Stochastic processes during transients in nuclear reactors

Steady state noise techniques are widely known and applied to the monitoring of neutron reacting system. This paper deals with the stochastic analysis of neutron chain systems (nuclear reactors or fissile system) which are changing in time from subcritical states reaching other subcritical, critical or hypercritical states due to an external parametric excitation. Two cases are analyzed: 1) without reactivity feedback, that is from a subcritical state to one with almost zero power, and 2) a supercritical excursion with thermalhydraulic feedback. Our goal is to check in case 1 if the usual noise techniques can be used to monitoring the reactivity changes and in case 2 how to calculate the variance of the power and energy released.