Physical analysis of spent nuclear fuel decay heat

Letnar, Gašper (author), Snoj, Luka (mentor), Schillebeeckx, Peter (comentor)

For the safe and reliable design of long-term spent fuel storage solutions, an accurate characterization of spent nuclear fuel is of paramount importance. Among the key parameters in this context is decay heat, the calculation of which requires coupled neutron transport and depletion simulations. In this work, an industry-standard two-dimensional modeling approach utilizing the Serpent Monte Carlo code is validated against 173 spent fuel decay heat measurements performed at the Central Interim Storage Facility for Spent Nuclear Fuel (CLAB). A comparison between the calculated and measured values yields a mean Calculation to-Experiment (C/E) ratio of 0.994(26) when utilizing the ENDF/B-VIII.1 nuclear data library, and 0.999(27) for the older ENDF/B-VII.1 library, demonstrating excellent global agreement between the modeling methodology and experimental benchmarks. In addition to evaluating the total decay heat, this work also compares the correction, required due to the escape of high-energy photons from the calorimeter, with the values reported in the literature. This is achieved through an independent, decoupled computational scheme combining neutron and photon transport simulations in Serpent with localized electron transport simulations in MCNP. Comparison of escaped power yields a global 11 % to 14 % underprediction, likely due to modeling limitations. Application of photon escape correction to the total decay heat power yields a marginal improvement in the net decay heat comparison, resulting in an updated mean C/E value of 0.996(26) for the calculations using the ENDF/B-VIII.1 library.

• Language: English
• Keywords: Neutron transport, SNF, Reactor, Decay heat, Nuclear physics, Monte Carlo
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2026
• PID: 20.500.12556/RUL-186875
• COBISS.SI-ID: 290403331
• Publication date in RUL: 06.09.2026

Secondary language

• Language: Slovenian
• Title: Fizikalna analiza zakasnele toplote izgorelega jedrskega goriva

Abstract

Za varno in zanesljivo načrtovanje rešitev za odlaganje jedrskih odpadkov je natančna karakterizacija izgorelega jedrskega goriva izrednega pomena. Eden izmed pomembnejših parametrov v tem kontekstu je zakasnela toplota, ta pa je v večini primerov določena le zgolj preko zgorevalnih preračunov. V tem delu validiramo dvodimenzionalen pristop k modeliranju gorivnih elementov z Monte Carlo programom Serpent na primeru 173 gorivnih elementov, katerih zakasnela toplota je bila izmerjena v sklopu Švedskega centralnega prehodnega skladišča za izgorelo jedrsko gorivo (CLAB). Pri preračunu s knjižnico jedrskih podatkov ENDF/B-VIII.1 opazimo povprečno razmerje med izračunano in izmerjeno (C/E) zakasnelo toploto 0.994(26), ter 0.999(27) pri izračunih s knjižnico ENDF/B-VII.1. Kar v povprečju nakazuje na odlično ujemanje med meritvijo in izračunom, kljub poenostavljeni modelski geometriji. Na koncu določimo še prispevek k meritvi zakasnele toplote, zaradi pobega fotonov iz merilnega kalorimetra in ga primerjamo z vrednostmi iz literature. V ta namen najprej karakteriziramo spekter fotonov za vse izračunane primere z uporabo jedrskih podatkov ter z preračunom transporta elektronov v programskem paketu MCNP. Nato pa z preračunom transporta fotonov iz geometrije kalorimetra določimo prispevek k celotni izmerjeni zakasneli toploti. Primerjava pobegle moči fotonov nakazuje na sistematsko podcenitev tega prispevka tudi do največ 20 %. Vendar pa primerjava med izračunano in izmerjeno zakasnelo toploto, z uporabo menjenega popravka, malce izboljša globalno vrednost C/E in sicer v primeru ENDF/B-VIII-1 knjižnice iz 0.994(26) na 0.996(26).

• Keywords: Izgorelo jedrsko gorivo, Zakasnela toplota, Reaktor, Transport nevtronov, Jedrska Fizika, Monte Carlo