Radionuclides in reactor core components and structural materials

6 Radionuclides in reactor core components and structural materials 

In addition to the nuclear fuel rods with the cladding as discussed above, the reactor core also contains structural materials such as spacers, springs, bolts etc., as well as the fuel assembly upper and lower end structures, in which radionuclides are also produced by neutron activation reactions. Quite in contrast with the fuel itself, no generally valid data on the activity inventories of these structural materials can be established. The main reason for this are the differences in composition of the materials used by different manufacturers, in particular as regards impurities such as cobalt (which is the source element of Co which is frequently the predominant radionuclide in irradiated structural materials). Another important fact is that the standard codes for activity calculations such as Origen, Korigen or Anisn usually are quite accurate for the active zone of the reactor core, whereas in the outer regions (where the fuel assembly end pieces are located) only approximate values can be obtained, due to the steep axial decrease in neutron flux. 

The fuel assembly spacer grids are fabricated either of Inconel or of Zircaloy materials, depending on fuel assembly type and manufacturer. While the central grids are located in the region of maximum neutron flux, the upper and the lower 

BWR fuel assemblies are surrounded by a fuel channel made of Zircaloy-2. The radionuclides generated here and their activity concentrations are on the same order of magnitude as in the fuel rod claddings. 

The upper and lower end pieces of the fuel assemblies made of stainless steel are in the region of steeply decreasing neutron flux and, therefore, show comparatively low radionuclide inventories. According to investigations reported by Fischer (1987), the most important radionuclide induced in the stainless steel material (Stahl 1.4541) is Co, which shows an activity concentration on the order of 500 MBq/g in the upper end piece of a PWR fuel assembly irradiated to a bumup of 32.2 MWd/kg HM (heavy metal) and a corresponding value of about 300 

---