One should be able to take a mass of fuel, either initial moles or grams of U235+U238 in UO2, and determine the energy on the basis of atomic fraction and mass. One can assume 95% TD of UO2 or about 10410 kg/m3 (10.41 gm/cm3), then remember to convert mass of UO2 to mass of U.
I've seen different conversion ratios with a range of something like 9.38 to 9.75 GWd/tU per 1% fima, and some use a mid-range value of 9.5 GWd/tU/fima. The factor depends on the neutron energy spectrum and the proportions of fast fissions in U-238 and in U-235/Pu-239/u-241. The Pu isotopes come from the conversion of U-238 to Pu-239/-240/-241 via neutron capture and subsequent beta decays. The energy release from Pu isotopes is closer to 205 MeV per atom, while that of U-235 is slightly less than 200 MeV. I believe Studsvik-Scandpower has published some papers on the subject.
As one example Nuclear Hydrogen Production Handbook, in section 10.2.1.3 (page 223 in the textbook), the authors describe a test in ATR in which fuel achieved burnups of 11.5–19.6% FIMA (108–184 GWd/tU), which gives a conversion ratio of ~9.39 GWd/tU = 1% FIMA), but that's in ATR with the particular type of fuel.
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