reactor the arrows 32 show the general flow path that the coolant takes through the reactor the reactor coolant flows downwardly through the annulus 30 to the lower flow distributor 26 which distributes the coolant to the core 22 the coolant traverses the length of the fuel elements from which heat is transferred and the coolant is discharged via outlet nozzles 21 to the reactor coolant loops in order to monitor changes in reactor material properties as described herein before surveillance specimen assemblies 40 only one of which is shown are installed in the annulus 30 between the thermal shield 31 and the inside wall of the shell 12 at circumferentially spaced intervals as is best shown in fig2 and 3 each specimen assembly includes a holder tube 41 which is an assembly of tubular segments and support brackets integrally attached by welding or other means the holder tube 41 includes lower 33 intermediate 34 and upper 35 tubular segments having circular cross sections and a lower bracket 42 an intermediate bracket 43 and an upper bracket 44 the lower bracket 42 is integrally attached to the lower end of the holder tube 41 while the intermediate bracket 43 is integrally fixed between tubular segments 33 and 34 and the upper bracket 44 is integrally fixed between tubular segments 34 and 35 a plurality of perforations 51 are bored through.