Also, fission power has advanced a long way since the 1970s in both safety, power availability, and reactor size. The biggest issue with its use has always been more political then safety related. BT has the advantage of having cheap space lift options and far more advanced materials to store waste fuel then we do now, as well as power controls and storage materials. So fission power in any power's hands is viable, but fusion simply gives you more bang for your buck on weight, power, and reaction mass. I
I can easily see any world with access to large amounts of fissile materials moving that way until they invent and perfect fusion power, and maintaining their fission powered units as second line equipment. A TRO of fissile powered vehicles and mechs would be a fin side project.