Understanding Nuclear Power, Part I: Whirlwind Nuclear Physics

wikis.lib.ncsu.edu

by Ian Pollock

Introduction

This post is the first in a series on nuclear fission power, intended to provide the background knowledge to understand what is at stake in all major aspects of the nuclear power debate — science and engineering, safety and health, economy and environment.

Energy policy may turn out to be by far the most important issue of our time. Given this, it is crucial that policy-makers and an informed public understand the relative costs and benefits of all the power generation methods that are on the table. Unfortunately, discourse about nuclear power in particular is plagued by wild misinformation. This discourse is heavily politicized, thanks in part to the cold war, and is riddled with fallacies arising from ignorance of the relevant science, heavily influenced by fear. This series of posts is meant to try, in some small way, to correct that.

I make no bones about the fact that I am pro-nuclear. One of the aims of this series of posts is to argue that, after taking into consideration the risks and drawbacks of nuclear fission energy, we would still be crazy not to expand its use substantially. That argument will have to wait for the final post in the series.

However, there is a more important purpose to these posts, which even critics of nuclear power generation should be willing to embrace. Namely, to move toward a saner discussion of nuclear power. To be clear, no policy decision is one-sided; there are reasonable objections to expanding nuclear generation. However, they are discussed less than they should be, partly because discourse is often derailed by a lot of very silly ideas (or worse, unstated assumptions!) from pop-culture floating around in the public imagination — to take one example, the widespread idea that a standard nuclear plant could blow up in a nuclear explosion, complete with a mushroom cloud. I want to help clear these myths out of the way once and for all.

If, dear reader, we still disagree at the end of this series, then I want our disagreement to at least be substantive!

This first post will give an extremely brief outline of basic nuclear physics concepts and jargon. Future posts will expand on specific aspects of this outline when they become relevant to the discussion.

Before starting, I wish to disclaim that although I work in the engineering profession, I am not a nuclear engineer, so my opinions on this subject should be taken as those of an informed layperson.

The nucleus

Atoms are usefully pictured as consisting of a small, dense, central nucleus, surrounded by a comparatively large cloud of very tiny, fast-moving electrons. Broadly speaking, the perceived size of an atom is determined by how much area the electron cloud covers. In comparison to the atom’s overall size, the size of the nucleus is typically extremely tiny — about 1/100,000th of the atom’s overall dimension.

The nucleus itself is composed of both protons, which carry positive charge, and neutrons, which carry no charge. The electrons orbiting the nucleus are negatively charged. Protons and neutrons have nearly the same mass, which is about 1,800 times more than the mass of an electron.

Electrical charge is clearly not the whole story in explaining the structure of atoms. For one thing, since electrons are electrically attracted to the protons in the nucleus, one would expect that they should quickly spiral down into the nucleus and stick to the protons. The fact that they do not do so finds its ultimate explanation in quantum mechanics.

Likewise, one would expect the protons in the nucleus to repel each other so violently that the nucleus would fly apart. Since it does not do so, there must be another force acting on the nucleons (protons and neutrons). This force is called the strong nuclear force. It is both extremely strong and extremely short-range, attracting both protons and neutrons to themselves and to each other; it is the balance of the electrical forces and strong nuclear forces in a nucleus that determines whether it is stable or not.

Because neutron numbers are more or less irrelevant in chemistry, the chemical elements are named based on the number of protons they contain, regardless of the number of neutrons. For example, Uranium (symbol U) has 92 protons (and therefore, 92 electrons). For the purposes of chemistry alone, it does not matter how many neutrons Uranium has, for its chemical properties will be virtually identical. However, for nuclear physics, the number of neutrons becomes very important.

Accordingly, nuclear physics identifies a particular atom not only by its chemical name but also by its atomic mass number, which is simply the count of all nucleons in that atom. For example, the most common type of Uranium has 238 nucleons (92 protons + 146 neutrons). But there are other types of Uranium that have the same number of protons; therefore the same chemical properties, but different numbers of neutrons. These varieties are referred to as isotopes of Uranium. Standard nuclear jargon is to identify an isotope with the chemical name followed by the mass number; hence the most common isotope of Uranium is called “Uranium-238” or “U-238.”

Nuclear physicists find it convenient to chart all the possible combinations of protons and neutrons in the Chart of the Nuclides, which is a very simple plot of the number of protons versus the number of neutrons showing which species are stable or unstable, along with their other properties (“nuclide” refers to any unique combination of protons and neutrons).

The neutron-to-proton ratio is a key piece of information. For lighter nuclei, n/p≃1 provides stability, but as one approaches heavier nuclides, stability can only be achieved with a ratio of n/p≃1.5. Observe the subtly downward-curving “line of stability” on the chart of the nuclides. A moment’s perusal of this chart will also show you that if I were to pick, at random, a certain number of protons and a certain number of neutrons, the nuclide resulting from their combination would almost certainly be unstable. This will become important.

Binding energy, fusion, fission, decay

A stable nucleus has tightly-bound nucleons that are difficult to separate from each other. Nuclei may be usefully characterized by their binding energy, which represents the amount of energy it would require to dissociate the nucleus into its constituent protons and neutrons. High binding energy means “tightly bound.”

If you were to dissociate a nucleus into its protons and neutrons, you would discover an interesting fact. Namely, that if you were to weigh all the protons and neutrons in isolation and add up their weights, they would be slightly heavier than the fully assembled nucleus — the whole weighs less than the sum of its parts. This difference in mass is called the mass defect. There is a familiar relation between the mass defect (difference in weight disassembled vs. assembled) and the binding energy (energy required to disassemble). If we symbolize the mass defect as Δm, and binding energy as ΔE, and use the symbol c for the speed of light (~300,000 km/second), then we find that ΔE=Δmc². Mass is also a form of energy, as that famous equation shows, and the mass defect is just another way of writing the binding energy.

As a general rule, both light elements (like Helium) and heavy elements (like Uranium) have low binding energy and relatively unstable nuclei, while elements of medium weight (like Iron) have high binding energy and therefore very stable nuclei. This is suggestive of two ways of getting energy out of nuclear interactions: creating medium-weight elements by fusing light elements together (nuclear fusion — a worthy subject for another occasion), and creating medium-weight elements by smashing heavy elements apart (nuclear fission). It turns out that the ideal way to smash a heavy nucleus is to bombard it with neutrons. However, not all heavy nuclei break apart easily this way, and crucially, not all release neutrons when they do. Neutron release is important because it permits the chain reaction, allowing the process to sustain itself indefinitely, or even accelerate. Those fissionable nuclides which can sustain a chain reaction are called fissile. Currently, the two most important fissile nuclides are Uranium-235 and Plutonium-239. Controlled fission (steady chain reaction) is the key to nuclear power, while uncontrolled fission (exponentially accelerating chain reaction) is the key to the nuclear bomb.

Even when left to their own devices, however, radioactive nuclides do not merely sit placidly. Because they are unstable, they will tend to undergo radioactive decay — which essentially means ejecting particles from the nucleus — and the more unstable they are, the more readily they will do this. If I have a sample of Plutonium-239 that weighs 1 kg now, I can predict that half of it will have radioactively decayed in about 24,000 years, and half of that in another 24,000 years, and so on. Hence, we say that Pu-239 has a half-life of 24,000 years. Half-lives vary wildly depending on the stability of the nuclide. For example, Uranium-238 has a half-life of about 4.5 billion years — the age of the earth — while Helium-5 has a half-life of only 10^-21 seconds, roughly the time it takes to transition from never having heard of an Ikea product, to desperately needing it.

The relative absence of radioactive materials in the world around us is due, not to non-radioactive material being “more natural” than radioactive material, but rather to survivorship. Radioactive materials have decayed into non-radioactive ones — our (mostly) non-radioactive world is what’s left after everything else has decayed.

The next post will look at decays more closely, especially with regard to their health effects.

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Recommended sources:

- Hyperphysics section on nuclear physics.

- Richard Muller’s fantastic lectures “Physics for Future Presidents,” available on YouTube: Radioactivity 1, Radioactivity 2, Nukes 1, Nukes 2 & Review.

- David Bodansky, “Nuclear Energy: Principles, Practices and Prospects.” 2004, Springer.