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The purpose of this study is to use real options theory to answer the following question: Is it necessary, in France, to invest in new nuclear power units or should some of the existing units be decommissioned? The theoretical model developed establishes two price thresholds which determine investment or decommissioning rules for a regulated risk-neutral firm which does not know the future price of its input. It also provides an empirical reading of past choices in construction of French nuclear power plants. The main finding is that, on a certain number of theoretical and empirical assumptions, it is optimal at present is to leave French nuclear power capacity unchanged. Other more general findings follow from the theoretical model. Thus an increase in uncertainty facilitates investment, defers decommissioning and extends the range of input prices for which there is no change in capacity.