This paper describes a retrospective measurement of neutron activation rates within steel standpipe penetrations of the pressure circuit of the Wylfa nuclear power plant. This was carried out in support of neutron dose calculations. Samples were taken from a length of irradiated thermocouple cable and counted for the fast neutron reactions ⁵⁴Fe(n,p) ⁵⁴Mn, and ⁵⁸Ni(n,p) ⁵⁸Co, and the thermal neutron reactions ⁵⁹Co(n,γ) ⁶⁰Co, ⁵⁸Fe(n,γ) ⁵⁹Fe, and ⁵⁰Cr(n,γ) ⁵⁰Cr. By isolating clean sheath material and performing elemental analysis, it was possible to obtain absolute neutron flux data from the measurements over a broad range of locations.The measured data were compared with results obtained from a Monte Carlo calculation performed with the code . Structure observed within the measured and calculated thermal neutron flux distributions was used to confirm the position of the thermocouple samples relative to the reactor geometry. The measurements show that the calculations consistently over-predict fast and thermal neutron reactions. It is concluded that the model predicts spectra without significant bias, and consequently, that the standpipe dose recommendations are similarly over-predicted.