As these machines are often abandoned once the tunnel is drilled, a nuclear power source would likely not be a good idea, for risk of contamination as
the machine decays over the years after abandonment.
For instance, there is still one of these TBMs buried in a chamber adjacent to the English Channel tunnel, that was abandoned, encased in concrete,
and left to rot, so that the TBM coming from the other direction could finish the job. I'd be glad that wasn't a nuclear powered TBM.



