Uptake of niobium by cement systems relevant for nuclear waste disposal: Impact of ISA and chloride

Abstract

The isotope Nb-94 (t(1/2) = 2.04 .10(4) a) is produced during the operation of nuclear reactors due to the neutron activation of Nb-93 present in reactor vessels. Related waste streams will be disposed of in cement-based repositories for low and intermediate level wastes (L/ILW). The retention of niobium by cement was investigated using a combination of Nb-95 and natural Nb-93. Sorption experiments assessed also the impact of isosaccharinic acid (ISA) and chloride on Nb retention, both expected in specific L/ILW. Sorption experiments revealed a strong uptake of Nb by cement. Isotopic exchange with Nb-93 in HCP partly explains Nb-95 uptake, although sorption on C-S-H is tentatively proposed as main retention process. Sorption of Nb is weakly affected by chloride, but decreases importantly at [ISA] >= 10(-3) M reflecting the formation of (Ca-)Nb(V)-ISA complexes. This work provides key inputs to assess Nb-94 retention in the Safety Case context of nuclear waste repositories.