Uncertainty assessment on the calculated decay heat of the ASTRID basic design core based on the DARWIN-2.3 package

Abstract A good knowledge of the decay heat of the various elements of the core (fissile and fertile zones, structures, sodium…) as well as the associated uncertainties is critical for the safe operation of a nuclear facility, but also at the design stage as in the case of the ASTRID technological demonstrator. For this reactor, the amount of decay heat is all the more important as it will strongly impact the design of the dedicated EPur (Evacuation de la PUissance Residuelle = disposal of the decay heat) system. The uncertainties on the decay heat calculations of a sodium fast rector that are currently used at CEA were defined several decades ago and are now considered very conservative, because of the method that was used in their evaluation. They may thus be penalizing for the design and operation of ASTRID. These past uncertainty assessments were based on a semi-empirical approach - a combination of physical considerations and experimental results - and on calculations using conservative methods and obsolete nuclear data libraries that need to be updated. We have used up-to-date calculation tools in order to reduce these uncertainties: ERANOS-2.2 for the neutronic calculations and DARWIN-2.3 for the depletion calculations. The uncertainty on the decay heat is estimated with the CYRUS tool, which performs the propagation of all the relevant nuclear data uncertainties (decay constants, branching ratios, cross sections, independent fission yields). For some parameters, we have completed the JEFF-3.1.1 uncertainties database with values from other nuclear data libraries, for instance the branching ratio for the decay of 137g Cs to 137m Ba, of 90g Kr to 90g Rb and the branching ratio for the capture of 241g Am to 242g Am. The resulting 1σ uncertainty on the decay heat calculation never exceeds 2.6%, which shows a major improvement compared to the previous uncertainty evaluation that could reach 6.6%. Additional sources of uncertainties will have to be considered in the future in order to evaluate the full uncertainty on decay heat calculations: the reactor operation conditions variations, the accuracy of the nominal power measurement and the uncertainty on the neutron flux calculated with the new APOLLO3® code system, which will be dedicated to the ASTRID neutronic calculations.