I do not think that is really quite the case. Radio-manganese is not a fission product. Where it has been found to be released it has come not from the fission but from some equipment associated with the device. The physical half-life of radio-manganese is only 291 days compared to 28 years for strontium 90. Its life is very small, and therefore I would have said, in considering this Question, that there are almost minimal risks.