This is essentially an ion engine, but with negative power output, zero thrust, modified exhaust, and using LH2 as fuel. I assumed that LH2(since deuterium is not a fuel in game) is entirely comprised of deuterium.
The reactor provides ~19 megawatts per second, and can continue to provide this for 248 years. Use this when you have photon drives or need to fuel nuclear-electric propulsion.

Does not need any mods, can be used on mobile.

Calculations(if you really want to see them) are below

3-4 mEV for D-D fusion

mEV to joules = 1.602 *10^-13 joules
2 deuterium nuclei needed

3*10^26 deuterium nuclei in 1kg of deuterium

So 1.5*10^26 fusion events(ideally) can take place
* 3(low estimate for energy)

* 1.602*10^-13(mev to joules)

7.209e13 joules in 1 kg of deuterium
72,090,000,000,000
*0.000226524557

16,330

So at 0.000226524557 kg/second

16,330,155,314 joules / second

ideally for a power in the range of megawatts/second(Im pretty sure that float rounding of integers made the numbers slightly off)

2.2652455E-7 fuel(kg/second)
0.0000002652455*72,090,000,000,000
19,121,548.095 watts
19.121 megawatts