Nuclear Waste is the radioactive chemical that a Mekanism Fission Reactor leaves behind, at 1 mB for every mB of Fissile Fuel that’s burned. It can be turned into Plutonium or Polonium, which are both needed for late game crafting, or it can be left to decay in a 
Radioactive Waste Barrel.
Spent Nuclear Waste is a different chemical, and it comes from making Plutonium Pellets and Polonium Pellets. If you’re wondering what to do with it, the answer is nothing, as no recipe uses it. It has to be piped into Radioactive Waste Barrels, which will slowly destroy it.
How to Get It
The only source of Nuclear Waste is a running Fission Reactor. It builds up in the reactor’s waste tank and gets pushed out of any 
Fission Reactor Port that’s been set to output waste with a 
Configurator. A reactor burning 1 mB/t will make 20 mB of it every second, so it adds up quickly.
Waste can’t be ignored either, because a full waste tank doesn’t stop the reactor. It’ll keep burning and the extra Nuclear Waste is released as radiation in the middle of the reactor. That’s why the waste line should be built and tested before the reactor is ever switched on.
| Chemical | Comes From | Used For | Decays in a Barrel |
|---|---|---|---|
| Nuclear Waste | Fission Reactor, 1 mB per mB of fuel | Plutonium and Polonium | Yes |
| Plutonium | Isotopic Centrifuge, 10 mB Nuclear Waste each | Plutonium Pellet | No |
| Polonium | Solar Neutron Activator, 10 mB Nuclear Waste each | Polonium Pellet, Antimatter in the SPS | No |
| Spent Nuclear Waste | Making either pellet, 1,000 mB each | Nothing | Yes |
All four of these are radioactive, and a normal Chemical Tank will refuse to take any of them. They’re only able to sit in the reactor, in Pressurized Tubes, in Radioactive Waste Barrels and in the tanks of the machines that process them. So 
Barrels are your buffer, and tanks aren’t an option.
How to Use It
Nuclear Waste has two uses, and both of them shrink it down by ten. An 
Isotopic Centrifuge will turn 10 mB of it into 1 mB of Plutonium, whereas a Solar Neutron Activator turns 10 mB into 1 mB of Polonium. The activator needs daylight and open sky above it, so it won’t do anything at night.
Plutonium Pellet
To make a 
Plutonium Pellet, a 
Pressurized Reaction Chamber needs 1,000 mB of Plutonium, 1,000 mB of Water and one 
Fluorite Dust. That’s 10,000 mB of Nuclear Waste for a single pellet. Each one that’s made also leaves 1,000 mB of Spent Nuclear Waste in the machine’s output tank.
Plutonium Line
Plutonium Pellets go into 
SPS Casing and the larger 
QIO Drives, and they’re how burned fuel gets recycled. A pellet put through a 
Chemical Injection Chamber becomes 4 
Reprocessed Fissile Fragments, which are worth 2,000 mB of fuel each. The full loop is on the Fissile Fuel page, and it returns 80% of what was burned.
Polonium Pellet
A 
Polonium Pellet is made the same way, with 1,000 mB of Polonium in place of the Plutonium, and it leaves the same 1,000 mB of Spent Nuclear Waste. These are the ones you’ll need the most of, as they go into SPS Casing, 
Supercharged Coils, 
Fusion Reactor Frames and every piece of the MekaSuit.
Polonium Line
Polonium is also the chemical that the Supercritical Phase Shifter turns into Antimatter, at 1,000 mB for each mB, and that Antimatter is what the 
Antiprotonic Nucleosynthesizer runs on. So a base that’s heading for Antimatter will want most of its Nuclear Waste going to 
Solar Neutron Activators, with only a little sent to the centrifuge.
Radioactive Waste Barrel
The Radioactive Waste Barrel is crafted from 4 
Steel Ingots in the corners and 4 
Lead Ingots on the sides, with the middle left empty. It holds 512,000 mB of one radioactive chemical and it won’t accept anything that isn’t radioactive. Tubes only connect to the top and the bottom of it.
Each Barrel will destroy 1 mB of whatever is inside it every second, which comes to 3,600 mB an hour. That’s slow, as a full one would take about 142 hours to empty itself, and it only cancels out a burn rate of 0.05 mB/t. So a reactor that’s running at 1 mB/t would need 20 of them just to break even.
So Barrels aren’t a good way to get rid of Nuclear Waste from a busy reactor, and it’s better to turn it into Polonium or Plutonium first, which cuts it down to a tenth. They’re the right place for Spent Nuclear Waste though, as that only arrives 1,000 mB at a time with each pellet.
Stacking and moving waste
When a Barrel is full, anything extra that’s piped in will overflow into the Barrel directly above it, so a tall stack fed from the bottom works as one big store. Sneak and right-click a Barrel with a Configurator to make it active, and an active Barrel will push its contents into whatever is below it.
Plutonium and Polonium can be stored in a Barrel but they’ll never decay in there, which the chemical’s tooltip will tell you. That makes a Barrel a safe buffer between the centrifuge or activator and the Pressurized Reaction Chamber. Sneak and right-click with an empty hand to see what’s inside one.
Breaking a Radioactive Waste Barrel releases everything in it as radiation on that spot, and the same goes for a machine or the last tube of a line that’s holding a radioactive chemical. A full Barrel of Nuclear Waste makes a source that takes about 11 hours to fade, which is covered on the radiation page.
A Waste Line That Works
A simple layout is to run a Pressurized Tube from the reactor’s waste port into the bottom of a stack of Barrels, then pull from that stack into your Solar Neutron Activators and one Isotopic Centrifuge. The Barrels soak up the waste at night when the activators are idle, and anything that’s left over will slowly decay.
Then send the Spent Nuclear Waste from the Pressurized Reaction Chamber to its own separate stack of Barrels and leave it there. It’s worth wiring a 
Fission Reactor Logic Adapter on Critical Waste Level as well, which turns on when the reactor’s waste tank reaches 90%. Once the waste is handled, the next thing to understand is radiation itself.
FAQ
There isn’t a recipe that uses it at all, so the only thing to do is to pipe it into Radioactive Waste Barrels and leave it there. Each Barrel will hold 512,000 mB and destroy 1 mB a second, and you’ll only make 1,000 mB of it for each pellet, so a small stack of them is enough to keep up with it.
The quickest way is to use it, as an Isotopic Centrifuge or a Solar Neutron Activator will turn 10 mB of it into 1 mB of Plutonium or Polonium. Whatever you don’t need can be left to sit in Radioactive Waste Barrels, which destroy 1 mB a second each. It can’t be dumped safely, as dumping it will only release it as radiation instead.
A normal Chemical Tank will refuse every radioactive chemical while radiation is turned on, and that covers all four of the ones on this page. They’ll only go into Radioactive Waste Barrels, Pressurized Tubes and the machines that are able to use them, like the Isotopic Centrifuge, and a 
Dynamic Tank won’t take them either.
Plutonium and Polonium are both immune to decay, so a Barrel will hold them for as long as you like without losing any. Only Nuclear Waste and Spent Nuclear Waste are destroyed over time. That means it’s safe to use Barrels as storage for the two you actually want to keep, as well as a bin for the two you don’t.
