An experiment has been conducted in which a 180-mm thick reactor pressure vessel (RPV) wall has been simulated using eighteen 10-mm slices and irradiated under test reactor conditions to investigate the through wall attenuation of neutron embrittlement. Attenuation of neutron radiation damage through the wall of an RPV is a process that involves a changing neutron flux spectrum. The effect of the changing spectrum has not been fully studied to define the change in fracture toughness properties through the RPV wall. One low copper content base metal and one high copper content Linde 80 weld metal have been irradiated in various positions through the simulated wall to allow quantification of an improved experimentally-based embrittlement attenuation model. Comparisons are made of predicted attenuation changes in toughness properties with measured fracture toughness and Charpy V-notch results for the high copper content weld metal and the low copper content plate. The predictions of through-wall attenuation follow the practice defined in ASTM E 900-02, in which the attenuation of high energy neutron fluence (E>1 MeV) is projected based upon displacements per atom (dpa) change through the wall thickness. The resultant degree of material damage (Charpy V-notch 41 J transition temperature, T41J) using this dpabased fluence change is estimated also using the ASTM E 900-02 embrittlement model. The irradiation-induced shift in T41J (ΔT41J) is typically assumed to infer the shift in fracture toughness transition temperature to be used for structural integrity assessments for the reactor pressure vessel. This assumption will be checked by measuring Master Curve fracture toughness properties for the high copper content weld metal and the low copper content plate.