Clinoptilolite
The clinoptilolite (CLINO) used in this study was sourced from the same deposit in the Mojave desert as that currently used in the SIXEP plant, Sellafield. The material was kindly supplied to the ISL group by Sellafield ltd. The density was measured at 2.222 ± 0.002 g cm-3 via helium pycnometry. The stated average particle size was 628 μm. An average particle size of 0.7 mm
± 0.1 mm was measured across 150 particles using Vernier callipers.
Ion Exchange
Ion exchange of the CLINO was performed using non-active isotopes of Cs and Sr. CLINO (1 kg) was placed in a solution of deionised H2O (1.25 L, 18.2 MΩ) with CsNO3 (11.2 g, Sigma-Aldrich, 99.99%) and Sr(NO3)2 (0.55 g, Sigma-Aldrich, ≥99%). This was left at room temperature for 28 days and agitated for one minute, once every seven days. The Cs and Sr exchanged clinoptilolite (CSXC-CLINO) were washed with deionised H2O (5 x 2L washes, 18.2 MΩ) and dried overnight in a 90 °C oven. After drying, the simulant was created through the addition of 10 wt % quartz sand (SiO2 > 99.5%) as per the physical description of the waste stream at Sellafield, obtained from the NDA’s waste stream data sheet [160]. Table 6.1 gives
the bulk clinoptilolite compositions before and after ion exchange, as measured using XRF on a Phillips PW2404 XRF Axios with ~10 g of powdered material. This exchange process represents both a significantly higher residency time and proportion of both Cs and Sr than found in the effluents and waste streams on the Sellafield site. The loading of 1 wt % Cs was chosen as the lowest concentration of Cs readily quantifiable by SEM-EDX and XRF analysis.
CPS testing for selection of additives and processing parameters
Five 1 g pellets of CSXC-CLINO were made using the CPS method in a uniaxial press with 13 mm hardened steel die at a pressure of 30 MPa. These pellets were pressed with the addition of NaAlO2 at 0 wt%, 2.5 wt%, 5 wt%, 7.5 wt% and 10 wt%. These pellets were sintered at 1300 °C for two hours in an electric muffle furnace with set ramp rates of 10 °C min-1.
Batching and Canister Preparation
Canisters were created according to the coin and tube design described in Section 3.3. Prior to canister loading the clinoptilolite was calcined at 700 °C for 6h. Samples were top filled with simulant to determine the approximate mass required to fill the canister. Once the approximate volume of simulant required to fill the canister was determined, the material was prepared prior to filling by the addition and mixing of either 5 wt% anhydrous NaAlO2
(Sigma-Aldrich >99.95%) or 5 wt% anhydrous Na2B4O7 (Sigma-Aldrich, >99%). The samples produced by HIPing the CSXC-CLINO simulant with these additives are identified from this point as CLINO-NaAlO2 and CLINO-BORAX respectively. The components were mixed by shaking in a polyethylene bag before top filling the canister.
No grinding was performed to mimic an industrial production line, avoiding the creation of fission product containing dusts. The canister lid was utilised to uniaxially press the powders in order to obtain a higher packing density. The canisters were sealed by autogenous TIG welding before being baked out under vacuum. The bake out of the canisters was performed at 300 °C to a vacuum level of ~3 Pa. Once vacuum was achieved the evacuation tube was crimped to seal the canister. A second crimp was applied and broken above the sealing crimp.
This was immediately sealed with a weld. The volume of the canister was recorded by measuring using the principle of water displacement.
Oxide
Table 6.1 - Composition of clinoptilolite before (CLINO) and after exchange (CSXC-C) with Cs(aq) and Sr(aq) at room temperature for 28 days (excluding SiO2 added to create simulant) with comparison to the active loading of radio Cs and Sr as seen in waste stream stored at Sellafield [160]. Compositions of CSXC-NaAlO2 and CSXC-BORAX as calculated from batch and as measured by XRF and ICP-MS after HIPing. * indicates measured composition, errors approximately 5 % of stated value. The exception is B2O3 where the error is substantially higher, potentially ± 100% of stated value as will be discussed later. Theoretical density stated measured on pulverised products via helium pycnometry, monolithic density measured in triplicate using the Archimedes methodology.
Hot Isostatic Pressing (HIPing)
HIPing was performed at the University of Sheffield’s research HIP facility using an AIP-630H HIP. Processing was performed using a molybdenum furnace with an argon pressurising medium at 1200°C, a 100 MPa pressure was applied for 2 hours with 10 °C min–1 heating and cooling rates. The volume of the canister was measured after processing via the displacement of water.
Sample Retrieval
Powder samples were retrieved by sectioning the welds of the canister lids with an abrasive saw, pressure was applied to the canister walls using a vice. This pressure created significant fragmentation in the processed wasteform, allowing the collection of fragmented and powdered material. Further size reduction was performed using a hardened steel percussion mortar. The powders obtained by this process were separated into appropriate size fractions by sieving.
Larger monolithic samples were obtained by sectioning the lids and walls from the canister using an Isomet 5000 linear precision saw fitted with a 0.9 mm diamond blade. The canister was then mounted for sectioning using a combination of hot wax mounting and vice mounting to minimise the potential fracturing effects due to excessive clamping forces. The retrieved monolith was further sectioned to obtain appropriate monoliths using an Isomet low speed saw fitted with a 0.3 mm diamond wafering blade. Monoliths were prepared for investigation by grinding using P1200 grit SiC papers and polishing with 6, 3, 1 and ¼ μm diamond pastes.
Static Aqueous Durability Experiments
PCT type protocols were performed with a particle size of 106-180 μm and a SA/V ratio between 2150 m–1 and 2200 m–1 for PCT. MCC-1 experiments were performed on samples polished to a 0.25 μm finish with a SA/V of 10 m-1. Both used a temperature of 90 °C and 18.2 MΩ H2O.
SPFT Experiments
The methodology utilised to perform SPFT is described in more detail in Section 3.10.3 and 3.10.4. A detailed study of the aqueous durability of both CSXC-NaAlO2 and CSXC-BORAX was performed in this chapter. A matrix of the experimental conditions is provided in Table 6.2.
The methodology used is outlined in Section 3.10.4. SPFT was performed using the following pH values at 90 °C; 2, 4, near neutral (~6), 9 and 11 with an approximate log(Q/S) of -7.0.
Two other suites of experiments were performed on each wasteform. These were to study variations in temperature and log(Q/S) at pH 4. The first alterations were in log(Q/S) which were performed at 90 °C at pH 4 with log(Q/S) values of approximately -6.4, -6.7, -7.0 and -7.25. The variations in temperature were made at pH 4 and log(Q/S) -7.0 with variations in temperature of 50 °C, 70 °C and 90 °C.
Table 6.2 – Matrix of conditions used during SPFT testing of CSXC-NaAlO2 CSXC-BORAX.