High temperature water radiolysis

In operational reactors it is necessary to select conditions such that the radiolytic decomposition of the water is suppressed, and this is achieved most effecively when the radiation chemistry of water under reactor conditions is understood. Thus, as with water radiolysis imder normal ambient conditions, one needs to know (a) the yields (g-values) of the primary products formed in reaction (1)  

This chapter begins with a brief sununary of the scheme for water radiolysis, followed by a description of the chemical systems used to obtain radiation chemical yields, or G-values, and rate constants at elevated temperatures that are pertinent to this scheme for both high and low LET, in H2O and DjO. Next, there is a section showing how the data can be accommodated in a simple spur difiusion model, and finally Arrhenius parameters for a number of reactions of more general interest are presented and discussed. 

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Buxton GV. (2001) High temperature water radiolysis. In Jonah CD, Rao BSM (eds.), Radiation Chemistry — Present Status and Future Trends, pp. 145-162. Elsevier, Tokyo. 

Potential fusion appHcations other than electricity production have received some study. For example, radiation and high temperature heat from a fusion reactor could be used to produce hydrogen by the electrolysis or radiolysis of water, which could be employed in the synthesis of portable chemical fuels for transportation or industrial use. The transmutation of radioactive actinide wastes from fission reactors may also be feasible. This idea would utilize the neutrons from a fusion reactor to convert hazardous isotopes into more benign and easier-to-handle species. The practicaUty of these concepts requires further analysis. 

T o grasp the chemical condition of water in the pressure vessel, direct measurement is practically impossible because of high pressure, high temperature, and intense radiation. In order to predict the concentrations of water decomposition products, a computer simulation should be applied. This idea was found in 1960s [1-3]. To perform the simulation, both a set of G-values for water decomposition products and a set of reactions for transient species are necessary. For these two decades, much effort has been made in Sweden, Denmark, United Kingdom, Canada, and Japan to evaluate the G-values and rate constants of the reactions at elevated temperatures up to 300 °C, and now there are practically enough accumulated data. There are several reviews of water radiolysis at elevated temperatures [4-7] and examples of practical application of the radiolysis in reactors [8,9]. 

Theoretical Calculations of High-Temperature Radiolysis of Water... 

A pulse radiolysis study of Mn-SOD from B. stearothermophilus has indicated a fast outer sphere mechanism of the interaction of O2 with the metals at low O2 concentrations (5.6X109 M s 1) and a slow inner sphere mechanism at higher O2 concentrations (4.8X107 M-1 s l). The latter mechanism is becoming more important at high temperatures.78 NMR studies support an outer sphere mechanism by which O2 interacts with the Mn3+ through Mn-bound water.79 The proposed reaction for the fast cycle is ... 

The temperature dependence of the absorption spectrum of the solvated electron has been recorded not only in water but also in alcohols (Fig. 3). Measurements are performed using nanosecond pulse radiolysis with a specific cell for high temperature and high pressure in a temperature range up to around 600 K depending on the solvent. Indeed, by increasing the temperature, the decay of solvated electrons becomes faster for example, this decay is much faster in alcohols than in water, so, the data obtained with nanosecond set-up are limited at lower temperatures for alcohols compared with water. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

Lin M, Katsiuniu-a Y, Muroya Y, He H, Wu G, Han Z, Miyazaki T, Kudo H. (2004) Pulse radiolysis study on the estimation of radiolytic yields of water decomposition products in high-temperature and supercritical water Use of methyl viologen as a scavenger. /Phys Chem A 108 8287-8295. 

Wu G, Katsumura Y, Muroya Y, Li X, Terada Y. (2001) Pulse radiolysis of high temperature and supercritical water Experimental setup and observation. Radiat Phys Chem 60 395-398. 

It was some ten years before any systematic program of pulse radiolysis at high temperature had begun when Christensen and Sehested [75] had available a pulse radiolysis cell that enabled measurements to be made up to 320 °C and 140 bar. This work focused on the radiation chemistry of water at elevated temperatures because of its relevance to the radiation chemistry occurring in the primary cooling circuits of pressurized water reactors used for electricity generation. 

We recently reported (3) that ultrasonic irradiation of alkaline oxic aqueous solutions of bivalent sulfur, S(-II), at 20 kHz resulted in the rapid oxidation of S(-II). The observed distribution of the oxidation products was similar to that reported for y-radiolysis of S(-II). The ultrasound-induced oxidation of S(-II) in alkaline solutions was attributed to the reaction of HS" with OH radicals (3). These radicals form during ultrasonic irradiation of water as a result of the high-temperature decomposition of water vapor inside the hot cavitation bubbles (1,2). Although the experimental results were qualitatively consistent with our proposed mechanism, some questions remained unanswered concerning the amount of OH released into the aqueous phase and the existence and relative importance of additional oxidants. 

The explanation of this behavior, which has paramount importance to the nuclear industry, was given by Allen [3]. After World War II, Allen depicted a model of water decomposition under radiation that considers the production and consumption of Hj. The key role of OH, H2 and O2 involved in the chain as a carrier or a breaker is clear (see Inset). Within this chain reaction, the reaction between H2 and O2 (which is thermodynamically favorable) takes place in water at high temperature only in the presence ofa catalyst such as copper or silver cations. In the radiolysis of water, the reaction can take place at room temperature in the presence of free radicals which form the molecular products H2, H2O2 and O2 at the first step inside the tracks or the spurs. Subsequently in the bulk of the solution, the free radicals which have escaped recombination in the tracks recombine as molecular products into water. The molecular products are formed in the nanosecond range and their recombination takes place in the millisecond range. 

The requirement, then, for reactor coolant chemistry is to measure up to 300 C in light and heavy water the primary yields for low and high LET radiation, and the rate constants of the spm reactions. It is also of intrinsic interest to test the spur-diffusion model for water radiolysis over a wide range of temperature. 

The self reaction of the hydrated electron (R4 in table 1) is an important contributor to g(H2) in water radiolysis [1]. Its rate constant has been measured by Christensen and Sehested [22] in alkaline solution. They found that kobs increased with temperature up to 150 °C and then decreased at higher temperatures, the value at 250 °C being about the same as that at room temperature. However, they also foimd kobs at high temperature to be independent of pH (room temperature values) for pH > 10, but to increase sharply below pH 9. These findings were interpreted [22] in terms of reactions (12) and (13) ... 

Material and structural issues to be addressed are primarily related to the potential for corrosion and stress corrosion cracking under irradiation at the high temperatures and pressures associated with the SCWR. Materials for cladding and structural components must be identified and tested to demonstrate their performance in thermal and fast-spectrum reactors. Radiolysis and water chemistry at supercritical conditions must be investigated to understand the effect on reactor materials. Specific material properties to be investigated include dimensional and microstructure stability, and strength, embrittlement, and creep resistance characteristics of the materials. 

The radical reactions with water should appear in chemical kinetic models to assure a correct analysis of kinetic data for processes in hot compressed water. The example of reaction (15.19) shows that the high temperature rate constants reported from pulse radiolysis measurements may require re-evaluation. 

G. Wu, Y. Kalsumura, Y. Muroya, X. Li, Y. Terada, Pulse radiolysis of high temperature and supercritical water experimental setup and e-observation Radiation Physics and Chemistry, 60 (2001) 395-398. 

G. Wu, Y. Katsumura, et al., Pulse Radiolysis of High Temperature and Supercritical Water Experimental Setup and e -Observation, Radiation Physics and Chemistry, Vol. 60, 395-398 (2001)... 

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