Coupled neutronics–thermal–thermal expansion analysis of a heat pipe-cooled reactor using the SARAX code

Heat pipe cooled reactors are perceived as promising for providing power supply in remote grids owing to their compactness and safety properties. In this context, thermal expansion plays an important role in inducing sufficient negative reactivity during operation and accident conditions, and thus must be considered in neutronics analysis of reactors of this type. In this paper, a coupled neutronics–thermal–thermal expansion calculation framework is developed within the core calculation module of the SARAX code for heat pipe reactor analysis, with the Megapower reactor as a case study. The coupled framework consists of three main solvers, as follows. The neutronics solver solves the neutron transport equation on arbitrary triangular prism geometry, providing the neutron flux and power distribution. The thermal solver, which includes heat pipe simulation, solves heat conduction in hexagonal geometry and provides temperature distributions for both hexagonal and triangular prism regions. The thermal expansion solver computes axial and radial expansion ratios, repositions mesh points, and updates the geometry accordingly. The axial expansion is assumed to be uniform within each layer, whereas radial expansion is represented by the non-uniform repositioning of mesh points, with their positions determined using the spring method. The three solvers are coupled through a Picard iteration scheme on a unified mesh system. Based on the coupled solution, the reactivity feedback from the temperature distribution is calculated to be −714 pcm, while that from thermal expansion is −640 pcm, yielding a total negative reactivity of −1354 pcm. The temperature distributions of the fuel and matrix were relatively uniform inside the core region, with only a minor impact on the power distribution. The maximum axial expansion reached 2 cm at the top, assuming the bottom layer remains fixed. The maximum radial shift of position in the fuel region was around 5.3 mm at the corners. These results demonstrate the effectiveness of the coupled method in capturing multiphysics feedback effects and thus provide a reliable tool for the neutronics analysis of heat pipe cooled reactors.