Simulation of the Transient Response of Ionization Chambers to Bias Voltage Perturbations

Abstract

Ionization chambers (ICs) are used in reactor protection instrument channels for monitoring neutron flux levels. These neutron sensors may degrade during the operation of the reactor through a change in their fill-gas characteristics. The comparison of the simulated and measured transient IC response to bias voltage perturbations can lead to the identification of these mechanisms. Once the mechanisms are identified, their impact on instrument channel response can be assessed by parametric studies.

The charge transport model for such an identification and assessment process consists of three coupled nonlinear parabolic differential equations. The initial conditions for these equations are found by solving for the steady-state charge distribution in the IC fill gas prior to bias voltage perturbation. The space-time charge distribution in the IC is determined by a fully explicit-semi-implicit numerical scheme.

The model is implemented to determine the transient response of a N₂- and a xenon-filled IC to a 500- V bias voltage perturbation. In this implementation, good agreement is observed between the predicted and measured responses, with substantial improvement over the previously proposed models. The comparison of the numerical scheme to the interactive continuous system modeling program technique used in the previous studies indicates a twentyfold reduction in the number of time steps required for the simulation of a 5-ms transient. The model is also capable of quantifying the effect of fill-gas impurities on the transient IC response.