This document contains the accepted poster abstract for NuMat 2026: The Nuclear Materials Conference, scheduled to be held in Canada from September 21–24, 2026. Presentation Type: Poster Presentation Abstract Reference Number: 7 Conference Track: Track 7: Characterization of Irradiated Fuels and Materials, and Techniques Sub-category: Electron microscopy of irradiated materials Abstract: The High Burnup Structure (HBS) in irradiated UO₂—characterised by sub-micron grain subdivision (~0.1–0.3 µm), fission gas bubble arrays, nanoscale amorphous clustering, and athermal uranium redistribution at effective diffusivities of ~10⁻¹⁸ cm²/s—remains mechanistically unresolved. These anomalies emerge sharply beyond ~60–70 GWd/tU at temperatures (~700 K) where purely Arrhenius bulk diffusion is negligible, implicating a non-thermal energy source beyond standard thermal-spike models. We propose HBS formation is driven by a sub-picosecond (~10⁻¹³ s) spherically symmetric mechanical pressure pulse (SPP) generated at nuclear scission. Arising from a fraction of the mass-defect energy release prior to primary Coulomb-driven fragment acceleration (~168 MeV TKE), this channel provides the missing mechanical work. We estimate the SPP reaches peak stresses of ~1–3 GPa within ~5 nm of scission, decaying over a mechanical coherence radius of ~0.5–1 µm. Crucially, sub-grain nucleation at the observed ~0.1–0.3 µm scale is governed by competing defect sink densities within this pulse envelope, successfully decoupling the nucleation scale from the absolute coherence radius. Three falsifiable experimental tests are proposed. First, using muscovite mica as a high-resolution morphological proxy, AFM of ²⁵²Cf spontaneous fission tracks isolates pressure-specific lateral branching nodes predicted by the SPP. Second, comparative HRTEM of ²⁵²Cf and equivalent-LET heavy-ion tracks at matched stopping powers (~20 keV/nm) discriminates scission-specific amorphous core morphology from purely electronic deposition. Third, synchrotron SAXS/WAXS strain mapping (beam size <50 nm) around fission tracks in UO₂ probes the predicted compressive annular zone, distinguishing the SPP strain profile from the monotonic tensile signatures of thermal-spike models. This escalating programme—from proxy morphological screening to quantitative UO₂ strain characterisation—offers a rigorous pathway to validate or falsify the SPP hypothesis.