It would be next to impossible for them to keep it secret.
When North Korea developed its nuclear weaponry program, it asked for assistance from the Soviet Union and China. Both nations refused. Even with regards to peaceful nuclear energy, it needed assistance and training help from other nations.
The nature of North Korea precludes significant technological advancement on the scale of the rest of the world. You're correct in stating that you'd need deuterium, and for that you need processing heavy water plants. And for that, you need training.
They have to train scientists to master the art of nuclear fusion.
They not only have to beat everyone else to nuclear fusion, but they'd need to train people on processes like the Girdler sulfide process and manufacturing of the heavy water plant itself. It's no secret why someone would want a heavy water plant, and it's easy to see from space. That in itself would be a red flag to anyone who was on the lookout.
If they had to keep it hidden, they'd need to train/hire architects, designers, construction crews, and anyone with the skills to do the necessary excavations to hide it from satellites.
If they did all of this, and managed to hide nuclear fusion reactors and heavy water plants, and hired and trained only North Koreans or at least foreigners sworn to secrecy (or forced into slavery) all they have is energy. They have to utilise that energy. It would be an extremely slow process to keep everything hidden and also master the utilisation of that energy.
By the time they got anywhere, everyone else would already have gained the technology to counter any sort of headway that North Korea has made.