Nuclear Fission in Power Plants and Bombs
How exactly do today’s nuclear power plants produce energy? As we’ve already said, the basic answer is through fission of large atoms such as uranium. But to understand the process better — and to understand the differences between nuclear power plants and nuclear bombs — we must look at the process in a little more detail. We’ll focus on uranium, which is the fuel commonly used in today’s nuclear power plants . Uranium was also the material used in the atomic bomb that destroyed Hiroshima in World War II. We’ll look at the process through a series of questions and answers.
Q: How does the fission used in nuclear power differ from natural radioactive decay?
A: Remember that nuclear fission releases energy as a result of the splitting apart a large atomic nucleus. Fission is one form of radioactive decay , which means that it can occur naturally. But while natural radioactive decay releases energy that can generate heat, it is not enough heat to operate a power plant. Therefore, if we are to make use of nuclear energy, we need to somehow ramp up the rate of fission from what it would be naturally. In essence, then, the goal of a nuclear power plant is to induce fission to occur in far more nuclei than would occur naturally.
Q: What is a “chain reaction” and why is it important to nuclear energy?
A: Think about playing dominoes. If you stand the dominoes far apart from one another, then even if one falls down, it won’t affect the others. But if you carefully place the dominoes, you can make each one knock down others, which is the essence of a chain reaction. (You can find many amazing videos of domino chain reactions, such as this one.) This same idea helps explain how nuclear fission can be induced. In natural radioactive decay, the fission of one atom does not affect others around it. But in a nuclear power plant, the atoms are packed together in a way that allows the fission of one to cause the fission of others, thereby sustaining the reaction and generating far more heat and energy that would be generated otherwise.
Q: How does a nuclear “chain reaction” work?
A: In a domino chain reaction, the dominoes are carefully arranged so that every domino that falls hits at least one other domino. But the fission of an atomic nucleus sends the pieces out in random directions, so one fission won’t generally cause another unless there is some multiplier effect. Figure 1 below illustrates what actually happens with the form (isotope ) of uranium called uranium-235 (U-235) . Notice that the fission is induced by a neutron, and each U-235 nucleus that splits apart sends out either two or three more neutrons. If these hit other U-235 nuclei, then those can also undergo fission, leading to a chain reaction.
(left) If the individual atoms of U-235 are not packed closely together, the neutrons from one fission will simply escape without affecting other atoms.
(right) But if U-235 atoms are closely packed (and if the neutrons are moving relatively slowly), then the fission of one atom can cause the fission of two or more others, creating a chain reaction.
Q: Why uranium and not most other elements?
A: If you think about how the nuclear chain reaction works (Figure 1), you’ll realize that a nuclear fuel must have two key characteristics: (1) it must undergo fission fairly easily, and (2) it must undergo this fission in a way that can sustain a chain reaction by causing the fission of two or more other nuclei. Uranium-235 is one of the rare substances that meets both criteria.
Q: So can we just dig up uranium from the ground and use it for nuclear power?
A: No. Uranium does exist naturally in the Earth (see Figure 5.1.3–8), but most of this uranium is not uranium-235. Instead, natural uranium is a mixture of a very small amount of uranium-235 with a much larger amount of uranium-238 . In fact, natural uranium is more than 99% uranium-238, and this form of uranium cannot be used as a nuclear fuel (because it does not meet the two criteria above).
Q: What is “enrichment” and why is it important?
A: Look again at Figure 1 above. The key to a chain reaction is having uranium-235 atoms packed fairly closely together. Because this is not the case in natural uranium (since it is more than 99% uranium-238), the uranium that we dig out of the ground must be processed in a way that separates the two isotopes so that the mixture becomes “enriched” in uranium-235 . In order to be useful as nuclear fuel, the enrichment must raise the amount of uranium-235 in the mixture to about 4% (rather than the less than 1% found in nature).
Q: So can we generate energy simply by making some enriched uranium?
A: Not quite; it’s still a difficult process. The uranium mixture used in nuclear reactors, with about 4% uranium-235, still won’t sustain a chain reaction without extra help. For example, it requires setting up the reactor with a “neutron source” to start the reactions, and it also requires a carefully designed mixture of other materials to help slow the neutrons down (because fast-moving neutrons will simply escape). Indeed, that’s why the pellets of enriched uranium that are used as reactor fuel can be handled safely and easily when they are outside of the reactor core. These “extra” requirements for sustaining a chain reaction explain why building a nuclear reactor is a complex engineering project.
Q: What makes a nuclear bomb different from a nuclear reactor?
A: Look again at Figure 1, and notice that even in a sustained chain reaction (like that which occurs in a nuclear reactor), many of the neutrons escape without causing other nuclei to undergo fission. But now imagine: what if you could enrich the uranium far more, say to 90% uranium-235, and then somehow squeeze this enriched uranium so that the uranium-235 atoms end up being extremely close together. Further, imagine that you make a ball of this squeezed, enriched mixture that is massive enough (so-called “critical mass”) so that hardly any of the neutrons escape. If that all happened, the chain reaction could suddenly rage out of control, causing nearly all the uranium-235 atoms to undergo fission in just a fraction of a second. That is essentially what happens in a nuclear bomb.
Q: Could a nuclear power plant explode as a nuclear bomb?
A: No, absolutely not. There is simply no way that the 4% enrichment mixture of the fuel used in nuclear power plants could ever undergo the kind of sudden and uncontrolled chain reaction that makes a nuclear bomb. The absolute worst thing that can happen in a nuclear power plant is an accident leading to what is usually called a “meltdown,” in which the nuclear fuel becomes hot enough to cause melting of some of the core material. This can lead to a release of dangerous radiation, which is what has happened in some famous accidents including Chernobyl and Fukushima. It is also possible for a meltdown to lead to steam explosions (which happened at Chernobyl). But, again, there is no possible way for a nuclear power plant accident to cause a nuclear explosion.
