A sample holder assembly for a laser flash apparatus for measuring a thermal conductivity of a pebble-bed, the assembly comprising: - a tubular sample container configured to be mounted on a sample carrier tube for the laser flash apparatus, wherein the sample container has open top and bottom; - a bottom disc configured to receive a pebble-bed of a predetermined thickness, and disposed in the sample container to block the open bottom of the sample container and configured for delivering a laser energy from a laser flash unit of the apparatus to the pebble-bed; and - a top disc configured to be disposed on the pebble-bed and in the sample container to block the open top of the sample container and configured for receiving heat from the pebble-bed to transfer the heat upward; - wherein the assembly further comprises a tubular load ring having an outer diameter smaller than an inner diameter of the sample container so as to press the top disc toward the bottom disc to pack the pebble-bed, - wherein the sample container has an annular inner step extending inwardly from an inner face of the container, wherein a bottom face of the annular inner step of the sample container is configured to be mounted on the sample carrier tube such that the sample holder assembly is mounted on the sample carrier tube, - wherein the assembly further includes a quartz disc disposed on a top face of the annular inner step, wherein the bottom disc, the pebble-bed and the top disc are sequentially stacked on the quartz disc, - wherein the assembly further includes a side ring disposed between outer side faces of the bottom disc, the pebble-bed, the top disc, and the load ring and an inner face of the sample container, wherein the side ring is configured for blocking heat transmission from the packed pebble-bed to the inner face of the container, wherein the side ring is made of quartz, - a tubular sample container configured to be mounted on a sample carrier tube for the laser flash apparatus, wherein the sample container has open top and bottom; - a bottom disc configured to receive a pebble-bed of a predetermined thickness, and disposed in the sample container to block the open bottom of the sample container and configured for delivering a laser energy from a laser flash unit of the apparatus to the pebble-bed; and - a top disc configured to be disposed on the pebble-bed and in the sample container to block the open top of the sample container and configured for receiving heat from the pebble-bed to transfer the heat upward; - wherein the assembly further comprises a tubular load ring having an outer diameter smaller than an inner diameter of the sample container so as to press the top disc toward the bottom disc to pack the pebble-bed, - wherein the sample container has an annular inner step extending inwardly from an inner face of the container, wherein a bottom face of the annular inner step of the sample container is configured to be mounted on the sample carrier tube such that the sample holder assembly is mounted on the sample carrier tube, - wherein the assembly further includes a quartz disc disposed on a top face of the annular inner step, wherein the bottom disc, the pebble-bed and the top disc are sequentially stacked on the quartz disc, - wherein the assembly further includes a side ring disposed between outer side faces of the bottom disc, the pebble-bed, the top disc, and the load ring and an inner face of the sample container, wherein the side ring is configured for blocking heat transmission from the packed pebble-bed to the inner face of the container, wherein the side ring is made of quartz,