On the transient models of the VITAS code: applications to the C5G7-TD pin-resolved benchmark problem

• Published in: Nuclear science and techniques 2023-02, Vol.34 (2), p.28-57, Article 20
• Main Authors: Xiao, Wei, Yin, Han, Liu, Xiao-Jing, He, Hui, Zhang, Teng-Fei
• Format: Article
• Language: English
• Subjects: Beam Physics; Nuclear Energy; Particle Acceleration and Detection; Particle and Nuclear Physics; Physics; Physics and Astronomy
• ISSN: 1001-8042; 2210-3147
• DOI: 10.1007/s41365-023-01170-x

This article describes the transient models of the neutronics code VITAS that are used for solving time-dependent, pin-resolved neutron transport equations. VITAS uses the stiffness confinement method (SCM) for temporal discretization to transform the transient equation into the corresponding transient eigenvalue problem (TEVP). To solve the pin-resolved TEVP, VITAS uses a heterogeneous variational nodal method (VNM). The spatial flux is approximated at each Cartesian node using finite elements in the x - y plane and orthogonal polynomials along the z -axis. Angular discretization utilizes the even-parity integral approach at the nodes and spherical harmonic expansions at the interfaces. To further lower the computational cost, a predictor–corrector quasi-static SCM (PCQ-SCM) was developed. Within the VNM framework, computational models for the adjoint neutron flux and kinetic parameters are presented. The direct-SCM and PCQ-SCM were implemented in VITAS and verified using the two-dimensional (2D) and three-dimensional (3D) exercises on the OECD/NEA C5G7-TD benchmark. In the 2D and 3D problems, the discrepancy between the direct-SCM solver’s results and those reported by MPACT and PANDAS-MOC was under 0.97% and 1.57%, respectively. In addition, numerical studies comparing the PCQ-SCM solver to the direct-SCM solver demonstrated that the PCQ-SCM enabled substantially larger time steps, thereby reducing the computational cost 100-fold, without compromising numerical accuracy.