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| Fig. 1: A potential energy diagram for a chemical reaction with and without a catalyst. (Source: Wikimedia Commons) |
One of the most serious problems with nuclear fission power usage is the generation of 70 kton of highly radioactive waste annually. [1] While this waste is currently stored onsite at the many nuclear generation stations across the globe, the radiative power held in the material is given off as waste heat. For the first 10 years the waste heat is so great that the waste is stored under constant cooling. [1] If the energy emitted from the spent nuclear fuel as alpha particles as well as β and γ rays could be utilized for practical purposes, it is a potential win-win scenario.
One mechanism by which the remaining energy in spent nuclear fuel can be effectively utilized is to provide the energy needed to catalyze chemical reactions. γ and β rays are particularly effective because their main mechanism of interaction with matter is through excitation of electrons, which are the main actors in chemical conversions. [2] The interactions of α, β, and γ rays with catalyst materials which perform chemical reactions have been studied in a variety of ways with many promising results. Here I present a review of some of the studies of potential uses of the radiation from spent nuclear fuel in heterogeneous catalysis.
A catalyst is a substance that accelerates a chemical reaction without being consumed. Chemical reactions of all kinds proceed through all sorts of highly energetic excited states without the presence of a catalyst. A useful way of conceptualizing a chemical reaction is with a potential energy diagram which plots the energy of the involved chemicals as a function of their progress in reacting (see Fig. 1). Catalysts in effect lower the highest energetic barrier of a chemical reaction through many mechanisms. β and γ ray-driven chemical reactions are promising because the transition state energy in a catalytic reaction typically involves an excited electron with energy equal to or higher than the transition state energy, which are easy to produce with high enough energy with β and γ rays.
Radiation-assisted chemical reactions (radiocatalytic reactions) are typically studied in three ways. Homogeneously, where catalyst molecules are dissolved in a solution that is then exposed to radiation and through 2 different types of heterogeneous (different phase) experiments. In heterogeneous radiocatalysis, catalyst solids can be exposed to some sort of radiation prior to introduction of reactant molecules or concurrent with exposure to the reacting molecules. A useful metric in discussing catalytic rates in relation to radiation exposure is the G ratio. The G ratio is defined as the number of reactions that are measured per 100 eV of radiation exposure. This metric allows many different researchers to normalize results across a wide variety of experiments.
Nanoparticle Ssynthesis: Nanoparticles made of transition metals are useful in catalyzing chemical reactions due to the high surface area afforded by their small dimensions. This has been studied as recently as 2008 when Roy and coworkers developed a method of AuPd alloy nanoparticle synthesis with γ radiation. [3] They use these nanoparticles as effective catalysts for groundwater pollutant remediation. Many other researchers continue to contribute to this field and have developed synthetic techniques for transition metal colloidal particle suspensions of catalytic metals and alloys from Nickel to Rhenium using both radioisotopes and external sources of γ radiation to make nanoparticles. [4]
N2O Lysis: A simple probe reaction that was one of the first gas phase reactions attempted with radiocatalysis was N2O lysis. In this reaction N2O is broken into N2 and O2 gas. Many metal oxide semiconductors are inert surfaces to the reaction under normal conditions, but after being exposed to many different types of radiation, inert metal oxides such as silica and alumina become active. [5] What is most remarkable about this system is that the G ratio of approximately 15-50 is independent of radiation type from α particles to recently spent nuclear fuel. The authors demonstrate a dependence on the metal oxide with alumina being the most active.
Nitrogen Fixation: The reverse reaction of N2O Lysis, nitrogen fixation with oxygen is potentially more useful as a potential fixed nitrogen source in forming nitrate for fertilizer is the fixation of Nitrogen to various nitrogen oxides. Gamma radiation as well as epithermal neutrons have shown to allow this reaction to proceed high rates on silica-based catalysts. [2]
Methane Radiolysis: Supported uranium catalysts have been shown to be active for methane radiolysis when exposed to beta radiation at a G ratio of ~9 in both heterogenerous and homogeneous systems. [6] This could be useful in chemical synthesis as methane is the least active hydrocarbon for functionalization and with radiolysis, up to 10% conversion to ethane, H2, and C3 hydrocarbons was observed, all of which are more valuable molecules.
Ammonia Production:Ammonia production by the Haber-Bosch method is important as a supply of fertilizer for the worlds population from Nitrogen from the air and hydrogen from fossil sources. The conditions of the reaction are intense ( > 35 bar and > 450°C) and require an Fe-Ru catalyst without radiation yet with a γ radiation source, production with a G ratio of around 1 is possible on bare alumina. [7] This turns an undesirable waste stream of unused γ rays into a potential foodsource.
Methanol Synthesis: Another useful synthesis enabled with radiation is methanol synthesis from CO and H2, for which Co γ radiation allows for a G ratio of 1 at atmospheric pressure and at moderate temperature (160°C) on ZnO. [8] This reaction would not proceed to high conversion at atmospheric pressure, which would open potential opportunities for lower pressure fuel synthesis with γ radiation.
Pollution Removal: Remediation of polluted water supplies is another area where radiation can assist a catalytic reaction. Recent reports show the effect of metal oxide materials acting as sensitizers to gamma radiation creating highly oxidizing species which can remove carbonaceous contaminants via oxidation to carbon dioxide. [9,10] The authors quantify conditions including necessary radiation dose required to remove some dye compounds that effectively model typical organic contaminants.
There are many challenging chemical reactions that require catalysis and are accelerated with radiation that could easily be obtained from spent nuclear fuel. Moreover, energy in the form of γ rays, β rays, and energetic neutrons is more targeted than thermal energy and can directly add energy necessary to perform some useful chemical transformations. All geopolitical concerns aside, it would make obvious sense to use the energy in the high energy radiation waste for chemistry as has been demonstrated in these selected studies. Unfortunately, given increased proliferation risks and safety dangers associated with processing of spent nuclear waste, it is unclear that the modest gains in chemical reaction rates presented in these studies give enough reason to pursue using this waste stream as a chemical energy source.
© Andrew Riscoe. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. G. Croff, M. S. Liberman and G. W. Morrison, "Graphical and Tabular Summaries of Decay Characteristics for Once-Through PWR, LMFBR and FFTF Fuel Cycle Materials," Oak Ridge National Laboratory, ORNL/TM-8061, January 1982.
[2] R. Coekelbergs, A. Crucq, and A. Frennet, "Radiation Catalysis," Adv. Catal. 13, 55 (1962).
[3] K. Roy and S. Lahiri, "In situ γ;-Radiation: One-step Environmentally Benign Method to Produce Gold-Palladium Bimetallic Nanoparticles." Anal. Chem. 80, 7504 (2008).
[4] A. Abedini et al., "A Review on Radiation-Induced Nucleation and Growth of Colloidal Metallic Nanoparticles," Nanoscale Res. Lett. 8, 474 (2013).
[5] G. R. A. Johnson, "The Nitrous Oxide Radiation Dosimeter," J. Inorg. Nucl. Chem. 24, 461 (1962).
[6] F. W. Lampe, "High Energy Electron Irradiation of Methane. Remarks on the Reaction Mechanism," J. Am. Chem. Soc. 79, 1055 (1957).
[7] N. Getoff, "Radiation-Induced Synthesis of Ammonia from Nitrogen and Water," Nature 210, 940 (1966).
[8] T. I. Barry and R. Roberts, "Effect of Gamma-Radiation on the Synthesis of Methanol over Zinc Oxide," Nature 184, 1061 (1959).
[9] D. Şolpan et. al, "High-Energy Irradiation Treatment of Aqueous Solutions of Azo Dyes: Steady-State Gamma Radiolysis Experiments," Radiat. Phys. Chem. 67, 531 (2003).
[10] G. A. Zacheis, K. A. Gray, and P. V. Kamat, "Radiation-Induced Catalysis on Oxide Surfaces: Degradation of Hexachlorobenzene on γ-Irradiated Alumina Nanoparticles," J. Phys. Chem. B 103, 2142 (2002).
