The Fermi Chronicles - Part 9: Nuclear Fission

This is where the magic occurs. Nuclear fission. In the last installment, I wrote about neutron scattering. This will be important here. In fission, when a neutron strikes an already unstable atom, most specifically uranium, it gets into such a highly excited state that the nucleus actually splits into two nuclei. This split is called fission. A byproduct of the fission results in the emission of 2 or more neutrons that will collide with other uranium atoms and the chain reaction continues.

The chain reaction is a necessary part of the picture so that the fission reaction is self-sustaining. Since the nuclear force is unimaginably large being that's it's in such a small space, a good chunk of energy is given off in the process of fission. That energy can be harnessed to generate electricity. Or to blow things up.

Now not all elements can be fissioned in this way. There are two types of material that are very important to understand - fissile material and fissionable material. And there a huge difference between them and that difference resides in the behavior of neutron. I didn't get into this before, but there are 2 types of neutrons - fast neutrons and thermal neutrons. The difference between the two is speed. Fast neutrons come right out of the fission reaction. If they start hitting other things, though, they slow down and become what are known as thermal neutrons. Thermal neutrons aren't powerful enough to fission some materials, but can fission other. This will become extremely important in nuclear reactors that we harness for power.

Fissile material: this is material that is susceptible to fission by any neutrons whatsoever - fast or thermal. Very few materials are fissile. Examples of fissile materials are U-235, U-233 and Pu-239. And that's about it!

Fissionable material: this material is susceptible to fission by either fast or thermal neutrons. A small subset of fissionable material, is fissile material. Most fissionble material, however, requires fast neutrons for fission to occur. Examples of fissionable material that is not fissile are Th-232, U-238, and Pu-240. These materials are far more common than fissile materials!

The cause of the difference between fissile material and the rest of the fissionable domain is in the critical energy necessary to fission the nucleus. This varies by material. Now, you might think that since we only have a finite supply of fissionable material and that only a small fraction of fissionable material is fissile, then we're screwed. You would be wrong, though. We can create more fuel! But that's for another episode.

Previously:
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 8: Neutron Interaction

OK, before getting into nuclear reactions and nuclear fission, I have to talk about the enabling mechanism for such. For nuclear fission to occur, there must be a neutron that comes into the nucleus of an atom. I already covered neutron radiation before, and won't repeat myself here (see prior entries on that subject). In any case, necessary for a self-sustained nuclear reaction is a neutron hitting the nucleus and emitting another neutron that will hit another nucleus etc. This is known as neutron scattering. Without the self-sustained neutron generation, no sustained nuclear reaction will occur.

Now, the scattering can be elastic or non-elastic (inelastic). An elastic scattering occurs when the neutron strikes a nucleus with some exchange of kinetic energy as the same neutron is redirected elsewhere, like billiard balls. In an inelastic collision, the neutron is actually absorbed by the nucleus and then a new neutron is discharged but with less kinetic energy, leaving the target nucleus 'excited' (yes, there are puns all over the place here). The 'excited' nucleus then releases the extra energy by means of gamma radiation. Here is an animation of neutron scattering on YouTube. Note the low-energy neutrons emitted now and again in the animation:

Now you have the tools so that we can discuss actual nuclear fission in the next installment! Neutron scattering as an important concept for fission.

Previously:
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life

I've already discussed in this blog what radioactivity is - the means by which unstable atoms get stable. Unstable atoms are not always isotopes. That is, they exist only in radioactive form. Such atoms are typically large. Small atoms, such as hydrogen, helium, etc., tend to be stable and not radioactive (albeit they have radioactive isotopes). When you look at the periodic table of elements, there are some big dudes out there that have so many nuclides in the nucleus that they are never fully stable. Unstable atoms are always radioactive - they emit radiation (in various forms and various quantities) to become stable. Uranium, most commonly U-238, is one of these large dudes that have a huge number of protons and neutrons packed in its center (238 to be exact). Therefore, uranium and its isotopes are all radioactive. When uranium, most commonly U-238, gives off radiation, it becomes something else. Uranium, by the way, is the heaviest off all atoms that occur naturally. Everything on the periodic table bigger than uranium is synthetic, including plutonium that I will discuss in another post. Historically, these elements are pretty new on the discovered list. From DOE handbook 1019/1-93:

In 1896, the French physicist Becquerel discovered that crystals of a
uranium salt emitted rays that were similar to x-rays in that they were highly penetrating, could affect a photographic plate, and induced electrical conductivity in gases. Becquerel's discovery was followed in 1898 by the identification of two other radioactive elements, polonium and radium, by Pierre and Marie Curie.

Heavy elements, such as uranium or thorium, and their unstable decay chain elements emit radiation in their naturally occurring state. Uranium and thorium, present since their creation at the beginning of geological time, have an extremely slow rate of decay. All naturally occurring nuclides with atomic numbers greater than 82 are radioactive.

Element 82 (that's the atomic number meaning it has 82 protons) is lead. From there in numerical order are the naturally radioactive elements: polonium, astatine, radon (everyone has heard of this one, no?), francium (the wimpy atom), radium, actinium, thorium (symbol should be a big hammer and a lightning bolt), protactinium, and uranium.

Since all these elements are naturally radioactive, they will undergo radioactive decay - that is, they will give off radiation (either electromagnetic or particle or both) and become something else that is more stable. For instance, U-234 gives off alpha and gamma radiation and becomes thorium. Because of the alpha radiation, it is called alpha decay. Other processes include beta decay. Yet another process occurs when a proton captures an electron from orbit and becomes a neutron (weird). When a radioactive decay occurs, the nucleus takes time to stabilize, and is called an isomer in its excited stage.

How quickly radioactive materials decay to more stable elements and isotopes can best be quantified by it's half-life. After one half-life, half that material is now gone and became something else. Of the remaining 50%, half of that will be gone in another half-life, etc. Some atoms have a half-life of a fraction of a second, others, like uranium, are much longer, which is thankfully the reason that any uranium remains on this planet. Interestingly, plutonium-239 which is synthetic has a very long half life into the thousands of years.

Previously:
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"

The Fermi Chronicles - Part 3: Radiation Types

The Fermi Chronicles - Part 2: A week of training

The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 6: Atomic Structures

OK, some atomic basics before we get into nuclear reactions. I've covered a bit of the basics regarding radiation, which is a byproduct of an unstable atom going stable. Let me cover basic atomic structures so that I can get into nuclear fission in the next installment. The nucleus, or center, of an atom is where the atomic magic occurs. The nucleus is composed of protons, which have a positive charge, and neutron which have no charge at all. Both protons and neutrons are call nuclides since they reside in the nucleus of each atom. The number of protons in an atomic nucleus is called the atomic number, while the total number of nuclides in a nucleus (protons + neutrons) is called the mass number.

The force that holds nuclides together is one that none of us have any experience with - the nuclear force. It is extremely powerful but only works on a small scale. Other forces like gravity and electrostatic forces we do have experience with as we see them in the macroscopic world that we exist in. The nuclear force must be very strong since protons have a positive charge and would therefore repel each other. So a much stronger force is present that clumps them together.

Now, most stable atoms have the same number of positively charged protons in the nucleus, and negatively charged electrons in orbit. If not, then the atom is said to be ionized. Also, there is a balance between protons and neutrons, but it is not 1-to-1. There are slightly more neutrons than protons in bigger atoms and that increases as the atomic number goes up. When that gets out of balance due to relatively too many neutrons, the atom is an isotope. Isotopes are the different versions of the same atom with the same number of protons but different number of neutrons. Not all isotopes are unstable. Some are actually stable and remain in that state. It is the unstable isotopes that are radioactive - giving off radiation in various forms to become stable again. For example, hydrogen, denoted in the periodic table by H, has two stable isotopes, H-1 and H-2. H-2 has two nulides (one proton, one neutron) but is stable. Stable isotopes sometimes have names. H-2 is called deuterium (also designated as D-2). Another hydrogen isotope, H-3, is unstable (thus radioactive) and is called tritium (isn't that the stuff Dr. Octopus was using to form a miniature self-sustained fusion reaction in Spiderman 2?).

Some isotopes of certain elements are extremely common in nature. Uranium is one of those as there are three isotopes: U-238, U-235, and U-234 (and yes, that's a lot of protons and neutrons in each). U-238 is the most common, comprising 99.2745% of all uranium. U-235 comes in a distant second at 0.72%, while U-234 comes in dead last at a trace 0.0055%. Each of these isotopes have unique nuclear properties, but it is U-235 that is most important to nuclear applications, both for power generation and for nuclear weapons. But that's another story. Ready for a quiz now? Here are some common isotopes:Previously:

The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"

The Fermi Chronicles - Part 3: Radiation Types

The Fermi Chronicles - Part 2: A week of training

The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 5: Nuclear Waste Storage

One of the first things I toured at Fermi 2 after training week was the reactor building. In fact, I stood no more than 30 feet from the nuclear reactor core, which is encased in high-density concrete. The shielding is so good that my electronic dosimeter (ED) didn't pick up anything. Right next to the core is the spent fuel pool. The Nuclear Regulatory Commission allows only 2 storage mediums. From the NRC website:

There are two acceptable storage methods for spent fuel after it is removed
from the reactor core:

Spent Fuel Pools - Currently, most spent nuclear fuel is safely stored in specially designed pools at individual reactor sites around the country.

Dry Cask Storage - If pool capacity is reached, licensees may move toward use of above-ground dry storage casks.

The spent fuel pool is right next to the reactor. About 22 feet below the surface is the tray where the spent nuclear fuel is stored. It is clearly visible. Turns out that water is a great radiation shield, knocking out 90% of radiation for every 2 feet. So 2 feet above the fuel rods, the radiation is reduced to 10%, another 2 feet and the you only have 10% of that 10%, etc. After 22 feet, no detectable radiation whatsoever. Some of it is even glowing a deep blue! Here's a pic of a spent fuel pool I pulled off of Google:What is most amazing to me is that it's not much bigger than a large swimming pool you find in many neighborhoods, and that is all the nuclear waste at Fermi 2 from over 20 years of operation at 1.1 GW of power! That is simply an unbelievably small amount. It almost seems impossible. Any other source of energy for that kind of power over that kind of time period would literally produce mountains on top of mountains of waste. Why any environmentalist would oppose nuclear is beyond me. In fact, if you account for all the energy you will ever use in your lifetime for every purpose - transportation, heating, cooling, recreation, etc - the total nuclear waste produced would not fully fill a standard can of Coke!

And yes, it is highly toxic waste. Our guide told us that if you were to jump into the spent fuel pool, take a deep breath and dive under as fast as possible, you would be dead before you ever touched the bottom. Yikes! But here's my thought from an environmental viewpoint. The 2nd Law of Thermodynamics demands waste be produced from any process. It's inevitable. The question is, would you want a large volume of uncontrollable but less toxic waste or a very small volume of controllable but highly-toxic waste? I'll go with the latter any day!

What's perhaps even more interesting (at least to me) is that the spent nuclear fuel can be reprocessed, where most of it can be used again. Of course, Jimmy Carter put an end to that back in the 70's. So instead of reusing some of that waste as fuel, it just sits there while new fuel must be excavated and enriched. Thanks a lot, Carter!

Previously:
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"

The Fermi Chronicles - Part 3: Radiation Types

The Fermi Chronicles - Part 2: A week of training

The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"

Last installment, I wrote about different forms of radiation that are of concern over at Fermi. But that's not it for the radiation picture. It is far bigger than that. The radiation that I covered relevant to nuclear reactors were particle radiation (alpha, beta, neutron) and electromagnetic radiation (gamma). Fact is, gamma is only one of many electromagnetic radiation types. For a broader picture, here is the entire electromagnetic spectrum:Note that radio waves are electromagnetic in nature as are tv waves (anyone remember UHF and VHF?). They are relatively safe because the wavelength is extremely long and the frequency is low. Penetration depth is inversely proportional to the energy of that wave. Radio waves don't tend to penetrate obstructions very well. Radar is not far away. You might have thought that radar is this high-tech penetrating radiation, but it is not so. Radar is ideal in applications such as military apps since it tends to bounce off objects rather than penetrate them. The bounce-back is what tells an operator that something is there.Moving down the energy spectrum we come to infra-red radiation, which is the way some animals see (snakes being one example). After that, it's visible light, which is slightly penetrating (ever put a flashlight under your hand to see the bones slightly?). Then the UV bands. UV gets dangerous. It penetrates the skin to such a degree that the skin reacts. There are actually 3 bands of UV - UVA, UVB and UVC. The atmosphere and ozone layer filter out the UVB and UVC. You get a suntan (or burn) from UVA. UVB and UVC have more energy as they are higher in frequency and will begin to destroy the DNA molecule. That's bad. Fortunate that we live on a planet with such a well-designed atmosphere, no?

After the UV spectrum, we get into microwaves and x-rays which have very high penetrating depths. X-rays are familiar to most that have ever been to a hospital. X-rays in particular have so much energy (because of the very high frequency and very short wavelength) that they go right through you, and impact a film behind you that then gives a picture of your insides. Note that in all hospitals, the x-ray room is shielded and only you are present for the pic, probably donning a lead vest. Smile!

More energetic than x-rays are gamma rays that are typically produced by nuclear decay and nuclear reactions, including fission. It is the most energetic type of radiation (save the mysterious cosmic rays) and thus has the potential to do the most harm. But that doesn't mean that it will hurt you. That depends on the "dose" you get.

A "dose" is the total amount of radiation you are hit with over a period of time. From the American Nuclear Society:

We live in a radioactive world - humans always have. Radiation is part of our natural environment. We are exposed to radiation from materials in the earth itself, from naturally occurring radon in the air, from outer space, and from inside our own bodies (as a result of the food and water we consume). This radiation is measured in units called millirems (mrems).

The average dose per person from all sources is about 360 mrems per year. It is not, however, uncommon for any of us to receive far more than that in a given year (largely due to medical procedures we may undergo). International Standards allow exposure to as much as 5,000 mrems a year for those who work with and around radioactive material.

Click the above link for an interactive chart that will help you gage the dose that you get. So 360 mrems is the average. A worker at Fermi 2? 310 mrems. Not bad! In fact, I just took a tour last week of the reactor building and stood no more than 30 feet from highly toxic spent nuclear fuel and the nuclear reactor core. My total dose for that tour? 0.4 mrems. How was this measured? Well, when you go into radiologically-active buildings, you go through a process, part of which is the issuing of an ED (electronic dosimeter) that continuously monitors your exposure. You also carry around a TLD (thermoluminescent dosimeter) with you at all times. All of your radiation exposure is continuously logged. Pretty cool!

UPDATE (3/20/11):Via Instapundit: RADIATION DOSES EXPLAINED at XKCD (click for bigger pic)

Previously:

The Fermi Chronicles - Part 3: Radiation Types

The Fermi Chronicles - Part 2: A week of training

The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 3: Radiation Types

Radiation is simply the result of unstable atoms. Unstable atoms want to become stable (don't we all?) and do this by emitting radiation of various types. There are 4 major types of radiation that could be harmful and that are the focus at Fermi 2 - alpha, beta, gamma and neutron radiation.

Alpha radiation - this is the weakest of the 4 (there's more than just 4 in totality, but anyway). It is a real particle and is thus a type of radiation referred to as particle radiation. It is basically the nucleus of a helium atom (2 protons, 2 neutrons) but without the electrons. Usually, atoms have a balance between positive protons and negative electrons. A helium nucleus without electrons has a net positive charge, and it is called ionized. Because alpha particles are large, they are easily blocked by even a sheet of paper. The danger is if the particles are inhaled or ingested somehow. As it turns out, the DNA molecule has a slight negative charge, which will attract the positively charged alpha particle. Chaos ensues. Not good.

Beta radiation - this is also a form of particle radiation. Whereas alpha radiation is a clump of 2 protons and 2 neutrons, beta particles are loose electrons (or positrons), but unlike regular electrons they are discharged from the nuclear core, not from atomic orbit. Because of their much smaller size, beta particles travel much greater distances and are also much faster. Beta particles, just like alpha particles, are ionized (whereas the alpha particles carry a positive charge, beta radiation carries a negative charge. Except for positron emissions (positive electrons) that are positive, but I'll skip that for now). Beta particle have greater penetrating depth than alpha particles, but still cannot penetrate material such as plexiglass.

Gamma radiation - this stuff is worse than alpha or beta radiation because it is pure energy, and does not involve a particle with rest mass (but does involve a photon (not the Star Trek photon) since all electromagnetic radiation travels as both a particle and a wave). Thus, it has a far greater penetrating depth, like x-rays but even more powerful. It will go right through you damaging atomic structures as it passes through. Gamma rays can be shielded by thick lead barriers, but it's nothing that you will be wearing.

Neutron radiation - this type of radiation is emitted by nuclear fission of material such as uranium and is necessary for self-sustaining the fission chain reaction. Because neutrons don't have a charge, they are the most penetrating of the particles. Neutron radiation by itself would appear less harmful, but the problem is its interaction with matter. That neutron will eventually merge with the nucleus of an atom, for example, an atom inside your body, making it unstable. The unstable atoms then have a tendency to emit alpha, beta and gamma radiation to become stable again and things just go downhill from there.

Neutron radiation is a necessary byproduct of fission, while alpha, beta and gamma radiation are emitted by radioactive decay (and fission). In the next installment, I will talk more broadly about the different forms of electromagnetic radiation. People tend to freak out when they hear the word 'radiation,' but visible light is an example of a type of electromagnetic radiation, so is radio, and the old UHF and VHF tv frequencies, etc. So it's not all bad.Previously:
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post