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Description
One of the main challenges in the design of neutron scattering instruments is the design of their shielding structures. These must provide adequate radiological protection for the working area surrounding the instrument, while also being mechanically feasible to fabricate and having their structural capacity verified according to nuclear industry standards. This work presents the seismic verification criteria and methodology developed for the shielding and support structures of the ANDES instrument (Advanced Neutron Diffractometer for Engineering and Science), and illustrates its application through a representative case study.
The methodology is based on the equivalent static method, in which the seismic demand is represented by an equivalent static force derived from the peak acceleration of the floor response spectra (or the zero-period acceleration, ZPA, when the component's fundamental frequency exceeds 40 Hz), affected by a conservative static coefficient. Two seismic levels are considered, an Operating Basis Earthquake (SL-1) and a Safe Shutdown Earthquake (SL-2), each associated with a different service level and corresponding stress-intensity acceptance limits, following ASME Section III NF (Design by Analysis) criteria. In addition to stress verification, rigid-body motion checks (sliding and rocking/overturning) are performed following simplified ASCE 4 procedures, comparing the seismic shear and overturning moment at the support interface against the frictional and restoring resistance, respectively.
A key modeling challenge addressed in this work concerns the mechanical interaction between shielding blocks. Since the individual shield boxes rest on one another without rigid connection, two limiting modeling assumptions are possible: treating the interfaces as fully bonded, which overestimates stiffness and does not reflect the real load path between parts, or treating them as fully separated (unbonded), which increases flexibility and can amplify seismic demand disproportionately. The friction coefficient and the interlocking interfaces between surfaces (such as pins or coupling plates) play a central role in this assessment, since they govern whether adjacent blocks slide or move relative to one another under seismic acceleration. The work discusses how these considerations are combined in practice, using hand calculations to bound the problem and finite element models to refine the stress verification where geometry and boundary conditions are more complex.
Results are presented for the Polychromatic Beam Shielding (SS 0204) of ANDES as an example case. The results obtained show that the developed methodology allows the seismic behavior of the shielding to be characterized with an adequate level of confidence, combining analytical and numerical tools to identify the governing failure modes. These results illustrate how the combination of simplified analytical checks and finite element modeling supports a practical, conservative seismic qualification process for shielding components.
Key Words
Seismic Analysis, Design by Analysis, Finite Element, Rocking, Sliding