Behaviour of radionuclides in the presence of superplasticiser

Abstract

Superplasticisers improve the flow properties of fresh cement and offer undoubted benefits to the construction sector. There is concern in the nuclear industry, however, that the presence of a superplasticiser in grout or backfill cement may increase the solubility of radionuclides in the cementitious pore water and/or reduce their adsorption from solution. This paper describes the effect of a commercial, polycarboxylated, polyether comb type superplasticiser on the behaviour of selected metals in blended cements through a series of batch and monolith leach experiments.Results of batch experiments show that the presence of free superplasticiser in solution reduces uptake of nickel ( 63 Ni) and europium ( 152 Eu) by both blast-furnace-slag-and pulverised-fly-ash-modified ordinary Portland cement.Further, metal bound in the presence of free superplasticiser is readily remobilised on exposure to fresh cement solution. Conversely, metal uptake is almost complete and appears irreversible when exposed to hardened cements prepared with superplasticiser as part of the original mix. Monolithic slag cement samples prepared with superplasticiser suffer from bleed, with the surplus water containing a significant proportion of the metals added, including uranium and thorium. Digital autoradiography reveals heterogeneous distribution of radioactivity in the monoliths and demonstrates that the dissolved metals have not been effectively immobilised throughout the specimen. The mobility of thorium may indicate similar behaviour by other tetravalent actinide species, notably Pu(IV) and Np(IV).

Factors affecting the suitability of superplasticiser-amended cement for the encapsulation of radioactive waste

“…Although there are clear operational benefits to be gained from using SPs, it is important from a safety case perspective to understand their effect on the longterm behaviour of the encapsulated radioactive inventory. Research has shown that these high-molecular-weight, watermiscible, organic polymers have the potential to both enhance radionuclide solubility (McCrohon and Williams, 1997) and inhibit the uptake and incorporation of radionuclides by the cement matrix (Young et al, 2013). Further, it has been shown that proprietary SPs vary substantially in their complexation behaviour depending on the cement and experimental conditions (Young et al, 2013).…”

“…Research has shown that these high-molecular-weight, watermiscible, organic polymers have the potential to both enhance radionuclide solubility (McCrohon and Williams, 1997) and inhibit the uptake and incorporation of radionuclides by the cement matrix (Young et al, 2013). Further, it has been shown that proprietary SPs vary substantially in their complexation behaviour depending on the cement and experimental conditions (Young et al, 2013). Commercial SP formulations may also contain a number of adjuncts, including de-foaming agents, biocides, viscosity modifiers and other impurities, all of which may affect cement pore solution chemistry.…”

“…In addition, a bespoke, synthesised PCE comb SP, purified by dialysis to be monomer-and adjunctfree, was subjected to the same tests to shed light on how the purity of the SP products might be a factor in their applicability for radioactive waste encapsulation. A final series of experiments was carried out in which a 14 C-labelled SP was added to the cement pastes in order to determine whether the plasticiser has any effect on the amount of bleed water produced and/or inhibits homogeneous uptake of the chosen radionuclides, as suggested by Young et al (2013).…”

Editorial

“…They show that the use of cement replacement materials can improve resistance to chloride ingress. However, they caution that carbonation may release bound chlorides, so that whilst measurement of free chlorides is usually employed as a ready way of testing for corrosion potential of reinforced concrete structures, the bound chloride content should not be overlooked.Taking durability performance to an extreme, Young et al (2013) consider the application of cementitious materials for the encapsulation of radionuclides. They have investigated the sorption behaviour of a range of radionuclides onto ggbs-and pfa-bearing grouts in the presence of superplasticisers.…”

“…Taking durability performance to an extreme, Young et al (2013) consider the application of cementitious materials for the encapsulation of radionuclides. They have investigated the sorption behaviour of a range of radionuclides onto ggbs-and pfa-bearing grouts in the presence of superplasticisers.…”