Burnup

From Wikipedia, the free encyclopedia

This article is about nuclear fuel. For the Anime, see Burn Up!.

In nuclear power technology, burnup is a measure of the neutron irradiation of the fuel. It is normally quoted in megawatt days per ton (MWd/MTU), where ton refers to a metric ton of uranium metal or its equivalent.

The unit MWd/MTU is the (average) thermal output, multiplied by the time of operation, and divided by the mass of fuel involved. This gives a rough measure of the number of nuclear fission events that have taken place within the fuel.

The actual fuel may be uranium, plutonium, or a mixture of these or of either or both of these with thorium. This fuel content is often referred to as the heavy metal to distinguish it from other metals present in the fuel, such as those used for cladding. The heavy metal is typically present as either metal or oxide, but other compounds such as carbides or other salts are possible.

Generation II reactors were typically designed to achieve about 40,000 MWd/MTU. With newer fuel technology, and particularly the use of burnable poisons, these same reactors are now capable of achieving up to 60,000 MWd/MTU.

In a power station, high fuel burnup is desirable for:

Reducing downtime for refueling
Reducing the number of fresh nuclear fuel elements required and spent nuclear fuel elements generated while producing a given amount of energy
Reducing the potential for diversion of plutonium from spent fuel for use in nuclear weapons

It is also desirable that burnup should be as uniform as possible both within individual fuel elements and from one element to another within a fuel charge. In reactors with online refuelling, fuel elements can be repositioned during operation to help achieve this. In reactors without this facility, fine positioning of control rods to balance reactivity within the core, and repositioning of remaining fuel during shutdowns in which only part of the fuel charge is replaced may be used.

1 Fuel requirements
2 Waste
3 Proliferation
4 External links

[edit] Fuel requirements

In once-through nuclear fuel cycles such as are currently in use in much of the world, used fuel elements are buried whole as high level nuclear waste, and the remaining uranium and plutonium content is lost. Higher burnup allows more of the fissile ²³⁵U and of the plutonium bred from the ²³⁸U to be utilised, reducing the uranium requirements of the fuel cycle.

[edit] Waste

In once-through nuclear fuel cycles, higher burnup reduces the number of elements that need to be buried. However short-term heat emission, one deep geological repository limiting factor, is predominantly from medium-lived fission products, particularly ¹³⁷Cs and ⁹⁰Sr. As there are proportionately more of these in high-burnup fuel, the heat generated by the spent fuel is roughly constant for a given amount of energy generated.

Similarly, in fuel cycles with nuclear reprocessing, the amount of high-level waste for a given amount of energy generated is not closely related to burnup. High-burnup fuel generates a smaller volume of fuel for reprocessing, but with a higher specific activity.

[edit] Proliferation

Burnup is one of the key factors determining the isotopic composition of spent nuclear fuel, the others being its initial composition and the neutron spectrum of the reactor. Very low fuel burnup is essential for the production of weapons-grade plutonium for nuclear weapons, in order to produce plutonium that is predominantly ²³⁹Pu with the smallest possible proportion of ²⁴⁰Pu and ²⁴²Pu.