# Why isn't heavy water much heavier, and what use is it for fission reactions?

With confusion I experienced recently, I'd like to clarify some stuff for myself about nuclear energy, that I find are usually not properly explained.

What exactly is heavy water, just water where both hydrogen atoms have a proton and a neutron? Forgive my being naive but why does heavy water only have a slightly higher density of mass? I would expect it weighing around 2000 kilograms for a cubic metre, the double of normal water

And what do you do with it? It can slow down neutrons, but is this done to transform U-238 into plutonium or to create a fission reaction in low enriched uranium?

Please note that water is composed by two hydrogens and one oxygen. The stable isotope of oxygen has atomic number $8$, but atomic mass $16$, i.e. it is composed out of eight protons and eight neutrons. Thus in normal water the ratio of hydrogen contribution to the over all in mass is $2/18$.

When deuterium takes the place of hydrogen we have heavy water, and that means that two more neutrons are added to the original mass balance, an effect again of $2/18$ extra mass. So the water is more dense but by this small factor.