Building an online community for nuclear professionals should be easy for the International Atomic Energy Agency (IAEA).
The IAEA know exactly who to reach (nuclear professionals), they know exactly how many they want to reach (about 400). They know how to reach them (existing networks and contacts) and they have credibility to talk to them.
If you have all five of these, you should launch your community right away. If you don't, then you should get all five first before launching your community.