A Sub-Picosecond Scission Pressure Pulse as a Candidate Mechanical Energy Channel in Nuclear Fission: Energy Budget, Geometric Discriminators, and Three Falsifiable Tests Relevant to High Burnup Structure in UO₂

Context and Motivation: The High Burnup Structure (HBS) in irradiated UO₂ and athermal fission-enhanced diffusion are established phenomena driven by per-event energy deposition. While the underlying microstructural mechanism for HBS remains debated between polygonisation and recrystallisation models, both rely on the mechanical work extracted from accumulated fission-fragment tracks. Grounded strictly in empirical evidence from peer-reviewed studies of spent nuclear fuel, this paper investigates a novel, quantifiable mechanical energy channel capable of driving these athermal processes. The Scission Pressure Pulse (SPP) Hypothesis: This work hypothesizes the existence of a spherically symmetric mechanical pressure pulse (the Scission Pressure Pulse, SPP) launched precisely at nuclear scission. The SPP is characterized by a generation time of ~10⁻¹³ s, a peak stress of ~1–3 GPa within ~5 nm of the scission point, and elastic decay over a coherence radius of ~0.5–1 µm. Explicit energy bookkeeping demonstrates that this hypothesis is highly conservative. The SPP energy is calculated at ~10 to 100 eV per fission, representing a fraction of 10⁻⁷ to 10⁻⁴ of the total fission energy release. This value is completely imperceptible within the ~1 MeV experimental uncertainty of measured fragment kinetic energies. Cumulatively, however, it delivers mechanical work three to five orders of magnitude larger than the ~0.05 eV per fission of grain-boundary energy stored in a fully developed HBS subgrain network. Methodology and Proposed Tests: The defining geometric discriminator of the SPP is an isotropic monopole strain component centered on the scission vertex—a feature that, dictated by symmetry, cannot be produced by track-aligned cylindrical thermal spikes. To isolate this vertex-specific signature, this paper proposes three progressive experimental tests comparing ²⁵²Cf spontaneous fission against matched-stopping-power heavy-ion controls: Test 1: AFM screening of ²⁵²Cf tracks in muscovite mica to identify near-vertex morphological anomalies. Test 2: Matched-LET HRTEM comparisons to physically isolate the scission-specific signature at the fragment origin. Test 3: Differential ensemble strain measurements (using high-resolution XRD and SAXS) in UO₂ to detect the isotropic lattice-strain component. These tests are structured under a blind, pre-registered statistical design with pre-committed falsification criteria. Significance: Demonstrating the existence of the SPP would establish a necessary physical precursor for any SPP-based mechanistic account of HBS formation and athermal diffusion. Conversely, a null result under the proposed falsification criteria would definitively refute the hypothesis. Note: This preprint was developed from the accepted NuMat 2026 abstract titled, "Three Falsifiable Experimental Tests of a Sub-Picosecond Scission Pressure Pulse: Mechanistic Insights into High Burnup Structure in Irradiated UO₂."