Simulation of Melt Infiltration and Spreading using Moving Particle Semi-implicit Method

QC 2024-05-16

Abstract

During a core meltdown severe accident (SA) of light water reactors (LWRs), melt penetration in porous media (so-called melt infiltration) may occur due to melt relocation on top of a debris bed, or after dryout and re-melting of metallic particles containing steel and Zr, and then remelting of more refractory oxidic particles containing fuel and oxidized Zr in a hot zone of a debris bed. Both in-vessel and ex-vessel phases of SA progression may involve melt infiltration. For instance, if a debris bed in the lower head of reactor pressure vessel (RPV) is uncoolable, melt infiltration occurs due to corium debris remelting and can affect RPV failure mode and place. After the lower head failure, molten corium discharges from the RPV, leading to melt spreading in the reactor cavity if the cavity floor is dry or covered by a shallow water pool. In the case of deep water pool below the RPV, remelting of uncoolable debris bed can also result in melt spreading under the water. Clearly, modeling of melt infiltration and spreading are paramount not only to the prediction of severe accident progression, but also for the analyses of corium coolability and retention. In spite of their importance, insufficient work has been done in mechanistic modeling of the phenomena.

The objective of this doctoral thesis is to provide high-fidelity predictive capabilities for melt infiltration and spreading, which can be employed to substantiate the understanding of melt infiltration in the RPV and melt spreading in the reactor cavity. The focus of the work is to develop a computational code with multi-physics models for modeling specific phenomena important to melt infiltration and melt spreading. The models and code are based on the Moving Particle Semi-implicit (MPS) method, starting from two-dimensional representation and progressing with three-dimensional simulations.

The thesis first describes the MPS method which is a mesh free method suitable for free-surface flow. The governing equations including mass, momentum and energy conservation equations are discretized with particle interaction models, such as gradient model and Laplacian model. Furthermore, models for viscosity and phase change, surface tension and wettability are implemented in the code. Additional models of multi-phase flow, crust formation, and film boiling heat transfer are added for relevant applications.

The developed MPS code is then validated against various experiments for analyses of in-vessel melt infiltration, and ex-vessel melt spreading in dry and underwater conditions. In particular, three numerical studies have been conducted to investigate the capabilities of the MPS code for prediction of melt infiltration, melt spreading under dry condition and melt spreading under water. The key points from the numerical studies are as follows.  

• The MPS code is applied to predict melt infiltration phenomena in various particulate beds employed in the REMCOD experiments carried out at KTH-NPS with corium simulant materials. The wettability model is implemented, which is characterized by the contact angle between the melt and the debris bed. The REMCOD-E09-C4 and E09-C2 tests are calculated in which melt penetrates through hot debris beds with spherical particles and cylindrical particles, respectively.  Then, the MPS code is applied to simulate the REMCOD-E08-C4 test in which the debris bed temperature is below the melting point of melt, thereby the solidification occurs. The results suggest that the melt infiltration process in the experiment with cylindrical particles can be predicted by using Sauter mean diameter. Additionally, the simulation shows a good agreement with the tests.

• The MPS code is further extended and employed for the dry spreading test. A modified crust formation model has been proposed based on rigid body assumption to keep the dynamic shape of crust, which is appropriate for the crust formed at the top surface. Two tests conducted at KTH are simulated as one-dimensional and two-dimensional spreading schemes, respectively. These simulations concentrate on hydrodynamic motion and heat transfer while ignoring the associated phenomena such as MCCI and generation of gaseous concrete decomposition products. The results illustrate the capability of the MPS code predicting melt leading-edge progression and spread thickness in dry cavity.

• The MPS code capabilities are further extended to model the specific phenomena appearing during melt spreading over substrates under a water layer, such as multiphase flow and film boiling heat transfer. A smoothing scheme is proposed to replace the real properties of interfacial particles for multiphase flow. The film boiling heat transfer is calculated along the film boiling regime of the boiling curve. For validation, the updated MPS code is then employed to simulate the S3E-2MWS-Ox-1 and PULiMS-E9 tests, which are conducted at KTH. These simulations focus on investigating the thermal-hydraulic characteristics of 1D and 2D melt underwater spreading, and they predict the leading edge progression and terminal spread thickness well.

In general, the developed MPS code offers a novel approach to predict melt infiltration in the RPV and melt spreading in the reactor cavity with high fidelity. The insight from the simulations helps the understanding of corium coolability and retention.

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