Graduate Thesis Or Dissertation


Fuel Dispersal Under LOCA Conditions Public Deposited

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  • Most operating power reactors use oxide fuel in the form of uranium oxide (UO2) which is robust against high temperatures but suffers from poor mechanical performance at high levels of burnup. Currently, the maximum average burnup in light water reactors in the US is 62.5 GWd/t, which is partially imposed to prevent severe fuel damage during a hypothetical accident, most importantly the loss-of-coolant accident (LOCA). During a LOCA, high-burnup fuel has been observed to undergo extensive fragmentation, relocation, and dispersal through ruptured cladding, and this has the potential to impact both the coolable geometry and long-term cooling of a reactor during such an accident. This study seeks to understand the mechanisms which drive fuel dispersion. This is accomplished experimentally by subjecting test rods filled with a surrogate nuclear fuel to large pressure transients and observing the results in both a single rod and rod assembly configuration. Test rod pressures are measured in order to determine system thermodynamic time constants, and exiting fuel velocities are measured via high-speed cameras. An image processing technique is developed to reliably calculate particle velocities using Python’s TrackPy module, and these data are used to provide the input conditions for simulations. The two simulations developed here are an analytical one and numerical one; the numerical model uses an implicit Euler method which allows for real-time updating of the physics to determine collisions between particles and provide an estimate of in-core fuel distributions in an assembly. Verification and validation (V&V) are performed to provide a quantifiable comparison to experimental results. The results of this study provide an improved method for estimating fuel mass distribution in a reactor core during a LOCA.
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