Abstract
Metallic Waste (MW) is one type of low and intermediate radioactive waste. It is estimated that the current fleet of civil nuclear power reactors throughout the world would produce about 500 000 tons of this kind of waste in the near future due to their advancing age. MW comes mainly from spent fuel reprocessing facility, storage ponds and reactor primary circuit systems. MW typically consists of stainless steels or nickel alloys, which are the primary components of nuclear installations. Aluminum and beryllium belong to this waste category and are also called Reactive Metallic Waste (RMW).
The major risk involved in trying to encapsulate RMW is corrosion, resulting in hydrogen release. In a specific pH range, MW forms a stable protective oxide coating. It is therefore important to employ matrices that provide a favorable environment, reducing the reactivity of MW and thus the production of hydrogen gas. Ordinary Portland Cement (OPC) is not recommended for RMW encapsulation due to its extremely alkaline pore solution which favors corrosion. Moreover, the high content of free water may further contribute to H2 production due to water radiolysis. One of the potential matrices being considered for the encapsulation of RMW is Magnesium Potassium Phosphate Cement (MKPC), produced from the acid-base reaction between magnesium oxide (MgO) and dihydrogen potassium phosphate salt (KH2PO4). The lower pH ensures RMW remains inside the passivation range, while maintaining high performance characteristics, such as compressive strength and stability towards leaching.
To assess the applicability of MKPC as a containment matrix for radioactive RMW, it is important to evaluate its resistance to radiation, thus ascertaining the radiolytic production of H2, as well as its durability. In this study, a Cs-137 gamma source has been used to irradiate:
i. at 200 kGy samples with different curing ages, ranging from few days to 28 days, to shed light on the effects of irradiation during the curing period,
ii. at different doses samples with the same 28 days curing time (reference) to better understand the effect of the total absorbed dose,
iii. at 200 kGy samples with slightly different water to binder ratio to investigate the impact on radiolytic production of H2.
To evaluate possible radiation-induced modification of the structure and morphology of the MKPC, XRD and SEM-EDX have been used. Micro-gas chromatography analyses have been performed to determine the amount of radiolytic H2 released. The water immersion resistance of irradiated MKPC is also evaluated by a standard leaching protocol. Non irradiated MKPC samples have been tested as reference. The encouraging results obtained in this work further promote the use of MKPC for the encapsulation of RMW.
