I just finished a marvelous book called “How To Drive a Nuclear Reactor” by Colin Tucker. It’s about the engineering inside nuclear power plants and some operator duties. It’s a very good explanation in plain English. There were a few typos which was jarring, but forgivable. I suppose that’s what happens when you let an engineer be a writer. I am going to write a summary of the text so I can review it later if I ever need to.
The first topic covered in the book is the nuclear/atomic/particle physics. All of mater is made up of three “fundamental” particles. I put fundamental in quotes because even those particles are made of smaller things which are irrelevant for the subject at hand. These three particles of import are: protons, neutrons, and electrons. Protons and neutrons have roughly an equivalent mass and the electron has a hundred thousandth of the mass. A minuscule, negligible amount of mass for this topic. The proton has a positive charge. The electron has a negative charge.
The elements are classified by the number of protons in a lump. If you have a lump of two protons, then you have helium. If you have eight protons, then you have oxygen. These lumps are called a nucleus. These nuclei pick up electrons that float by. The positively charged protons attracts the negatively charged electron. Protons do not particularly like being lumped together. Since they all have the same charge, they find each other repulsive! They yearn to fly away at the soonest convenience. And they would fly away except for the fact that they are “glued” together with neutrons. Neutrons apply a force that is similar to gravity or electrical charge, except it is wayyyyy stronger but only works at verrrrry close range. Imagine if you took two magnets and glued them together North pole to North pole. Then you tap it just right. Pew! The two magnets would break apart and whiz away in opposite directions. This is a good metaphor or a helium nucleus because it only has two protons. Now imagine gluing 92 magnets together. You can imagine how much more fragile this configuration is. Helium usually has two neutrons to hold together the two protons. Oxygen usually has eight neutrons to hold onto eight protons. Iodine has about 73 neutrons to hold the 53 protons together. And uranium usually has 146 neutrons to hold 92 protons in place. You need more and more “glue” to hold onto these volatile protons.
Here’s the clever bit of physics that let’s us power light bulbs with neutrons. We actually don’t want Uranium that’s stable. When we find uranium ore in the ground, it’s in the most stable state with 146 neutrons. We want Uranium that is fragile. We want it to be balanced like an extra tall tower of Jenga blocks. We purposefully remove three neutrons from the lump. This is effectively like using less glue in our magnet pile. That way, it just needs a ever so gentle tap, and the whole thing splits apart. It’s like having dynamite with a very short fuse. We want it that way because we can put a bunch of uranium in one very dense pellet. Each fission of uranium nuclei will send neutrons firing off like bullets in every direction to hit other uranium nuclei. It’s like a truck full of dynamite. Each dynamite stick sets off another dynamite stick. Or like a Old Western movie where every poker disagreement in a saloon inevitably evolves into an old fashioned bar fight.
Now onto the engineering. You can imagine that a truck full of dynamite releases a lot of energy. But how to harness that energy? It boils down to (pun intended) a steam generator. Water passes by the explosions, it heats up, it creates steam, the steam pushes a fan, the fan is attached to an electric motor, the motor has coils of copper wire and magnets. When the magnets pass the coils of copper wire, electricity flows.
There are two predominant ways of creating steam for the generator. You can either use two water loops, or three. A water loop is a circuit of pipe. There is a pool of water, a pump that pushes the water through the pipe, then it returns to the pool and repeats. The technical term for the pool is “reservoir.” For a three loop system, we’ll call them A, B, and C. The uranium fissions are miniature explosions that release a lot of heat. The water in Loop A swishes and swirls around the uranium pellets. The heat is absorbed by the water. The water flows through a heat exchanger where it gives up it’s heat to Loop B. And then it flows back into the pool to repeat the process.. It adsorbs heat from the uranium, and then gives the heat to Loop B, absorbs heat from uranium, and then gives it to Loop B, etc. Loop B starts in a completely separate pool. The water never mixes with Loop A or C. It gets pumped out of the pool and goes through the heat exchanger and absorbs the heat from Loop A. It gets so hot that it immediately boils and turns to steam. When water evaporates, it expands by 400x. So it creates a tremendous gale force wind. This wind turns a fan blade. The technical term for these fans are turbines. The turbines are fixed to a shaft with an electric motor.
If you apply power to an electric motor, it turns, like a desk fan. If you turn an electric motor, you get electric power, like a wind mill. Inside of an electric motor are coils of copper wire and magnets. When a magnet passes a copper wire, it pushes electrons through the wire.
After the steam passes through the turbine, it gets condensed back into water and returned to the pool for Loop B.
Loop A and B are “closed loops” because it’s a complete circuit. Loop C is an “open loop” because they take sea water or lake water and return it. The lake or the sea is the reservoir. It gets pumped in to another heat exchanger, absorbs the heat from Loop B, and then returned to the sea or lake. That absorbing of heat is what condenses the steam in Loop B.
For a two loop system, there is no Loop A. It’s just Loop B that turns to steam and Loop C that condenses the steam. A three loop system is called a PWR (Pressurized Water Reactor) and a two loop system is called a BWR (Boiling Water Reactor). PWRs are safer because during a failure, usually only Loop A is affected and everything else is isolated. It’s easy enough to clean up. While a BWR is more efficient because heat exchangers are inefficient and it uses one less. But it’s more dangerous because the water that is turning to steam and powering the plant is also cooling the uranium pellets. If there’s not enough water, the pellets explode like a truck of dynamite. And you have radioactive steam on top of that.
Back to the uranium. It’s explosive. We need to be able to control it. We need a “neutron sink.” Just like a kitchen sink drains water no matter how much you have the faucet turned on, we need a way to absorb neutrons no matter how many explosions there are. When the uranium nucleus fissions (breaks apart) and shoots of neutrons like bullets, we need a punching bag dummy to absorb some bullets to tame the barfight, so to speak. There are two main ways of doing this. One is with graphite rods. Graphite is a special arrangement of carbon atoms. The carbon atoms are happy to absorb stray neutrons without breaking apart. The graphite rods are inserted in channels along side the uranium pellets and can be withdrawn an inch at a time as needed. The more they are inserted, the more neutrons they absorb. The second method is with boron dissolved in water Loop A. The boron is also happy to adsorb stray neutrons without breaking apart. The operators in the control room can also control the boron concentration in the water.
Those are all the juicy bits!