"The boiling water expands as it becomes a gas. The expansion creates tremendous pressure and forces the steam through a turbine — basically a series of fan blades mounted on a steel shaft — which rotates as the steam forces past the blades. After exiting the turbine, the water cools to its liquid state and is recycled through the boiling process."
"Got it," I said. "Water turning to steam spins the turbine. Then the steam is cooled again, like going through a car radiator, and continues through this boil and condense cycle indefinitely."
"Very good. Now… connected to one end of the turbine is a generator. The generator is a device that converts the mechanical energy present in the rotating turbine — the spinning fan shaft — into electrical energy. The generator transmits the electricity through various circuits to substations which meld it with the electricity already in the power transmission lines — the grid."
I nodded my understanding.
"The main difference between types of electric generating stations is the kind of fuel used to turn the water into steam. Gas plants use methane. Coal plants use mixtures of coal ore. Nuclear plants use uranium, though as soon as the reaction begins, plutonium is also present, and that also ‘burns.’ Obviously, each type of plant needs to burn its fuel in a different way."
Dana hadn’t actually told me anything that I hadn’t known for twenty years. But she was being thorough and I appreciated that.
I have always endorsed the theory that a sound understanding of the fundamentals leads to better execution of more advanced skills. That theory applies equally whether you’re involved in the military, academics, sports, business… just about anything.
Preparedness is next to godliness. Cleanliness isn’t actually next to anything.
I let her continue.
"For coal and gas, you light what amounts to a huge barbecue grill and surround it with water pipes. The fire heats the water in the pipes to make the steam. In the case of nuclear fuel, the uranium actually burns itself. So you don’t need to light it.
"By the time the nuclear fuel arrives at electric power plants, the uranium pellets have already been formed into rods and coated with a zirconium alloy. The zirconium is strong and reacts very little with the uranium, so it makes the fuel easier to move around. The fuel rods are typically about thirteen feet long and a centimeter or so in diameter. Long and thin.
"Fuel rods are bundled together in rectangular metal frames to give them structure and stability. A bundle might typically contain around 150 to 200 fuel rods. Collectively these framed bundles are called ‘fuel assemblies.’ A nuclear reactor in an electric plant might have up to 200 of these fuel assemblies in its reactor vessel at any given time."
"Just a moment, please." Now I was learning new stuff and I wanted to get it right. "What is a reactor vessel?"
"The reactor vessel is the oven of the reactor. The fuel assemblies are precisely racked in this oven space to produce maximum heat, with minimum risk that the nuclear reaction will get out of control.
"More terminology here," Dana continued. "Once placed in the reactor vessel, the collection of fuel assemblies is called the ‘reactor core.’ Normally the reactor core will be suspended in the center of a radioactively insulated space. The core will also have control rods strategically placed to interrupt the nuclear reaction if it needs to be reduced or shut down — like for repair or to adjust power output."
"I’m sorry. I need to stop you again. How do the control rods affect the nuclear reaction? I understand that they shut the reaction down, but how do they do that?"
Dana thought for a moment. I had another swallow of Bass.
"To answer that question, I need to first explain how nuclear fuel burns. You see, the uranium in the fuel assemblies is inherently unstable and is constantly releasing neutron particles into its surroundings. In the reactor core, the freed neutrons slam into uranium atoms in other fuel assemblies. Every time a neutron-uranium collision occurs, another uranium atom splits, releasing more neutrons, and so on. Every collision releases heat.
"As long as there are unstable atoms like uranium and plutonium in the area, the reaction is self-sustaining. To control the intensity of the reaction, ‘control rods’ are inserted between the fuel assemblies."
"What are these control rods made of and how do they work?" I asked.
"Control rods are typically encased in stainless steel and contain obscure elements, like gadolinium or hafnium, that will withstand bombardment by the neutrons without fissioning — that is, splitting apart. As the control rods intercept and absorb free-flying neutrons in the core, the reactor cools. The farther the rods are inserted, the more neutrons they absorb, and the cooler the reactor becomes."
"Okay. I think I understand the basic idea. We may have to come back to this for more details later. Okay?"
"Sure." She sipped her ale. After wiping excess condensation from the table with an already soggy cocktail napkin, she replaced her drink in the semi-dry spot.
"How are the reactor vessels protected from sabotage?" I asked. "What keeps someone from bombing them or flying a plane into them or something?"
"Well," Dana continued, "the reactor core is totally enclosed within a theoretically bomb proof containment structure. Many feet of steel-reinforced concrete. More steel. Then more concrete, etc. And no one is allowed in or out of containment except to conduct repairs or to refuel the reactor."
"You said ‘theoretically bomb proof.’ Only ‘theoretically’?"
"The engineers and the physicists are pretty sure… very sure actually… that the containment structures can survive conventional explosives, or even an airliner crash. But of course, no one has actually flown a commercial airplane into a containment structure. So their imperviousness remains theoretical.
"I don’t mean to imply that the structures are unsafe or anything. From all the data I’ve seen, the reactor containment towers seem pretty impregnable."
"Okay. Got it. What happens to the uranium fuel when it’s been burned up, for lack of a better term?"
"About every year-and-a-half, when the nuclear fuel is ‘spent,’ the reactor must be refueled. Usually about a third of the oldest fuel assemblies are removed from the reactor core and replaced by fresh fuel.
"Although the fuel that has been removed is considered ‘spent,’ it remains highly radioactive and the nuclear reaction still needs to be controlled. For this reason, the spent fuel assemblies are placed in a pool and surrounded by circulating water infused with boric acid — the ‘spent fuel pool.’
"The circulating borinated water is a closed loop. The water in the pool absorbs heat from the nuclear fuel, then dissipates it through a heat exchanger, like the car radiator you mentioned before. A continuous supply of river water provides the coolant.
"The spent assemblies remain in the pool until the utility gets government authorization to move them elsewhere. That was supposed to be Yucca Mountain. But there is no permanent storage site for spent fuel at present."
"Can regular water without boron be used to fill the pool?"
"If ordinary distilled water were the only coolant in the pool, you could keep the fuel cool, as long as the water kept circulating through the heat exchanger, but there would be little margin for error. The boron is the real ‘control rod’ in the pool.
"Boron absorbs the flying neutrons. Plain water wouldn’t slow the nuclear reaction, just carry the heat away. If the water stopped flowing for even a few hours, the pool could boil dry."
That was interesting.
"So what happens when the spent fuel pool gets full and there’s no more room for fuel assemblies?" I was catching on to the terminology.
"If the spent fuel pool is full, the reactor must be shut down and decommissioned. We were close to that at Prairie River a few years ago. The spent fuel pool at the plant was nearly full. Fortunately, the NRC, the Nuclear Regulatory Commission, allowed us to re-rack the fuel assemblies in the pool — basically, packing the fuel closer together.