AbstractThe impact of geometrical modelling and energy treatment on neutronics and activation results for materials with giant resonances, such as tungsten, is significant. Three-dimensional (3-D) neutronics calculations have been performed using the Monte Carlo codes MCNP and TRIPOLI to determine the self-shielding effect in tungsten layers in fusion environment. While excellent agreement exists between the two Monte Carlo results, the 187W production rate is overestimated by about a factor of 4 when a homogenised model is used with multi-group sampling. To correctly predict reaction rates in tungsten, 3-D continuous energy Monte Carlo calculations with layered heterogeneous modelling should be used. An effective reaction rate derived from the Monte Carlo pointwise results, which accounts accurately for resonance absorption effects, which should be used in the subsequent activation calculation.