Nuvia Ltd Dynamic Analysis of Irradiated Fuel Containers

Company Profile

Nuvia Limited (formerly NUKEM) has grown as a result of well planned mergers and acquisitions that can be traced to the very beginning of the nuclear industry. Bringing together nuclear experience and expertise from the UK and France, Nuvia has undertaken pioneering work in engineering, energy, nuclear R&D, design, build, operations and decommissioning. Nuvia is one of three businesses that form Freyssinet, a world leader in specialized civil engineering. With over 1700 staff and revenues in excess of 200 million Euros from operations worldwide, Freyssinet has a long history of involvement in the nuclear industry through worldwide use of its advanced engineering and specialised products in new build.


Background

Nuvia had been contracted to design & manufacture a new type of Prototype IF Container. To support the validation of this design, Nuvia commissioned IDAC to study the structural responses of a revised IF Container design subjected to a drop test using finite element analysis (FEA). The analysis had to be setup to simulate a fully loaded IF Container being dropped in an upright position from a height of 6.5 m.

Analysis

The IF Container assembly is shown in Figure 1. The 3D model provided to IDAC in Autodesk Inventor format consists of a Lid, Seal Plug, Quick Release Coupler, Top Section, Mid Section and a Base. ANSYS DesignModeler was used on the Inventor geometry for further defeaturing and preparation of an FE analysis. Earlier test had shown that the deformation and stresses in the region of interest, i.e. around the base, were highly symmetrical about the axis of the container. As a result, the smallest symmetric sector of 30-degree was used as the base geometry in the modeling.

For continuity of the structure, 3 representative weld geometries were also added to connect the Seal Plug to the Lid, the Top Section to the Mid Section, and the Mid Section to the Base. With the model finalized within ANSYS DesignModeler, it was then imported into ANSYS LS-DYNA for the transient dynamic analysis. All the parts/volumes within the container were 'glued' together so adjacent volumes shared common nodes at the boundaries and no bonded contact element was needed across the interfaces. The 3D solids were meshed using 3D explicit solid elements within ANSYS LS-DYNA. Regular, sweepable volumes were meshed using predominantly hexahedral and wedge elements (8-noded SOLID164). Irregular volumes such as the Seal Plug and the Lid were meshed using tetrahedral elements as seen in Figure 2.

For the Drop Test simulation a bilinear elastic-plastic material model along with a de-rated Youngs Modulus was used for the IF Container. In addition, all materials were assumed to have a stiffness (Beta) damping coefficient of 0.2. The advantage of using this type of damping is that it cancels out oscillatory motion at high frequencies i.e. ringing.

At the start of the simulation the bottom surfaces of the fully loaded IF Container were placed 1 mm above the rigid floor element. A drop velocity of 11292 mm/s was specified, which was calculated using a drop distance of 6499 mm with a zero initial velocity. With the existing mesh and standard gravity, an appropriate time step of 90 nano-seconds was used to resolve the shock waves propagating through the structure. The event was simulated up to a total time of 0.025 seconds starting at this drop position.

In determining the validity of the design, Equivalent (von Mises) stress was calculated as this provided an indication of how close the material within the structure was to the Yield Limit.

The IF Container design was revised with an increased wall thickness at the container base. The resultant contour plots in Figure 4 show the deflection during the transient run. While the walls do undergo deflection under the drop test, the analysis does show that the new design still fairs better than its predecessor. Following the accidental drop event from a height of 6.5 m, the nominal plastic strain in the lower wall section is less than 8%. The peak plastic strain in the base where it was impacted by the payload was 14%, which was still well within the assumed breaking strain of 26% at UTS. The thickened wall was only deforming with a maximum value of 1.24 mm outwards in the most critical location during impact. The equivalent stress at the end of the simulation was also found to be within the operable design limits of the assembly as seen in Figure 3.


Design Benefit

The aim of the analysis was to determine whether an accidental drop of the assembly would cause a loss of containment of the payload from the system. As the analysis shows, the design is safe such that there would be no leakage in a real world situation similar to the boundary conditions. In conducting a Drop Test analysis using FEM, Nuvia were able to validate their new design which should be going into production by the end of the year. Using ANSYS allowed Nuvia to create an iterative design that could be tested and yield results within a week itself.