Plutonium is an IC2 Experimental item crafted from nine Tiny Piles, used for MOX fuel and RTG pellets.
Guide
Add IndustrialCraft 2 Experimental to a Minecraft 1.7.10, 1.10.2, or 1.12.2 modpack; Plutonium does not exist in vanilla Minecraft.
Craft a Nuclear Reactor, place it, and prepare cooling cells, heat vents, and a power output line so the reactor can run safely.
Place an Isotope Fuel Rod directly adjacent to a Uranium Fuel Rod that is close to depletion; the Isotope rod will accumulate Tiny Piles of Plutonium over time.
Collect nine Tiny Piles of Plutonium and combine them in a 3x3 crafting grid to produce one Plutonium; the recipe is reversible if you need piles again.
Combine Plutonium with depleted uranium parts to craft MOX Fuel Rods for hot reactors, or craft RTG Pellets and load them into a Radioisotope Thermoelectric Generator for passive EU.
Raw Plutonium in your inventory does not damage you, but spent rods and reactor mishaps cause IC2 radiation; keep a Hazmat Suit nearby when handling fuel.
No, Plutonium is an inventory item from IC2 Experimental; you cannot place it as a block, only use it in crafting recipes.
Run an Isotope Fuel Rod next to a near-depleted Uranium Fuel Rod inside a Nuclear Reactor; it accumulates Tiny Piles of Plutonium that craft into full Plutonium.
Nine Tiny Piles of Plutonium fill a 3x3 crafting grid to produce one Plutonium, and one Plutonium decomposes back into nine Tiny Piles.
MOX Nuclear Fuel mixes Plutonium with depleted uranium parts; MOX rods produce more EU/t than uranium rods when the reactor runs at high heat.
Plutonium crafts into RTG Pellets that load into a Radioisotope Thermoelectric Generator, providing a constant passive EU output that slowly decays over time.