To improve input accountability and shipper/receiver differences at the front end of the reprocessing cycle, nondestructive assay (NDA) techniques are being investigated to quantify plutonium content in spent nuclear fuel. One of these particular NDA methods is x-ray fluorescence (XRF) which has the potential ability to directly measure plutonium content in spent fuelrods. A series of XRF measurements were performed on North Anna and Three-Mile-Island-1 (TMI-1) fuel rod segments at the Coupled-End-To-End (CETE) demonstration facility at Oak Ridge National Laboratory. Ongoing simulations are being performed to predict detector pulse height spectra for the North Anna and TMI fuels using MCNP5. These simulations have brought to attention the question of how the different XRF sources in spent fuel are contributing to the measured spectra and how MCNP5 models this phenomenon. This study investigates the results of MCNP5 transport calculations of XRF from gamma-ray and beta decay of fission product isotopes. The North Anna spent fuel measurements ranged in burn-up values from 62 GWd/MTU to 67 GWd/MTU and the fuel was 4.2 year old fuel. The TMI spent fuel ranged in burn-up from 27 GWd/MTU to 59 GWd/MTU and was 13.3 year old fuel at the time of measurements. Previous TransLAT simulations using limited declared power histories and the correlated burn-up values were performed to provide the isotopic content of the North Anna and TMI-1 spent nuclear fuel at the different burn-up values. These calculated isotopic contents were then used to calculate the fission product gamma and beta source strengths to be used in MCNP5. By considering two different aged PWR fuels and different final burn-up values, the complete range of the source term was considered. The fission product beta source term was approximated by a 317 multi-group beta spectrum comprised of the individual isotopic beta spectra ranging from 0 MeV to 3.17 MeV. This study involved simulating a cylindrical spent fuel pin with a surrounding HPGe detector. Several simulations were performed using the calculated beta source or gamma source; the source term calculations and simulated results are presented.