Page 193 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 177

of radical products decrease while the molecular G-values increase, which evidences the
formation of tracks. The higher G-values for water consumption and for molecular product
formation in irradiated water vapor, as compared to liquid water, show that spur formation
and spur reactions are of minor importance in the vapor phase. This is to be expected in
a less dense medium where the average distance between separate spurs will be larger. Also
the diffusivities will be larger in the vapor phase.
In the absence of solutes, reactions between radical and molecular species occur in the
bulk water. In pure water irradiated with ~ or X-rays, water is reformed via the reactions

H 2 + -OH--, H. + H20

H. + H202 --* H20 + .OH

9 OH + H.~H20

and no net decomposition of water is observed. The yields depend on the LET-value of the
radiation, as illustrated in Figure 7.6 for acidic and neutral water. Irradiation with higher
LET radiation, which have higher G-values for molecular than for radical products, causes
net decomposition of water.
In nuclear reactors water used as a coolant or moderator should be as pure as possible to
minimize dissociation during the time in the reactor. The formation of an explosive gas
mixture of H 2 and 0 2 must be carefully avoided in all reactors in order to prevent
accidents. Moreover the decomposition products of water can increase the corrosion of fuel
elements, structural material, etc. Many reactors use N 2 as a protective gas. In this case
the radiolysis can lead to the formation of HNO 3 unless suppressed by an excess of H 2
which preferentially yields NH 3. The pH of the water may be regulated by the H2(g )
pressure.

7.7. Aqueous solutions

In irradiated dilute aqueous solutions at concentrations < 0.1 mol/1, practically all the
energy absorbed is deposited in the water molecules. Hence, the observed chemical changes
are the result of the reactions between the solutes and the products of the water radiolysis.
With increasing solute concentrations, the direct radiolysis of the solute gradually becomes
important and the solute may also interfere with the spur reactions. The use of high
concentrations of solutes which react selectively with the radical products so-called
scavengers -- have provided experimental support for the existence of spurs.
In dilute solutions the chemical changes of a specific solute primarily reflect its reactivity
towards eaq-, H" and -OH. The hydrated electron eaq" is a strongly reducing species
(E 0 = -2.9 V) whereas the hydrogen atom is a less powerful reductant (E 0 = -2.3 V).
The H atom can be considered as a weak acid with a pKa of 9.6

e- aq + H+ ~--H 9