A 1080 grade aluminium reactor component, irradiated to various neutron fluences, has been examined using X-ray energy dispersive spectrometry (XEDS) on scanning and transmission electron microscopes (SEM and TEM) to determine the levels of radiogenic Si. Thermal neutron irradiation resulted in radiogenic Si levels up to 0.64 wt%. Quantitative SEM analysis of XEDS spectra, yielded a precision of 200 ppm, a lower limit of quantification of 300 ppm and a lower limit of detection of around 200 ppm Si. TEM analysis yielded software processed results in good agreement with SEM data at Si levels > 0.4 wt%, while manual processing of spectra improved the agreement between the techniques at lower Si levels. These techniques enable radiogenic Si levels in 1080 grade aluminium to be determined independently of the original impurity Si. From these silicon levels, thermal neutron fluences can be derived independently of fluences determined from a core flux model and exposure histories, even in the absence of an unirradiated control specimen.