Besides occasional unexpected repairs, generators require periodic scheduled maintenance. Nuclear units must be refueled every 12 to 18 months, and fossil fuel units require boiler maintenance outages of 3 to 6 weeks in length about every 12 to 18 months. Most maintenance is scheduled during seasons when the load is relatively low--typically in the spring and fall. But, there is always the risk of unseasonable "freak weather" that could increase demand unexpectedly during a scheduled maintenance outage.