Chemical Engineering Journal 149 (2009) 143–152 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Long-term performance of zeolite Na A-X blend as backfill material in near surface disposal vault R.O. Abdel Rahman a,∗ , H.A. Ibrahim a , N.M. Abdel Monem b a Hot Laboratory Center, Atomic Energy Authority of Egypt, P.O. 13759, Inshas, Cairo, Egypt b Chemical Engineering Department, Faculty of Engineering, Cairo University, Egypt article info Article history: Received 28 June 2008 Received in revised form 6 October 2008 Accepted 9 October 2008 Keywords: Radioactive wastes Backfill material Synthetic zeolite Mathematical models abstract This study investigates the feasibility of using synthetic zeolite Na A-X blend prepared from fly ash as near surface disposal backfill material. Tests were conducted at laboratory scale to evaluate the physical and chemical properties of the prepared zeolite. The zeolite density, porosity, and particle size distribu- tion were measured. The distribution coefficient (K d ) value of Cs ions was evaluated using batch sorption experiment in synthetic groundwater to simulate possible conditions for near surface disposal. The tran- sient behavior of the batch sorption experimental data were analyzed using Lagergren, Ho and Mckay, and Morris–Weber rate models to assess the controlling mechanism of the sorption process. It was found that the sorption process is chemisorption and controlled by diffusion mechanism. The dispersional behav- ior of Cs ions on the prepared material was studied using column experiment and the hydrodynamic dispersion coefficient was determined. To provide an overall functional performance of the proposed backfill material, the long-term behavior of the prepared zeolite has been evaluated using computer model. This model consists of two modules that has been developed to study the migration of Cs radionu- clides from bare cementitious waste form through the backfill. The study compares the release rate from bentonite–crushed rock mixture to that from the prepared zeolite. The result demonstrates that synthetic zeolite Na A-X blend shows a better performance in terms of radionuclide containment. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Near surface disposal of low- and intermediate-level radioac- tive wastes (LILRW) is intended to isolate these wastes from the accessible environment during a period sufficiently long to allow substantial decay of the shorter lived radionuclides and, in the longer term, to limit releases of the remaining radionuclides. To achieve this aim, the multi-barrier concept that relay on engineered barriers to augment natural barriers has been developed. The use of engineered barriers helps in ensuring that increasingly stringent design aims are satisfied to an appropriate level [1]. This concept helps in avoiding over-reliance on one component of the disposal system (i.e. natural barriers) to provide the necessary safety and allow for certain component to fail without compromising the over- all safety of the disposal system [2]. Engineered barriers may consist of a number of separate com- ponents, including structural walls, buffer or backfill materials, chemical additives, liners, covers, leachate collection and drainage ∗ Corresponding author. Tel.: +20 16 1404462; fax: +20 2 462 0796. E-mail addresses: karimrehab1@yahoo.com, alaarehab@yahoo.com (R.O. Abdel Rahman). systems, cut-off walls, gas vents and monitoring wells [3]. Buffer or backfill materials surround the waste packages in their emplace- ment at the disposal facility. Their principle functions are to provide structural stability of the disposal units, reduce the potential for subsidence, minimize the transport of radionuclides released from failed waste packages, and to limit the access of any infiltrating water to the waste packages [4–6]. Different backfill materials have been suggested, notably cement grouts, sand, bentonite, rock, gravel, and compacted clays [5–10]. Bentonite–sand and bentonite–crushed rock mixtures are commonly used in nuclear waste repositories due to their high- erosion resistance, and their mechanical and chemical stability [11–14]. Research has been undertaken concerning evaluation of physical, chemical, hydraulic, thermal, and mechanical properties for different backfill materials [15–22]. Compacted clays, especially those containing appreciable amount of bentonite, have shown problems with shrinkage and or desiccation cracking [17,23–27]. On the other hand, zeolites has been suggested as alternative mate- rial because of its high-cation exchange capacity and mechanical stability [28,29]. Hot Laboratory and Waste Management Center (HLWMC) estab- lished a research program to synthesize zeolite from fly ash. The aim of this program is to solve the disposal problem of fly ash by utilizing 1385-8947/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2008.10.011