Nuclear reactors are extremely reliable; and many of the world's reactors operate over 90% of the time, and the remainder of the time is mostly devoted to essential maintenance, which can be scheduled well in advance to take place when the demand is low. Unplanned breakdowns are increasingly rare. The unit capability factor, defined as the percentage of maximum energy generation that the plant is capable of supplying to the grid, is continually increasing. The number of unplanned shutdowns has fallen to about one in 7000 per day (Nuclear Issues 22, April 2000).
A disadvantage of nuclear reactors is that they take longer to come up to full power than other power stations, especially those using gas. Nuclear power stations are therefore best to provide the base load, with alternative sources such as gas power stations that can be rapidly activated when there is a sudden need.
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