Risk assessment in banking depends on two main aspects, (1) Probability of Default ("PD"), and (2) Loss Given Default ("LGD").
PD assesses how likely a default is, whilst LGD assesses how serious the loss to the bank will be if a default arises. Loans that have plenty of liquid collateral as security may have a high PD, but a low LGD and, therefore, be rated as lower risk to the bank.
If we apply this technique to the nuclear barge, I would have to say it has a medium PD (compared to other energy sources), as there have been recent examples of failure. On the LGD side, one would have to acknowledge the massive costs of nuclear plant failure, both to immediate people and property, but also to wider populations by way of environmental contamination. The LGD profile is only worsened by having the reactor on a barge, as a catastrophic failure could see radioactive fall out being placed directly into water, thus widening the spread radius.
I am not convinced that the PD-LGD balance is supportable in this instance.