via email from TJG...

Deuterium emits a telltale spectral fingerprint in the ultraviolet energy range. Why? An important, but often-overlooked process is radiation-induced isotope enrichment. In studies of electron-beam induced processing of ice surfaces, they have measured large isotope effects leading to enrichment of deuterium in the condensed phase.

I'm not talking about a neutron passing from the UV to the water molecule. It's likely that UV simply creates an environment where the hydrogen adds the extra neutron, or It could be a chemical process, or even electrolysis triggered by the UV. Ask yourself, where did that extra neutron come from ... we do know it's there.