Abstract A formulation for the calculation of reactivity using a recursive process is presented in this paper, as well as the treatment to reduce noise intensity that is found in the nuclear power signal. Using the history of nuclear power considered as the memory of such power and the filter exponentially adjusted with the least squares method, it is possible to reduce the nuclear power fluctuations without causing attenuation for the calculation of reactivity and with a smaller delay than that for low-pass filter of first order delay filter.