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 gasfuelled or a hydroelectrical 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 of nuclear electricity production as well as belgium and the slovak republic both more than of nuclear electricity production