Small-angle neutron scattering as a probe to decide the maximum limit of chemical waste immobilization in a cement matrix

Immobilization of low- and intermediate-level radioactive waste in a cement matrix is an important process in any nuclear industry. In this regard, setting the maximum limit of waste loading in cement is a crucial issue and one that significantly depends on the mesoscopic density fluctuations of the composite cement matrix. Small-angle neutron scattering (SANS), a technique that maps coherent neutron scattering length density fluctuation, is efficient in monitoring the degree of homogeneity of a condensed matrix. It is prudent, then, to use SANS to probe the degree of homogenization in a composite cement matrix upon immobilization of low-level non-heat-generating radioactive waste. In the present work, the effects of loading of simulated cerium waste on the mesoscopic structure of hydrated cement have been investigated by SANS complemented by scanning electron microscopy and mercury intrusion porosimetry. Utilizing the multiple-scattering phenomenon, the scattering mean free path has been estimated, which has a direct relevance to the degree of homogenization of the waste-loaded cement matrix as far as the neutron scattering length density is concerned. It has been revealed that the structure of hydrated cement becomes less consolidated because the mass fractal dimension decreases with an increase in waste loading. Scattering data unveil the existence of pores at two widely separated length scales. The size of the larger pores increases with loading beyond a certain concentration. However, no significant modification has been observed for the pores on the smaller length scale.