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?
 A: 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.
In this article the advantages of heavy water in reactors are given.

Using water as a moderator will absorb enough neutrons that there will be too few left over to react with the small amount of 235U in the fuel, again precluding criticality in natural uranium.

heavy water seems to solve the problem:

An alternative solution to the problem is to use a moderator that does not absorb neutrons as readily as water. In this case potentially all of the neutrons being released can be moderated and used in reactions with the 235U, in which case there is enough 235U in natural uranium to sustain criticality. One such moderator is heavy water, or deuterium-oxide.

The reason is that deuterium has already bound a neutron to the hydrogen atom, and thus fewer neutrons needed for criticality are absorbed.
