), then polonium-214 (0.0002 seconds), then lead-210 (22 years), then bismuth-210 (5 days), then polium-210 (138 days), and finally lead-206 (stable).  It can take other paths, but this is the most common, or most probable.  Each of these decay steps also releases radiation in the form of alpha or beta particles.

We skipped over an important concept in nuclear physics, which is half-life.  Half-life is a quantity that looks very specific and technical, until you examine it more closely and realize that it’s tied up in quantum physics.  Nuclear behavior is probabilistic, not deterministic (classical physics is deterministic), and it is impossible to say that any given atom will decay in a given amount of time.  Half-life is a way to get around this by taking an average of sorts.  Half-life is the amount of time it takes for half of a “sufficiently large” quantity of a radioactive isotope to decay into the next step on its chain (sufficiently large just means that a large enough number of atoms must be included for the most probable course to reasonably overbear the improbable outcomes that will inevitably occur – for a one kilogram sample of a radioactive isotope with a half-life of one year, there might be one atom that decays after a picosecond, and one atom that takes five billion years to decay, but half a kilogram will have decayed after one year).

With that, I think we have the basics of atomic structure and behavior down, at least sufficiently to proceed into talking about actual applications of this information, which is really what this post is intended to do.  If you have any further questions about the ideas covered (or not covered) here, please post them in the comments below, and I will do my best to answer them.

Nuclear Fission

In its most basic form, nuclear fission is just a fancy term by which to refer to the process of atomic decay that we described in the above section.  From the same root word as “fissure,” nuclear fission is the splitting of the atomic nucleus.  When people talk about “splitting the atom,” this is what they’re referring to, in all of its glorious Greek irony (atom means “not able to be cut”).  If you recall, I mentioned in passing when we talked about the radon decay change that each step was accompanied by the release of an alpha or beta particle, along with whatever protons or neutrons might have escaped.  There are all kinds of complexities of the different types of particles and energies involved that we could get into, but there is no need.  On a human level, you can think of each of these particles like the person who just let go of the cliff.  By the time they finally hit something, they’ve gained quite a lot of speed (kinetic energy), and they can transfer some or all of that energy into whatever they impacted.  When you have a lot of particles doing this, it manifests as heat.

Thus, it is clear now that energy is naturally released from the process of radioactive decay, and when viewed on a macroscopic scale, this mostly appears in the form of heat.  The heat effect can be quite potent, including keeping entire planets warm if they have large concentrations of radioactive elements in their cores or crusts.  Perhaps the most basic harnessing of the atom is something called an RTG (radioisotope thermoelectric generator), which takes advantage of the heat produced through the passive decay of a supply of radioactive material (the pile, in the parlance).  This technology is used on deep space missions that have inadequate access to the sun (which around the asteroid belt becomes too distant for modern solar panels to be very effective without being unmanageably large for most missions).  The Voyager probes, and more recently New Horizons, all used RTGs.  In an RTG, the natural heat energy produced as the result of radioactive decay is captured and converted into electrical energy that the spacecraft can use.

The “nuclear power” that most people refer to, though, is a little more complicated.  When most people say “nuclear power,” or “nuclear energy,” they are referring to nuclear fission power plants.  In order to generate electrical energy at scale through fission processes, something more potent than the slow, passive, low power outputs of an RTG is required.  It turns out that, under certain conditions, certain isotopes will release neutrons that, if the radioactive material is sufficiently dense, will collide with other nuclei and eject more neutrons, leading to a cascade reaction.  Think of it like our cliff-climber falling off of the cliff, and knocking off the climbers below them as they fall.  If uncontrolled, this can lead to a runaway nuclear reaction, causing ultra-high temperatures that will eventually melt down the whole reactor.  To prevent this, rods made of tungsten or other materials that can absorb excess neutrons are inserted into the radioactive fuel pile when the reaction starts to get too hot.

When you see pictures or films of nuclear reactors, the water is mostly to help absorb radiation around the reactor core.  This style of reactor uses the heat generated from the nuclear fission reactions to generate steam, which then turns a turbine to generate electricity, just like a coal or gas fired plant uses the heat from burning those materials to generate steam to turn a turbine.

There is no physical difference between this kind of fission reaction, and the nuclear decay we talked about earlier.  Choosing an isotope is based mostly on how easily and quickly it will decay, and how controllable it is.  Most fission power plants have traditionally used uranium-235.  When we talk about “enriching” uranium, it mostly means purifying it, and separating out the desired isotopes, which can be done in centrifuges because the isotopes have very slightly different masses.  “Weapons grade” uranium is just uranium of a different isotope that decays more readily and more easily begins a cascade reaction.  The disadvantage to using something like uranium-235 for fission power is that the decay product is still radioactive, and has a long half-life.  That means it will continue being radioactive for a long time before it reaches a stable state.

Two aspects of this system cause concern in the public at large, which in my opinion is not strictly reasonable.  One is the possibility of a runaway reaction.  While this can happen, and has in the past (the Fukushima disaster in Japan, for instance), modern reactor designs are far safer, and use far less fuel, than the older style.  New fission reactor designs are small and modular, instead of the bulky behemoths of the past, and therefore even if a runaway occurs, it will run out of fuel before it can become dangerous to a wider community.  The other is waste storage.  Regulations and/or physics prevent the reuse of spent nuclear fuel past a certain point, so material that is still radioactive and will remain so for a long time must be securely stored somewhere.  This is a valid concern, and careful steps must be taken to mitigate those risks.  It is the main reason why fission is not my preferred form of nuclear power.

Nuclear Fusion

Twenty five hundred words later, we return to the topic with which we began this post.  No, not my youthful arrogance and belief that I could one-up a fourteen year old fusion engineer if only I put in the time and effort.  Nuclear fusion is the power of the stars, the same reaction that keeps the sun and every other star in the universe (that we know of – it’s part of how we define a star) alight.  Fusion reactors can be powered by the most abundant element in the universe, produce no harmful byproducts, and can generate nearly unlimited quantities of power.  The catch: break-even fusion, or when the amount of power output by the fusion reaction exceeds the ignition energy, is a little like going to Mars – it’s been ten to fifteen years off since we landed on the Moon.

It took me longer than it should to understand the difference between fusion, and break-even (also called net energy positive) fusion.  Plain fusion is, while not easy to achieve, also not difficult, and we’ve been using it for various purposes now for decades.  In the rush of research into everything nuclear that followed the conclusion of World War II, fusion was an eager recipient.  So-called “hydrogen” bombs use a fission reaction to catalyze a hydrogen fusion reaction that massively increases the yield, and fusion reactors are used for experimental, medical, and industrial purposes today.  They are not used for power generation, because none of these designs output more energy than they put in – the reaction does not become self-sustaining.

Fission works for power generation because after you input the initial energy to start dislodging extra neutrons, those neutrons cascade down and dislodge more neutrons – the reaction is a self-sustaining one, and will only stop when the system runs out of fuel or the neutrons are absorbed, without needing any additional energy input.  We can fuse elements through a wide variety of methods, but it does not produce this cascade effect – each individual fusion reaction requires a new ignition energy.  But we’re getting a little ahead of ourselves.  Before we talk about fusion techniques, we should talk about what fusion is.

Just like fission, fusion’s process is in the name.  It is the fusing together of atomic nuclei.  When we talked about atomic structure and behavior, we mentioned that protons electrostatically repel each other, but in a nucleus they are bound together by the strong nuclear force.  Fusion is the process by which a nucleus is created.  The most basic example of this is hydrogen-1 fused with hydrogen-1 – that is, a proton being fused with a proton.  One proton plus another proton plus immense heat and pressure leads to a helium-2 nucleus, plus some extra energy (some mass is lost during the fusion process and converted into energy according to Einstein’s mass-energy relationship).  That’s fusion, and it’s the process by which all of the elements of the universe are created.  Stars fuse progressively heavier elements throughout their lives, balancing their gravitational pulls with the outward pressure from the reaction energy, until the elements are too massive for their conditions to fuse.  Heavier elements (past iron on the Periodic Table) are fused in supernovae or hypernovae explosions.

For fusion to succeed, the right conditions must be attained.  Specifically, there must be sufficiently high temperature and pressure to overcome the Coulomb Barrier (I said we would come back to that one).  The Coulomb Barrier is a product of the electrostatic force, and it keeps protons and nuclei apart.  In order for fusion to be achieved, the energy of the temperature and pressure trying to force the nuclei together must be greater than the energy of the Coulomb Barrier (there is an exception to this, called quantum tunneling, but it is beyond the scope of this discussion – suffice to say that, at a macroscopic level, quantum tunneling looks like a slight reduction in the ignition temperatures and pressures required for fusion).  It turns out that these temperatures and pressures are enormous.  The exact numbers vary depending on different combinations of elements and isotopes, and there can be some variability based on quantum tunneling and the exact conditions, but for proton-proton fusion, the temperature is somewhere around 20,000,000 K (35,999,540 F).  It is also possible to exchange between temperature and pressure, achieving fusion at low temperatures but very high pressures, or at high temperatures and low pressures (after all, pressure is related to the temperature and the volume).

Time is also a factor in fusion, and one of the major difficulties in achieving break-even. In order for the fusion reaction to succeed once the ignition temperatures and pressures are met, the nuclei must be kept in that condition for a sufficiently long time for the random motions of the energized nuclei to lead to collisions – that is, to fusion. This is based on root mean square laws, which were also discussed in Einstein’s Fridge. So the total fusion process is really temperatures, plus pressures, plus time.

To achieve those temperatures and pressures on Earth, a few methods have been developed.  The implosion method, practiced by the National Ignition Facility, involves compressing a pellet of deuterium-tritium fuel (hydrogen isotopes with one and two neutrons, respectively) using extremely high-powered lasers.  Magneto-Inertial fusion methods involve multiple different strategies, such as the Z-pinch or sheer flow stabilization, but they all involve (this is a gross simplification, but it works here) whirling superheated plasmas around fast enough to reach the adequate pressures.  There are also mechanical or piston based methods, which involve gradually increasing the pressure in a mechanically constrained chamber using pistons or other, physical compression systems.

In order for this form of fusion (thermonuclear) to be useful for power generation, it must start converting the energy released in the reaction into more reactions (called “ignition”).  To date, no one has achieved this state.  Whenever the proper conditions for fusion are achieved, instabilities quickly develop, and the system cannot sustain the reaction – there is insufficient time for the reaction to propagate.  Our present understanding suggests that instability is the main enemy of our fusion efforts. This does not change the fact that fusion is in many ways the “ultimate” method of energy generation, but it does mean that, for now, it remains like Mars: ten to fifteen years in the future, for the last fifty years.

There is, however, another form of nuclear fusion, which I feel obliged to mention.  Unfortunately, it might have even worse publicity than the word “nuclear.”  I’m referring, of course, to cold fusion.  There are people in the field who would decry me as a fraud just for saying the term.  There are also people who would demand to know why I didn’t mention it before now.  Cold fusion is hugely controversial, and makes periodic rises and falls in popularity as new experiments crop up claiming to have achieved net energy positive cold fusion.  So far, none have been successful.

Cold fusion is not actually cold, but it is room temperature, and compared to the millions of degrees Kelvin required for thermonuclear fusion, room temperature is very, very cold indeed.  Its main problem is historical.  In the days when people were first starting to realize that this fusion thing might be harder than everyone thought, an experiment was put forth purporting to have achieved fusion at room temperatures using a metallic lattice immersed in “heavy” water (that is, water in which the hydrogen atoms have been replaced with deuterium atoms).  Results from those who tried to replicate the experiment were intermittent at best, but one major experiment reported success, only to be later found fraudulent.  Thus, cold fusion became a byword for academic falsehoods, quacks, and charlatans, especially since no one could explain the physics of how such a thing could even be possible.

These days, theory at least suggests that something akin to cold fusion may be possible.  We talked earlier about electrons.  Electrons are a particular species of lepton in the Standard Model, and there are other species.  One of them, the muon, has the same electrical charge as an electron, but is more than two hundred times as massive.  As a result of its greater mass, if you replace the electrons in an atom with muons, they will orbit much, much closer to the nucleus.  This has the interesting effect of drastically reducing the Coulomb Barrier.  With the Coulomb Barrier lower, fusion requires less energy, potentially even as low as room temperatures.  Unfortunately, it is difficult and expensive to produce muons, and it is hard also to create muonic atoms (as opposed to electronic atoms), so while the theory behind cold fusion may be sounder than was originally believed, its practical implementation is as elusive as thermonuclear fusion’s.

Conclusion

Hopefully, this post has helped clear up something new for you involving nuclear energy.  Whatever your position on climate change, and particularly the role of the combustion of fossil fuels in the environment, we will eventually need more energy than can be obtained through chemical means, and “renewables” are too unpredictable (and battery technology inadequate and limited).  Nuclear fission provides a viable, short-term alternative, and nuclear fusion may be the ultimate energy source.  I mean, I suppose there’s antimatter, but that’s a topic for another time.

However, I’m not here to tell you what to think.  It would be enough for me if next time you run across something talking about nuclear energy, whether that’s in the news, or when you’re next having a conversation with a friend about z-pinched plasma flow (just me?), you understand enough to know it’s not all an indication of the coming nuclear apocalypse.  As always, if you have any questions or would like any more information on this topic, please ask in the comments below, or contact me through the site.  And who knows?  Maybe one of you will take what I’ve written here, and do what I never got around to doing: building a break-even fusion reactor in your garage.