Since nuclear energy has the lowest marginal (and the highest capital) costs among large conventional generating technologies, nuclear power plants should be operated at nominal power as much as possible in order to maximise the energy produced and recuperate the large investment costs. For an electrical utility with a diversified portfolio of generating plants, it is thus economically more attractive to require some flexibility from a coal, a gas-fuelled or a hydro-electrical power plant than from a nuclear unit. However, in some countries the share of nuclear energy in the electricity mix has become so important that the utilities have had to implement or improve the manoeuvring capabilities of the nuclear plants. This is the case of France (78% of nuclear electricity production) as well as Belgium and the Slovak Republic (both more than 50% of nuclear electricity production).