Fast-spectrum reactors operate in configuration that is highly sensitive to changes in core geometry, making them “vulnerable” to rapid reactivity increases during severe core degradation events. A Core Disruptive Accident is an extreme scenario in which fuel relocation may lead to a super-promptcritical power excursion and potential disassembly of the reactor core. While this sequence has been extensively studied for Sodium Fast Reactors, its behaviour in Heavy Liquid Metal Fast Reactors remains insufficiently understood due to fundamental differences in coolant properties and accident progression. This work presents a first comprehensive investigation of the physics and phenomenology of a Core Disruptive Accident in a Heavy Liquid Metal Fast Reactor, with application to the MYRRHA research reactor. A conservative bounding scenario is analysed using a purposefully developed multiscale, multiphysics modelling framework that couples Reactor Kinetics, Heat Transfer, and Fluid Dynamics across relevant geometrical scales. Owing to thermophysical properties of Heavy Liquid Metals, substantial amounts of liquid coolant remain present in the degraded core throughout the transient. As a consequence, heat transfer and coolant thermal expansion are demonstrated to provide a major contribution to the system’s reactivity feedback. Fuel melting, leading to significant system expansion, is identified as a key mechanism in terminating the reactivity transient. Parametric analyses indicate that the fission energy release is the most sensitive to the initial fraction of non-condensable gas in the core, the initial reactor power level, and the rate of reactivity insertion. The calculated fission energy release, normalised to nominal reactor power, ranges from approximately 10 to 160 J/W, remaining significantly lower than typical estimates for early Sodium Fast Reactors. Furthermore, it has been demonstrated that only a very limited fraction (well below 1%) of the released fission energy is converted into mechanical work, consistent with values typically reported for Sodium Fast Reactors. These findings suggest a favourable (inherent) response of Heavy Liquid Metal Fast Reactor to a Core Disruptive Accident.
This study investigates the application of minor actinides (MAs) as a cladding layer on thorium–uranium fuel to simultaneously mitigate excess reactivity at the beginning of the fuel cycle (BOC) and provide an effective pathway for MA transmutation. Four different MA loadings, represented by distinct cladding thicknesses, were applied to (Th, 233 U)O 2 fuel pellets and benchmarked against both conventional UO 2 and uncoated (Th, 233 U)O 2 fuels. Neutronic analyses were performed to evaluate burnup evolution, safety-related parameters, MA depletion, radioactivity buildup, and the spatial distributions of neutron flux and power. The results demonstrate that the incorporation of MAs into the cladding suppresses excess reactivity at BOC while enhancing fuel performance relative to UO 2 and uncoated thorium–uranium fuel. In addition, the approach reduces the accumulation of proliferation-sensitive isotopes such as 237 Np, 241 Am, and 244 Cm in spent fuel, while enabling efficient MA transmutation. Overall, the proposed concept provides a dual benefit of improved reactivity management and long-term reduction of radiotoxicity in the nuclear fuel cycle.
A U + 5 wt.% Al powder (written U-5Al), manufactured by centrifugal atomization, was used in this study to assess the feasibility of densifying UAl x + Al powder mixtures by Spark Plasma Sintering (SPS) and obtaining a final composition with a minimum content of UAl 2 that meets specifications of targets for 99 Mo production. The microstructure of the U-5Al powder was studied first and mainly consists of a mixture of UAl 2 primary dendrites and α -U phase. Three SPS conditions differing in terms of UAl x /Al volume ratio and dwell time at 800 K were tested. The sintered discs were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. A very quick transformation of the U-5Al particles into a mixture of aluminides (UAl 2 + UAl 3 + UAl 4 , referred to UAl x ) was evidenced: only a few minutes were sufficient to transform the U-based particles into a mixture of aluminides containing about 80 wt.% of UAl 3 . According to these first tests, this new highly versatile manufacturing process could advantageously replace the conventional method of producing targets, subject to technical adjustments and parametric studies.
FeCrAl alloys are promising candidate for accident tolerant fuel (ATF) cladding in light water-cooled nuclear reactors (LWRs) due to their excellent high-temperature oxidation resistance and mechanical strength. This study investigates the effect of silicon (Si) additions (0–8.00 wt.%) on the neutronic performance and microstructural evolution of FeCrAl alloys through combined Monte Carlo simulations and experimental characterization. Neutronic behavior was evaluated using OpenMC on an AP1000 pressurized water reactor assembly model, while microstructural and mechanical properties were assessed via optical microscopy, SEM/EDS, XRD, and Vickers hardness testing. The results show that incorporating Si slightly improves the neutron economy of FeCrAl cladding by reducing Fe content, though the infinite multiplication factor ( k inf ) remains lower than that of Zr-4, limiting fuel discharge to ∼830 days compared to ∼1160 days for Zr-4. Experimentally, increasing Si content refined the average grain size from ∼498 μm (0 wt.% Si) to ∼201 μm (8.00 wt.% Si), accompanied by a linear increase in hardness from ∼280 HV to ∼520 HV. XRD analysis confirmed lattice parameter shifts consistent with Si incorporation, while SEM/EDS revealed generally uniform elemental distribution with localized Si segregation at higher concentrations. These findings highlight a trade-off: Si enhances microstructural stability and mechanical performance but only partially mitigates the neutronic penalty of FeCrAl. Optimizing Si content in conjunction with reduced cladding thickness is proposed as a pathway to balance neutronic efficiency with mechanical robustness for ATF deployment.
