Here is a picture of an activated (or should I say irradiated?)uranium fuel bundle (which could hold anywhere from 60-80 fuel rods). There could be approximately 174 bundles in a reactor core. It all depends on the size of the plant. I don’t remember which plant this is from. I didn’t keep good records. But you can clearly see the Cherenkov blue glow. This is the cool part.
Cherenkov is when the charged particles are traveling faster than the speed of light through the water.
Basically, this is the order of disassembly of the reactor for refueling maintenance. First, they put the reactor in shut down mode. The insertion of all the control rods in the core will stop the nuclear chain reaction. There is always water in the core which is what the whole process is about. There are two kinds of nuclear power plants that I have worked at but there are several different others that I have no clue about. The two that I’m talking about are PWR’s (Pressurized Water Reactors) and BWR’s (Boiling Water Reactors).
Water in the core is heated by the nuclear chain reaction. This is turned into steam which turns the turbine which powers the generators which produces the electricity that you use every day. It’s so simple. But then not.
After the plant is shut down, then the workers will perform maintenance in all areas of the plant. The Refuel Floor is at the top of the Reactor Building in a BWR and on the main (top) floor in containment for a PWR. Depending on the type of plant (BWR or PWR) the disassembly begins. For PWR’s they remove the Reactor Head and set it aside somewhere on the floor. Then they fill the cavity with water. The reactor internals may be removed if I remember correctly for a PWR. This is placed on the cavity floor beside the core. After this they can start shuffling or replacing fuel bundles.
This picture was taken during a refueling outage. They pick up the fuel bundle with a mast from a refuel bridge over the reactor cavity always maintaining a certain amount of distance from the top of the water. The water is moderating the neutrons and shielding the radiation from the workers. The Operators then move the used fuel bundle to a Spent Fuel Pool where it sits for many years to decay. This is usually outside of containment in a PWR and the process in which they transport the bundles can be tedious. They would have to tilt the bundle horizontally in the water and move it through a transfer canal then upright it and place it in the Spent Fuel Pool.
In recent years we have begun Dry Fuel Storage where the fuel bundles are moved to a multipurpose container which is engineered to allow cooling effect while it decays and keeps any radioactivity from escaping to the atmosphere. Our oldest fuel rods would go into these containers allowing more space in the Spent Fuel Pool for more refueling outages. The used bundles are always kept under water until they are placed safely in these storage casks because it would be deadly if they were exposed.
During an outage the Control Room Operators figure out the matrix of the reactor core and how the assemblies should be arranged. Some fuel rods are shifted around the core while new ones replace old ones. This maximizes the efficiency of the chain reaction in the core. Sometimes when they are done refueling you can look down in the core and see a unique design from the shiny new rods and the darker old ones. I was trying to recall what the design is called artistically. I think it’s called Fractals. Don’t quote me. Someone sent me an email years ago with very cool pictures of geometrical repeated designs. But I can’t find them now.
In a BWR, the process is different. There are shield blocks on top of the cavity. These are removed. Then the Drywell Head (dome) is removed and the Reactor Pressure Vessel Head is removed which are placed on the floor. The Steam Dryer and then the Steam Separator are removed. These are moved into the Equipment Pit (filled with water). The Equipment Pit is directly beside the Reactor Cavity. On the other side of the cavity is the Spent Fuel Pool. There is a gate that is removed between the two areas which allow for the transfer of the fuel bundles.
Ok, I think that’s enough for today. I hope that I haven’t misspoken about anything here (maybe just the grammar, huh?). If anyone out there reading this is an expert on all of this don't hesitate to correct me. I’m on nights again and so very tired. It’s been a few years since I’ve worked an outage and I have the worst memory in the world. I’ve been out of the loop because I worked at a DOE site (Dept of Energy) doing various jobs. And then when I got hired on here permanently I became pregnant and therefore couldn’t, wouldn’t work in the RCA and then of course I went on maternity leave. It’s been actually 4 years since I worked the Refuel Floor.
That’ll be my next story. What it feels like to work on the Refuel Floor. To me, it’s like the most prestigious job there is for an HP. But that’s just me. I think it’s very exciting to work around the reactor core. The heart of the nuclear power plant. Thump, thump…thump, thump.
I’ve added a couple more pictures to this story. One is showing the Reactor Head of a PWR and the other is a better picture of the cavity pool and reactor core (PWR).
By the way, I’m not too worried about telling you guys all this stuff because it doesn’t compromise the security of the plant. It’s not top secret. You could find all this information out on the internet anyway. Now, if I were to tell you how security personnel do their job or the other security aspects of the plant then I would be in trouble. And it’s not like I’m telling you how to build a bomb. Which ironically you can find out on the internet also. Isn’t that sad?