| The radiolysis of water is one critical issue for the nuclear fuel cycle. The radiolytic products affect the corrosion of reactor materials, redox of actinides in reprocessing, safety of geological disposal of radioactive waste. The aqueous solution in nuclear industry is usually under extreme conditions, such as high temperate, high pressure, and strong acidicity. The physicochemical properties of aqueous solution under exterme conditions are obviously different from those under ambient condtions. For example, there is local density agumentation in supercritical water, pKa for ’OH is not linearly to 1/T in high temperature range, and the radiolysis of strong acid solution is more complicated than the radolysis of pure water. The study on radiolysis of aqueous solution under extreme condition is vital to maintain the safety of nuclear power plants, extend the life of nuclear power plant, and reduce the environmental impact, and provides fundamental reserch for the development of advanced nuclear system.Firstly we investigated the reactivity and pKa of the radical produced in the radiolysis of aqueous solution. Hydroxyl radical (’OH) is one of the most important water decomposition products because it has strong oxidative property. Hydroxyl affects the electrochemical potential in aqueous solution, thus has influence on the corrosion of structral materials in nuclear station. The study on pKa and reactivity of ·OH is important for the safety of nuclear plant and life extension of nuclear station. In high-temperature aqueous solution, pKa of ·OH deviates from the linear relation with 1/T obviously. However, its property when t>200℃ has not yet been clarified well due to technical difficulty. Therefore, in this work, reaction kinetics and pKa value of ·OH radical at elevated temperature over 325℃ have been investigated by means of nanosecond pulse radiolysis. Transient absorption spectra of formed radical and rate constants for the reaction with some solutes in aqueous solutions of benzoate, nitrobenzene, and carbonate etc., have been investigated. It is shown that, the reactivity increases rapidly with temperature when t> 300℃. In addition, temperature dependent pKa value (·OH/O·-) up to 300℃ has also been evaluated by direct trace and indirect competitive reaction method. The pKa of ·OH decreases with temperature when t<250℃ and rises with temperature when t>250℃.A supercrirtical water (SCW) is the water at a temperature and pressure above its critical point. SCW has many special properties. In supercritical aqueous solution, the local density around solute molecule deviates from the bulk density. This affects the reactions in supercritical water, especially high-selectivity or high-speed reactions. Till now the local density for radicals in SCW has not been studied. The investigation on the local density in SCW is important for the SCW oxidation and SCW reactor. In this work we investigated the solvents properties in high temperature/high pressure aqueous solution. Solvatochromic shift of 4,4’-bpyH" in aqueous solutions at elevated temperatures up to supercritical conditions and in various organic solvents with different dielectric constants, was investigated by pulse-radiolysis technique.4,4’-bpyH· shows a stronger solvent-solute interaction in water than in organic solvents, perhaps due to the dipole-dipole interaction and hydrogen bond between 4,4’-bpyH’and water. At 380℃, local density augmentation, namely ρlocal-ρbulk, in supercritical water becomes 280 kg.m-3 (ρbulk= 208 kg.m-3), and the density enhancement factor is 8.9. Density fluctuation maximizes when ρbulk is around 120 kg.m-3. Density inhomogeneity decreases as temperature rises, but is still remarkable at 400℃.In PUREX processing, aqueous nitrc acid is used to disslove the spent fuel. The seperation of U, Pu, and minor actinides is performed in nitric acid aqueous solution. The radiolytic products of aqueous nitric acid affects the reprocessing of spent fuel. For example, the produced nitrous acid has influence on the redox of actinides, thus block the seperation of acidnides. A complete kinetic model for the γ and α radiolysis of nitric acid aqueous solution, especially the model involving redox of actinides, will benefit the reprocessing of spent fuel. A novel kinetic model for the γ-and α-radiolysis of nitric acid aqueous solution was established, which takes the radiolysis of solute and solvent, namely the direct effect and indirect effect, both into consideration. The simulation results performed with MCPA FACSIMILE program fitted well with the experiments in literature and in this work on the y radiolysis (60Co as radiation source) and a radiolysis (accelator or 238Pu solution as radiation source) of nitric acid aqueous solution. This confirms the validity of this model. With this model, the radiolysis of both y radiolysis and a radiolysis of nitric acid and nitrate aqueous solution were simulated. The important reaction paths and intermediate products for the generation and consumption of nitrite ion were found. Interestingly, it is shown that the redox reactions involving Pu cannot be neglected in the α-radiolysis of the solution with 238Pu as the radiation source, where the reaction between Pu and H2O2 is the most important reaction among thos involving Pu. It is predicted that, in the a radiolysis of aqueous nitric acid with 238Pu solution as radiation source, the yield of nitrous acid will increases with the concentrations for Pu(Ⅲ) and H2O2.The original results are summarized as the followings:1) The optical measurement of ·OH is quite difficult. Till now the pKa of ·OH in water has only been measured with t< 200℃.With pulse-radiolysis technique and high-temperature high-pressure optical cell equipped with sapphire windows, the pKa and reactivity of ·OH in water with temperature up to 325℃2) It requires high time resolution to detect local density in SCW with radical. The local density effect for 4,4’-bpyH· radical in supercritical water was measured by using ns pulse-radiolysis technique;3) The radiolysis of aqueous nitric acid is quite complex. In this paper, one novel chemical reaction model was built for the y and a radiolysis of nitric acid and nitrate aqueous solutions, where the redox reactions of Pu are included. The simulation results fit well with the experiemental data in various conditions. The reacction mechanism in the radiolysis of aqueous nitric acid is analyzed with this model. |
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