In this paper, an optimization of the control rods parameters of a nuclear power plant is presented. Using a multi-physics simulator, the criteria of interest are computed in few minutes of computation and the optimization problem is solved thanks to a parallel asynchronous master-worker (1+lambda)-Evolutionary Algorithm. Two criteria are minimized independently: the first one based on the control diagram is a measure of the energy heterogeneity of the core, and a second one corresponding to the volume of effluents resulting of the use of the soluble boron. The main goal of this paper is then to analyze the solutions found by the optimisation. Moreover, it is known that the burnup of the core has a strong influence on the use of the soluble boron, so different starting points have been considered, showing that the evolution of the performances of the good solutions as regards the two criteria are not the same. Finally, this paper shows that two gathered mono-objective optimization are not satisfactory enough if one considers the burnup evolution, and that a bi-objective optimization must be considered to solve this problem.