Modelling and Simulation of Reactor Pressure Vessel Failure during Severe Accidents
Abstract: This thesis aims at the development of new coupling approaches and new models for the thermo-fluid-structure coupling problem of reactor pressure vessel (RPV) failure during severe accidents and related physical phenomena. The thesis work consists of five parts: (i) development of a three-stage creep model for RPV steel 16MND5, (ii) development of a thermo-fluid-structure coupling approach for RPV failure analysis, (iii) performance comparison of the new approach that uses volume loads mapping (VLM) for data transfer with the previous approach that uses surface loads mapping (SLM), (iv) development of a lumped-parameter code for quick estimate of transient melt pool heat transfer, and (v) development of a hybrid coupling approach for efficient analysis of RPV failure.A creep model called ‘modified theta projection model’ was developed for the 16MND5 steel so that it covers three-stage creep process. Creep curves are expressed as a function of time with five parameters (i=1~4 and m) in the new creep model. A dataset for the model parameters was constructed based on the available experimental creep curves, given the monotonicity assumption of creep strain vs temperature and stress. New creep curves can be predicted by interpolating model parameters from this dataset, in contrast to the previous method that employs an extra fitting process. The new treatment better accommodates all the experimental curves over the wide ranges of temperature and stress loads. The model was implemented into the ANSYS Mechanical code, and its predictions successfully captured all three creep stages and a good agreement was achieved between the experimental and predicted creep curves. For dynamic loads that change with time, the widely used time hardening and strain hardening models were implemented with a reasonable performance. These properties fulfil the requirements of a creep model for structural analysis.A thermo-fluid-structure coupling approach was developed by coupling the ANSYS Fluent for the fluid dynamics of melt pool heat transfer and ANSYS Structural for structural mechanics of RPV. An extension tool was introduced to realize transient load transfer from ANSYS Fluent to Structural and minimize the user effort. Both CFD with turbulence models and the effective model PECM can be employed for predicting melt pool heat transfer. The modified theta projection model was used for creep analysis of the RPV. The coupling approach does not only capture the transient thermo-fluid-structure interaction feature, but also support the advanced models in both melt pool convection and structural mechanics to improve fidelity and facilitate implementation. The coupling approach performs well in the validation against the FOREVER-EC2 experiment, and can be applied complex geometries, such as a BWR lower head with forest of penetrations (control rod guide tubes and instrument guide tubes).In the comparative analysis, the VLM and SLM coupling approaches generally have the similar performance, in terms of their predictability of the FOREVER-EC2 experiment and applicability to the reactor case. Though the SLM approach predicted slightly earlier failure times than VLM in both cases, the difference was negligible compared to the large scale of vessel failure time (~ s). The VLM approach showed higher computational efficiency than the SLM.The idea of the hybrid coupling is to employ a lumped-parameter code for quick estimate of thermal load which can be employed in detailed structural analysis. Such a coupling approach can significantly increase the calculation efficiency which is important to the case of a prototypical RPV where mechanistic simulation of melt pool convection is computationally expensive and unnecessary. The transIVR code was developed for this purpose, which is not only capable of quick estimate of transient heat transfer of one- and two- layer melt pool, but also solving heat conduction problem in the RPV wall with 2D finite difference method to provide spatial thermal details for RPV structural analysis. The capabilities of transIVR in modelling two-layer pool heat transfer and transient pool heat transfer were demonstrated by calculations against the UCSB FIBS benchmark case and the LIVE-7V experiment, respectively. The transIVR code was then coupled to the mechanical solver ANSYS Mechanical for detailed RPV failure analysis. Validation against the FOREVER-EC2 experiment indicates the coupling framework successfully captured the vessel creep failure characteristics.
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