Reactor Vessel is the structural shell block of the Industrial Craft 2 Pressure Vessel Reactor (Fluid Reactor), a multiblock that converts a six-chamber Nuclear Reactor from direct EU output into Coolant-heating mode. Industrial Craft 2 crafts Reactor Vessel on a 3x3 grid from 5 Lead blocks plus 4 Stone in a checkerboard pattern. One craft yields 4 blocks, and the same Reactor Vessel block also serves as the crafting ingredient for the three multiblock interface blocks. Each interface recipe wraps one inner block in 8 Reactor Vessel blocks: an Oak Trapdoor becomes a Reactor Access Hatch, a Universal Fluid Cell becomes a Reactor Fluid Port, and a Redstone becomes a Reactor Redstone Port. Reactor Vessel is not renewable, because Lead comes from finite lead ore, and the block stacks to 64 with a pickaxe as the harvest tool.

The Pressure Vessel Reactor assembles as a 5x5x5 cube of Reactor Vessel blocks wrapping a 3x3x3 hollow core that contains 1 Nuclear Reactor block surrounded by 6 Reactor Chambers. A full shell uses roughly 95 to 98 shell blocks plus at least one Access Hatch and one Fluid Port. When the shell forms around a running six-chamber Nuclear Reactor, the reactor stops emitting Energy Units (EU) directly and instead heats Coolant into Hot Coolant, which is piped through a Fluid Port to a Steam Boiler and Steam Turbine chain for more EU per uranium rod than direct EU mode. Reactor Vessel is blast-resistant, but at 85 percent core heat the shell blocks themselves melt into lava source blocks and a meltdown begins. Removing even one Reactor Vessel block while the reactor runs breaks the multiblock and shuts the reactor down until the shell is reformed.