Scheme for the radiolysis of water

According to the model, ionisation and excitation events and the resulting products Caq , H, H2, OH, H2O2 and occur in clusters called spurs. For low LET radiation the spurs are separated by large distances relative to their diameter for high LET radiation they overlap to form a continuous cylinder. The decomposition products then diffuse randomly and either react together or escape into the bulk solution. It is the competition between reaction and escape which determines the yields of the radical and molecular species extant when the spur processes are complete. The time for this completion is generally taken to be 10 -10 s and the yields at this time are known as the primary yields. The spur reactions are listed in Table 1. 

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. 

A suitable system for the measiuement of g(eaq ) and g(H) + g(H2) up to 200 °C comprises a deaerated solution containing 10 mol dm sodium nitrate and 5 x 10 mol dm sodium phosphite [2,3]. Steady-state radiolysis of this system produces nitrite ion and hydrogen, which are measmed at room temperature. G(N02 ) provides a measure of g(Caq ) and G(H2) is equated with g( + g(H2). Pulse radiolysis can also be used to measure g(eaq ) but a limitation is that the molar absorption coefficient, e, of the species formed has to be known at the temperature being studied since the measurable quantity is G . Deoxygenated solutions containing l,l -dimethyl-4,4 -bipyridinium ion (methyl 

A convenient way of measuring g(H2) is by -radiolysis of degassed solutions of sodium nitrite because the nitrite ion scavenges all the primary radicals, Caq , H and OH, very efficiently [5]. Elliot et al. [3] obtained g(H2) by extrapolation of a plot of G(H2) vs. the cube root of the nitrite ion concentration to zero concentration to allow for scavenging of the precursors of H2 in the spurs (see Table 1) [6]. 

The yield of the hydroxyl radical can readily be obtained by pulse radiolysis of aerated hexacyanateferrate(II) solution by measiuing the yield of the product hexacyanateferrate(in), formed in reaction (3), whose molar absorption coefficient is well known (fi 2o = 104 moT [3])  

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Table LI Reaction scheme for the radiolysis of water. For reaction between like species, the value of k and not 2k is given [53]...

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