Tech Briefs

FSI Analysis in Nuclear Power Plant Assessments

The use of finite element codes and CFD codes for the separate analyses of structural and fluid power plant components is of course well established. However, it can be of great importance to perform also FSI analyses, in which the fully coupled interaction between structural and fluid components is taken into account. A decoupled analysis can lead to conservative but also to significant non-conservative results.

We present here briefly some results obtained in the FSI analysis of a Forsmark nuclear power plant. These results have been furnished to us by Forsmarks Kraftgrupp AB, Sweden.

We consider the vessel shown in the figures. The movie above shows the inside of the finite element model.

An important question to be addressed by the analyst is whether the fluid should be modeled as a full Navier-Stokes fluid, whether a nonlinear inviscid fluid assumption is sufficient, or whether the fluid can be assumed to be an acoustic fluid (inviscid and small deformations). To answer these questions, ADINA can be used directly with these various assumptions for the fluid, and hence the effect of the assumptions on the predicted response can be assessed.

For the analysis case considered here, it has been found that assuming the fluid to be an acoustic fluid is adequate, but the fully coupled fluid structure interactions must be modeled. The figure below shows a comparison of typical results obtained in another analysis with the corresponding experimental data.

Comparison of results using FSI vs. without FSI, against measured results

The FSI analysis of the vessel when subjected to a pipe break was performed using

  • Implicit dynamic analysis, time step size 10-5 seconds, step size chosen to accurately integrate the highest frequency of interest in the structure

  • 15000 time steps

  • Linear analysis. Linear structural elements, linear potential-based fluid elements (½ρv2 effect is neglected)

  • Solution time: 3 hours on Linux x86_64 computer (2.66 GHz processor), one processor used. 1 GB memory required

Model used

Mesh used, with about 65000 nodes, consisting of shells, beams and fluid elements

The movie below gives the dynamic displacement magnitude and predicted dynamic mean stress intensity in the core shroud as a function of time.

The effectiveness of ADINA is evident. Firstly, various complex to simplifying assumptions for the structure and for the fluid can be made, and the effect of these assumptions can be studied. Secondly, each of the required analyses can be performed in an efficient manner.

Core shroud — dynamic stress intensity Pm

For the dynamic analysis pursued here, the final fully coupled FSI model could be solved on a PC within only 3 hours computing time. With such low computing time, the model can be used to efficiently make parametric studies, considering, for example, various loading conditions and boundary conditions.