Preliminary Study on Three-Dimensional Characteristics of Modular High Temperature Gas-Cooled Reactor

Abstract

The two-dimensional (2D) system analysis codes are usually used to design and analyze the modular pebble-bed High Temperature Gas-cooled Reactors (HTR). However, in some cases, three-dimensional (3D) characteristics, for example the 3D power distribution and temperature distribution due to a part of control rods movement, also need to be concerned. In this paper, based on the 200 MWe Pebble-bed Modular High Temperature gas-cooled Reactor (HTR-PM) design, a 3D system analysis code was used to analyze some three-dimensional characteristics in steady state, as well as the transient process caused by the asymmetric movement of control rods in the side reflectors. Under normal operations of HTR-PM, the control rods are inserted at certain heights and the primary coolant enters the Reactor Pressure Vessel (RPV) from the outer pipe of the coaxial hot gas duct. As a result, the fuel temperature has a relative circumferential deviation of about 1.64% on the outside of the pebble bed, while a very small deviation at the center of the bed. The temperature distribution of RPV is significantly affected by the position of the helium inlet as well. The analysis results showed that, during the transient process caused by the withdrawal of two control rods, the relative circumferential deviation of the fuel particle temperature on the outside of the pebble bed would increase to around 8.47%. However, because of heat conduction in relatively long distance, the movement of the control rods has very little effect on the circumferential temperature deviation of the fuel particles at the center of the bed. These 3D characteristics of HTR under different conditions demonstrate the good applicability of 2D and 3D system codes and will provide support for more accurate safety analyses in the future.