In this work, we study the method of matrix formulation when the neutron density presents noise. An exponential filter to reduce fluctuations in the calculation of reactivity is proposed. Neutron density fluctuations are simulated by assuming that its values are around an average density value with a Gaussian distribution. The results are presented for an exponential form of the neutron density and several standard deviations, with two different sample sizes for filtering that produce delays in the calculation of reactivity and with a first