Such structural movements can arise as thermally induced expansion movement (as in EBR-I: Section 2.5.5) and through a thermal model they can be related to temperatures in the core. Otherwise, the structural move­ment may arise from suddenly released thermal restraint or from seismic forces that can cause a movement of fuel, fuel support, and/or control material.

A structural analysis based on equivalent spring-mass systems gives the accelerations of various components for given seismic frequencies. Pessi­mistic vibratory modes give maximum deflections from which reactivity changes can be calculated on a steady-state basis. This can be done for vertical movement or radial movement of the fuel assembly considered as a simple supported beam. Such calculations will give pessimistic values since the fuel pins will be well supported by grids or wire wrap spacers and assemblies are usually close-packed.

Typical seismic movements might be in the range 0.01-0.1 in. resulting in reactivity changes of up to 100 and generally well within acceptable reactivity steps for adequate system protection to be guaranteed. These considerations apply equally to all three systems under discussion.