Complex sentence: Since this substance reacts with heat and is water soluble (and in fact is made up of part water molecules), it was well equipped to expand and harden, due to the intense heat of the radioactive water which was cooling the reactor.
Split into simpler sentences: Since this substance reacts with heat, is water soluble (and in fact is made up of part water molecules). It was well equipped to heat expand and harden, due to the intense heat of the radioactive water which was cooling the reactor.