Bentonite clay is used as buffer and backfill material which form engineered barrier in the spent fuel repository. The buffer material surrounding the copper canister will be exposed to gamma and neutron radiations, especially during the first few hundred years after closure of the repository. The redox states of the structural iron in montmorillonite, the dominant mineral in bentonite, have been shown to be sensitive to such irradiation in experimental studies using suspended bentonite. The change of the redox states of the structural iron will result in change of the charge density in montmorillonite mineral structure, which in turn can possibly influence some of the physicochemical properties of bentonite clay, such as swelling. In this study, the focus is on such radiation effect on compacted bentonite.
Molecular dynamics simulations show that the water radiolysis products H2O2, HO• and HOO• are neither strongly interacting nor strongly excluded from the interlayer regions of montmorillonite, although the latter two radicals show some preference to surface regions avoid of isomorphic substitution sites and high density of counter-cations. The same species also show significant affinity to the neutral clay edges modelled.
The changes in the structural Fe(II)/Fe(III) levels due to exposure to gamma radiation for compacted and water-saturated bentonite in general agree with the results in previous experiments obtained using clay dispersions. It generally shows that Fe(II) levels increase in place of Fe(III) with increasing gamma dose.
Investigations by means of isotope-ratio mass spectroscopy (IRMS) using isotopically labelled and non-labelled water show evidence of significant gas formation induced by gamma radiation. The effect of high levels of oxygen present potentially pre-irradiation in some samples should further be investigated before drawing conclusions regarding the gas formation of O2 and CO2.
In review of SKB’s reporting of safety after closure of the spent fuel repository in the framework of license application, SSM pointed out that the effect of gamma radiation on the redox states of structural iron of montmorillonite in bentonite clay remained to be an issue that needed further investigation. SSM considered that this is a mechanism that can possibly affect the swelling and other properties of the bentonite clay by changing the charge density in the mineral structure of montmorillonite.
Need for further research
The results of this study confirm previous reports that gamma irradiation under anoxic conditions reduces Fe(III) to Fe(II) in montmorillonite mineral and increases radiochemical yields of hydrogen. However, this study also revealed practical difficulties with the methodology used and highlighted limitations in investigating the radiation chemistry of heterogeneous and complex geochemical systems. To overcome some of these hurdles, new synchrotron-based experiments need to be undertaken. Such experiments would not only allow for on-the-fly irradiation and characterisation due to the high flux rates and brilliance at modern synchrotron facilities but also resolve the existing discrepancy in measured Fe(II)/Fe(III) levels between traditional wet chemistry methods (as used in the current study) and previous synchrotron studies that did not take radiation chemical effects into consideration.