A Monte Carlo Study of Reactivity Equivalence in Fresh Fuel Storage and Spent Fuel Pool Criticality Analysis: Applicability and Limitations for Use in Storage Capacity and Criticality Safety Calculations

Sammanfattning

The nuclear industry’s transition toward higher fuel enrichments, aimed at improving fuel cycle economics and reducing waste volumes, requires rigorous criticality safety assessments to justify the use of existing storage infrastructure. A widely used licensing approach, the Reactivity Equivalence Method, assumes that fuel assemblies with higher enrichment and burnable absorbers are safe for storage if their infinite multiplication factor k_inf in a simplified core geometry matches that of a licensed reference fuel. This thesis evaluates the validity of this method for a VVER-1000 fuel assembly containing gadolinium rods within Fresh Fuel Storage Container and Spent Fuel Pool (SFP) configurations, utilizing continuous-energy Monte Carlo (MCNP 6.2) simulations. The study demonstrates that the Reactivity Equivalence Method is both non-conservative and unreliable when applied to these storage environments. Numerical results indicate that 'equivalent' gadolinium-bearing assemblies yield consistently higher effective multiplication factors k_eff in storage geometries than the reference fuel. Detailed spectral analysis identifies two fundamental physical mechanisms driving this failure: a non-linear competition between U-235 fission and Gd absorption in softer neutron spectra, and a spatial thermal flux inversion driven by external moderation. This flux inversion causes a reversal in the effectiveness ranking of absorber patterns, leading the simplified core model to misidentify the bounding worst-case configuration. Additionally, in the SFP, competitive absorption between internal gadolinium rods and external borated storage racks further diminishes absorber worth, compounding the safety margin erosion. Consequently, this work concludes that criticality safety analyses for systems involving spectrally sensitive absorbers must utilize high-fidelity 3D models that accurately capture the specific spectral and spatial conditions of the storage environment.