How did you solve it?
Just find the RulesMD.ini in Ra2 directory. Open and look on the soviet buildings list. If you find Nuclear reactor in the list, add: AIBuildThis=yes
If you like to protect it with wall, you can also add: ProtectWithWall=yes. Then CPU AI will build nuclear reactor with walls around.
I did like this and worked. But didn't work at online i think.