Complex sentence: This was the world's first reactor to use heavy water as a neutron moderator, something that had been unavailable when CP-1 was built but which was now becoming available in quantity thanks to the Manhattan Project's P-9 Project.
Split into simpler sentences: This was the world's first reactor to use heavy water as a neutron moderator. It had been unavailable when CP-1 was built, but was now becoming available in quantity thanks to the Manhattan Project's P-9 Project.