I haven't worked specifically in nuclear production, but I have ample experience in system development where environmental safety (and human safety for that matter) is paramount. A lot of the development I have done in my career has been for use in this type of environment - whether it be Oil & Gas, Hydro-Electric production and could even be used in nuclear facilities, although I've yet to have that final honour - thankfully, perhaps.
The large majority of these types of systems are developed using SCADA systems and some form of HMI control system - which is what they call the GUI in industrial systems. This is usually an IDE built on a system designed purely for this purpose - CygNet, Wonderware, iFix or FactoryLink or similar.
Whenever you're coding for this type of environment, your first concern is failsafe. I will simplify to demonstrate my point (at the risk of being chastized by the SCADA community), but a system like this is controlled largely by hardware with safety limits hard-wired, firmware controlled and then software controlled.
The hard-wired limits are the outside boundaries of safety. In the event that firmware or software fails and these limits are breached, the system automatically shuts down. For instance on an oil pipeline this might mean closing a valve on a well to prevent an explosion at one end, or may mean venting excess to atmosphere or a burner if necessary.
Firmware limits are usually predetermined safety limits, considered safe for general use to push the system to.
Software is then used by an operator who will tweak the system to get the best possible performance or to meet other business targets - i.e. most power, coolest operating temperature, optimal performance etc.
In the event that anything fails, the underlying system takes over and operates safely. This means that in the event that the application were to fail catastrophically, the firmware built in the hardware controls can still operate the system safely. In the event that the firmware fails the hardware faults safely - i.e. shutting the system down to prevent environmental or human catastrophy.