WASTE MANAGEMENT | SPECIAL REPORT
There are new stringent economical requirements on spent fuel assemblies and stricter safety rules related to core transient, extended storage and deep geological repository. As a result, there have been calls for an improved understanding of the estimation of spent fuel decay heat, which underpins designs for spent fuel handling, transport, storage and reprocessing or long- term disposal and storage. The NEA group’s also saw the need to reduce
uncertainties and biases in measurements and calculations. Many uncertainties on input parameters can now be conveniently obtained, so there is a global effort to use these to reduce uncertainty. In addition, a larger experimental database is needed for validation. Most measurements for SNF decay heat come from the Central Interim Storage Facility for Spent Nuclear Fuel (Clab) in Sweden, with additions from two decommissioned US facilities. These measurements do not cover all the characteristics of current spent fuel, and in any case there is a risk in using experimental values that come from a single facility as possible correlations between measured values are not mitigated with other experimental sources. The NEA group recommended:
Above: Assumptions on spent fuel decay heat have not kept pace with changes in fuel design and use. Source: ANS
Bottom right: New experimental initiatives are needed in order to diversify measurement facilities, increase redundancy and provide public data. Source: US DOE
Understanding is key to economics
Decay heat spent fuel is one of the main quantities of interest in characterising handling, storage and disposal of high-level nuclear waste, with implications for the safety and economics. It is also important for safeguards and fuel cycle decisions. Depending on the cooling period considered, decay
heat can be up to 7% of the total reactor heat source for a commercial light water reactor (LWR). For days after operation the heat can damage primary and secondary safety barriers if not properly managed. During cooling time of days or years the heal levels are important in safe handling and storage, and the design of pools, casks and surface storage must account for this potential hazard. Longer term, decay heat must be considered in repository design. The main parameters in quantifying decay heat are:
● The integral fuel irradiation level or burn-up (MWd/t), obtained from core simulators and experimentally checked with gamma or neutron measurements.
● The initial fuel composition, enrichment and initial mass of actinides, provided by the reactor operator. ● The cooling time, provided by the reactor operator.
It may also include: ● the irradiation rate (in MW/t) during reactor operation (especially important immediately after shutdown and during fuel unloading);
● the use of burnable poisons; ● other manufacturing and irradiation parameters (such as dimensions, densities, temperatures, void).
In their first century of storage, spent fuel may be moved more than once, both from place to place and from cask to canisters. New calculations are needed for the stockpile of spent fuel, with new validation based on new (or re-analysed) decay heat measurements. The existing set of decay heat measurements
are limited. All recent (and new) measurements are performed on a limited set of spent fuel assemblies. Because they come from a single facility, redundancy and independent checking are impracticable.
44 | February 2026 |
www.neimagazine.com
● Extending the current experimental database. New measurements on different fuel types, possibly at a different facility, would greatly improve the situation.
● Improving theoretical understanding and nuclear data. Even if the methods to calculate decay heat are well known, their application may vary from code to code. Procedures may differ between institutes.
● Linked to the previous recommendation, it is necessary to separate cases where a best-estimate code and standard methods are applied. Different methods contain different levels of approximations, and users need to be aware of possible drawbacks of the method they use.
● An estimation of differences between reference values and a value from a procedure should be considered along the complete calculation chain. Clear definitions of underlying assumptions and the mathematical method used should be included.
Other features helping validate measurements include: repeating measurements, both at the same facility and measuring similar assemblies in different facilities; comparing different code implementations of the same method; using ‘blind benchmarks’; and trying to quantify the impact of different modellers performing the calculations. ■
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