The energy produced by a nuclear power plant is proportional to the irradiation to which the fuel in the reactor core is subjected. This relationship is expressed by the "burn-up rate". One tonne of uranium is able to cover the electricity requirements of a city with a population of 50,000 for three years. In a PWR reactor, uranium releases 33 gigawatt-days (792 million kilowatt-hours) of energy per tonne in standard operating conditions. Electricity companies express power output from their plants in terms of gigawatt-days rather than kilowatt-hours. One gigawatt-day equals 24 million kilowatt-hours, i.e. 1 million kilowatts during 24 hours.
As the fuel supplies energy, its fissile uranium-235 is depleted. Eventually, it must be removed from the reactor. Power generators and engineers seek to delay this requirement in order to increase the power generated and hence the burn-up. This goal can be achieved by more highly enriching the fuel with uranium-235. However, the irradiation resistance of fuel cladding that will be irradiated for extended periods must be enhanced.
The composition of the fuel upon removal from the reactor depends on the burn-up, as illustrated in the example above, which shows the composition of two spent fuels, one enriched to 3.5% and used to generate 36 gigawatt-days/tonne, the other enriched to 4.95% and used to generate 60 gigawatt-days per tonne.
The second fuel generated 88% more energy. This near-doubling of the burn-up results in a proportionate increase in fission products and actinides. The amount of plutonium increased by only 27%, however. This is because an extended stay in the reactor allows time for the plutonium formed in the fuel to be subsequently consumed by fission. The plutonium thus consumed contributes to the power output.
Prolonged irradiation periods (4 years or more) offer benefits from a plant operation perspective, as they enable reactors to consume less uranium-235 and produce less plutonium for a given energy output. Companies such as EDF are increasingly adopting these longer fuel cycles as a means of reducing their plutonium inventories.
Certain limitations prevent burn-up levels from being pushed very far in water reactors. Above 50 gigawatt-day/tonne, alternative alloys to zircaloy-4 must be used, to prevent corrosion of cladding in water, resulting in the release of radioactive gaseous fission products. At these levels, structural strain in fuel assemblies becomes apparent, creating a risk of control rods becoming jammed.
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