Thank you @paulromano 
Referring to the posts you made [1] [2], I would like to know if I understood the calculation. The power of fuel (in watt) could be calculated by:
P (watt) = ( F * Pr * (vf / f) * c ) / ( Q * keff )
with
F = tally fission-q-recoverable (eV/source)
vf = tally nu-fission (neutron/source)
f = tally fission (fission/source)
Pr = known reactor power (J/s)
Q = 3.2E-11 J/fission (for U-235)
keff = neutron multiplication factor (neutron/source)
c = 1.6E-19 watt.s/eV
Is my interpretation correct?
Best regards,
Fadil