Assessment of geopolymer material for the manufacture of a nuclear fuel core catcher

Abstract

Nuclear accidents such as the ones that occurred at Three Mile-Island and Chernobyl, have led to the requirement for enhancements in the development of various reactor materials, in order to maintain safety in the nuclear industry. During these types of nuclear criticality events, attained residual corium materials need to be comprehensively immobilized, to restrict the potentially harmful effects. Accordingly, research is needed for the development of new core-catcher materials that are more reliable against the release of radionuclides and hazardous aerosols into the environment, in the event of a nuclear accident. For this investigation, the applicability of geopolymer materials, produced from naturally occurring aluminosilicates through alkaline activation, was evaluated as potential candidate material for nuclear core-catching application. The aluminate species contained in the geopolymer matrices have been documented to afford an extremely robust compound lattice and accordingly is deemed a viable candidate to surpass the effectivity of standard OPC-type (Ordinary Portland Cement) compounds in the reduction of the potential release of radioactive isotopes and aerosols into the environment. The aim of this study was to develop and structurally evaluate OPC and geopolymer matrices and to use the OPC results as baseline standard for the envisaged geopolymer species to attempt to supersede current OPC core-catching application. This was done by assessing the durability of different OPC samples in comparison with various geopolymer matrices when exposed to the ASTM standard; water absorptivity, sulphuric acid resistance and compressive strength testing protocols. From the results, geopolymeric superiority was established and subsequent recommendations were made for optimized geopolymer mixtures that are ideal for core-catching applications. In this study the water to cement ratios (w/c ratios) for ambient cured OPC baseline pastes were varied between 0.31-0.40 and were subjected to air, water and plastic sealed curing. It was observed that increasing the w/c ratio, as well employing ambient curing conditions resulted in extremely porous samples with lower compressive strength values. However, OPC samples cured in sealed plastic bags with a 0.31 w/c ratio resulted in optimum compressive strength of (30.94 (33) MPa), a low percentage weight (% wt.) loss due to sulphuric acid digestion (83.95 (33)%) and lower sorptivity rates relative to other OPC samples. For the second part of the investigation, various fly ash based geopolymers with either silica fume or mineral clay additives and activated with various alkaline solutions were identified for the proposed application of the study. From the pre-screening process it was observed that plastic sealed cured fly ash based geopolymers (F-GlP) activated with 14 M NaOH/ Na₂SiO₃ solutions, displayed increased durability relative to the other fly ash/mineral clay geopolymeric mixtures. These samples were then subjected to various ASTM testing protocols to substantiate their geopolymer superiority over the OPC analogue mixtures. It was found that, contrary to OPC samples, increasing the alkaline solution to binder ratio (AS/B ratio) in geopolymers improves the durability of the geopolymers samples. As a result, F-GlP samples with a higher AS/B ratio (0.40) resulted in an optimum compressive strength of 35.1 (30) MPa, an insignificant degree of % wt. loss when exposed to sulphuric acid (1.28 (11)%) and low sorptivity rates. Accordingly, it is therefore evident that not only did the F-GlP mixtures significantly outperform the OPC idealized baseline values, but it was also possible to identify the 0.40 AS/B ratio as a mixing formulation for the proposed application of this study.