The present disclosure provides 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 disposed in the sample container to block the open bottom of the sample container and configured for delivering a laser from a laser flash unit of the apparatus to a pebble-bed; the pebble-bed packed on the bottom disc to a predetermined thickness; and a top disc 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.

The present disclosure provides 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 disposed in the sample container to block the open bottom of the sample container and configured for delivering a laser from a laser flash unit of the apparatus to a pebble-bed; the pebble-bed packed on the bottom disc to a predetermined thickness; and a top disc 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.

A method for measuring position of a translational movable element of a nuclear reactor, includes emitting kN optical signals, k and N being natural numbers greater than or equal to 1, by kN so-called emitter optical fibres included in N probes resistant to a primary medium; receiving the kN optical signals by N tracks resistant to a primary medium, each track receiving k optical signals, the tracks having reflecting surfaces and diffusing surfaces; receiving, by mN so-called receiver optical fibres, m being a natural number greater than or equal to k, included in the N probes, the kN optical signals reflected or diffused by the N tracks; converting the kN optical signals received by the mN receiver optical fibres into binary code.