Van Tam Bui

Application of polymers for the long-term storage and disposal of low- and intermediate-level radioactive waste

Abstract Research carried out at the Royal Military College of Canada on the effects of mixed fields of radiation on high polymer adhesives and composite materials has shown that some polymers are quite resistant to radiation and could well serve in the fabrication of radioactive-waste disposal containers. A research program was launched to investigate the possibilities of using advanced polymers and polymer-based composites for high-level radioactive waste management on one hand and for intermediate- and low-level radioactive waste disposal on the other hand. Research was thus conducted in parallel on both fronts, and the findings for the later phase are presented. Thermoplastic polymers were studied for this application because they are superior materials, having the advantage over metals of not corroding and of displaying high resistance to chemical aggression. The experimental methods used in this research focused on determining the effects of radiation on the properties of the materials considered: polypropylene, nylon 66, polycarbonate, and polyurethane, with and without glass fiber reinforcement. The method involved submitting injection-molded tensile test bars to the mixed radiation field generated by the SLOWPOKE-2 nuclear reactor at the Royal Military College of Canada to accumulate doses ranging from 0.5 to 3.0 MGy. The physical, mechanical, and chemical effects of the various radiation doses on the materials were measured from density, tensile, differential scanning calorimetry, and scanning electron microscopy tests. For each polymer, the test results evidenced predominant cross-linking of the polymeric chains severed by radiation. This was evident from observed changes in the mechanical and chemical properties of the polymers, typical of cross-linking. The mechanical changes observed included an overall increase in density, an increase in Young’s modulus, a decrease in strain at break, and only minor changes in strength. The chemical changes included differences in chemical transition temperatures characteristic of radiation damage. All the changes in these properties are characteristic of the cross-linking phenomenon. For the glass-fiber-reinforced polymers, the results of the tests evidenced minor radiation degradation at the fiber/matrix interfaces. Based on these results, any of the investigated polymers could potentially be used for disposal containers due to their abilities to adequately resist radiation. This allowed proceeding one step further into determining a potential design framework for containers for the long-term storage and disposal of low- and intermediate-level radioactive waste.

Solvent effect on the equilibrium polymerization of α-methylstyrene

Abstract The Flory-Huggins interaction parameter χ has been determined from static vapour-pressure measurements for four binary mixtures: α-methylstyrene in p -dioxane or cyclohexane, and poly(α-methylstyrene) in p -dioxane or cyclohexane. For the first system, measurements were carried out at 25 and 40°C, yielding values between 0.12 and 0.18 for χ. The second one was investigated at 30, 35 and 40°C leading to χ values between 0.50 and 0.58. The values at 40°C for the third system lie between 0.40 and 0.46 when the polymer volume fraction varies from 0.10 to 0.35. The last system, also at 40°C, has χ values of 0.49 and 0.60 for polymer volume fraction of 0.10 and 0.27, respectively. These parameters were used to compute the equilibrium compositions of monomer and polymer, at any solvent concentration, for the polymerization of α-methylstyrene in different solvents.

Interaction parameters and the equilibrium anionic polymerization of α-methylstyrene in tetrahydrofuran

Flory–Huggins interaction parameters, χ12, have been determined for α-methylstyrene + tetrahydrofuran and tetrahydrofuran + poly(α-methylstyrene) mixtures from static vapour-pressure measurements. For the first system, experiments were performed at five temperatures varying from 20.0 to 40.0 °C and yield values of χ between –0.30 and –0.40. Experimental data for the second system were obtained at 25.0 and 35.0 °C, leading to χ values slightly greater than 0.40. These results are applied to the data obtained for the equilibrium anionic polymerization of α-methylstyrene in tetrahydrofuran and used for the computation of the free energy of polymerization, ΔG1c. Values of ΔG1c obtained in this fashion are found to be in good agreement with previously obtained results.

ADVANCED POLYMER COMPOSITES FOR THE FABRICATION OF SPENT NUCLEAR FUEL DISPOSAL CONTAINERS

Abstract In Canada, the spent nuclear fuel disposal method proposed is to permanently isolate the spent fuel in deep underground vaults carved in stable granite rock formations within the Canadian Shield, with the integrity of the isolation to be assured for a minimum period of 500 yr. The present work aims at determining the feasibility of using a consolidated composite material made of an advanced polymer called PEEK (Poly Ether Ether Ketone) and continuous graphite fiber to fabricate a container designed to isolate the spent nuclear fuel from the biosphere for such very long time periods. The research focused on submitting the PEEK-based composite material to a thermal and radioactive environment comparable to, and, in some aspects, more aggressive than, the conditions of exposure in the disposal vault. The changes to the physical, mechanical, and chemical properties of the material following prolonged exposure were then determined. The simulation of the environment was achieved by irradiating numerous test specimens in a mixed radiation field produced by a SLOWPOKE-2 nuclear research reactor at controlled ambient temperatures ranging from ˜20 to 75°C. The specimens were characterized via several methods: tensile and flexural testing, differential scanning calorimetry, scanning electron microscopy, and wide-angle X-ray scattering. The results confirmed that the PEEK-based composite material was resistant to exposure to high radiation doses (1 MGy), at temperatures between ˜20 and 75°C. The mechanical and other properties were barely affected, with values rarely exceeding 1σ of the properties of nonirradiated samples, suggesting that the PEEK–graphite fiber composite material can indeed be considered as a very good candidate for this demanding application.

Interaction parameters and the equilibrium cationic polymerization of tetrahydrofuran in benzene

Flory–Huggins interaction parameters, χ12, have been determined for tetrahydrofuran–benzene, benzene–polytetrahydrofuran and tetrahydrofuran–polytetrahydrofuran mixtures from static vapour-pressure measurements. Values of χ lie between –0.18 to –0.23 for the first system in the temperature range 10.0–35.0 °C. The second and third systems were investigated at 35.0 °C and χ values vary from 0.15 to 0.32, and 0.11 to 0.19, respectively, as the polymer volume fraction varies from 0.32 to 0.12. The results have been applied to the available data for the cationic equilibrium polymerization of tetrahydrofuran in benzene and have been used for the computation of the free energy of polymerization, ΔG1c. The need for a ternary interaction parameter of the type χmsp for the ΔG1c expression is discussed.