Radiation in Mekanism is released when a radioactive chemical escapes, like Nuclear Waste from a broken tube or a Fission Reactor that melts down. It sits at the spot where it was released, and anything that goes near it will build up a dose, which causes damage and drains hunger for a long time afterwards.
If you’re here to get rid of radiation, the honest answer is that you’ll have to wait, as it halves about every 23 minutes and nothing you can craft will speed that up. A small leak is gone in around 4 hours and a big one takes closer to 12. The same wait applies to the dose on your character, and a Hazmat Suit only stops it getting worse.
Where Radiation Comes From
Only four chemicals are radioactive, which are Nuclear Waste, Spent Nuclear Waste, Plutonium and Polonium. While they stay inside a reactor, a tube, a machine or a 
Radioactive Waste Barrel, they give off nothing at all. A source only appears when one of them gets out, and these are the ways that’ll happen.
Breaking a Radioactive Waste Barrel, or a machine with a radioactive chemical in its tank like an 
Isotopic Centrifuge or a 
Solar Neutron Activator, spills what’s inside. So does breaking the last Pressurized Tube of a line that’s holding one, or emptying a tank of it with a 
Gauge Dropper.
A Fission Reactor running with a full waste tank will release the extra Nuclear Waste at the middle of the reactor. The worst case is a meltdown, which releases everything in the waste tank at 50 times its normal strength.
How strong a source is depends on how much got out. Nuclear Waste and Spent Nuclear Waste are worth 0.01 Sv/h for each mB, Plutonium is 0.02 and Polonium is 0.05, so 1,000 mB of spilled Nuclear Waste makes a 10 Sv/h source. Spills on the same block will add together.
Uranium Ore, 
Uranium Ingots, 
Yellow Cake Uranium and Fissile Fuel aren’t radioactive, and neither are the finished pellets, so you’re safe to mine and carry all of those. An intact reactor is safe to stand beside as well. The one exception is being inside a reactor while it’s burning, which will irradiate you.
How Far It Reaches and How Long It Lasts
The reading drops off quickly with distance, as it’s divided by the distance squared. A 10 Sv/h source will read 0.1 Sv/h from 10 blocks away, and at 80 blocks it stops counting altogether. So moving your base isn’t needed for a small spill, but you’ll want to stay well back from it.
How long it lasts
Every source fades by itself with a half-life of about 23 minutes, and it’s removed completely once it drops under 0.00001 Sv/h. Because it halves each time, a spill that’s a thousand times bigger only lasts a few hours longer. The area has to be loaded for the time to count.
| What Got Out | Source Strength | Time Until It Is Gone |
|---|---|---|
| 1 mB Nuclear Waste | 0.01 Sv/h | 3.8 hours |
| 100 mB Nuclear Waste | 1 Sv/h | 6.4 hours |
| 1,000 mB Nuclear Waste | 10 Sv/h | 7.7 hours |
| A full Radioactive Waste Barrel (512,000 mB) | 5,120 Sv/h | 11.1 hours |
| A meltdown with 80,000 mB in the waste tank | 40,000 Sv/h | 12.3 hours |
Nothing that’s placed in the world will block radiation or soak it up, so walls, Water and 
Lead don’t help. You also can’t pick the source up, as it isn’t a block. All you’re able to do in survival is keep away from it until the 
Geiger Counter says it’s gone.
What Radiation Does to You
While you’re standing in radiation, your character builds up a dose, which is measured in Sv and shown by a 
Dosimeter. The dose is what hurts you, not the radiation in the air, so the effects will carry on after you’ve walked away. Mobs and animals are affected in the same way, whereas players in creative mode take none.
| Dose | What Happens |
|---|---|
| Under 0.00005 Sv | Nothing |
| 0.0001 Sv | Hunger drains faster and you take half a heart now and then |
| 0.01 Sv | The same, but far more often |
| 1 Sv | Half a heart of damage most of the time |
| 10 Sv | Over a heart of damage with each hit |
| 100 Sv or more | More than 2 hearts with each hit, almost constantly |
It doesn’t take long to pick up a bad dose. Standing one block from a 10 Sv/h spill for a single second will add about 0.003 Sv, which is already enough to hurt often. Your dose halves every 23 minutes as well, so dropping from 1 Sv to a harmless level takes about 5 and a half hours of play.
Clearing a dose
There’s no food, potion or item that removes a dose. In our reading of the code, dying should clear it, as the dose isn’t carried over when you respawn, but we haven’t been able to confirm that in game. So you’ll be keeping food on you and staying topped up on health until it wears off.
How to Get Rid of Radiation
In a survival world there are only two things you can do, which are to stop the leak and then wait. Fix the broken tube or shut the reactor down first, because a source that’s still being fed will never fade. After that, keep out of range and check on it with a Geiger Counter every so often.
Commands
If cheats are on, or you’re an operator on the server, Mekanism has commands that clear it straight away. The first one below removes every radiation source in every dimension, and the second one clears the dose from yourself or from the players you name.
There’s also a reduce command that takes an amount off a player’s dose, and a get command that reads the level at a position. Radiation can be switched off completely in Mekanism’s general config as well, which stops sources and doses and lets normal Chemical Tanks hold the waste. That setting is on the config page.
Protection
The Hazmat Suit is the early protection, and each piece blocks a share of the dose you’d pick up. The 
Hazmat Mask blocks 25%, the 
Hazmat Gown 40%, the 
Hazmat Pants 20% and the 
Hazmat Boots 15%. With all four on that’s 100%, so you won’t gain any dose at all.
It won’t heal a dose that you already have though, and it gives no armor, so put it on before you walk in. Later on the MekaSuit is able to do the same job, but only with a 
Radiation Shielding Unit installed in every piece, as each unit covers the same share as the Hazmat piece for that slot.
Geiger Counter and Dosimeter
These are the two tools for checking on radiation, and they’re both cheap. The Geiger Counter is made from 4 
Lead Ingots around a 
Basic Control Circuit, whereas the Dosimeter uses a 
Redstone Dust in the middle instead.
Right-click with the Geiger Counter and it’ll show the radiation where you’re standing in Sv/h, along with how long the strongest source nearby has left before it’s gone. A reading of 100 nSv/h is the normal background level, so that one means the area is clean.
The Dosimeter shows the dose on your own character and how long it’ll take to wear off, and you’re able to right-click a mob or another player with it to read theirs. If you have a MekaSuit, the 
Geiger Unit and 
Dosimeter Unit will put both readings on your screen. With the tools in hand, the next thing to make is the Hazmat Suit.
FAQ
Radiation in the world only goes away with time, as each source halves about every 23 minutes until it disappears. A small leak takes around 4 hours and a full 
Barrel or a meltdown takes 11 to 12. With cheats on, the command /mek radiation removeAll will clear every source at once.
The dose on your character fades at the same rate as the radiation does, halving every 23 minutes, and there’s no item that cures it. An operator can clear it with /mek radiation heal. A Hazmat Suit won’t remove it either, as it only stops you from picking up more.
Uranium Ore, 
Raw Uranium, Uranium Ingots and Yellow Cake Uranium are all safe to hold, and so is Fissile Fuel. Radiation only comes from Nuclear Waste, Spent Nuclear Waste, Plutonium and Polonium, and only when they escape from a tube, Barrel, machine or reactor.
Blocks don’t stop radiation at all, so a Lead wall or a room full of Water makes no difference. The only things that lower it are distance, as it falls off with the square of how far away you are, and time. It can’t be detected past 80 blocks from the source.
