This report "Implementation of photonuclear attenuation in EGSnrc'', supplements the upcoming publication: Ali et al, ``Towards a more-realistic estimate of the uncertainty of photon cross sections at radiation therapy energies''. Report CLRP-12-01 presents our implementation of photonuclear attenuation in EGSnrc (without modelling secondary particles). We hope that the details in this report aid colleagues in carrying this effort forward.