Uncertainty Driven Approach for Enhanced Criticality Safety Studies
Résumé
Accurate nuclear data is essential for the safety and efficiency of nuclear systems. In criticality safety assessments, uncertainties on nuclear cross-sections can significantly impact safety margins. Plutonium-239 (239Pu) is a crucial isotope in nuclear systems, and uncertainties in its cross-section data can lead to significant discrepancies between calculated and experimental effective multiplication factors (keff). In this paper, we introduce a methodology to optimize the 239Pu cross sections by directly utilizing the uncertainty information from the covariance matrix of resonance parameters. Instead of perturbing the cross sections themselves, we perturb the resonance parameters based on their covariance data, without making any assumptions or relying on expert judgment. We use the newly developed SCOOBY module, integrated into the IRSN in-house nuclear data processing code GAIA, to perform random sampling of these resonance parameters. This approach generates new cross sections that fully reflect the inherent uncertainties as defined by the covariance matrix delivered by evaluators in nuclear data evaluations. Using Simple Random Sampling and Monte Carlo simulations, we compute keff for benchmarks sensitive to 239Pu cross sections from the PU-SOL-THERM (PST) series of the International Criticality Safety Benchmark Evaluation Project (ICSBEP). By optimizing the resonance parameters through a chi-square minimization against experimental keff values, we significantly reduce the discrepancies between calculated and experimental results. To ensure that the optimized cross sections are robust and generally applicable, we validated them against an expanded set of PST benchmarks from the ICSBEP database. The results confirmed consistent improvements beyond the initial sample. This approach provides a systematic and unbiased way to optimize and improve nuclear data, enhancing the accuracy and reliability of criticality safety assessments. It also offers valuable insights for nuclear data evaluators and can be extended to other important isotopes in nuclear safety.
