As much as I know, nuclear safety systems are fully independent from the plant's scada system
and built using the same safety design guidelines as the commercial aircrafts are. This
usually exclude network. These safety systems have no real user interface anyway.
None of the nuclear plant engineers I have met thus far wanted their systems to be connected
to the internet. Even when the network ventures out of the plant to a nearby office, I have
seen the (dedicated) link being encrypted using military equipment.
There is a trend however within the scada communauty: VPN access for remote maintenance. The
most cautious (i.e. most customers) keep the link disconnected. It is connected on request
from an identified source, after some level of management approval (i.e. it is a bit of a pain
to work with, except when this is the customer who calls you first..).