Sorption of Plutonium and Americium on Repository, Backfill and Geological Materials Relevant to The JNFL Low-Level Radi
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SORPTION OF PLUTONIUM AND AMERICIUM ON REPOSITORY, BACKFILL AND GEOLOGICAL MATERIALS RELEVANT TO THE JNFL LOW-LEVEL RADIOACTIVE WASTE REPOSITORY AT ROKKASHO-MURA G M N BASTON, J A BERRY, M BROWNSWORD, T G HEATH, C J TWEED AND S J WILLIAMS AEA Technology, 220 Harwell, Oxfordshire, UK ABSTRACT An integrated programme of batch sorption experiments and mathematical modelling has been carried out to study the sorption of plutonium and americium on a series of repository, backfill and geological materials relevant to the JNFL low-level radioactive waste repository at Rokkasho-Mura. The sorption of plutonium and americium on samples of concrete, mortar, sand/bentonite, tuff, sandstone and cover soil has been investigated. In addition, specimens of bitumen, cation and anion exchange resins, and polyester were chemically degraded. The resulting degradation product solutions, alongside solutions of humic and isosaccharinic acids were used to study the effects on plutonium sorption onto concrete, sand/bentonite and sandstone. The sorption behaviour of plutonium and americium has been modelled using the geochemical speciation program HARPHRQ in conjunction with the HATCHES database. INTRODUCTION The work reported in this paper forms part of the performance assessment of a planned second-phase extension to the JNFL low-level radioactive waste repository at RokkashoMura. The sorption of plutonium and americium on repository, backfill and geological materials has been very widely studied in many countries (see eg reference 1). However, for the JNFL performance assessment, sorption data were required for these radioelements on JNFL's proposed engineered barrier materials and site-specific geological materials. Data were also required to quantify the effects of certain organic degradation products on have previously been shown to increase radioelement solubility sorption as similar solutions 2
and reduce sorption .. An integrated programme of batch sorption experiments and mathematical modelling has therefore been carried out. Sorption on samples of concrete, mortar, sand/bentonite (85%:15%), tuff, sandstone and cover soil has been investigated. Work with concrete, mortar and sand/bentonite was carried out at pH -12; that with cover soil at pH 8, while that with sandstone and tuff was undertaken at both pH 8 and pH 12, since the rocks surrounding the repository could be exposed to pH 12 porewater emanating from the concrete structure. In addition, specimens of bitumen, cation and anion exchange resins, and polyester were chemically degraded. The resulting degradation product solutions, alongside solutions of humic acid (acting as a typical natural organic material) and 2-C-(hydroxymethyl)-3-deoxy-D-pentonic acid, (iso-saccharinic acid, ISA, a major complexant produced by the alkaline degradation of cellulose 2) were used to study the effects on plutonium sorption onto concrete, of the organic sand/bentonite and sandstone. Aerobic conditions for the degradation materials were chosen as a "worst case", based on earlier work2 , where
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