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Effect of sodium nitrate on radiolytic gas production of Portland-based materials containing blast furnace slag (MATRICE project)

David Chartier 1 
1 LCBC - Laboratoire d’étude des Ciments et Bitumes pour le Conditionnement
DE2D - Département de recherche sur les technologies pour l'enrichissement, le démantèlement et les déchets
Abstract : Arising from spent fuel reprocessing or national defence, nitrates are often encountered in nuclear waste such as decontamination slurries and concentrates. As this kind of waste is often conditioned by solidification in cement-based materials, it is of interest to assess its effect on the radiolytic gas production of cemented wasteforms. Indeed, nitrates are known to have a significant effect on water radiolysis and cement-based materials contain water, both as free water in the porosity and bound water of the hydrates (calcium silicate hydrates for instance). Water of cement-based materials is known to produce H2 under irradiation emitted by the waste and previous works report H2 production drop when nitrates are present in the mixing water. The aim of the present work is to focus on the effect of sodium nitrates on gas production of cement-based materials made of Portland cement with or without addition of blast furnace slag. Indeed, Portland cement is widely used to perform the solid-ification of nuclear waste and its partial substitution by blast furnace slag is relevant to decrease hy-dration heat or to improve compatibility with waste such as ion exchange resins. NaNO3 strongly affects gamma radiolytic gas production of Portland-based materials (with or without BFS) by reducing H2 production but other gas are generated: mainly O2 in the case of pure Portland binder and mainly N2 in the case of addition of BFS. The low production of O2 during gamma irradia-tion of cement pastes containing BFS and NaNO3 is probably linked to the presence of sulfide ions (S2-) in the slag (reductive conditions prevail in blast furnace to produce iron). Sulfide ions are thus released in pore solution during hydration of BFS and its partial oxidation into polysulfide radicals S2- , S3- and S4- were responsible for the well-known green-blue coloration of concrete and mortars con-taining alkali-activated BFS. This coloration remains in anoxic condition that prevails inside materials whereas it disappeared within a few weeks on the surface where atmospheric O2 is available and dry-ing facilitates its ingress in the material. Thus, it is conceivable that O2 produced by nitrate radiolysis may be trapped by polysulfide and sulfide oxidation within the material. This assumption is strength-ened by the fact that a discoloration is observed after irradiation. Complete discoloration of the ce-ment paste irradiated at high dose (20 MGy) is even observed. Nevertheless, this discoloration is not related to detectable mineralogical changes since X-ray diffraction pattern of the material was not significantly modified by irradiation. As expected for such cement paste, C-S-H, hydrotalcite, ettring-ite and Portlandite are the main phases detected by X-ray diffraction. The involvement of sulfide (which originates from BFS) in the absence of O2 production under gamma irradiation in the presence of NaNO3 has been strengthened by performing an experiment of addition of calcium sulfide (CaS) to a Portland paste (water/cement = 0.4) containing NaNO3 or not. This addition of CaS to Portland ce-ment without BFS is intended for to mimic sulfide brought by BFS. With such addition (2.25%m.CaS/cement, which is a representative amount of sulfide in BFS) to Portland paste contain-ing 2.74%m.NaNO3/cement, no significant O2 production was noticed whereas H2 production is low-ered compared to the paste with CaS but without NaNO3 as expected by the occurrence of NaNO3. Reduction of H2 production due to NaNO3 has also been evidenced under alpha irradiation with mor-tars containing alpha emitters.
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Submitted on : Friday, September 9, 2022 - 4:00:33 PM
Last modification on : Saturday, September 10, 2022 - 3:49:15 AM

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  • HAL Id : cea-03773976, version 1

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David Chartier. Effect of sodium nitrate on radiolytic gas production of Portland-based materials containing blast furnace slag (MATRICE project). NUWCEM 2022, May 2022, Avgnon, France. ⟨cea-03773976⟩

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