Question

Explain how a nuclear power plant functions and what safety precautions must be taken to ensure the safety of the power plant, its employees, and the environment.

Answer 1

The core of a nuclear power plant is made of a mixture of fuel, moderator and control rods. The fuel rods are made of pellets of uranium oxide, which has been enriched slightly in the U-235 isotope of uranium, from the natural abundance of about 0.7% to about 3%. (Enrichment of uranium is the major technological obstacle to developing nuclear power plants.)

Each U-235 nucleus can undergo a fission reaction, in which it splits into two "daughter" nuclei, for example krypton and barium, and releases a great deal of energy. The fission reaction will be provoked if the U-235 nucleus absorbs a neutron, and the products of the reaction also include 3 more neutrons, so a "chain reaction" is possible in which the first neutron causes 1 fission, which releases 3 neutrons that cause 3 more fissions, which release 9 neutrons, which cause 9 more fissions, which release 27 neutrons, which cause 27 more fissions, and so on. This is called a chain reaction. At each step enormous amounts of energy are released, principally as gamma and X-rays, plus a lot of kinetic energy in the recoiling daughter nuclei and the speeding neutrons.

In a nuclear bomb, you want the chain reaction to grow as fast as possible, to release as much energy as fast as possible. Needless to say, this is highly undesirable for a power reactor, where you want the chain reaction controlled. You do this with the control rods, which contain (or are made of) elements that absorb neutrons without fissioning, like boron or cadmium. By adjusting how close the control rods are to the fuel rods, you can keep a lid on the chain reaction and keep the power production stable.

The moderator is made of a substance that slows down neutrons. It's necessary because the neutrons emitted by fissioning nuclei are going too fast to be absorbed by other U-235 nuclei. Without the moderator, these neutrons would just speed out of the core and the chain reaction wouldn't take place. Water is a common moderator, because it can also serve as a coolant, but old Soviet nukes used graphite.

Much of the energy released by the chain reaction is converted, by repeated collision with other atoms and molecules, to heat. This heat is typically used to raise the temperature of a "working fluid" to a very high temperature. The working fluid is then allowed to expand and cool by spinning a turbine, which generates electricity. Actually, there are usually two working fluids, one "inner" fluid that makes intimate contact with the reactor core (and which therefore becomes radioactive) and another "outer" fluid that only makes contact with the inner fluid and turbine, and which does not become radioactive. Not all of the heat can be converted to electricity (because of the Second Law of Thermodynamics) so some must still be simply thrown away. That means the reactor also needs cooling. (it also needs cooling when no power is being generated.)

Answer 2

Now let's think about the dangers:

(1) Radioactivity. The core generates gamma rays, X-rays, and high-speed subatomic particles (alpha and beta radiation). All of these can break chemical bonds (do radiative damage) to materials and flesh, and in addition can cause other substances to become radioactive themselves, compounding the damage. Low levels of exposure to radioactivity over time can lead to mutations in the DNA and cancer. Higher levels destroy DNA entirely and can kill. Very low levels appear to stimulate the immune system and can actually be good for you, oddly enough.

The core of a nuclear reactor is intensely radioactive, and a person right next to it would absorb a fatal dose of radiation in less than 1 minute. So obviously you need to protect your workers from getting anywhere near the core. There are also materials you can put around the core to block its radiation. Heavy metals, like lead, will work, but so will thicker pieces of ordinary building materials like steel and concrete, as well as water.

You obviously also need to provide the workers with some way to work on the core without being exposed to it, which usually means some kind of robotic or remote-control machinery.

(2) Excess heat. If the chain reaction is not sufficiently controlled, it can generate far more heat than the working fluid and coolant can absorb, which will raise the temperature of the core and nearby materials quite high quite fast, easily enough to melt them. This is the famous "nuclear meltdown" you'll hear described in the media. It's to be feared largely because melting the core and nearby structures will destroy their integrity, possibly leading to a disruption of the containment of the core described above which you need to shield everyone from the core's radioactivity.

(3) Unwanted chemical reactions. The materials out of which you have to make components of the core, to keep them stable in the presence of lots of neutrons and intense radioactivity, have the unfortunate property of chemical reacting with water to form hydrogen gas when the temperature gets sufficiently high. Hydrogen gas is extremely explosive when combined with air, so when the temperature gets too high you run the risk of a chemical explosion, which also threatens containment.

In the case of (2) and (3) you can see the critical issue is preventing a runaway chain reaction and/or providing sufficient emergency cooling that you can absorb unwanted extra heat, to keep the temperature down. So you need to design your reactor so that it has methods of controlling the chain reaction if it gets out of control, and making sure that your cooling is not interrupted, even under emergency or strange conditions.

The two most destructive nuclear accidents were illustrations of the failure of each of these needs: in the case of the Chernobyl accident, the design of the reactor was stupid, in that if the chain reaction went a little bit out of control, the way the reactor was designed would accelerate the deviation, and the temperature would rise hundreds of degrees in milliseconds, far faster than human or even machine reaction time. As a consequence, the temperature of the Chernobyl core rose so high so fast it turned all the cooling water abruptly to steam, causing an explosion that blew off the top of the containment building, and also set fire to the graphite moderator, ensuring that a steady stream of highly radioactive gases was released out of the top of the reactor for days.

In the case of the Fukishima Dai-ichi reactor, a tsunami took out not only the regular power needed to run the cooling pumps, but also the backup generators designed to provide power in an emergency to run the pumps. (The generators were behind a seawall, but it wasn't high enough.) Without cooling, the temperature of the core rose high enough to cause the chemical reactions that generated hydrogen, and there was a hydrogen explosion that ruptured containment, also releasing radioactive gases.

These are obviously the most extreme dangers to the workers (and local population), but every day when you work with an intensely radioactive object, a reactor core, you need to be careful about exposure to radiation not only from the core, but from anything that has come into contact with the core, because radioactivity is "contagious" in the sense that one radioactive object can turn another radioactive. That means there have to be strict procedures that keep track of which tools, machinery, clothing et cetera has been exposed to radioactivity.

Furthermore, you do not know when you are exposed to radiation, so you can't rely on workers keeping away from it through common sense -- like you can count on steel workers staying away from molten steel. Radiation is invisible, and absorbing harmful doses causes no sensation. (Absorbing very large, promptly fatal doses does, apparently, cause mild strange sensations, but that's no help.) So typically you need to control access to various parts of the plant carefully, and you need to have workers wear monitoring devices that can track the radiation they receive, and then you need to have strict limits on how much radiation they can absorb per day, per year, and per working lifetime.