in which the supporting portions and the oscillation prevention means are fixed there is a fear that the stress may undesirably exceed the limit value unless a sufficiently large distance is preserved between the thermally expansible rails and the supporting portions and the oscillation prevention means this problem however is overcome by the arrangement of the embodiment shown in fig5 the problem of lack of stability and precision can be overcome by decreasing the distance this embodiment therefore can effectively be used in the case where the circumstances do not allow preservation of a sufficiently large distance it is possible to use as the oscillation prevention means a thermal expansion absorbing construction using a spring which acts rigidly against dynamic load such as that caused by an earthquake but absorbs static expansion such as thermal expansion this arrangement facilitates the design of mounting position or the like when a multiplicity of circumferential rails are used in stages fig6 is a perspective view of another embodiment of the invention in which the same reference numerals are used to denote the same parts or members as those in fig3 in this embodiment a plurality of reinforcement supports 89 are suspended from the circumferential rail 82 and are provided at their lower ends with oscillation prevention means 86 by using such reinforcement supports it is possible to construct the rail apparatus using only one circumferential rail in the embodiments described hereinbefore each vertical rail is integral with the supporting member which is fixed at its one end to the gamma ray shielding wall this however is not exclusive and the supporting member fixed at its one end to the gamma ray shielding wall may.