A method of providing nuclear fuel pellets for a fuel assembly of a Boiling Water Reactor, BWR, comprising a method of determining 235 U enrichment levels of the fuel pellets wherein the enrichment levels are determined in dependence of the result of simulations performed by a computer-implemented simulation method such that crud deposition does not occur under pre-determined boundary conditions, the computer-implemented simulation method being a computer-implemented simulation method of predicting local concentrations of constituents in coolant water anywhere along fuel rods within any fuel assembly mechanical design of a BWR potentially resulting in crud deposits on said fuel rods, the method is based on a sub-channel approach of predicting local mass fluxes of vapour and liquid in coolant water anywhere along fuel rods within any fuel assembly mechanical design of a BWR for given steady-state or transient boundary conditions, the sub-channel approach is based on the solution of mass, momentum and energy conservation equations for the vapour phase and the liquid phase, the liquid phase is represented by more than one field variable, and is specifically represented by three fields, with the vapour phase as a fourth field, consisting of droplets, a liquid base film, and disturbance waves, wherein the method comprises solving individual mass, momentum and energy conservation equations, for said four fields, and solving the necessary closure relations describing the transfer of mass between the said fields, completed by mass conservation equations for constituents dissolved in the liquid phase and transported by each of the said fields individually, and wherein the method comprises: - simulating steady-state or transient boundary conditions, including inlet coolant water flow into said sub-channels, the coolant water flow may have a predetermined flow velocity variation, - analysing predefined parameters of said disturbance waves and base film, including wave velocity, wave frequency and base film thickness, and - analysing liquid base film thickness between consecutive passing disturbance waves, to calculate local instantaneous impurity concentrations based on said simulated boundary conditions, the calculation is made for each fuel rod of the fuel assembly, wherein, for each fuel rod, the method further comprises comparing said calculated local instantaneous impurity concentration to a crud compound precipitation limit, and during the time said concentration is higher than said precipitation limit, crud is considered to have occurred, wherein the method further comprising simulating local concentration of crud constituents when varying enrichment of one or many fuel pellets of one or many fuel rods in order to reduce crud. - simulating steady-state or transient boundary conditions, including inlet coolant water flow into said sub-channels, the coolant water flow may have a predetermined flow velocity variation, - analysing predefined parameters of said disturbance waves and base film, including wave velocity, wave frequency and base film thickness, and - analysing liquid base film thickness between consecutive passing disturbance waves, to calculate local instantaneous impurity concentrations based on said simulated boundary conditions, the calculation is made for each fuel rod of the fuel assembly, wherein, for each fuel rod, the method further comprises comparing said calculated local instantaneous impurity concentration to a crud compound precipitation limit, and during the time said concentration is higher than said precipitation limit, crud is considered to have occurred, wherein the method further comprising simulating local concentration of crud constituents when varying enrichment of one or many fuel pellets of one or many fuel rods in order to reduce crud. <EOS>