While many coal fired plants are able to fire gas or oil as well, the modern gas turbines in gas-fired combined cycle plants do not allow duel fuel capability (both gas and oil). However, they may be adapted (requiring additional system modifications like blade coating changes) if the frequency of occurrence and the duration of problems is large enough to justify such investments. Generic plant design problems may make a power system vulnerable when the type of generation plants is not very diversified, for instance in France two types of nuclear units form the majority of the production assets.