Reactions in cement encapsulated nuclear wastes: need for toolbox of different cement types

Abstract

Reactions that occur between a radioactive waste and a cement matrix being used for encapsulation can compromise the integrity of the resulting wasteform. The present paper describes some of the interactions that can occur within Portland cement based systems. It highlights the fact that to achieve successful encapsulation of the wide range of waste types that make up the legacy or historic wastes present in the UK, a toolbox or formulary of different cement systems with differing chemistries is needed. Some potential alternative cement systems are discussed.

Identification of the hydrate gel phases present in phosphate-modified calcium aluminate binders

“…In the CAC sample cured for 180 days, weight losses corresponding to dehydration/decomposition of the hexagonal metastable phases are not identified, and instead the peaks corresponding to dehydration of AH 3 and C 3 AH 6 are considerably more intense compared with those observed in specimens cured for 7 days. This indicates that the conversion of CAH 10 and C 2 AH 8 is essentially complete after 180 days of curing, in agreement with the diffraction data discussed in section 3.1. The residual mass loss in the low temperature range (below 200°C) for the 180 day cured sample can be assigned to the alumina-rich gel phase produced as a byproduct of conversion, as discussed above.…”

“…Depending on the duration and temperature of curing, the metastable hexagonal phases (CAH 10 and C 2 AH 8 ) undergo a conversion process whereby they dissolve and re-precipitate as the stable cubic hydrogarnet (C 3 AH 6 ), along with gibbsite and/or disordered aluminium hydroxide (AH 3 ). As a consequence of the differences in densities between the metastable and stable hydrate phases, this conversion can result in a significant loss of solid volume fraction within the hardened material, leading to a dramatic reduction in the loading capacity, and sometimes loss of mechanical integrity of the monolith [1].…”

“…These properties have made them suitable for use in several key applications [3], including as a biomaterial for bone and dental repairs [4], refractory concretes [5,6], specialised mortars for structural applications [1], carbonation resistant geothermal well applications [7], and more recently immobilisation of metallic radioactive wastes [8][9][10][11] and toxic metal solutions [12].…”

Hydration and properties of sodium sulfate activated slag

“…On the other hand, the addition of alkaline additives would increase the pH of the activated slag. Although the increased pH would be preferred for construction industry due to the enhanced stability of the reinforcing steel, it may cause concerns in some nuclear waste immobilization applications, as the increased pH could lead to the reactions between activated slag with certain metallic wastes, such as aluminium, causing expansive corrosion of metals and excessive generation of hydrogen, along with attacks on zeolites and glass which may exist in certain waste forms [37].…”

Evolution of phase assemblage of blended magnesium potassium phosphate cement binders at 200° and 1000°C

“…Their properties also include: low water demand, low drying shrinkage, and high early compressive strength 1 , which has favoured application of MKPCs to radioactive waste encapsulation 2 . MKPCs have been extensively researched at the Argonne National Laboratory (ANL), USA, to encapsulate wastes that are not compatible with standard cementation (blended Portland cement) such as nitrated wastes and Pu contaminated materials [3][4][5][6][7][8][9] .…”