Neutronic Investigation of Y 2 Sn 2 O 7 and Y 2 TiO 5 as Alternative Cladding Materials in a VVER-1200 Reactor Using the Serpent Monte Carlo Code

Accident-tolerant cladding for water-cooled reactors requires materials capable of withstanding severe transients, including loss-of-coolant accidents (LOCA), station blackout (SBO), and loss of heat sink, while preserving acceptable neutron economy. This study investigates the neutronic feasibility of two yttrium-based ceramic oxides, Y2Sn2O7 and Y2TiO5, as alternative fuel cladding materials in a VVER-1200 reactor using full-core continuous-energy Monte Carlo simulations with Serpent 2.1.31. A realistic VVER-1200 core model with 163 fuel assemblies, enrichment zoning, and gadolinia-bearing fuel was employed, and the effect of cladding replacement was isolated by preserving all non-cladding parameters of the reference E110 alloy design. The computational methodology was supported by code-to-code comparison with OpenMC using ENDF/B-VII.1 nuclear data. The results indicate that both candidate materials impose negative reactivity effects over the entire depletion interval, but with markedly different severity. Relative to the reference core, Y2Sn2O7 caused a moderate beginning-of-life reduction in ���� from about 1.16 to 1.13, while Y2TiO5 lowers it to about 1.10. The corresponding reactivity penalties are approximately −2200 pcm for Y2Sn2O7 and −5000 to −3700 pcm for Y2TiO5 over depletion. Although the thermal neutron spectrum remains generally preserved, Y2TiO5 induces greater spectral hardening and a larger neutronic departure from the reference case. The analysis uses a fixed-loading depletion history; results beyond the calculated criticality crossings are treated as exploratory numerical continuations rather than predictions of sustained reactor operation. These findings identify Y2Sn2O7 as the more promising of the two investigated ceramic cladding options from a neutronic perspective, and they provide reactor-specific evidence supporting its further evaluation in coupled thermal-hydraulic, structural, and safety analyses for VVER-1200 accident-tolerant fuel design.