Reactor Pressure Vessel
The Reactor Pressure Vessel block is part of the multiblock Pressure Vessel Reactor. It is used as a 'shell' block, to fill the space not occupied by Hatches, Fluid Ports, or Redstone Ports. It also used in the crafting recipe for all of the mentioned blocks.
The Pressure Vessel Reactor is built by surrounding a full nuclear reactor with a 5x5 shell of Reactor Pressure Vessel blocks. The whole reactor, with 4 Reactor Fluid Ports, 1 Reactor Access Hatch, and 1 Reactor Redstone Port, contains 140 Reactor Pressure Vessel blocks, or 175 Lead Ingots.
Reactor Access Hatch
Reactor Fluid Port
Reactor Redstone Port