The French Evolutionary Pressurised Water Reactor (EPR), the largest nuclear reactor in the world is designed to produce 1,600 megawatts of electrical power output (1,600 MWe). However the conversion of nuclear energy to electrical energy is not very efficient.
A basic problem with all kinds of large nuclear and fossil-fuelled power station which generate electricity from heat via the carnot cycle, is that only about one third of the power station's total heat energy can be converted into electrical energy.
The EPR reactor must generate 4,300 megawatts of thermal power (4,300 MWt) to produce 1,600 MWe of electrical power. The remaining 63% of nuclear heat energy is lost to the reactor cooling water system, which discharges the waste heat energy into the sea which acts as a heat sink. Alternatively waste heat can be dispersed into the atmosphere via reactor cooling towers.
Reactor cooling and disposal of heat energy has an important technical influence on deciding the best location for siting a nuclear power station. Energy companies normally build several nearly identical nuclear reactor units at the same site, doubling or trebling the total electricity output of the nuclear power station.
How much nuclear power do we have in the UK?
Nuclear power stations supply 11 gigawatts (11GWe) of nuclear generating capacity, 15% of the 73 GWe total maximum power station capacity in Britain today. However in practice nuclear power stations produce rather less than this theoretical maximum. Nuclear power generated only 13% (48 terawatt-hours, TWh) of the 369 TWh of electricity consumed in the UK during 2008. The UK has a total installed generating capacity of 73 GWe, mainly provided by conventional coal and gas-fired power stations and nuclear power stations.
Britain does not use all of its 73 GWe of maximum power station capacity, as peak winter demand is only around 59 GWe. Depending on the time of year, the electricity network today contains about 20 - 24% of excess generating capacity needed as a back-up safety margin to meet temporary demand surges plus a smaller amount of stand-by spinning reserve capacity in case one or more large power stations shuts-down unexpectedly.
Under some circumstances this could theoretically trigger a power blackout from a sudden collapse in the UK's power grid. Such a mass blackout event occurred in north eastern United States on August 14th, 2003 affecting 55 million people in the US and Canada. All but one of Britain's 10 operating nuclear power stations are scheduled to close by 2023 and two of these are planned to close at the end of 2010.
Britain's existing 11 GWe nuclear fleet is planned to be replaced with modern PWR reactors, as existing AGR and Magnox stations reach the end of their 30-year operating lives. However rising electricity demand during this period means that the overall percentage of nuclear energy in the UK electricity mix might actually fall unless extra nuclear power stations are built beyond a simple replacement programme.