Spent Fuel – Basic Notes

These are just a few basic notes about spent fuel waste from nuclear power stations. It assumes a basic understanding of the nuclear fission process (see Basic Nuclear Physics workshop), however, I have tried to provide links to the relevant sections within this blog. This post is designed as a basic introduction for the discussion on the Implications of High Burnup Fuel.

What is Spent Fuel Waste

The  fuel for a nuclear power station is Uranium. This undergoes a processes called fission within the reactor to produce heat. After a certain period the useful isotope of Uranium (U-235) gets used up and the build  up of some of the products of the fission process poison the nuclear reaction. Only about 4% of the theoretically possible  energy is actually used before the fuel has to be replaced.

The Amount of Waste

The mass of waste produces is about the same mass of fuel that you put into the reactor. Note quite since some of the mass is converted to energy (Einsteins famous E=mc2 equation) or lost as radiation outside the reactor.

This is about 33T of Uranium (37T of UO2) for Sizewell B (see Spent Fuel at Sizewell).

Burnup

The mass of waste depends on how much energy you can get out of a certain mass of fuel. This is called the ‘burnup’ and is measured in Gigawatt Days per Tonne of Uranium (GWd/TU). One Gigawatt Day is the amount  of energy you get from producing one Gigawatt of power for one day – similarly to the way Kilowatt Hours are used to measure energy (see Whats a Watt).  The energy output used here is the total thermal output (GWth) rather than the electrical output (GWe) which is only about a third of the total thermal output (see Using Energy and Thermal Efficiency).

Over the years the burnup for most reactors has been steadily increasing – for a PWR it has increased from about 25GWd/TU to about 45GWd/TU. The EPR reactors proposed for Sizewell and Hinkley are expected to run at even higher burnup rates – 60GWd/TU.

Radioactivity of Spent Fuel

Uranium is a alpha emitter and fresh fuel rods can be handled reasonably easily without much special protection. However the fission process creates a large number of fission products that are highly radioactive. Spent fuel is extremely radioactive and standing next to a used fuel rod will kill in minutes.

Decay Heat

The radioactive decay of the fission produces also produces heat. If the heat is not removed by adequate cooling then the temperature of the fuel will increase to dangerous levels. The fuel therefore has to place in cooling ponds after being taken out of the reactor. The amount of heat produced decreases with time as the radioactive fission products decay.

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