  The plant efficiency of a nuclear fusion power plant is considered. During
nominal operation, the plant efficiency is determined by the thermodynamic
efficiency and the recirculated power fraction. However, on average the reactor
operates below the nominal power, even when the long shutdown periods for large
maintenance are left outside the averaging. Hence, next to the recirculated
power fraction, the capacity factor must be factored in. An expression for the
plant efficiency which incorporates both factors is given. It is shown that the
combination of high recirculated power fraction and a low capacity factor
results in poor plant efficiency. This is due to the fact that in a fusion
reactor the recirculated power remains high if it runs at reduced output power.
It is argued that, at least for a first generation of power plants, this
combination is likely to occur. Worked out example calculations are given for
the models of the Power Plant Conceptual Study. Finally, the impact on the
competitiveness of fusion on the energy market is discussed. This analysis
stresses the importance of the development of plant designs with low
recirculated power fraction.
