I wrote about this pool a year ago when TEPCO (Nuclear Power and The Witch Hunt) made video footage available that showed that this pool was undamaged, and had the fuel within it was also intact, contrary to some rather inflammatory statements from former NRC Chair Jaczko.
In the past week, TEPCO has begun unloading the fuel from this pool. Such an effort represents a major milestone in progress on the site. By unloading this pool, Unit 4 will no longer pose any risk of radioactive releases. TEPCO can more fully concentrate its resources on the far more difficult challenges of cleaning up units 1, 2, and 3.
TEPCO provided photos and videos from the site of the first fuel bundles being removed. However, there is little explanation of what is actually happening. I’ve seen a number of questions regarding this effort. Let me provide a little clarification.
A little Background
Unit 4 was in a refueling outage when the earthquake and subsequent Tsunami struck. During this outage, they were required to replace some equipment inside the reactor vessel itself. In order to do that, the core was completely emptied of nuclear fuel. In addition, fresh fuel planned to be loaded after the repairs were complete had been staged into the pool for moving into the reactor later.
Nuclear fuel for light water reactors is loaded into long thin tubes of zirconium alloy sometimes called “fuel rods.” These rods are about ½ inch in diameter and 12-13 feet long. Because they are so long and skinny, they are pretty flexible by themselves, so they are grouped into bundles for strength and convenience. Think of single piece of paper vs. a phone book. For Boiling Water Reactors (BWRs) like Fukushima, these rods are then grouped into squares of 60-100 rods that are 6 inches in diameter and 14 feet long. BWR fuel also has something called a channel added to it. This box surrounds the fuel and helps to guide the water flowing through the reactor.
Nuclear fuel that hasn’t been loaded into the reactor and bombarded with neutrons is pretty benign stuff. When I worked at GE, I walked through an area we called “the forest” where bundles hang awaiting packing into special boxes for shipping. Unirradiated uranium gives off an occasional alpha particle (the nucleus of a helium atom) which can’t even really escape the bundle. Wearing gloves and regular clothes is more than adequate protection.This fuel is designed such that it can’t start spontaneously fissioning. It requires both a source of neutrons and some water to get the whole thing going, one without the other won’t start the chain reaction, that’s why the fuel is so easy to work with.
What we see
So, in these photos you see a long dark object, that looks a bit like a piece of cast iron dangling from a crane. That is a fuel bundle with a channel on it. In some pictures, you can see a metal cap on the bottom that looks like there’s a handle on it. That’s the lower tie plate that all of the rods are resting in. Some of them are actually screwed into that plate to hold the whole thing together.
The full radiation gear is NOT because that bundle is hanging there. This is fuel that was never placed into the reactor. This means that it is pretty much just like the fuel in that forest. The other fuel in the pool isn’t really affecting them either. The water in the pool keeps the radiation to a minimum. I’ve stood above spent fuel pools without getting even a little radiation from it. They are wearing full protection suits because to get to that platform and work requires moving around on the site and there’s still a great deal of radioactive contamination.
In some pictures they are spraying a bundle down with water and wiping it down. Not completely clear to me why, but an educated guess would say that the water is fresh and they are rinsing off the contaminated pool water (back into the pool as they are standing over top of it. By then wiping the bundle down, they are eliminating any radioactive particles that might have gotten into the pool water and settled on the bundle from events at the other reactors. Yes, it really is that simple decontaminate a surface. Wash it off.
Once they’ve done that, the bundles are being covered with a plastic sleeve. That is frequently done with fresh fuel when it is shipped to keep bits of stuff from getting into the bundles. It’s actually quite possible that bundle could be stored dry in a simple box as it needs neither the cooling nor the shielding that is needed by the exposed fuel.
They have probably 150 or so of these fresh bundles to move out before they start working on the more difficult exposed bundles. Those do require significant shielding and careful planning to remove and transport.
Tell me, what do you think of TEPCO’s progress and how they might go about the next steps?