My uninitiated mind would think that it would be the other way around.
I was watching a video about nuclear power. The guy being interviewed was wearing safety glasses and nitrile gloves while holding a uranium fuel pellet. Then the camera pans to a screen showing the robot handling spent fuel in the bottom of a 40-foot deep pool of heavy water. The pool is in a room behind a big red door with every “do not enter” warning imaginable. I would think the fuel would be less radioactive coming out than going in.
In: Chemistry
Someone please correct me if I’m wrong, but if I remember right:
Radioactive materials are generally described according to their half-lives–the amount of time it takes for half of the material to undergo radioactive decay, and become something else, releasing radiation in the process. Uranium is one of the more stable radioactive elements, and uranium fuel tends to be a mix of this uranium and its more unstable isotopes.
Through the nuclear reactor setup, a lot of uranium is brought close together, and the radioactive decay process is kicked into high gear, making a *lot* of it decay much quicker, as to release usable energy in high amounts. At the end of this decay, you also get some amount of other, even more unstable (lower half-life) materials. Stuff that is factors higher in danger to handle. So, you pull the fuel out, and this accelerated decay is stopped–but a lot of what remaining, despite being proportionally small, is this significantly more naturally radioactive elements now remaining. Stuff that still has to decay, but is decaying really fast in comparison to uranium (but, might still be many many years). In natural uranium the amount might be so small as to not be of an additional danger. In spent nuclear fuel, for a time it’s going to be *very present* and hazardous to handle, until enough of it decays away.
The spent fuel rods contain some fission products that are very radioactive, like Iodine-131 and Barium-140. These have short half-lives, so they decay to other, less radioactive isotopes. That’s why the spent fuel rods are kept in water, to cool them and shield the radiation. After a while, the radiation decreases as the more radioactive isotopes decay away.
Some of those isotopes are particularly dangerous. Strontium-90 mimics calcium, the body absorbs it into the bones. So you’ve now got a radioactive substance in your bones.
Uranium-235 fuel decays by alpha decay. Alpha particles are big and charged so your general “everyday” PPE will protect you sufficiently. After being in a reactor for a few years there’s all sorts of other byproducts, including those that break down via gamma decay. Gamma rays will go through quite a lot more materials, and so require quite a bit more in the way of protection.
Basically it works like this. When something’s radioactive, it means that it is actively changing into something else. If you left a block of uranium around long enough, it wouldn’t be uranium anymore. Meanwhile, if you left a block of iron sitting around until the end of time, it would still be iron.
Uranium is actually pretty stable. If it wasn’t, we wouldn’t have any. You have to think that almost all the radioactive stuff on Earth has been here for a very, very long time. When a radioactive material breaks down into other materials, it tends to form very unstable stuff that is breaking down rapidly (perhaps only a decade).
Because the whole point of radioactive power generation is to promote more fission, you want more radioactivity. To this end, they look for naturally findable uranium-235 which is much less stable than uranium-238 which is most uranium. Taking a big pile of uranium and extracting as pure a 235 batch as possible is called enrichment.
Thing about refined uranium for reactors is that it’s actually pretty stable for a radioactive element. Half life is something like 700million years.
It’s why it’s able found in ore in decent quantities instead of already having decayed long ago. The decay can also be induced in a chain reaction by gathering enough of it near each other so that neutrons from one decay are likely to hit other atoms and cuse them to decay. This allows a reactor to controllably accelerate the decay and generate the amount of heat we want by adjusting the uranium concentration, total volume, and absorbing or reflecting neutrons.
Nuclear waste though has a much shorter half life. It decays much faster and produces more radiation in the short term. And while we have a way to accelerate decay, we do not have a way to decelerate them.
imagine it like that glow sticks you have to crack to start glowing.
Before you crack it, it’s has unreleased energy, but isn’t releasing (almost) any of that.
Once you crack it, it glows really bright. That’s when you use it.
After a while, it’s glow is too low to be useful, but still much more before you cracked it
others have provided some good answers, there is one thing I’m not seeing here though. An assembled fuel rod is very heavy, and contains LOTS of fuel. The pellets that come out of the pulverization and refinement process on the other hand are very small.
Think of radiation level being a multiple of certain things: the total mass of material, and the makeup of the material (another’s comment about fission products applies here). A tiny pellet that weighs a couple grams isn’t big enough. You could at least handle it for a moment without worry. I wouldn’t be taking any souvenirs though.
When removing fuel rods from the bundle, each one is very large and also flooded with fission products. You wouldn’t be able to break it into pieces safely, so it goes in a tank for 80 years.
The other answers are essentially correct but missing a description of how nuclear fission works to provide power in nuclear reactors and how it forms a lot of other radioactive isotopes/atoms. Elements are defined by the number of protons in the atom but the same element can contain atoms with different amounts of neutrons and each of these different forms are called isotopes. The basic idea is that a nuclear reactor takes uranium and splits it into radioactive isotopes, extra neutrons, and a ton of energy. The extra neutrons keep the reactor going.
Splitting uranium atoms in a nuclear reactor, called fission, produces isotopes of smaller atoms than uranium. The uranium usually splits into particles of different sizes that are unstable – they undergo radioactive decay, which is different from fission. The split isn’t always the same so a whole range of radioactive elements are formed. The radioactive decay produces a lot of radiation and new isotopes that can decay further until eventually it reaches a stable isotope. With time the radioactivity of the spent fuel decreases but unfortunately some of the isotopes remain dangerous for many thousands of years.
The key to why the uranium fuel isn’t as radioactive is that there isn’t much decay going on until you put the fuel in a reactor and start a fission chain reaction..
The fuel works by turning into other stuff.
When it’s spent it has turned from something safe into something that isn’t safe.
Also you can touch fresh food (sandwich) with bare hands but you shouldn’t touch spent food (poop) after your body (reactor) has used it for fuel
I think the main misunderstanding that OP has that the other posts miss is that the radioactivity is not what fuels a nuclear reactor and isn’t what is “spent” in the reactor. What is fueling it is energy left over from the mass of the fuel when splitting it. It’s just that the material that is easy to split in a reactor is also radioactive. And the byproducts generally even more radioactive.
Latest Answers