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# Burn-up unit of measure

## Burn-up unit of measure

(OP)
In old publications from the 80's and 90's I find the burn-up expressed in "a/o".
For example it reads that a certain fuel rod has reached a burn-up of 6.2 a/o.
What does it mean ? How can I convert this a/o in MWd/kg HM ?

Replies continue below

### RE: Burn-up unit of measure

If it is an old text (or textbook) that is not clear, are you sure you are not reading a "percent" % symbol or "o/o" ?

### RE: Burn-up unit of measure

(OP)
Yes, it is "a/o".
Searching on the internet I found that "a/o" means "total atom per cent". A colleague of mine told me that "a/o" corresponds to what is called "%FIMA", that is, "fissions per initial metal atom": the percentage of atoms of heavy metal having undergone fission. This includes also the fissionable nuclei.
If you google after "total atom percent" you can find a little bit of information.

### RE: Burn-up unit of measure

I know nothing of this subject. However, found this on the net:

https://en.wikipedia.org/wiki/Burnup

From the Measurement of burnup section of the Wiki write up:

Converting between percent and energy/mass requires knowledge of κ, the thermal energy released per fission event. A typical value is 193.7 MeV (3.1×10−11 J) of thermal energy per fission (see Nuclear fission). With this value, the maximum burnup of 100%, which includes fissioning not just fissile content but also the other fissionable nuclides, is equivalent to about 909 GWd/t. Nuclear engineers often use this to roughly approximate 10% burnup as just less than 100 GWd/t.

Hope this helps.

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