SPECIAL REPORT | WASTE MANAGEMENT Some like it hot
Understanding decay heat is key to managing economics and safety in spent fuel management. But databases are based on standard PWR fuel, which has lower enrichment and burn-up than fuel assemblies now being used. An NEA group recommends action
IN THE DECADES SINCE the first light water reactors first went into operation the design and use of their fuel has evolved far from the original. Among the changes are higher initial fuel enrichment and higher average assembly burn-up in connection and extended reactor cycles. Initial enrichment has been steadily increased to 5.0% and the assembly average burn-up may be above 70 MWd/kgU. This is a significant shift in practice. Before 2000, initial fuel enrichment rarely exceeded 3.8% and burn-up was below 55 MWd/ kgU. However, assumptions on spent fuel decay heat have not kept pace with these changes in fuel design and use.
In addition there are novel fuel variants, some in
operation, such as mixed oxide fuel (MOX), and some at different stages of development towards regular use, such as enriched reprocessed fuel (ERU), accident- tolerant fuel (ATF) and high-assay low-enriched uranium fuel (HALEU). Some countries have MOX fuel in storage
from past use. Additional minor actinides means the decay heat of MOX fuel is higher than uranium dioxide. No direct decay heat measurements are available at the MOX or ERU assembly level, so it is not feasible to validate calculations with direct measurements. One group working on this issue is Subgroup 12 of the
NEA Working Party on Nuclear Criticality Safety, which began work in 2022 on questions related to spent fuel decay heat. The group asked how well the decay heat of spent fuel be estimated and what are the main quantities of interest for such an estimation? Additionally, the group studied questions linked to uncertainties, tolerance intervals and user needs and it reviewed experimental methods, physics-based calculation practices and codes were also presented with specific details. The group’s conclusions were published in a report, Summary of the NEA Assessment on Spent Nuclear Fuel Decay Heat for Light Water Reactors, A Report by Subgroup 12 of the NEA Working Party on Nuclear Criticality Safety, published in 2025. The report says precise knowledge of the decay heat for these parameters directly affects the safety and economy of the back end of the fuel cycle.
Its main recommendation was for new measurements of SNF decay heat, based on a calorimetric approach. The current experimental database measurements of standard PWR uranium dioxide fuel for lower initial enrichment and burn-up values should be compared with today’s fuel assembly characteristics. It said no public experimental data are available regarding decay heat are available for MOX, VVER or Candu fuel types. The group proposed new experimental initiatives in order to diversify measurement facilities, increase redundancy and provide public data. The group’s second recommendation is to improve
the current understanding of calculation methods by providing detail on the impact of relevant input parameters and their uncertainties. Comparisons between methods, practices and validation approaches would help understand discrepancies between methods applied in various institutes, The group advocated machine learning as a resource
for defining new measurements, optimising information gains and controlling costs.
Efforts to understand in more detail the heat decay of spent fuel: ● International Atomic Energy Agency (IAEA) Co-ordinated Research Project on Spent Fuel Characterisation (2020-2024) (IAEA, 2025)
● EURAD (2019-2024) (European Union, 2025), the European Union Joint Strategic Programme of Research and Knowledge Management dedicated to supporting radioactive waste management studies,
● Transnational projects between the US Electric Power Research Institute (EPRI) and Sweden’s Nuclear Fuel and Waste Management Company (SKB), which will provide information on new decay heat measurements.
The decay heat of MOX fuel is higher than uranium dioxide and no direct decay heat measurements are available at the assembly level, so it is not feasible to validate calculations. Source: Foro Nuclear
www.neimagazine.com | February 2026 | 43
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