Experimental investigation of fission gas release from spent nuclear fuels under conditions expected in a deep geological repository
Authors: König Tobias , Bohnert Elke , González-Robles Ernesto , Herm Michel , Iglesias-Pérez Luis , Metz Volker , Walschburger Arndt , Geckeis Horst 
Published: September 2023

Abstract. In Germany, as well as several other countries (e.g.
Sweden, Finland and Switzerland), the disposal of spent nuclear fuel (SNF)
in deep geological formations, intertwined with a resilient multi-barrier
concept, is considered to be the favourable option for the safe and profound isolation of such wastes from the biosphere. However, the intrusion of
groundwater, followed by a failure of canisters and loss of cladding
integrity, has to be considered in the long-term evaluation of such an
underground repository. Therefore, a thorough process understanding of
SNF dissolution, radionuclide source terms and alteration processes is
necessary in order to evaluate the performance of SNF in the repository
system. The aforementioned processes can be assigned into two steps: (i) a
fast, initial release of radionuclides, located in accessible structures of
the SNF upon cladding failure, and (ii) a slower, long-term release,
resulting from the dissolution of the fuel matrix itself (Ewing, 2015). During the first stages of breaching of the container and the resulting contact of
groundwater with the waste form, a fraction of radionuclides, located in
easily accessible parts of the SNF such as grain boundaries or the
pellet-cladding gap, will be released relatively quickly. This fraction is
comprised of mostly volatile radionuclides, such as 79Se, 129I,
135Cs, and the fission gases Kr and Xe. Especially the release of
fission gases from SNF is often correlated to the release properties of
safety-relevant radioisotopes such as 129I or 135Cs and is, therefore,
an indicator of their release rates. In particular, 129I shows a
release behaviour similar to the fission gases. Within our contribution, we provide results regarding the release of fission
gases during the dissolution process of two different types of SNF, i.e.
UOX (50.4 GWd tHM-1) and mixed-oxide (MOX) (38.0 GWd tHM-1, where HM denotes heavy metals)
fuels, under simulated geochemical conditions, as they are expected in a
deep geological repository. Both examined fuel types were irradiated in
commercial pressurised water reactors in Switzerland and Germany during the
1980s. The shown data were compiled over more than a decade during
experimental campaigns performed at the Institute for Nuclear Waste
Disposal at Karlsruhe Institute of Technology (KIT-INE) within the framework of several
international programmes. Results obtained in our study indicate that,
independent of the type of SNF, a comparable, fast initial release of
fission gases is evident in all experiments. However, throughout the
experiments, this prompt release slows down significantly, although a
continuous release can be observed. Furthermore, the obtained data on
fission gas release are compared to the release behaviour of iodine and
caesium. An attempt is made to identify and qualify the role of fission gas
release rates as indicator of the radionuclide source term for SNF by
comparing and interpreting respective data collected over various
experimental campaigns.