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NF-PRO – Studies on Long-Term Stability of Spent Fuel
Vincenzo V. Rondinella, Detlef Wegen, Thierry Wiss, Daniel Serrano-Purroy,
Joaquin Cobos-Sabate, Dimitrios Papaioannou, Jean-Pol Hiernaut, Jean-Paul Glatz
European Commission, Joint Research Centre, Institute for Transuranium Elements,
P.O. Box 2340, 76125 Karlsruhe, Germany
Summary
In the frame of institutional research activities and as contributions to the Integrated Project
NF-PRO, a series of experimental campaigns was carried out at the Institute of Transuranium
Elements (ITU), a Joint Research Centre of the European Commission. Within NF-PRO, the
experiments were included in the WP 1.4 "Evolution of spent fuel prior to water ingress (normal
evolution scenario) and during the transient period (early failure scenario) and impact on
radionuclide release". Source term investigations to determine the grain boundary inventory
of MOX fuel, as an important component of the instant release fraction (IRF), were
performed. For these studies an innovative approach combining Knudsen cell effusion
measurements and leaching experiments was implemented. A further objective of WP1.4 was
to improve knowledge on the microstructural properties of spent fuel and their possible
changes in the scenarios considered. The mechanical stability of spent fuel is important
because it affects the surface area of fuel potentially available for leaching. The effects of
alpha decay damage on the mechanical stability of spent fuel were investigated using alpha
doped UO2 and (U,Pu)O2. The techniques used were SEM/TEM combined with annealing and
DSC measurements. Finally, potential alteration processes of the spent fuel in an early failure
scenario, namely during the transient phase between initial breach of the canister and water
ingress in it, were investigated. This work involved exposing fuel rodlets containing
intentionally manufactured defects in the cladding to a water-saturated atmosphere containing
argon and/or hydrogen to simulate the atmosphere inside the canister emplaced in a geologic
repository. The condition of the fuel inside the cladding was evaluated at the end of the
experiments by SEM/EDS while gas and liquid phase were examined by mass spectrometry.
1. Grain boundary inventory and instant release fractions for MOX
An estimation of the grain boundary inventory of radionuclides belonging to the so called "labile
fraction" was performed for ~39 GWd/t SBR mixed oxide fuel (MOX) (BNFL - now Sellafield Ltd)
[1-2] adopting a combined approach including effusion and leaching tests. Very scarce or no data
were previously available concerning grain boundary leaching/inventory for MOX fuel, after the
work of Gray et al in the’90 in PNNL [3-5], more recently extended on UO2 fuel by Hanson et al.
[6], Roudil et al. [7], Garisto et al. [8], and reviewed by Johnson et al. [9]. The experimental results
obtained in this work indicate that for the species showing fractional release higher than U, i.e. Rb,
Sr, Mo, Cs, Tc, Ba, fractions typically �1 % are released from grain boundary if only the amounts
released after removing pre-existing oxidized surface layers on the sample are considered. By
including also the amounts released during preliminary “washing” cycles, the highest observed
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