Burn-up (nuclear technology)

With burnup ( English burnup ), also specific burnup , which is in a power reactor produced heat energy per mass of the nuclear fuel , respectively. This physical parameter burn-up is the (local) measure for the energy yield of the fuel. As a general term, burn-up also refers to the accompanying phenomena, such as changes in the nuclide composition of the fuel or the aging and wear of the fuel elements .

definition

The amount of burn-off can be defined in different ways. Usually the quotient of the total heat energy released by the nuclear fuel up to a certain point in time and its mass is considered. The burn is then ${\ displaystyle E}$ ${\ displaystyle m}$ ${\ displaystyle B}$

{\ displaystyle B = {\ frac {E} {m}}}

.

In most cases, this means the mass of the fuel before the start of energy production, i.e. generally that of the fresh fuel. Instead of the SI unit J / kg , MWd / kg is usually used, ie “megawatt days per kilogram”. Usually U for uranium or SM for heavy metal is added to the unit as a clarifying addition; this specifies as a reference the originally existing nuclear fuel-metal, without the oxygen of the oxide, the structural parts (cladding tubes etc.) and the other fissile parts created during operation. So it is not the usual term heavy metal . Since 1000 MW = 1 GW and 1000 kg = 1 t, the unit of measurement can also be written GWd / t SM without a difference. ${\ displaystyle m}$

"Burn-up" without any further details often means the highest achievable burn-up or the burn-up achieved when discharging from the reactor.

General

The burn-up in light water reactors was continuously increased in the past from an initial 20 MWd / kg SM to over 60 MWd / kg SM today. A high burn-up is desirable because it does this

the amount of highly active waste is reduced,
the cost of changing fuel is reduced, and
the risk of proliferation is reduced (with higher burnup, plutonium becomes less and less interesting for military use).

However, the demands on the fuel rod cladding also increase with the burn-up , since they are subject to aging processes during operation. A higher enrichment of the fresh fuel is also required. The resulting higher reactivity excess at the beginning of the fuel cycle must be compensated for with increased use of neutron absorbers .

Mass densities of heavy metal nuclides depending on the burnup for a pressurized water reactor

The development of the fuel with increasing burnup is shown in the figure using a simulation of the frequency of some relevant isotopes in the fuel. It is based on a pressurized water reactor with UO ₂ fuel enriched to 4% (not MOX ). Most of the original fuel ^{nuclide 235} U is consumed (“burned”). Transuranic elements such as plutonium are generated and in the later process contribute to the reactor performance themselves. In addition to the nuclides shown, the fission products also accumulate in the fuel. Together these effects influence the reactivity , which decreases with higher burnup.

The term 'spent fuel' should not be confused with 'burned up'.

Other definitions

In addition to the size specified in MWd / kg SM, the ratios FIMA ( fissions per initial metal atom ) and FIFA ( fissions per initial fissile atom ), usually given in percent, are commonly used: Would be in a fuel 3.3% ²³⁵ U and 96.7% ²³⁸ U as many splits take place as ²³⁵ U atoms were initially present, the burn-up would be 3.3% FIMA or 100% FIFA. A specification in% FIFA is particularly suitable for comparing burn-ups at different original degrees of enrichment.

Typical values

Today, in light water reactors, burn-ups of around 40–55 GWd / t SM are achieved on average. Peak burnups of individual fuel assemblies of up to 105 GWd / t SM have been documented from Swiss plants. For pressurized water reactors, average burn-ups of up to 75 GWd / t SM are aimed for by means of improved fuel elements. In Magnox reactors and in the Canadian Candu reactors , the discharge burn-offs are lower because of the lower initial enrichment, but in the unit “% FIFA”, especially with Candu reactors, they are higher than with conventional reactors

Much higher burn-ups can be achieved in high-temperature reactors and in breeder reactors . Research expects new reactor concepts to even result in a discharge burn-up of up to 500 GWd / t SM. B. from the gas turbine developed by General Atomics in 2007 - Modular Helium Reactor (GT-MHR)

Individual evidence

↑ http://www.world-nuclear.org/nuclear-basics/glossary.aspx

↑ ^a ^b R. Zahoransky (Ed.): Energy technology . 7th edition, Springer 2015, ISBN 978-3-658-07453-1 , page 109

↑ - ( Memento of the original from January 25, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.vpe.ch
↑ ANP, magazine from Framatome ( Memento from November 23, 2008 in the Internet Archive )
↑ http://www.world-nuclear.org/info/inf33.html

[1] ttp://www.world-nuclear.org/nuclear-basics/glossary.aspx

[Zaho-2] R. Zahoransky (Ed.): Energy technology . 7th edition, Springer 2015, ISBN 978-3-658-07453-1 , page 109