Project No. 14-6309 Zeolite Membranes for Krypton/Xenon Separation from Spent Nuclear Fuel Reprocessing Off-Gas Fuel Cycle Research and Development Sankar Nair Georgia Institute of Technology Patricia Paviet, Federal POC Robert Jubin, Technical POC Final Project Report (Oct 2014-Dec 2017) Project: Zeolite Membranes for Krypton/Xenon Separation from Spent Nuclear Fuel Reprocessing Off-Gas (Project 14-6309, PICS WP NU-14-GA-GT-0201-04, CID NE0008298) List of Graduate and Undergraduate Students Supporting Project Yeon Hye Kwon, South Korea, Chemical Engineering, expected graduation in 2018, pursuing Ph.D. Byunghyun Min, South Korea, Chemical Engineering, expected graduation in 2019, pursuing Ph.D. Shaowei Yang, China, Chemical Engineering, Post-doctoral Fellow. Kiwon Eum, South Korea, Chemical Engineering, graduated in 2016, Ph.D. Emily Benjamin, USA, Chemical Engineering, graduated in 2016, B.S. Christine Kiang, USA, Chemical Engineering, expected graduation in 2018, pursuing B.S. Abstract The overall focus of this project is to develop and understand SAPO-34 zeolitic membranes that can separate mixtures of radioisotope krypton-85 and xenon released as off-gases during used nuclear fuel recycling. The primary advantage of separating 85Kr from Xe is to reduce the volume of radioactive waste for storage. The second advantage is the revenue generated from the sale of high-purity Xe. Zeolite membranes are attractive because of their much lower energy requirements relative to cryogenic distillation, and their high resistance to radiation degradation. We report the detailed study of silicoaluminophosphate zeolite SAPO-34 materials and membranes for this application, due to hypothesized favorable molecular sieving properties. In the 3-year Mission Support project, we developed a novel, high-performance, low- energy intensity, lower-cost zeolite membrane process for Kr/Xe separation during SNF processing; and investigated the underlying molecular adsorption and transport processes in both ‘idealized’ and ‘realistic’ operating conditions to develop reliable synthesis-structure-property relationships for such membranes. Adsorption and diffusion measurements on SAPO-34 crystals indicate their potential for use in Kr-Xe separation membranes, but also highlight competing effects of adsorption and diffusion selectivity. SAPO-34 membranes synthesized on α-alumina substrates via steam-assisted conversion seeding and hydrothermal growth are characterized in detail, with Kr permeances 26 GPU and ideal Kr/Xe selectivities >20 at 298 K after thickness reduction. Post-synthesis cation exchange shows large (>50%) increases in selectivity at ambient or slight sub-ambient conditions. In addition, we confirm that SAPO-34 membrane is stable under radiation exposure and the impact of radiation exposure on membrane performance would not be substantial. We also successfully synthesized hollow-fiber SAPO-34 membranes with the same performance levels as the disk-type and tubular membranes. This important development will allow a very compact and low-cost Kr/Xe separation system. Finally, a detailed process calculation for techno-economic analysis was performed by integrating Maxwell-Stefan model into cross-flow membrane system, in order to estimate the required number of membrane stages and the total cost. Background of Program Objectives Nuclear energy is an efficient and economical means of providing carbon-free energy, but requires effective management of radioactive waste products [1, 2]. As part of the overall process for recycling spent nuclear fuel (SNF), it is necessary to develop technologies of advanced and cost-effective SNF recycle and waste minimization for long-term energy security. Recycling of zircaloy (the second largest mass in SNF) has been investigated to maximize reuse and minimize waste volume requiring geological disposal. This involves an integrated process (e.g., Figure 1), including chemical zircaloy decladding, removal of fission products by voloxidation, and dissolution. separate the off-gases formed during the fission of uranium and plutonium in nuclear power reactors [3]. Separating and capturing each gas is critical to reduce the volume of radioactive Figure 1: Process diagram for off-gas treatment. Red arrows indicate solid and aqueous streams (uranium and plutonium), blue arrows are gas streams (process off-gas). Kr/Xe separation, which is the main focus of the proposed work, is highlighted at the bottom left. waste for storage. The long-lived radioisotope 85Kr and stable 136Xe are released as process off- gas and Kr/Xe separation is challenging because they are chemically inert and physically very similar. Based on data from a US national laboratory, the Kr/Xe mixture is approximately 10/90 by volume [4] and hence the separation of Kr would reduce the volume of radioactive 85Kr waste by a factor of 10. One way of separating them is by cryogenic distillation [3, 5] wherein they are condensed out as pure streams at very low temperature (Kr boiling point: -153°C, Xe: -108°C). However, cryogenic distillation has intensive power requirements and a large system volume, resulting in high capital, operating, and disposal costs. Membrane-based separation using radiation-stable materials is an attractive alternative [6, 7], if membranes with sufficient separation properties are successfully fabricated and scaled up. Chabazite (CHA) framework type zeolites have a nominal pore size of 0.38 nm, which is in between the kinetic diameters of Kr (0.36 nm) and Xe (0.396 nm), and it has been recently shown that silicoaluminophosphate CHA (also referred to as SAPO-34) membranes are good candidates for Kr/Xe separation [7, 8]. Main Objectives and Results Our proposed work was divided into 5 main objectives. In this report we describe our accomplishments in each objective over the project performance period . Objective 1: Understanding and control of intrinsic Kr and Xe adsorption/diffusion properties of SAPO-34. The main goals of this work were to synthesize SAPO-34 crystals of appropriate dimensions and obtain the adsorption and diffusion properties of Kr and Xe as a function of temperature and pressure from a well-defined set of SAPO-34 materials of tuned SixAlyPzO2 (x + y + z = 1) compositions and containing exchanged metal cations. Due to limited information on intrinsic Kr and Xe adsorption and diffusion properties which determines the membrane performance, we synthesized SAPO-34 crystals and obtained its adsorption and diffusion parameters using pressure-decay measurement over 10 – 400 kPa and at 308 – 343 K. Figure 2. Single-gas Kr and Xe adsorption isotherms on (a) H-SAPO-34 at different temperatures and (b) H-SAPO-34 and ion-exchanged M-SAPO-34 materials. Symbols: experimental data, dashed lines: Langmuir fits. SAPO-34 crystals were synthesized from a reactant molar composition of 1.0 Al2O3:1.0 P2O5:0.32 SiO2:1.0 TEAOH:1.6 CHA (cyclohexylamine):52 H2O with aluminum hydroxide (Al(OH)3) as the Al source. The average crystal sizes and standard deviations are 10.7±2.3 µm (SAPO-34-CHA) and the edge dimension of the cuboid crystals is used as the effective crystal diameter, since this achieves the required equivalence of surface area-to-volume ratio with a spherical crystal of the same diameter. The crystals were immersed in the ethanol solution of acetate salts of monovalent (Li+, Na+, and K+) cations for ion exchange. Single-component gas adsorption isotherms of Kr and Xe were collected and fitted to Langmuir model (Equation 1) by a global fit of the isotherms over all temperatures (Figure 2a), 푞 푏 푝 푞 = 푠푎푡 (1) 1 + 푏 푝 where q is the adsorbate concentration in the sample, qsat the saturation coverage, p the pressure and b the Langmuir constant which can be expressed in terms of the enthalpy of o adsorption (ΔHads) and the pre-exponential entropic factor b = exp (ΔSads/R). The enthalpy and entropy of Xe adsorption are larger than those of Kr, leading to the stronger adsorption of Xe. Table 1 also shows the M-S diffusion parameters obtained from the kinetic uptake data. The diffusion activation energy of Kr is much lower than that of Xe which is consistent with the kinetic diameter of Kr and Xe in relation to the crystallographic pore size of SAPO-34. Table 1. Langmuir Adsorption Parameters and Maxwell-Stefan Diffusion parameters for Kr and Xe in H- SAPO-34 0 qsat -ΔHads b0 D MS Ea Gas (mmol/g) (kJ/mol) (*10-6 kPa-1) (*10-10 m2/s) (kJ/mol) Kr 6.1 ± 0.2 16.1 ± 0.3 1.4 ± 0.3 2.5 ± 0.3 13.9 ± 1.0 Xe 4.2 ± 0.1 18.3 ± 0.4 3.8 ± 0.2 2.0 ± 0.1 23.6 ± 0.8 Adsorption isotherms on H-SAPO-34 and the three ion-exchanged M-SAPO-34 materials were measured at 298 K over 10-300 kPa (Figure 2b) which showed all the materials preferentially adsorb Xe over Kr. Table 2 summarizes the textural properties obtained by N2 physisorption at 77 K, and the results of fitting the Kr and Xe adsorption isotherms with the Langmuir model. The BET surface area and micropore volume decrease monotonically (by 10 – 36 %) as the cation size increases (H+ < Li+ < Na+ < K+). The Langmuir constants (b) are not strongly influenced by ion exchange, indicating that the van der Waals interactions between the noble gas atoms and SAPO- 34 are not substantially affected by the cations. This is consistent with a previous report that lithium cation exchange does not notably change the heats of adsorption of CO2 and CH4 in SAPO-34 [9]. However, the Kr and Xe saturation capacities (qsat) decrease in the same order as the micropore volume, consistent with the decreased pore volume available as larger sizes and numbers of cations are introduced. Table 2. Gas adsorption and textural properties of SAPO-34 materials as obtained from Kr and Xe adsorption (298 K) and nitrogen physisorption (77 K) isotherms. H-SAPO-34 Li-SAPO-34 Na-SAPO-34 K-SAPO-34 BET surface area [m2/g] 480 429 323 306 Micropore volume [cm3/g] 0.23 0.21 0.20 0.18 qsat Kr 6.7 6.1 5.6 5.2 [mmol/g] Xe 5.2 4.8 4.4 4.1 b Kr 9.0 8.3 8.2 8.2 [10-4 kPa-1] Xe 39 37 36 39 The bulk compositions of the as-synthesized H-SAPO-34 and cation-exchanged M-SAPO- 34 (M = Li, Na, K) materials were determined using ICP-MS (H-SAPO-34 and Li-SAPO-34) and EDX analysis (H-SAPO-34, Na-SAPO-34, and K-SAPO-34) of the Si, P, and Al content.