The kilonova AT2017gfo that resulted from the merger of two neutron stars has provided new insights into the rapid neutron capture process that is responsible for producing many of the nuclei that are heavier than iron. As with supernovae, progress in understanding kilonova spectra can be achieved either by using simplified models to connect spectral features with particular elements, or by attempting to construct detailed simulations that capture all of the relevant physics. In the forward modelling approach, we require a theoretical simulations of the merger and ejection physics, r-process nucleosynthesis, radioactive energy deposition, and radiative transfer. We plan to calculate synthetic spectra for a three-dimensional merger and r-process nucleosynthesis simulation using the ARTIS Monte Carlo radiative transfer code. Here, we describe current progress in developing the code to handle energy deposition from beta- and alpha-decay reactions and thermalisation of decay particles.