POINT KINETICS AND COUPLED NEUTRON TRANSPORT-CFD MODELING OF CRITICALITY EXCURSIONS IN FISSILE SOLUTIONS

摘要

The UK version of the CRITEX code is used to analyze criticality transients in Uranyl nitrate fissile solutions in the TRACY experimental facility. The CRITEX model consists of a point kinetics model of neutronic behavior coupled to a 1D thermal hydraulic model of a cylindrical system. The model is found to estimate the magnitude of the first power peak well. It is also found to be capable of representing subsequent oscillations if the bubble rise speed is suitably adjusted. The need to adjust the rise speed prevents the code from having a fully predictive capability, however with appropriate benchmarking the code is a useful tool for parametric studies. Imperial College's FETCH code consists of a coupling of the EVENT transient radiation transport code to the FLUIDITY CFD code. The EVENT code is an even parity Pn code for transient neutron and photon transport. The FLUIDITY code is a CFD code capable of handling multi-phase fluid flows in 2D and 3D. This includes modeling of the nucleation, growth and transport of radiolytic gas bubbles. The FETCH code is used to model axisymmetric transients in fissile solutions in the TRACY experimental facility. The use of a 2D CFD calculation allows inhomogeneous phenomena to be studied. The power is found to be deposited heterogeneously in the fluid, resulting in an inhomogeneous distribution of radiolytic gas bubbles, which is subsequently homogenized by natural convection. The mechanistic modeling in the FETCH code holds the promise of a fully predictive capability for criticality excursion analysis, based on the modeling of bubble nucleation, growth and advection by the liquid. Further development of the modeling is required and this is discussed in the paper. The multi-dimensional analysis provides important insights into the transient behavior.