Like any mechanical system with moving parts, nuclear power stations can stop working if something fails. Otherwise they can run for years at a time. Most are stopped every 18 months or so for refuelling, usually combined with inspection and maintenance, but these outages can be scheduled for times when total demand can be met by other plants which are still running.
Amongst other low-carbon energy sources, hydro and geothermal can also produce energy more or less indefinitely. Hydro accounts for all or part of most of the lowest carbon electricity systems in the world. However hydro generation plants may be dependent on seasonal rainfall patterns, and are increasingly at risk from the effects of climate change which can disrupt these patterns. Hydro stations fuelled by glacial meltwater are also at risk.
Besides hydro, another major source of reliable energy is biomass. It is used particularly in Britain and some other European countries due to its classification by the EU as renewable. It should in theory be the burning of fast-growing crops such as miscanthus or the tops and trimmings of trees harvested for timber, but increasing demand from the UK and EU has led to large quantities of whole trees being felled for biomass. Apart from the effects on local biodiversity and ecosystems, the decades-long period between emissions of CO2 by the burning of biomass and its theoretical eventual re-sequestration by forest re-growth produces a spike in atmospheric CO2 at a critical time for global warming when we need to be reducing emissions as far as possible. Although applying Carbon Capture and Storage to biomass power stations (as is being trialled at Drax in Yorkshire) can largely eliminate immediate emissions from burning biomass, and eventually result in negative emissions, there are still immediate emissions from the fossil fuels burned to fell and transport trees, dry them and convert them to pellets, and transport the pellets across the Atlantic to European power stations.
The most commonly used other renewables - wind and solar - produce power only intermittently, in proportion to the speed of the wind and the intensity of sunshine. Thus although the energy they produce when they are generating, like hydro and geothermal, has zero marginal cost (i.e. it doesn’t cost any more to produce energy than not to produce it), it is difficult and expensive to incorporate into a system such as a national grid which needs to produce energy when it is demanded.
Another source of renewable energy is tidal. Its output is intermittent but predictable; it varies between peak output and nothing twice each day, and its peak power varies over the 14-day cycle of spring and neap tides. This also causes difficulty and expense incorporating it into a system demanding reliability.