S. P. Ladan1, 2, V. I. Borysenko1, 2
1 Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, 12, Lysohirska st., Kyiv, 03028, Ukraine
2 Kyiv Academic University, 36, Akademik Vernadskyi blvd., Kyiv, 03142 Ukraine
DOI: doi.org/10.31717/2311-8253.25.2.5
Abstract
The article presents an analysis of decay heat in spent nuclear fuel, focusing on methodological and regulatory approaches based on international standards ISO 10645:2022 and ANSI/ANS‑5.1-2014. The paper evaluates and compares decay heat estimations derived from multiple approaches: the methodology described in the Ukrainian national standard SOU NAEK 099:2023, analytical models based on ISO/ANS formulas and Monte Carlo simulations performed using the SCALE6.1.3. The results reveal that the ISO and ANS models, when appropriately applied, provide a high degree of accuracy. For time intervals ranging from 10 to 1200 days after reactor shutdown, the deviation between the ISO-based calculations and SCALE simulations remains within 5%, validating the use of ISO/ANS models for practical reactor and fuel cycle applications. The study also discusses the structure and implementation of the ISO 10645:2022 methodology, including its handling of time-dependent reactor power profiles and isotope-specific decay constants. Calculations were carried out using scientific computing tools based on Python programming language. These methods allows flexible modeling and rapid assessment without detailed fuel assembly data. This capability highlights the practical advantage of ISO/ANS-based models for integration into reactor monitoring and safety systems, particularly under time-limited conditions Special attention is paid to the contributions of various nuclides to decay heat over time, including 239U, 239Np, and 134Cs, as well as the limitations of simplified empirical models that assume a constant power, as is the case in SOU NAEK 099:2023. These simplifications are shown to introduce notable biases, particularly during early cooling periods and for assemblies with non-standard operational histories. Overall, the article advocates for the adoption of internationally harmonized standards in Ukrainian nuclear practice. The demonstrated reliability of ISO/ANS models and their compatibility with computational tools make them suitable for use in routine evaluations of spent fuel decay heat, promoting safer fuel management and alignment with global best practices.
Keywords: decay heat, spent nuclear fuel, SCALE, SOU NAEK 099:2023, ISO 10645:2022, ANSI/ANS‑5.1-2014.
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