It's because he's a steam engine.
Name: Nuclear Man
Designation: Robot Master (DW2N-059)
Weapon Get: Nuclear Sludge
Other weapons: Nuclear Smog, Nuclear Burn
Background: ... Makes you wonder why I created him ehhh...
Nuclear Man is an experimental Robot Master, harassing the power of nuclear energy. However, some miscalculations laid to the nuclear reactor to almost meltdown levels, hence the only way was to install a complex coolant system, hence the additional control rods in his shoulders and special coolant fiuld tanks on his back. Also, to prevent accidental leakage, a special buster cover is created, that doubles as a reticale when the buster is in use. Besides his coolant system, Nuclear Man also has reinforced armour to further reduce external damage to his reactor.
Nuclear Man's main weapon, Nuclear Sludge, fires out a blob of nuclear waste. If it hits the floor, the resulting puddle will stay for the rest of the battle, creating reduced space for his enemies to move around. Nuclear Man can also create radioactive smog from his top to damage enemies (Nuclear Smog), as well as temporarily shut down his coolant system to unleash a powerful heat wave (Nuclear Burn).
Character: Calm and collected... However... Can be rash with power.
Weakness: It seems Nuclear Man is very dependant on his coolant system... Those tubes... Engine Slicer... (There's a secondary weakness... Remember how Nuclear Sludge creates nuclear waste puddles that stay throughout... They can be removed... With strong winds...)
You get this weapon from defeating Nuclear Man. Megaman (technically I should say borockman, but I'm lazy...) fires out a blob of nuclear waste, that can leave a trail of sludge puddles for surviving enemies to walk into, resulting in additional damage.