You really don't understand this.
EDIT: actually, I withdraw my comment. In fact it was I that didn't understand. I read your comment as "we could use it as a fusion reactor." A fission reactor would in fact work as the power source as well. You are correct.
Though, the big downside of a fission reactor in space is that you need giant radiators to cool it. In many applications it's probably easier just to use more solar panels and fewer radiators. At least solar panels don't generate tons of waste heat. You still need radiators, but 1 watt of solar power requires one watt of radiators. With a reactor, as its a heat engine, 1 watt of electric power requires 4 watts of radiators.