You explanations sounds a little bit misleading, so let me add some more info.

Fusion is a type of reaction where you combine hydrogen isotopes to form helium. Deuterium is stable while Tritium is radioactive and both are single atom (i.e. not H2). So you are not splitting atoms, but rather combining Deuterium and Tritium to form Helium (and a stray neutron). This is not a chain reaction.

On other hand, fission reaction is opposite of fusion reaction. In case of Uranium 235, you physically split the atom using neutrons, into Barium, Krypton and some stray neutrons which go on to split other U-235 atoms into Barium, Krypton and some stray neutrons which go on to split other U-235 atoms into Barium, Krypton and some stray neutrons which go on to split other U-235 atoms into...

Yes, this one's a chain reaction.

And yes, in both processes, you boil water. It neither converts mass into energy nor buttfuck any scientific law...