The next step would likely be deuterium, deuterium, avoiding the extra neutron.

However, the Carbon 13 isotope is heavier because it has an extra neutron in its nucleus.

A hydrogen atom with an extra neutron (atomic mass of 2) is called deuterium(H).

It has two extra neutrons, since the deuterium is Hydrogen plus a neutron.

But occasionally, they have an extra neutron, giving them an atomic weight of 13.

A tiny fraction is oxygen-18, which is a slightly heavier atom because it contains two extra neutrons.

This turns the uranium-235 into uranium 236 as the extra neutron is added to the 235 protons and neutrons already in the nucleus.

Thus, we can imagine why an extra neutron can cause nuclear stability and make a nucleus radioactive.

The mass of heavy water is about 10% more than that of regular water due to the extra neutron in deuterium.

The extra neutron needed for achieving criticality would be provided by an external source - a particle Accelerator.