A big difference between radionuclides and helium is that radionuclides are only needed in a relatively "small" quantity (producable by fission). Nuclear fusion

A big difference between radionuclides and helium is that radionuclides are only needed in a relatively "small" quantity (producable by fission). Nuclear fusion would create helium but in very small quantities. Having 1TWe of fusion reactors would only yield 5grams of helium/second. That is not much.