The objective of our research programme is to characterise and quantify the multiscale deformation in a high chrome martensitic power plant steels (P91/P92/MARBN). The work to be undertaken in this proposal aims to examine to what extent in-situ neutron diffraction (ND) observations can identify the inelastic response of materials at elevated temperature under cyclic loading. It is expected to implement a strong link between ND measurements and modelling simulations in order to quantify the evolution of lattice strain and dislocation density during cyclic plastic deformation.