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Development of innovative binders for the stabilization / solidification of low- or intermediate-level radioactive wastes containing aluminium
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Acronym: DABSRAWAL
Number / Date of the contract: C2-01 / 2012-03-01
Capacitati Modul III, Parteneriat IFA-CEA
Project Manager: Corneliu TURCANU (Ro); Celine CAU DIT COMES (Fr)
Partners: IFIN-HH, CEA Marcoule/ DEN / DTCD / SPDE / L2ED
Starting date / finishing date: 2012-03-01 / 2015-02-28
Project value: 480000 RON
Abstract: Abstract: It has long been common practice to stabilize and solidify low- and intermediate-level radioactive wastes using cement: (i) the process is relatively simple and inexpensive, (ii) properly set compositions are characterized by good self-shielding and high compressive strength, and (iii) the high internal pH of cement systems precipitates and thus confines many radionuclides. Cementitious materials intended for radioactive waste encapsulation usually include substantial amounts of Ordinary Portland Cement (OPC) in their formulation. However, wastes produced by nuclear activities are very diverse and, under certain circumstances, chemically react with cement phases or mixing water, thus reducing the quality of the product. For instance, the dismantling of old nuclear reactors generates a large volume of LLW and ILW wastes, some of them containing aluminium. Aluminium is a reactive amphoteric metal, readily forming a protective oxide layer on contact with air or water. This layer is generally regarded as stable in the pH range 4-10. However, in a strongly alkaline medium, such as that encountered in conventional cementitious materials based on OPC, this layer is soluble, resulting in continued corrosion with associated liberation of hydrogen and subsequent formation of expansive metal hydroxides, in addition to calcium-based aluminosilicate hydrates. As a result, using Portland cement, or a composite cement (OPC blended with blastfurnace slag and/or fly ash) to encapsulate wastes containing aluminium is prohibited. Some alternative cementing systems have different chemistries that may be more compatible with aluminium. The project aims at investigating alternative cement systems to provide innovative solutions for the solidification and stabilization of problematic historic wastes containing aluminium metal. The objectives are threefold: (i) compare the potential of calcium sulphoaluminate, magnesium phosphate, calcium phosphate and magnesium silicate binders to mitigate the release of hydrogen by a solidified waste form due to corrosion of aluminium, (ii) assess the feasibility to reduce the corrosion of aluminium still further by the addition of a corrosion inhibitor in the mixing solution, and (iii) develop, for the two most promising cementitious systems selected from steps (i) and (ii) a formulation checking the desired criteria for waste conditioning. The project will involve multi-disciplinary research groups with skills in materials science, physico-chemistry, analytical chemistry, and process engineering. Step 1: Screening of binders (30.11.2012) Pastes will be prepared with cements from the four investigated systems (calcium sulfoaluminate, magnesium phosphate, calcium phosphate, and magnesium silicate), with or without aluminium. The phase assemblage of the hardened materials will be characterized using X-ray diffraction and thermogravimetry. The pore solution of the material will be extracted using pressure and analysed by ICP-AES, pH-metry and ionic chromatography. The production rate of hydrogen by samples containing aluminium will be investigated periodically sampled and analysed for its hydrogen content using gas chromatography. At the end of the trials, the transition zone between metal and cement paste will be characterized using scanning electron microscopy, EDS analysis, and X-ray diffraction; the different cementitious systems will be ranked as a function of their capability to mitigate aluminium corrosion. Step 2: Investigation of corrosion inhibitors (30.11.2013) The corrosion rate of aluminium will be measured as a function of the nature and concentration of the inhibitor added to the synthetic pore solution of the most promising binders identified in step 1 and binders will be investigated. The influence of the inhibitor on the properties of the hardened material (strength, volume stability) will also be assessed. Finally, aluminium rods will be encapsulated using the most promising cements and inhibitors, and the materials will be characterized as previously described in step 1 (hydrogen production rate, solid phase assemblage, microstructure and composition of the transition zone between the binder and the aluminium rod). Step 3: Design of a cement formulation checking the criteria for industrial application and final disposal (30.11.2014) The third step of the project will aim at designing a pumpable, flowable and self-levelling grout using the most promising binders and inhibitors. The material will also have to check the specifications for a final disposal of the waste packages in a surface repository. The investigated recipes will be tested at the laboratory scale and optimized for their cement, water, and sand contents. The elaborated materials will be characterized using the conventional tools of civil engineering. Particular attention will be paid to the heat output at early age, in order to keep an acceptable thermal excursion when the grout will be cast in large volume canisters. Experimental matrices will be prepared and disposed in simulated (fast damaging) and normal repository (Romanian National Repository for Radioactive Waste Baita-Bihor) conditions. Step 4: Experimental data correlation in order to validate the optimum matrix for conditioning low-or intermediate-level radioactive waste containing aluminium (28.02.2015) In the last step there will be made a detailed evaluation of the results obtained in order to choose and validate the optimum recipe for preparing the innovative materials for conditioning low-or intermediate-level radioactive waste containing aluminium. Also the aspects that require further investigation will be pointed out, sketching new possible ways to approach them and opportunities for future collaboration.

THE STAGES OF THE PROJECT AND DELIVERY DATES
1. Screening of binders (2012-11-30)
2. Investigation of corrosion inhibitors (2013-11-30)
3. Design of a cement formulation checking the criteria for industrial application and final disposal (2014-11-30)
4. Experimental data correlation in order to validate the optimum matrix for conditioning low-or intermediate-level radioactive waste containing aluminium (2015-02-28)
RESULTS

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