CriSAA: A Simulation Code for Evaluation of Criticality Transients Involving Low Source Start-Up

A code called CriSAA used for criticality excursion evaluation and analysis is developed by coupling a two-dimensional thermal-hydraulic code and a zero-dimensional neutron kinetics code. The generalized semi-Markov process simulation method is improved in the neutron kinetics code, which makes it suitable for the transient cases of both stochastic and intense neutron fields. Pulse withdrawal experiments and ramp feed experiments performed on the Transient Experiment Critical Facility (TRACY) are simulated with CriSAA. The transient characteristics of isolated and open criticality excursions on TRACY are studied. The simulation fits well with the experiments. Stochastic phenomena of the criticality excursions under a weak source case are studied. The statistics characteristics, including time and energy to first peak, power of first peak, and total energy, are shown. This work provides an advanced approach for criticality excursion evaluation and analysis.