Nuclear power is an established and reliable way of generating electricity. In normal operating conditions, given a sufficient supply of fuel, 75% of the UK's nuclear capacity can be assumed to be available to help meet peak demand for electricity. These power stations were mostly designed and built in the 1970s and '80s; the next generation is expected to be even more reliable.
Nine of the UK's ten existing nuclear power stations are scheduled to close by 2023. Some of these may continue operating after their scheduled shutdown date, provided the Office for Nuclear Regulation (ONR) is satisfied that this is safe, but eventually they will need to be replaced. The assumed availability of the new nuclear plants proposed to replace them is around 90%.
However, no power station generates electricity 100% of the time. There are periods when nuclear power stations operate at reduced levels or need to be shut down for refuelling and maintenance. Most shutdowns are planned: for example, for refuelling, routine inspections, maintenance, or safety tests. Because they can be foreseen, planned shutdowns can take place when demand is expected to be lower. Some reactors can continue to operate at 20–40% of full capacity while being refuelled – a process which typically takes three or four days, and is repeated about every six weeks.
Around every three years, a reactor must be shut down for statutory inspection and maintenance. Permission to restart is required from the ONR following a thorough review.
As with all types of electricity generating plant, unplanned shutdowns can occur when the station is forced to shut down either by its control system – an automatic shutdown – or by the plant operator taking the decision to manually shut down as a result of a suspected fault in the plant. A precautionary approach is used for the shutdown systems and operating regimes of nuclear power stations. As a result, unplanned shutdowns can be due to relatively small changes in instrument readings and other 'external' issues such as indicated faults in backup systems.
These are the reliability challenges facing nuclear energy. Read about the possible solutions.