When all operational reactors are shutdown, the UK will have a graphite waste legacy of 99,000 tonnes; this waste contains a variety of radionuclides and will fall under the Intermediate Level Waste classification for which the UK does not have a current disposal policy. Characterisation of the end of life radionuclide inventory is required before any policy can be developed. This paper investigates a combined programme of experimental and simulated work has been undertaken to investigate this. The tritium, carbon-14 and cobalt-60 content of a trepanned sample from Wylfa Magnox Reactor 1 have been experimentally determined using beta liquid scintillation counting and gamma spectroscopy. The WIMS9A reactor code and FISPACT-2007 neutron activation software have also been used to calculate this samples' inventory considering only a model which is isolated from the reactor circuit. Comparison between experimental and calculated results has shown that the original impurity levels are sufficient to explain the end of life activity, without additional consideration of contamination from other materials in the reactor circuit, in this type of simulation. Additionally, the calculations show that the production of 14C from 14N is approximately equal to that produced from 13C.