[en] The core of a large HTGR (1100 MWe) is contained in a thick reinforced concrete pressure vessel. The core is made up of 577 hexagonal graphite columns (about 8,000 blocks) and the circumference of the core consists of 36 irregularly-shaped graphite reflectors (about 450 blocks). All blocks are arrayed horizontally with a 4 millimeter clearance between each block and are stacked vertically in 13 layers. It is expected that seismic behavior of the core can be extremely complicated due to inter-block collisions. The paper describes the vibration test and its simulation analysis of the two-dimensional vertical slice model which was conducted in 1975 as a follow-on research for the one-dimensional vibration test in 1974. A vertical slice (25 block columns and 2 reflector columns) across the center of the core was selected, and the forced vibration test of a ¹/₅ scale model was performed by using a shaking table to obtain such dynamic properties as frequency characteristics and vibration modes of the core, lumping and collision patterns of blocks and the response distribution of reflector reactions. Together with the test, a computer code called COLLAN2-V was developed to simulate the test results. Consequently, it was clarified that the collision behavior of the core could be analyzed by the computer