Modeling and simulation of fuel dispersal during the loss-of-coolant accident
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Authors
Moharana, Avinash
Issue Date
2024-08
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Mechanical engineering
Alternative Title
Abstract
As the nuclear industry explores new fuel designs to accommodate increased burnup, studying fuel behavior during loss-of-coolant accidents (LOCA) is essential for ensuring the safe operation of light-water reactors. In the event of cladding rupture during a postulated LOCA in a pressurized water reactor, fuel particles, along with fission gases, can be expelled into the reactor core from the fractured fuel rod. This expulsion, known as fuel dispersal, poses thermal hydraulic and radiological challenges, as the transport and resulting mass distribution of fuel fragments within the reactor pressure vessel can affect long-term cooling. The initial fuel dispersal phenomenon is significantly influenced by the high-pressure ejection characteristics of the fuel fragments, the size and shape of the ruptured cladding, and the fuel rod depressurization history during LOCA transients. Depending on the location of the burst orifice relative to the quench front, the dispersal event represents an intricate three-phase flow and heat transfer phenomenon, where high-temperature fuel particles carried by the fission gases induce phase changes within the the narrow subchannels of the fuel assemblies. Given the unique nature of this flow problem, the current study develops a dedicated computational framework to predict the mass distribution and cooling of dispersing fuel particles, facilitating post-accident management of the fuel assemblies. Firstly, considering the scale of nuclear reactor applications, a continuum three-fluid model is proposed for simulating the transport of solids within the reactor core. With high-temperature fuel fragments within the liquid media, nucleation sites inducing phase changes are dispersed within the flow domain. Coupled with the fact that the transient dispersal event occurs on different time scales than other three-phase flow applications, this study derives a time-averaged three-fluid flow model without losing generality. The assumptions regarding the continuum treatment of the solid phase and the modeling of fuel dispersal behavior are incorporated to simplify the governing equations and derive applicable closure relations. The computational validation of the model was conducted using adiabatic experimental results obtained from ongoing research at Oregon State University, focusing on characterizing fuel dispersal behavior during simulated LOCA conditions. Settlement characteristics of the solids, quantified by the probability distribution of equivalent particles, closely matched the probability density functions reported in experimental studies.
To gain insights into the governing flow regime characteristics during a dispersal event, a discrete phase modeling framework was developed to track solid particles in a Lagrangian manner within the gas-liquid flow. A two-way coupling algorithm was proposed, where interfacial and buoyancy forces are updated for the particle phase based on their instantaneous positions within the computational domain. Simultaneously, the forcing terms due to particle loading within a computational cell are updated through a source term for the gas-liquid two-fluid flow. This modeling framework was validated using adiabatic experimental results from the literature, and the particle settlement characteristics were found to be in good agreement.
Lastly, the transport of fuel particles within a scaled 5 × 5 lattice of a pressurized-water reactor rod bundle geometry was modeled through a two-fluid Eulerian framework. The required boundary conditions were evaluated from the fuel performance code BISON in a postulated large-break LOCA scenario. The modeling framework considered solid fuel particles as granular matter, interacting with the gaseous dry steam phase and fission gases through the governing interfacial momentum exchange between the participating fluids. The simulation results provided the volume fraction of the solids obtained at the bottom surface of the enclosing tank geometry.
The key highlights of this study include a detailed derivation of a three-fluid flow modeling framework, a theoretical description of fuel dispersal behavior, and simplifying assumptions necessary to arrive at a set of governing equations and appropriate closure models for interpenetrating three-phase flow during a simulated fuel dispersal event. Additionally, the study demonstrates a three-fluid coupling algorithm for tracking inert point masses interacting with gas-solid flows. Finally, it discusses key factors affecting fuel dispersal behavior within reactor rod bundle geometry during a postulated LOCA event in a PWR.
Description
August 2024
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY