For this reasons, nuclear power plants pose more stringent requirements in term of the stability (voltage and frequency permissible variations) and availability of external power supply. For instance, nuclear power plants require redundant electrical connections with the existing grid, which makes for higher connection costs compared with a thermal plant of equivalent size. Attempting to work in “island mode”, i.e. a reactor producing its own electricity for cooling, is a difficult challenge in normal times and would be inappropriate and potentially dangerous in an emergency situation.