Deuterium: A Hydrogen Engine Expansion

I seek to feed two birds with one scone here:

• Fix the Hydrogen Engine's perceived violation of thermodynamics by re-classifying it as a fusion reactor, and
• Make it a viable end-game method of power generation, without making fission reactors obsolete

By default, Hydrogen Engines simply consume regular Hydrogen; this process, whilst inefficient, does produce net power -- far less than either a battery or even a small fission reactor can provide, despite taking up 3x the space (on a large grid, at least).

What I propose is the ability to extract Deuterium from Ice, either using an extremely energy-intensive process such as electrolysis (an upscaled O2/H2 generator would be nice, as an aside), or by having our current fission reactors produce small amounts of Deuterium over time if given an optional ice supply (possibly also receiving a small power output boost, as they have active cooling).

The latter option here would require fission reactors as a production step and not take away any of their current utility, and the former, depending on power consumption, would require prohibitively large infrastructure to generate power by other means.

Providing Deuterium fuel to a Hydrogen Engine should boost its output considerably, making it a step between a small and a large reactor.

This may raise the issue of the Hydrogen Engine being cheap to build, in which case an advanced version could be made that requires more exotic components, such as superconductors.