Scheduling of maintenance and refueling shutdowns for the pressurized water reactors at Electricite de France

For the operation manager, the main difference between a classical thermal unit and pressurized water reactor is constituted by the refueling: a PWR must be completely shut down during several weeks (maintenance operations are placed on this period) to be refueled. So, the increasing part of nuclear based generation in the total electricity output by Electricite' de France has raised this question: how to schedule the output dates for about 50 reactors, so that: - global generation is provided at lower costs, - constraints such as staff and equipment available, the supplying of new fuel elements, the transmission network are respected. EDF has then developed two models: - PLANUM, whose aim is to optimize a deterministic shutdown schedule; the method used in Lagrangian relaxation, - HERMES, which uses stochastic relaxation to take into account the forced outages of the reactors. It gives then schedules adapted to this random factor. These two tools provide complementary information to the manager: PLANUM, for short-term decisions in the shutdown dates and HERMES for a longer-term vision of the optimal management of the PWRs