or the spider assembly may hereafter be referred to as a mounting assembly the neutron source rod assembly 50 principally comprises a plurality of rods 52 at least one of which is a neutron source rodlet assembly 10 and a support 54 mounting assembly the support 54 has a spider shape which is generally similar to that used in the prior art for moveable neutron absorber rod assemblies with the exceptions noted below the support 54 which can therefore be referred to as a u201c spider assembly u201d principally comprises an upper hub 56 whose longitudinal center axis c is intended to be oriented vertically when the neutron source rod assembly 50 is arranged on a nuclear fuel assembly in a nuclear reactor core fins or flukes 58 which extend radially outward from the hub 56 and which are distributed angularly in a substantially regular manner about the axis and coupling systems 60 for mounting the rods 52 on the support 54 at least one of the coupling systems 60 mates with a mounting coupling 70 on the upper end plug 48 of the neutron source positioning rodlet assembly 12 one such coupling 70 intended to mate with a hold down assembly is shown in fig6 the support 54 is produced from a metal such as stainless steel which can withstand a high radiation exposure the upper hub 56 has a hollow cylindrical shape with a circular base it comprises a lower portion 62 from which.