Modelling of fast source transients in PDS-XADS with VARIANT-KIN3D

In a system with an external neutron source such as an Accelerator Driven System (ADS), transients can be induced by a fast variation of the source amplitude. In this paper we present new capabilities of the VARIANT-KIN3D code for analyses of these transients. The code solves the time dependent neutron transport equation by using the spherical harmonics (PN) and simplified spherical harmonics (SPN) methods. Recently a more accurate time discretization scheme and an automatic time step control option have been implemented in KIN3D. In the paper we consider a fast transient in a 3D HEX-Z model representing the PDS-XADS (an ADS designed by ANSALDO). We consider responses of several detectors, placed at different axial and radial positions. The same transient is simulated with different options for angular discretization of the neutron transport equation. As expected, we observed that angular discretization influences the transient results. The influence is observed with respect to both time and spatial distributions of the detector rates. The new code version is able to deal with fast source induced transients and therefore may serve as an interesting tool for investigating the ADS physics.